A1: A Distributed In-Memory Graph Database

04/12/2020 ∙ by Chiranjeeb Buragohain, et al. ∙ 0

A1 is an in-memory distributed database used by the Bing search engine to support complex queries over structured data. The key enablers for A1 are availability of cheap DRAM and high speed RDMA (Remote Direct Memory Access) networking in commodity hardware. A1 uses FaRM as its underlying storage layer and builds the graph abstraction and query engine on top. The combination of in-memory storage and RDMA access requires rethinking how data is allocated, organized and queried in a large distributed system. A single A1 cluster can store tens of billions of vertices and edges and support a throughput of 350+ million of vertex reads per second with end to end query latency in single digit milliseconds. In this paper we describe the A1 data model, RDMA optimized data structures and query execution.

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1. Introduction

The Bing search engine handles massive amounts of unstructured and structured data. To efficiently query the structured data, we need a platform that handles the scale of data as well as the strict performance and latency requirements. A common pattern for solving low-latency query serving is to use a two-tier approach with a durable database for the ground truth and a caching layer like memcached in front for read-only serving. The Facebook TAO datastore(Bronson et al., 2013) is a sophisticated example of this architecture using the graph data model. But there are some elements of the design that introduce problems. First, systems like memcached expose a primitive key-value API with little query capability. Therefore complex query execution logic is pushed into the client, rather than the database itself. Second, cache consistency is hard to achieve and such systems guarantee only eventual consistency. Finally, there is no atomicity for updates which leads to data constraint violations. For example, in TAO one can have partial edges between two objects with the forward link existing, but no backward link. In Bing, we have a huge set of diverse data sources that need be stitched together with real-time update requirements. Therefore we wanted to move beyond an eventually consistent cache and into a more capable transactional database system.

In representing structured data, the relational and the graph data models have equivalent capabilities, though with different ease of expression(Robinson et al., 2013)

. Our choice of the graph data model is a natural match for much of Bing data including core assets like the knowledge graph

(Google, ). Therefore we designed A1 to be a general purpose graph database with full transactional capabilities. Transactions in a distributed system frees up application developers from worrying about complex problems like atomicity, consistency and concurrency control, and instead allows them to focus on core business problems(Corbett et al., 2012). A1 also exposes a query language which simplify application development by moving query execution into the database. Our query language doesn’t attempt to be as comprehensive as SQL and instead focus on the core capabilities needed by the applications using A1. The primitives we support are general enough that multiple classes of applications can start using A1 with little difficulty. Another key characteristic of A1 is that it is a latency-optimized database. Since search engines like Bing have a fixed latency budget to render pages, all queries issued to the backend by the search engine come with a corresponding latency budget (typically 100ms). If a query takes more than 100ms to execute, then the results of that query will simply be discarded. That means that the availability of the system is measured by its latency, not by its error rate —if a system’s 80th percentile latency is 100ms, the system’s effective availability is only 80%. Therefore having tight control over tail latency is a key requirement.

Cheap DRAM and fast networks with Remote Direct Memory Access (RDMA) are the two major hardware trends that enable A1. We are deploying machines with hundreds of gigabytes of DRAM, so a set of racks can hold more than 200 TB of memory. This is sufficient for most applications to keep their data in memory and to avoid accesses to secondary storage. Until recently, RDMA has been mostly in the province of exotic high performance computing networks, but now it has become a commodity technology easily deployed in cloud data centers. The RoCE (RDMA over Converged Ethernet) networks we use offer a round-trip latency less than 5 microseconds, bandwidths of 40Gb/s and message rates approaching 100 million messages per second. Note that running RDMA in data centers is still not an easy endeavor and we will have more details on this later. The combination of in-memory storage and RDMA allows A1 to achieve single digit millisecond latencies for queries that access thousands of objects across multiple machines.

This paper makes three contributions. First, we describe the design and implement of A1 on top of the FaRM distributed memory storage system (Sections 2, 3). Next in section 4, we show how A1 is integrated into a more complex system in Bing with replication and disaster recovery. Finally, we evaluate the applications built on top of A1 and their performance (Section 5 and 6). A key part of our journey in building A1 has been the evolution of a research prototype like FaRM into a production system. The learnings on this path will be described throughout the paper.

2. System Architecture

A1 has a typical layered architecture with networking at the bottom and query processing at top, as depicted in Figure 1.

Figure 1. Layers of the A1 architecture

The four lowest layers of the stack together form a distributed storage platform called FaRM (Dragojević et al., 2014, 2015; Shamis et al., 2019). FaRM provides transactional storage and generic indexing structures, while the rest of A1 provides graph data structures and a specialized graph query engine. The bottom RDMA communication layer provides primitives like one-sided RDMA read/write and a fast RPC implementation. The distributed memory layer exposes a disaggregated memory model where the API enables one to allocate/read/write objects across a cluster of machines. These objects are replicated so that single machine or rack failure never leads to any data loss. Given a handle to an object, a single one-sided RDMA read is sufficient to retrieve the object. The transaction engine provides atomicity, failure recovery, and concurrency control. FaRM also exposes basic data structures like B-trees. We use these layers to build a database that exposes the graph data model. The query processing works directly on the graph storage, but it also leverages aspects of the distributed memory platform and communication to scale out and coordinate execution of queries.

Before we get into the details of the graph storage, it is worthwhile to understand the lower FaRM layer in a bit of more detail. We refer the reader to the existing literature (Dragojević et al., 2014, 2015; Shamis et al., 2019) on the implementation of FaRM and instead focus here on building applications on top of FaRM.

2.1. A Brief Tour of FaRM

FaRM is a transactional distributed in-memory storage system. It is worthwhile unpacking these adjectives. FaRM uses a set of machines in a datacenter and exposes there combined memory as a single flat storage space. The storage API exposed by FaRM is very simple: every storage object in FaRM is an unstructured chunk of contiguous memory. Objects are uniquely identified by a 64bit address or pointer and can range from size 64 byte to 1MB. All object manipulations happen in the context of a transaction. For durability FaRM replicates all data 3-ways.

FaRM uses RDMA capable NICs (Network Interface Cards), which are becoming commodity in modern data centers for cross-machine communication. RDMA enables the ability to read/write the contents of a remote machine’s memory with low latency ( within a rack) and high throughput. RDMA achieves this low latency in three ways: first in the local machine, the RDMA library bypasses the OS kernel and talks directly to the NIC. Second, in the remote machine, the memory is accessed by the remote NIC directly without involving the CPU. This is known as a one-way read/write. Finally, TCP features like reliability and congestion control are all implemented within the NIC and the network switches which reduce the load on CPU further. Of course, taking an ordinary storage system and simply porting its network layer to RDMA doesn’t always result in high performance. FaRM optimizes its whole stack including replication, transaction protocol and data structures to leverage RDMA at every layer to provide a high performance storage system.

A FaRM cluster is a set of machines each running a FaRM process. One machine is designated as a Configuration Manager(CM) whose purpose is to keep track of machine membership in the cluster and data placement. The memory of each machine is split into 2GB chunks known as regions. Objects are allocated within a region and every region is replicated 3-ways in-memory for fault tolerance. Replication is done using a primary-backup mechanism and all reads/writes are served from the primary only which ensures consistency of all operations. The the 64-bit address of an object essentially consists of two 32 bit numbers: the region id which uniquely identifies the region and the offset within the region where that object is located. The CM is responsible for determining which machines are part of the cluster (i.e. membership) and region metadata: allocation of regions to machines. Given a FaRM address, the CM metadata can be used to find the machine which hosts the primary copy of the region and then we can use RDMA to directly read the contents of the object by using the offset. All reads and writes happen in the context of a transaction. The transaction protocol is a variant of two phase commit with multiple optimizations for RDMA. For example, reads are always done using one-sided RDMA reads which bypass the CPU. Similarly data replication happens using one-sided writes, again bypassing the CPU. In production deployments, we deploy FaRM machines across at least three fault domains. A single fault domain consists of a set of machines which share a common critical component like a network switch or power supply. Therefore all machines in a single fault domain may become inaccessible in case of a hardware failure. By replicating data across three fault domains, we ensure that no single component failure can lead to loss of more than one copy of the data.

std::unique_ptr<Transaction> CreateTransaction();
ObjBuf* Transaction::Alloc(size_t size, Hint hint);
Addr ObjBuf::GetAddr();
ObjBuf* Transaction::Read(Addr addr, size_t size);
ObjBuf* Transaction::OpenForWrite(ObjBuf *buf);
void Transaction::Free(ObjBuf *buf);
Status Transaction::Commit();
Status Transaction::Abort();
Figure 2. FaRM API

The FaRM API(Figure 2) exposes a set of basic operations on objects: allocation, reading, writing and freeing them. The ObjBuf object referred to in the API is the wrapper around the FaRM object. Reads return an ObjBuf object which holds the data for the object read. The operations must be executed in the context of a transaction which provides programmers with atomicity and concurrency control. FaRM transactions provide strict serializability as the default isolation level using multi-version concurrency control(Shamis et al., 2019). Every transaction has a timestamp associated with it and this timestamp ensures a global order among all the transactions in the system.

Note that the Alloc API takes a Hint parameter. The hint is used to determine where to allocate the object: by default we allocate the object in the local machine where the API is invoked. The more useful option is to pass the address of an existing object in the hint —in that case we attempt to allocate the object in the same region in which the existing object exists. Since the region is our unit of replication, if two objects are allocated in the same region, they are guaranteed to be on the same machine in spite of machine failures. The hint is advisory only: in case the region doesn’t have enough space, the allocator will find another place to allocate it.

Here is an example of atomically incrementing a 64 bit counter which is stored in FaRM:

Status status = COMMITTED;
do {
  std::unique_ptr<Transaction> tx = CreateTransaction();
  ObjBuf *rbuf = tx->Read(address, sizeof(uint64_t));
  uint64_t value = *(uint64_t*) buf->data();
  value++;
  Objbuf *wbuf = tx->OpenforWrite(rbuf);
  memcpy(wbuf->data(), &value, sizeof(value));
  status = tx->Commit();
} while (status != COMMITTED);
Figure 3. Atomic increment of a counter using FaRM API

In this example(Figure 3), we read a FaRM object identified by address and extract the value stored in it. The ObjBuf object is a local immutable copy of the object. To modify it, we need to create a writable copy which we do with the OpenForWrite API. Once all the objects have been modified, we commit the transaction which atomically makes the update. The reason we have the loop here is that FaRM transactions run under optimistic concurrency control and hence may abort under conflict and it is necessary to retry them. Note that in this model all transaction writes are buffered locally. The OpenForWrite operation doesn’t cause any remote operations, it merely creates a modified buffer and stores the updates locally. The Commit operations pushes writes to remote machines, performs concurrency checks and finally commits the data.

2.2. Design Principles

Applications like A1 are integrated with FaRM using what we call the coprocessor model. In this model, A1 is compiled into the same executable as the FaRM code and is part of the same address space. So calling the FaRM API (Figure 2) is as simple as making a regular function call. As part of being a coprocessor, the application needs to integrate with FaRM’s threading model, which we will talk about later. The availability of transactions in the FaRM layer proved to be a great engineering productivity boost in building A1. When writing A1, the following principles guided our development:

  • Pointer linked data structures: The standard way to build data structures in FaRM is to use FaRM objects connected by pointers, e.g. linked lists, BTrees, graphs etc. Since dereferencing a pointer generally require an RDMA read (unless the object is hosted on the local machine), we optimize the layout and placement of data structures to reduce the number of pointer dereferences. For example, we prefer arrays to store list-oriented data instead of traversing linked lists. BTrees with high branching ratio works well for search structures, and we use the tuple address,size as the pointer which indicates both the address and size of the RDMA read to access the data stored in the object.

  • In-Memory Storage : Since the cost of memory is high compared to SSD, we need to be frugal with storage. Typically A1 is used as a fast queryable store with non-queryable attributes stored in cheaper storage systems. For example, if we are storing the profile of an actor in A1, the photo of the actor will not be stored in A1 itself.

  • Locality: RDMA reduces latency, but there is still a 20x-100x difference between accessing local memory vs. remote memory. Therefore, at object creation time, we attempt to co-locate data that is likely to be accessed together in the same machine. Similarly, at query time, we ship query to data to reduce the number of remote reads. When we reallocate any object, we keep its locality intact by passing the old object’s address into the Alloc call.

  • Concurrency: Since FaRM transactions run under optimistic concurrency control, it is critical to avoid single points of contention. For read-only queries, we use snapshot isolation to ensure that updates to data does not delay or block read-only operations. When we run a distributed query, all objects across the cluster are read as of a single consistent snapshot version and those versions are not garbage collected until the query runs to completion.

  • Cooperative Multithreading: Recall that we compile the application with FaRM itself into a single binary. Inside the FaRM process, coprocessors must run using cooperative multithreading to share compute resources. At startup, we allocate a fixed number of threads and affinitizes them to the cores. FaRM code and the application code (i.e. the coprocessor) share these threads. Coprocessors use a fixed number of fibers per thread to achieve cooperative multithreading. All FaRM API calls which touch remote objects are asynchronous, but the use of fibers hide the asynchrony and gives the application writer the illusion of writing synchronous code.

Figure 4. A1 cluster deployment

Figure 4 shows the full physical deployment of a single A1 cluster. Clients access the A1 cluster by making RPC calls to the A1 API. The RPC calls are routed by a software load balancer (SLB) to a set of frontend machines. The frontend machines are stateless and mostly perform simple routing and throttling functions. The RPC request is forwarded by the frontends to the backend machines where it is processed. The backend machines makes up the FARM cluster, and each machine runs a combined binary of FaRM and A1. All query execution and data processing happens on the backend machines, utilizing RDMA communication. Communication between client and the cluster uses the traditional TCP stack which has higher latency. However, our target workloads are complex queries with many reads and writes, so the latency between the client and the backend is typically immaterial to the total execution time.

3. Data Structures and Query Engine

Using a graph to model data is nothing new –entity-relationship databases have been in use for a while and have enjoyed renewed popularity recently. A1 adopts the property graph model: a graph consists of a set of vertices and directed edges connecting the vertices. The vertices and edges are typed and can have attributes (also known as properties) associated with them. The type for the vertex/edge defines the schema of the associated attributes. In contrast to typical property graph models such as Tinkerpop(Foundation, ) or Neo4J(Inc, ), we choose to enforce schema on attributes to improve data integrity and performance. An example will clarify the model. Consider the relationship between a film and an actor as shown in Figure 5.

Figure 5. Simple graph example

We introduce two types of vertices: Film and Actor. The Actor attributes are name, origin and DOB; while the film attributes are name, release date and genre. The attributes in the schema are comparable to column definitions in traditional relational databases. Microsoft Bond(Microsoft, ) is a language for managing schematized data, similar to Protocol Buffers(Inc, ). In Bond, our schema looks like as follows:

struct Actor {                    struct Film {
    0: string name;                 0: string name;
    1: string origin;               1: string genre;
    2: date birth_date;}            2: date release_date;}

Next we introduce the edge type Acted. that stores data like the name of the character played by the Actor. So the edge data schema will look like

struct Acted {
    0: string character;}

By using Bond, A1 inherits the Bond type system with primitive types like integers, floats, string, boolean and binary blobs. Since Bond allows composite types (arrays and maps) and nesting of structs, A1 can support a richer type system than typical relational database.

A1 organizes customer data in a hierarchy: the top level of the hierarchy is a tenant and it is the default isolation container. Two tenants can’t see each others data. A tenant may have one or more graphs and every graph contain a set of types. A graph contains a set of vertices and edges and every vertex/edge must belong to one of the types defined within that graph. The analogy between relational data model and the A1 model is presented in table 1.

A1 Graph Type Vertex/Edge Attribute
Relational Database Table Row Column
Table 1. Analogy between relational database entities and A1 entities.

When declaring a vertex type, the user must also define one of the attributes as a primary key, which must be unique and non-null. Every type by default comes with a sorted primary index defined over the primary key. Edge types do not require primary keys and there are no indexes on edges. It is also possible to declare secondary indexes on vertex attributes. There are no requirements on uniqueness or nullability on secondary index attributes.

Within a graph, to uniquely identify a vertex, we need to specify the tuple . Using the type, we can identify the relevant primary index and then retrieve the vertex by using the primary key in the index. Edges can’t be identified directly except through the vertices to whom they are attached. An edge is uniquely identified by the tuple . This implies that given two vertexes, there can only be a single edge of a given type.

In terms of APIs we support the usual CRUD APIs on objects like vertices, edges, types and graphs. We divide the APIs into two classes: control plane APIs which manipulate bulk objects like graphs and types and data plane APIs which manipulate fine grained objects like vertices and edges. In addition, we expose a set of transaction APIs: CreateTransaction, CommitTransaction and AbortTransaction. The CreateTransaction API creates a transaction object which can be used to group multiple data plane operations into a single atomic transaction. If a transaction is not specified for a data plane operation like CreateVertex, a transaction is implicitly created for that operation and committed at the end of the call. Unlike data plane operations, control plane operations cannot be grouped under a transaction. Each control plane operation executes under its own transaction.

3.1. Catalog

A1 roots all data structure in the catalog. It is a system data structure which returns handles to objects like tenants, graphs, types, indexes, BTrees etc. The catalog is fundamentally a key-value store where the key is the name of the object and the value is a pointer to all the data needed to access the object. For example in the case of a BTree, the catalog maps the name of the BTree to the FaRM address of the root node of the BTree. Once we have the root node of the BTree, we create an in-memory object called a BTree proxy which allows us to lookup/manipulate the BTree contents. The catalog itself is stored in FaRM and hence materializing a proxy from the BTree name can be an expensive operation —it involves multiple remote reads to map the name to the root node and then potentially reading the root node itself for any BTree metadata. To reduce load on the catalog and as well as avoid remote reads in materializing proxies, we cache proxies in memory once they are materialized. Once cached, data plane operations like CreateVertex can use them without incurring the overhead of looking up the catalog separately. The cache has a fixed TTL to ensure that we don’t use stale proxies. When the TTL expires, the cache checks if the underlying object has changed: if it has then we refresh the proxy, if it hasn’t then we simple extend the TTL and continue to use the proxy.

Figure 6. Vertex and primary index

3.2. Vertices and Edges

The storage format for vertices and edges are dictated by how they are accessed. For a vertex, we can look it up using either an index or through edge traversal. Since traversal queries are much more frequent than index lookups, we optimize for that. A vertex is stored as two FaRM objects: a header object and a data object as shown in Figure 6. The vertex header contains the type of the vertex, pointers to data structures that hold edges associated with the vertex, and a pointer to the data associated with the vertex. As the vertex is updated with new edges or new data, the header content changes, but the pointer to the header itself remains unchanged. We call this pointer the vertex pointer. The vertex data is stored in a separate variable length object and serialized in Bond binary format. Since the data for a vertex is always schematized, the data representation is very compact and efficient to deserialize. Since vertex data and header are looked up together most of the time, we use locality to store both of them in the same region.

Looking up a vertex from its primary key is a multi-step process. First, we look up the vertex pointer (address of the vertex header) from the index which is a BTree. We cache internal BTree nodes heavily(Dragojević et al., 2014) and in most cases this lookup requires one RDMA read rather than . Once the vertex pointer is found, we need two consecutive RDMA reads to read the header and then the actual data. If the vertex is being read during a traversal, then we can bypass the index lookup and we need only two consecutive reads.

Figure 7. Vertex, edge lists and half-edge

Unlike vertexes which can be uniquely identified by the vertex pointer, edges are not stored in a single unique FaRM object. This is again dictated by how edges are used inside A1 queries. Given an edge from vertex to vertex , if we delete we’d like to ensure that the edge pointing from does not remain dangling. To achieve this, we store the edge as a 3 part object as shown in Figure 7. First we associate two edge lists with each vertex: an incoming edge list and an outgoing edge list. The edge appears twice: once as an entry in the outgoing edge list of and once in the incoming edge list for . We call the entry that appears on the edge list a half-edge. The outgoing half-edge for consists of the tuple edge type, pointer, data pointer while the incoming half-edge for consists of the tuple edge type, pointer, data pointer. The data pointer field points to a FaRM object that holds the data associated with the edge. In this example, if we delete , then by inspecting its incoming edge list, we know that there is an edge pointing to it from and we can go to and delete the entry in as well. The edge list data structure needs to satisfy a few constraints —first, since a single vertex can hold millions of edges associated with it, the data structure needs to be scalable. Next, given an edge characterized by the source vertex, destination vertex and edge type, we should be able to lookup/insert/delete the edge quickly.

To satisfy these requirements, we actually use two different implementations of the edge list. For small number of half-edges, all half-edges are stored as an unordered list in a single FaRM object of variable length. As the number of edges increase, we resize the FaRM object in a geometric progression until we reach around 1000 edges. For vertexes with more than 1000 edges, we store the edges in a global BTree where the key is the tuple src vertex pointer, edge type, dest vertex pointer and the value is the edge data pointer. As long as the half-edges are stored in a single FaRM object, we use locality to locate that FaRM object with the associated vertex. Empirically we have found that for our current use cases, 99.9% of the vertexes contain fewer than 1000 edges. Given this edge layout, once a vertex is read, enumerating its edges requires just one extra read as long as the number of edges is small. Due to locality, this read is often simply a local memory access.

Although we take pains to allocate vertices and edge lists together using locality, we do not attempt to enforce locality between different vertices. In case of immutable or slowly changing graphs, it is possible to run offline jobs to pre-partition a graph so that vertices connected together end up close to each other. But we have avoided going down this route since it imposes considerable burden on our customers to do the offline graph partitioning. Also as updates happen, the original partitioning may no longer make sense. Instead we believe it is the responsibility of the database to simplify the application developer’s experience and provide acceptable performance. We currently place vertices randomly across the whole cluster and use locality to push query execution to where data resides. Looking at sample query executions, we have found that this strategy can be highly effective (95% local reads) and we will discuss more in section 6.

3.3. Asynchronous Workflows

Recall that APIs like DeleteGraph or DeleteType are asynchronous. For example, calling DeleteGraph transitions the graph from state Active to Deleting, but the storage and resources associated with the graph is not freed synchronously. Instead an asynchronous workflow is kicked off which deletes all the resources associated with the graph and finally frees the graph itself. Before a graph can be deleted, all types associated with the graph is deleted. For a type to be deleted, we delete all the indices associated with the type: both primary and secondary. When the primary index is deleted, we delete the vertices at the same time.

The asynchronous workflows run within the A1 process using what we call a Task execution framework. Tasks are units of work that can be scheduled to execute in future: tasks are enqueued on a global queue that is stored in FaRM. We have a pool of worker threads on every backend machine that look for pending tasks and work on them. Since tasks are globally visible, any single task may be worked on any backend machine in the cluster. The worker threads are stateless and they save their execution state in FaRM itself. Once a task is scheduled, it is picked up by a worker thread. If the thread can finish the task immediately, the task is completed and deleted. Alternatively, if the task is bigger, the worker may reschedule the task to run in future or spawn more tasks to parallelize the execution. This is the pattern we follow in the DeleteGraph workflow: the DeleteGraph API call simply creates a task. When this task is executed by a worker, it spawns more tasks to delete all the types in the graph and waits for all those tasks to complete. The DeleteType tasks in turn execute for a long time since each type needs to delete all the vertices, edges and indexes associated with the type. Using this framework, we are able to harness the entire cluster’s resources to execute long running workflows. To ensure that the workflows do not interfere with real-time workload, the worker threads run at a low priority.

3.4. Query Execution

A1 workloads are dominated by large read-only queries which access thousands of vertices, and small updates that read and write a handful of vertices. When a query/update operation arrives at the frontend, it is by default routed to a random backend machine in the cluster. There are cases where more complex routing is required and we will discuss it later in the section. When an update operation arrives at a backend machine, that machine becomes the transaction coordinator for the operation. All read and writes are executed on that machine using RDMA to access remote data. Writes are made durable during transaction commit using one-sided RDMA writes. Queries are executed a little differently: the backend machine where the query arrived first, is designated as the coordinator for that query, which drives the execution of the query, but the bulk of the query execution work is distributed across the cluster. We designate the machines where query execution happens at the instigation of the coordinator as workers.

To understand the A1 query language and its execution, let’s take as an example, a knowledge graph of films, actors and directors. If an actor appears in a film, then the film is connected to the actor with an outgoing edge of type film.actor. Similarly the director and the film is connected with an edge of type director.film. The A1 query language known as A1QL is similar to MQL: the Metaweb Query Language (Freebase, ). Let’s consider the two-hop query that asks for all actors that worked with Steven Spielberg. in A1QL, the query is written as shown in Fig.8.

{ "id" : "steven.spielberg",
  "_out_edge" : { "_type" : "film.director",
      "_vertex" : {
          "_out_edge" : { "_type" : "film.actor",
              "_vertex" : {
                  "_select" : ["*"]
              }}}}}
Figure 8. A1 query to retrieve all actors that have worked with Steven Spielberg

Every A1 query is a JSON document with each level of nested JSON struct describing a step in the traversal with the starting point at the top level document. In this query, the top level struct specifies the starting vertex as the vertex with primary key steven.spielberg and we use the id field to look up the director from the primary index. The next level specifies that we should traverse an outgoing edge (_out_edge) of type film.director to a film. The next level describes that we should traverse out on an edge of type film.actor to from the film to arrive at the actor vertex. At the last level, the select(*) clause indicates that we should return all values.

Figure 9. Physical A1 query execution. The query is to retrieve all actors that have worked with Steven Spielberg (Fig.8.

Now let us see how the example query from Figure 8 is executed. The query coordinator parses the query to derive a logical plan and then generates a physical plan. A1 doesn’t have a true query optimizer: most of the queries submitted to A1 are straightforward and executed without any optimization. In A1QL the user can supply some optional optimization hints. If they are supplied, then they are used in creating the physical execution plan from the logical plan. Building a true optimizer is currently work in progress. Queries are built on top of a few basic operators like index scan, predicate evaluation against a vertex/edge data and edge enumeration for a given vertex. The step by step query execution is shown in Figure 9. In this execution, the coordinator starts by instantiating a transaction and choosing the transaction timestamp as the version which will be used for all snapshot reads. Next it does an index lookup to locate the Steven Spielberg vertex and then from the vertex, enumerates all neighboring half-edges of type film.director.

The edge enumeration gives the coordinator a list of vertex pointers for all the films (see Figure 7). In the next step, we need to look up all the actor edges from those film vertexes. Since the edge list is co-located with the vertex, it is more efficient to execute the task of edge enumeration at the actual location of the vertexes. Therefore the coordinator maps the vertex pointers to the physical hosts which are the primary storage hosts for the corresponding vertex. Mapping pointers to physical hosts is a local metadata operation with no remote accesses. Operators like predicate evaluation and edge enumeration are shipped to the machine hosting the vertex via RPC so that it can be evaluated without invoking a remote read. When we have multiple vertex operators to be processed at the same machine, we batch the operators together per machine to reduce the number of RPCs. Although query shipping is the norm, if the number of vertexes operators to be shipped are too small we avoid the RPC overhead by evaluating the operators locally using RDMA reads.

Each worker receives RPCs from the coordinator and instantiates a new read-only transaction at the timestamp chosen by the query coordinator. This ensures that all the query reads form a consistent global snapshot across the entire distributed graph. The typical operator that executes in a worker are predicate evaluation which applies predicates against vertex data and edge enumeration for the vertex. Note that both of these operations do not require any remote reads assuming locality applies. During edge enumeration, any edge predicate is also applied. Once edge enumeration is finished, the results are a set of vertex pointers for the next hop of the traversal, i.e. a set of actor vertices. These vertices are shipped back to the coordinator where they are aggregated, duplicates removed and repartitioned by pointer address to run the next phase of the traversal. Once the whole query completes, the results are aggregated by the coordinator and returned to the client. Since we keep the entire state of the query in the memory of the coordinator, we are vulnerable to queries which require a working set bigger than the coordinator’s available memory. Implementing disk spill for such a case is infeasible since our goal is to be a low-latency system. Currently we simply fast-fail queries whose working set grows too large —in future we plan to dedicate regions in the cluster for spilling intermediate query results. Fast-fail is an acceptable option since very large queries typically will not be finished within its time budget anyway.

If the final result set is too large to return in a single RPC, the coordinator does not return the full result set and instead return partial results and a continuation token. Rest of the results are cached in the coordinator and can be retrieved by the client by supplying the continuation token in the next request. The continuation token encodes the coordinator host’s identity in it. When a request for result retrieval using a continuation token is received by a frontend, the frontend decodes the coordinator’s identity and forwards it to the correct machine so that rest of the results can be returned. The coordinator caches the results only for a limited time (typically 60 seconds) to conserve resources —the client is expected to retrieve all the results in that time. If the cache times out or the coordinator crashes before all the results are returned, the client is expected to restart the query. Since our typical query execution lifetime is measured in less than a second, this is not a big concern.

4. Disaster Recovery

Although FaRM replicates data in memory 3-ways, there are situations where data can be lost such as power loss to an entire datacenter or coordinated failure of 3 replicas. Therefore any system built on FaRM needs to have a disaster recovery plan. A1 implements disaster recovery by replicating all data asynchronously to a durable key-value store known as ObjectStore which is used by Bing. We will not go into details of ObjectStore except to say that it supports the abstraction of tables with each table containing a large number of key-value pairs. Both keys and values are schematized using Bond. Writes to ObjectStore made durable by replicating every write 3-ways into durable store.

Since the replication from A1 to ObjectStore is asynchronous, in the event of a disaster, ObjectStore may not contain all the writes committed to A1. To deal with this data loss, our recovery scheme supports two types of recovery: consistent recovery and best-effort recovery. In consistent recovery we recover the database to the most up to date transactionally consistent snapshot that exists in ObjectStore. With best-effort recovery, we do not guarantee that the recovered state of A1 will be transactionally consistent, but the database itself will be internally consistent. Let’s take an example to illustrate this. Suppose we have a single transaction in A1 that adds two vertexes, A and B and an edge from A to B. We take a few example scenarios:

  • We succeed in replicating A and B, but the edge is is not replicated. In that case after consistent recovery, A1 will not contain any of A or B or the edge. On the other hand, best effort recovery will recover both A and B, but there will be no edge between them.

  • We succeed in replicating A and the edge, but not B. Again, consistent recovery will treat this as a partial transaction and will ignore the edge and A. Best effort recovery will recover A and notice that the other end of the edge B is missing and will not recover the edge. Therefore the database will be internally consistent —no dangling edges, but not transactionally consistent.

Best-effort recovery therefore always recovers the database to a state which is at least as up to date as consistent recovery and in almost all practical cases, to a more up to date state.

For every graph, we create two tables in ObjectStore to durably store the data: the vertex table stores all the vertexes regardless of the vertex type, while the edge table stores all the edges. When an update request arrives at A1, we apply the update to A1 and also insert a log entry for the update to a replication log transactionally. The replication log is itself stored in FaRM with the usual 3 copy in-memory replication guarantee. As soon as the update transaction commits, we attempt to replicate the update in replication log to ObjectStore synchronously with the customer request. If the replication effort succeeds, then we delete the log entry and acknowledge success to the client. If the replication effort fails, we have an asynchronous replication sweeper process that scans the replication log in FIFO order and flushes the unreplicated entries to ObjectStore and if successful, delete the entry. We closely monitor the age of entries in the replication log to make sure we do not have too many entries in there —in ideal case, the replication log should be empty except for ongoing update transaction entries. In case of a disaster, the entries in the replication log which were not replicated to ObjectStore synchronously are the ones which will be permanently lost.

When we replicate entries from replication log to ObjectStore, we need to make sure that entries are applied in the same order as the transaction order in A1, i.e. if we stored value in vertex and then store value in V, then regardless of delays or failures in the replication pipeline, eventually when all updates are flushed from the replication log, ObjectStore must reflect value as the final value. This is achieved differently for consistent recovery and best-effort recovery. Recall that in FaRM, every write transaction is assigned a global commit timestamp which imposes a global order among all transactions that occurred in the system. In best effort recovery, every row in the vertex or edge table has a timestamp field which corresponds to the timestamp of the FaRM transaction responsible for that update. When a new update comes in, we compare the timestamp of the existing row in ObjectStore with the update’s timestamp. If the update’s timestamp is newer, then the update is a later transaction and we store the update into the row. On the other hand, if the update is older than the existing content of the ObjectStore table row, then this update is a stale update and we can discard it. For create operations, we unconditionally create the new row, while for delete operations we create a tombstone row with the delete timestamp. The tombstone entry is removed either when the row is recreated with a newer timestamp or by an offline garbage collection process which removes all tombstones older than a week. For the sake of efficiency, we do not explicitly do a read-modify-write to implement this protocol: ObjectStore exposes a native API that accepts a timestamp version and achieves this is a single roundtrip. Note that this update process is idempotent: if a replication log entry is flushed multiple times, the outcome is not changed.

Consistent recovery works a little differently: in this case we treat ObjectStore as a versioned datastore. Instead of just storing keyvalue rows in the ObjectStore table, we augment the key by the transaction timestamp version to get the row (key,timestamp)value. Since ObjectStore supports iterating over keys in sorted order, given a key, it is easy to find all versions of that key or the latest version of the key. When an update comes in with a given timestamp, we always insert it into ObjectStore. For deletes, a tombstone entry is inserted. Again, this protocol is idempotent. To recover to a consistent snapshot from this durable versioned store, we need to find a timestamp value below which all updates in A1 are also reflected in ObjectStore. To do this, A1 continually monitors the timestamp of the oldest unreplicated entry in the replication log () and stores this value to ObjectStore durably. Clearly when is made durable in ObjectStore, all writes that have timestamp smaller than are also durable in ObjectStore. On recovery, we read the value of and recover using the snapshot corresponding to this timestamp.

5. A1 in Bing

In this section we will first look at Bing’s use of A1 and then focus on our experience bringing A1 into production use.

A1 is designed to be a general purpose graph database and there are multiple applications in Bing that runs on top of it. In this paper, we focus on a single use case: knowledge graph. We have already encountered a few knowledge graph serving example scenarios. The knowledge graph is generated once a day by a large scale map-reduce job. There are real-time updates to the knowledge graph as well. The original Bing knowledge graph stack was a custom-built system with immutable storage and regular key-value store. This prohibited real-time updates and could not handle more complex queries within the latency constraint. A1 addresses both of these shortcomings and increases the overall flexibility of the system.

In A1, the knowledge graph is designed with a semi-structured data model. All entities whether they are films, actors, books or cities are modeled as a single type of vertex named entity with all attributes stored as a key-value map. This is a choice necessitated by the fact that the number of different types of entities in a knowledge graph is vast (tens of thousands) and their attributes are constantly changing because we add more and more information to entities. On the other hand, we strongly type the edges since the edge types are typically fixed and there is little data associated with edges. In practice, we have found that weak typing of vertices do not lead to significant query slowdowns while enabling more flexible data modeling. Since A1 storage is expensive, only queryable attributes of an entity are stored in A1 while non-queryable attributes like image data are stored elsewhere.

Bing receives human generated queries like “Tom Hanks and Meg Ryan movies” which are translated to A1QL queries. The translation step is non-trivial since it requires us to map strings like “Tom Hanks”, “Thomas Hanks” or even just “Hanks” to the unique actor entity Tom Hanks that we all know and love. We will not go into the complexities of query cleaning and query generation here in this paper. The results of the queries are joined with data from other sources to render the final page view. For example in this query if we return the vertex corresponding to the film You’ve Got Mail, the rendering pipeline pulls together image data for the movie (e.g. movie poster) and generates the final page. Overall, A1 improves the average latency of the knowledge serving system by 3.6X and enables significantly more complex queries.

5.1. RDMA in the Data Center

RDMA originated in rack scale systems and is a difficult protocol to work with in the large data center networks. Since RoCEv2 doesn’t come with its own congestion control, we use DCQCN (Zhu et al., 2015) to enforce our own congestion control and fairness. We handle a lot of the protocol level instability by defensive programming around communication problems. FaRM is able to recover very quickly from any host/network level failures which ensure that users do not notice networking hiccups. In addition to RDMA Read and Write, we also make heavy use of RDMA unreliable datagrams (UD) for clock synchronization and leases. In general we have been able to achieve latencies less than 10 microseconds within a single rack and less that 20 microseconds across racks with oversubscribed network links.

5.2. Opacity and Multiversioning

In building A1, we enhanced FARM from the version described in (Dragojević et al., 2014, 2015) (denoted FARMv1) with several features into what we will call FARMv2.

The isolation guarantee provided by FaRMv1 transactions is serializability, but combining serializability with optimistic concurrency control can lead to certain well known problems. For example, consider two transactions: reading a linked list consisting of two items , and deleting from the list concurrently. Suppose the execution interleaving of the transactions is the following:

  1. reads and gets the pointer to .

  2. deletes and commits.

  3. dereferences the pointer to which is now pointing to invalid memory. The application reads the invalid content of and panics.

Since executions of and are serializable, will abort once it attempts to commit, but even before that, the application will conclude erroneously at the last step above that the data it has read is corrupt. The solution to this problem is known as the opacity(Guerraoui and Kapalka, 2008) property which guarantees that even transactions that will eventually abort (e.g. ) are serializable with respect to committed transactions (e.g. ) and hence will no longer cause application inconsistencies at runtime.

Optimistic concurrency control can often lead to high abort rates for large transactions. A1 is an OLTP system which combines small update transactions (touching a handful vertices/edges at most) with much larger read-only queries which can read many thousands of vertexes in a single query. Since optimistic concurrency control does not acquire read locks, the large queries are susceptible to conflict with updates and hence abort frequently.

FaRMv2 solves both of these problems by introducing a global clock which provides read and write timestamps for all transactions. These timestamps provide a global serialization order for all transactions. In addition FaRMv2 implements MVCC (multi-version concurrency control) which ensures that read-only transactions can run conflict-free with update transactions. For details on the implementation, we refer the reader to FaRMv2 paper(Dragojević et al., 2015). The fact that all transactions can be ordered globally using their write timestamp is also used in our disaster recovery solution which we discussed in section 4.

5.3. Fast Restart

Data stored in FaRM can be made durable(Dragojević et al., 2015) using SSD for storage and using non-volatile RAM (NVRAM) for transaction log durability. But since A1 runs on commodity machines with no NVRAM, the durability problem needs to be solved differently. There are two different durability problems that we address. First, if we lose power to the entire data center, clearly all data in memory in A1 will be lost. We consider this as a disaster scenario and implement disaster recovery, which was described in section 4.

A software outage in 3 machines across 3 failure domains can occur during deployment or due to a bug. In the case that these 3 machines hosts the 3 replicas of a single region, a total loss of that region will occur. This implies losing parts of the graph or index, and should be considered catastrophic.

We protect against this by implementing a feature known as fast restart. In FaRM, the memory where the regions are allocated do not belong to the FaRM process itself: instead we use a kernel driver known as PyCo, which grabs large contiguous physical memory segments at boot time. When the FaRM process starts, it maps the memory segments from the driver to its own address space and allocates regions there. Therefore, if the FaRM process crashes unexpectedly, or restarts, the region data is still available in the driver’s address space and the restarted process can grab them again. Note that fast restart doesn’t protect against the machine crash or power cycle because in that case the machine will reboot and the state held in the driver’s memory will be lost. In FaRMv1, only the data regions were stored in PyCo memory. As part of fast restart, we moved all data needed to correctly recover after a process crash to PyCo memory —this includes region allocation metadata and transaction logs. Recall that the configuration manager (CM) is responsible for determining which regions are hosted in which machines. In case of any machine failure, if the CM detects that all replicas for any region has been lost, it pauses the whole system and all transactions are halted. In case of accidental A1/FaRM process crash, our deployment infrastructure automatically restarts the process. Therefore the CM waits to see if the failed process or processes will come back to life and if they come back it initiates recovery of that region’s data including all blocked transactions. Overall, fast restart has cut down the downtime for A1 cluster by an order of magnitude.

6. Performance Evaluation

To evaluate the performance of A1 experimentally, we use a graph consisting of 3.7 billion vertices and 6.2 billion edges, which is generated from the film and entertainment knowledge base containing 22.9 billion RDF triples with 3.7 billion entities. Graph vertices represent entities and have several attributes, and edges do not have data attributes. On average every vertex had a payload of 220 bytes. Although the average vertex degree is small, the skew in vertex degree distribution is very large and some vertices have degrees larger than ten million.

We use a cluster of 245 machines and measure end-to-end response time from a client in the same datacenter as the A1 cluster. Every machine has two Intel E5-2673 v3 2.4 GHz processors, 128GB RAM and Mellanox Connect-X Pro NIC with 40Gbps bandwidth. A1 uses 80GB of the available RAM for storage. The total storage space available in the machines is 245*80GB/3 = 6.5TB –the factor of 3 is for 3x replication. Our data occupies 3.2TB of the total available space. The machines are distributed across 15 racks and four T1 switches connect the racks. ToR (Top of the Rack) switches provides full bisection bandwidth between machines in a single rack, while T1 switches use oversubscribed links between racks. Therefore, most of the cross-machine traffic uses oversubscribed links. Vertices are distributed at random across the machines, and therefore 99.6% (=244/245) of a vertex neighbors are on a remote machine. We report the average and P99 (the 99th percentile) latency for a few multi-hop queries which represent various types of graph queries.

Id A1QL
Q1
{ "id" : "steven.spielberg",
  "_out_edge" : { "_type" : "director.film",
      "_vertex" : {
          "_out_edge" : { "_type" : "film.actor",
              "_vertex" : {
                  "_select" : ["_count(*)"] }}}}}
Q2
{ "id" : "character.batman",
      "_out_edge" : { "_type" : "character.film",
          "_vertex" : {
              "_out_edge" : { "_type" : "film.performance",
      "_vertex" : {
          "str_str_map[character]" : "Batman",
          "_out_edge" : { "_type" : "performance.actor",
               "_vertex" : {
                   "_select" : ["_count(*)"] }}}}}}}
Q3
{ "id" : "steven.spielberg",
  "_out_edge" : { "_type" : "director.film",
      "_vertex" : {  "_type" : "entity",
          "_select" : ["name[0]"],
          "_match" : [{
              "_out_edge" : { "_type" : "film.actor",
                  "_vertex" : {
                      "id" : "tom.hanks"
                  }}},
              { "_out_edge" : { "_type" : "film.genre",
                  "_vertex" : {
                      "id" : "action"
                  }}}] }}}}
Q4
{ "id" : "tom.hanks",
  "_out_edge" : { "_type" : "actor.film",
      "_vertex" : {
          "_out_edge" : { "_type" : "film.actor",
              "_vertex" : {
                  "_out_edge" : { "_type" : "actor.film",
                      "_vertex" : {
                          "_select" : ["_count(*)"] }}}}}}}
Table 2. Queries used to evaluate A1 performance

We focus on the following set of specific queries and see how the system performs. The actual representation of the queries in A1QL is in Table2.

  • Q1: Count actors who have worked with Steven Spielberg.

  • Q2: Count actors who have played Batman.

  • Q3: Action movies with Steven Spielberg and Tom Hanks.

  • Q4: Count number of films by actors who have worked with Tom Hanks.

Figure 10. Average and P99 latency for Q1.

The first query, Q1, asks for all actors that have worked with the director Steven Spielberg. This translates to a simple 2-hop query where we look up all films for Spielberg and then for those films find all actors that have acted in them. Figure 10 depicts the average and P99 latency in for Q1, which reads a total of 49 vertices in the first hop (films by Spielberg) and 1639 vertices in the second hop (actors in those films). The total number of edges visited were 1785: this number is larger than the number of vertices since multiple edges could point to the same end vertex. By parallelizing all these reads across the cluster, we were able to complete this query in less than 8ms on average and 14ms at p99 at 20000 queries/second. Note the tight spread between the average and P99 latencies which is a consequence of the focus on latency for A1.

The total number of raw FARM objects read during the query is 3443 out of which only 163 are remote. In other words, we achieve more than 95% local reads through query shipping to workers.

Figure 11. Total RDMA read latency (microseconds) for different number of read operations.

Figure 11 shows the distribution of RDMA latencies as a function of number of reads done. Recall that we ship the vertex predicate evaluation and edge enumeration to workers. So if a worker lands with a bunch of vertices which are remote than it has to do one or more RDMA reads to get all the data. Figure 11 shows the total time in microseconds doing RDMA reads vs the total number of reads done and the trend is roughly linear. Average read times for RDMA was 17us.

Q2, is deceptively simple, but more complex in its implementation: find all actors who have played Batman. This maps to the following traversal where we first look up the entity Batman and then all movies in which this entity appears. For each of the movies, we look up the performances of all actors and then filter those performances by name of the character (Batman) and then the actor for that performance. This translates to a three-hop query from character to film to performance to actor (Figure 12.

Figure 12. Average and P99 latency for Q2.

Query Q3 (Figure 13) represents a more complex pattern of graph exploration. The query is to find all Spielberg movies which belong to the War movie genre and stars Tom Hanks. Here the graph pattern we are interested in is a star pattern where the center is the movie and the movie is connected to three entities: Spielberg as director, War as genre and Tom Hanks as an actor. A similar query is to find all comedies starring both Ben Stiller and Owen Wilson.

Figure 13. Average and P99 latency for Q3.

To evaluate maximum throughput of the system, we carried out a test using Q4. For a given actor, Q4 finds all actors he/she has worked with and finds films starring them. This maps to a three-hop traversal query from actor to films to actors (co-stars) to their films. The goal of Q4 was to stress the system by exploring a large number of vertexes rather than being a realistic user query. On average, Q4 accesses 24,312 vertices with 33ms latency for throughput (1000 queries/second). We pushed the cluster to 15,000 queries/second and at this throughput this query executes 365MM vertex reads/second across the cluster, i.e. 1.49MM vertex reads per second for every machine in the cluster.

Figure 14. Latency vs throughput for different cluster sizes (10, 15, 35 and 55).

Finally, to understand the scalability characteristics of A1, we created clusters of 10, 15, 35, and 55 machines in the same network configuration as used for the larger cluster. We used a smaller dataset of 23 million vertices and 63 million edges. This dataset was distributed uniformly across the machines, and then ran a set of 2-hop queries. For each cluster, we measured the latency at different query loads, as shown in Figure 14. As expected, the usable throughput (below a given latency) correlates to the cluster size; Latency of queries below the capacity threshold is mostly flat as the cluster grows. The clusters have the same network topology, so this is as expected. For larger clusters, the expected benefit is not just scalability of throughput, but also capacity for bigger datasets.

7. Related Work

Graph databases are not new in the database world and in recent years, they have been experiencing great interest in industry: some notable efforts in the open source space include Neo4J

(Inc, ), Apache Tinkerpop(Foundation, ) and DataStax Enterprise Graph(Inc, ), while AWS Neptune(Amazon.com, ) and CosmosDb(Inc, ) are prominent cloud based offerings. All of these systems are disk based and apart from DataStax Enterprise Graph and CosmosDb, none of them are distributed. Traditional commercial databases like Oracle and SQLServer also now support the graph data model and associated query capabilities.

Graph data has been represented in various ways using RDF triples as well as property graph like model. RDF triples have been stored directly in relational stores (Chong et al., 2005) or stored in more efficient columnar formats (Abadi et al., 2007). Storing RDF data in relational stores allows one to take advantage of the existing depth of the relational technology. Since A1 was built ground up as a new system and the FaRM data model was highly conducive to building linked data structure, we opted to go with a property graph model rather than RDF or relational. Moreover, we have found that most of our customers prefer the property graph model in modeling their data.

Trinity(Shao et al., 2013; Shao and Wang, 2012) from Microsoft Research is the system closest to A1 in terms of its use of in-memory storage and horizontal scalability. But compared to A1, Trinity lacks transactions and not comparable in terms of performance. Facebook’s TAO(Bronson et al., 2013) and Unicorn(Curtiss et al., 2013) are two horizontally scalable systems which are deployed at large scale in production. TAO’s query model is much more restricted than A1 in that it’s not meant for large multi-hop queries and it doesn’t offer any consistency or atomicity guarantees. Unicorn is built more as a search engine with very limited OLTP capability, but highly efficient exploration queries like A1. Since TAO and Unicorn are disk based, their storage capacity is much larger than A1’s.

LinkedIn’s economic graph database (Carter et al., 2019) is a very high performance graph query system designed for low latency queries similar to A1. It scales up vertically and can answer lookup queries in nanoseconds while A1 operates in microseconds. Overall, A1 has taken the approach of using cheap commodity hardware to scale out while taking advantage of RDMA to keep query latency low, while the LinkedIn database relies upon fixed sharding and specialized hardware to achieve its performance.

As the price of RAM has fallen, building distributed in-memory storage systems(Ousterhout et al., 2015) for low-latency applications has become very attractive. The combination of RAM storage and RDMA networks is a newer development and research systems like FaRM(Dragojević et al., 2014, 2015) and NAMDb(Zamanian et al., 2017; Ziegler et al., 2019) have shown the advantages of using RDMA to build scale-out transactional databases. NAMDb has studied in detail the performance benefits of building remote pointer based data structures like B+ trees over RDMA. The challenges of designing data structures optimized for remote memory has been considered by Aguiler et.al. (Aguilera et al., 2019, 2017). RDMA has been used to build high performance RDF engines(Shi et al., 2016) and file systems as well(Cao et al., 2018). But the adoption of RDMA in industry has been limited by the fact that it is hard to ensure proper fairness and congestion control in large data center deployments with commodity hardware(Zhu et al., 2015). We believe the example of A1 will augment the case for wide-spread adoption of RDMA in cloud data centers.

8. Conclusion

Building a generic database is a complex problem. A1 was designed to work in a space with huge data volume, wide variety of data sources and update frequencies and strict requirements to perform queries with very low latency.

Distributed systems are complex to program and operate, and we chose to implement transaction support to hide the complexities of availability, replication and durability in the face of machine failure. The connected nature of graph data made it even more important to ensure correctness at any time. In our experience, the developer productivity was high due to the support of transactions. Furthermore, the natural property graph model was intuitive and powerful to use for building search-oriented applications in Bing.

FaRM and A1 utilizes the benefits of RDMA to a great extent, and the performance achieved makes more complex question answering possible at scale, and within latencies acceptable for interactive searching. FaRM was originally designed to support relational systems, but our work also shows that it is general enough to be considered a very efficient programming model for low-latency systems at scale.

9. Acknowledgements

A1 is built on top of the work by the FaRM team: in particular we’d like to thank Aleksandar Dragojevic, Dushyanth Narayanan, Ed Nightingale and our product manager Dana Cozmei. Getting A1 into production would not have been possible without the help and support of the ObjectStore team: Sam Bayless, Jason Li, Maya Mosyak, Vikas Sabharwal, Junhua Wang and Bill Xu. The feedback we received from our customers in Bing was invaluable in guiding our roadmap and features. We would also like to thank our anonymous reviewers whose feedback improved the paper in multiple ways.

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