Serverless computing [1, 2] represents a natural next step of the “as a service” and resource sharing trends in cloud computing. Specifically, “function as a service” offerings such as AWS Lambda allow developers to write blocks of code with well-defined entry and exit points, delegating all aspects of execution to the cloud provider. Typically, these blocks of code are stateless, reading from and writing to various “state as a service” offerings (databases, message queues, persistent stores, etc.).
Standard serverless deployments are characterized by asynchronous, loosely-coupled, and event-driven processes that touch relatively small amounts of data . Consider a canonical example that Amazon describes: an image processing pipeline such that when the user uploads an image to a website, it is placed in an S3 bucket, which then triggers a Lambda to perform thumbnail generation. The Lambda may then enqueue a message that triggers further downstream processing. Most serverless applications are user facing, even if users are not directly involved in the processing pipeline.
This paper explores serverless architectures for a completely different use case: large-scale analytical data processing by data scientists. We describe Flint, a prototype Spark execution engine that is completely implemented using AWS Lambda and other services. One key feature is that we realize a pure pay-as-you-go cost model, in that there are zero costs for idle capacity. With Flint, the data scientist can transparently use PySpark without needing an actual Spark cluster, instead paying only for the cost of running individual programs.
The primary contribution of our work is a demonstration that it is indeed possible to build an analytical data processing framework using a serverless architecture. Critically, we accomplish this using cloud infrastructure that has no idle costs. It is straightforward to see how workers performing simple “map” operations can execute inside Lambda functions. Physical plans that require data shuffling, however, are more complicated: Flint takes advantage of distributed message queues to handle shuffling of intermediate data, in effect offloading data movement to yet another cloud service.
Ii Background and Design Goals
Our vision is to provide the data scientist with an experience that is indistinguishable from “standard” Spark. The only difference is that the user supplies configuration data to use the Flint serverless backend for execution. In this context, we explore system performance tradeoffs in terms of query latency, cost, etc.
Currently, Flint is built on AWS, primarily using Lambda and other services. All input data to an analytical query are assumed to reside in an S3 bucket, and we assume that results are written to another S3 bucket or materialized on the client machine. The AWS platform was selected because it remains the most mature of the alternatives, but in principle Flint can be re-targeted as other cloud providers have similar offerings.
One major design goal of Flint is to provide a truly pay-as-you-go cost model with no costs for idle capacity. This needs a bit of explanation: as a concrete example, Amazon Relational Database Service (RDS) requires the user to pay for database instances (per hour). This is not pay as you go because there are ongoing costs even when the system is idle. Therefore, this means that one obvious implementation of using RDS to manage intermediate data would violate our design goal. In general, we cannot rely on any persistent or long-running daemons.
Note that this is a challenging, but also worthwhile, design goal. In a cloud-based environment, there are a limited number of options for Spark analytics. One option is to offload cluster management completely to a cloud provider via something like AWS EMR, which starts up a Spark cluster for each user job. The downside is that a lot of time is wasted in cluster initialization.
The alternative is to manage one’s own Spark clusters in the cloud (on EC2 instances). There are, of course, tools to help with this, ranging from the UI of Databrick’s Unified Analytics Platform to full-fledged orchestration engines such as Netflix’s Genie. Even if cluster startup and teardown were completely automated (and instantaneous, let’s even say), the fact remains that the organization pays for cluster instances for the entire time the cluster is up; charges accumulate even when the cluster is idle. For large organizations, this is less of an issue as there is more predictable aggregate load from teams of data scientists, but for smaller organizations, usage is far more sporadic and difficult to estimate a priori.
In contrast, we believe that serverless analytics with pay-as-you-go pricing is compelling, particularly for ad hoc analytics and exploratory data analysis. This is exactly what our Flint prototype provides.
Iii Flint Architecture
The overall architecture of Flint is shown in Figure 1. At a high-level, Spark tasks are executed in AWS Lambda, and intermediate data are held in Amazon’s Simple Queue Service (SQS), which handles the data shuffling necessary to implement many transformations.
To maximize compatibility with the broader Spark ecosystem, Flint reuses as many existing Spark components as possible. When a Spark job is submitted, the sequence of RDD transformations (i.e., the RDD lineage) is converted into a physical execution plan using the DAG Scheduler. The physical plan consists of a number of stages, and within each stage, there is a collection of tasks. The Task Scheduler is then responsible for coordinating the execution of these tasks. Spark provides pluggable execution engines via the SchedulerBackend interface: Spark by default comes with implementations for local mode, Spark on YARN and Mesos, etc. Flint provides a serverless implementation of SchedulerBackend; everything else remains unchanged from standard Spark. The primary advantage of this design is that we can reuse existing Spark machinery for query planning and optimization, and Flint only needs to “know about” Spark execution stages and tasks in the physical plan.
The Flint SchedulerBackend (hereafter “scheduler”), which lives inside the Spark context on the client machine, is responsible for coordinating Flint executors to execute a particular physical plan. The scheduler receives tasks from Spark’s Task Scheduler, and for each task, our implementation extracts and serializes the information that is needed by the Flint executors. This information includes the serialized code to execute, metadata about the relationship of this task to the entire physical plan, and metadata about where the executor reads its input and writes its output. When this serialization is complete, the scheduler asynchronously launches the Flint executors on AWS Lambda, with the serialized task as part of the request. After a Flint executor has completed its task, the scheduler processes the response. Once all tasks of the current stage complete, executors for tasks of the next stage are launched, repeating until the entire physical plan has been executed.
Iii-a Flint Executor
A Flint executor is a process running inside an Amazon Lambda function that executes a task in a Spark physical plan. Since the startup latency of a Lambda invocation is small once the function has been “warmed up” (more discussion later), each Lambda instance only processes a single task. This is different from standard Spark, where executors are long-running processes.
Once a Flint executor is initialized, it first deserializes the task information from the request arguments. From the input partition metadata, the executor creates an input iterator to read from the appropriate input partition. For the first stage in a plan, this iterator will fetch a range of bytes from an S3 object. For most other stages, the input iterator will fetch from a designated SQS queue (discussed in detail below).
Once the input iterator is ready, it is passed to the deserialized function (i.e., code to execute) from the task as an argument; this yields the output iterator. If the task is in the final (result) stage of the execution plan, there are two possibilities: If the final action on the RDD is saveAsTextFile, outputs are materialized to another S3 bucket; otherwise, the results are materialized within the executor and passed back to the scheduler (for example, if the data scientist calls the collect action).
When a task is part of an intermediate stage, the execution plan requires the output to be shuffled so that all values for a key are placed in the same partition. The shuffling is part of the physical plan created by Spark; the Flint executors simply execute the task, and thus are not explicitly aware of the actual RDD transformation (e.g., if the shuffling is part of reduceByKey or join, etc.). Since the execution time of a Lambda invocation has a limit of 300 seconds, it is not possible to guarantee that the Flint executors from the previous stage are still alive to pass data to executors running tasks from the next stage. Thus, we need some external data store to deliver the intermediate output. Flint uses Amazon’s Simple Queue Service (SQS) for this purpose.
Once an executor of a task belonging to an intermediate stage has computed the output iterator, the hash partition function (or custom partition function if specified) is used to decide which partition each output object will be assigned to. The executor groups objects by the destination partition in memory. However, if memory usage becomes too high during this process, the executor flushes its in-memory buffers by creating a batch of SQS messages and sending them to the appropriate queue for each partition. After all output data are sent to SQS queues, the executor terminates and returns a response containing a variety of diagnostic information (e.g., number of messages, SQS calls, etc.).
Once all tasks of the current stage are completed, executors for tasks in the next stage will be launched. These executors read from their corresponding SQS queues and aggregate data in memory. Results are passed to the iterator of the function associated with the task, as described earlier. Since we are using in-memory data structure for aggregation, memory forms a bottleneck. Due to the complexities of implementing on-disk multi-pass aggregation algorithms in the Lambda environment, we currently address this problem by increasing the number of partitions such that we do not overflow memory. This solution appears to be adequate, since it takes advantage of the elasticity provided by AWS Lambda.
Queue management is performed by the scheduler. Before the execution of each stage, the scheduler initializes the necessary partitions. Partition metadata (i.e., source and destination queues) are passed as part of the Lambda request. The scheduler also handles cleanup.
Iii-B Overcoming Lambda Limitations
The current Flint implementation supports PySpark, which counter-intuitively is easier to support than Scala Spark. The Flint SchedulerBackend on the client is implemented in Java, but the Flint executors in AWS Lambda are implemented in Python. This design addresses one of the biggest issues with AWS Lambda: the long cold startup time of function invocations. The first time that a Lambda is invoked (or after some idle period), AWS needs to provision the appropriate runtime container. Java Lambdas tend to have large deployment packages due to numerous dependencies, whereas Python Lambdas are significantly more lightweight; thus, they start up faster. Furthermore, in the default Spark executor implementation, to run PySpark code, data (from S3) is first read in the JVM and then passed to a Python subprocess using pipes. In Flint, we bypass this extra wrapper layer, and Python code is able to read from S3 directly. As we later show, this has significant performance advantages.
Another limitation of AWS Lambda is that execution duration per request is capped at 300 seconds. This leads to the failure of long-running tasks. In order to avoid this problem, we chain executors: if the running time has almost reached the limit, the Flint executor stops ingesting new input records. Then, the current state, including how much of the input split has been read, is serialized and returned to the scheduler, which launches a new Flint executor to continue processing the uncompleted input split from where the previous invocation left off. Since the function is already warm, the cost of using chained executors is relatively low.
A third limitation of Lambda comes from a number of resource constraints. Each invocation is limited to a maximum memory allocation of 3008 MB. Thus, it is important for the Flint executor to carefully manage in-memory data. There is a limitation of 6 MB on the size of the request payload for an invocation. This payload is used to hold the serialized task data, which is typically much smaller. However, for larger tasks we are currently implementing a workaround for this size restriction by splitting the payload into smaller pieces. These can be uploaded to S3, and the scheduler can direct the Lambda functions to fetch the relevant data to complete initialization.
Iv Experimental Evaluation
|Query Latency (s)||Estimated Cost (USD)|
|0||101 [93 - 109]||211||188||0.20||0.41||0.37|
|1||190 [186 - 197]||316||189||0.59||0.61||0.37|
|2||203 [201 - 205]||314||187||0.68||0.61||0.36|
|3||165 [161 - 169]||312||188||0.48||0.61||0.36|
|4||132 [122 - 142]||225||189||0.33||0.44||0.37|
|5||159 [142 - 177]||312||189||0.45||0.60||0.37|
|6||277 [272 - 281]||337||191||0.56||0.66||0.37|
We evaluated Flint by comparing its performance with a Spark cluster running the Databricks Unified Analytics Platform. The entire cluster comprises 11 m4.2xlarge instances (one driver and ten workers), with a total of 80 vCores (Amazon’s computation unit) of processing capacity. For AWS Lambda, we allocated the maximum amount of memory possible, which is 3008 MB. The developer can also configure the maximum number of concurrent invocations; we set this to 80 to match the Spark cluster, under the assumption that one Lambda invocation roughly uses one vCore. AWS is not completely transparent about the instances running AWS Lambda; documentation refers to a “general purpose Amazon EC2 instance type, such as an M3 type” without additional details. Thus, this is the best that we can do to ensure that all conditions consume the same hardware resources. In all cases, we used the latest version of the Databricks runtime, which is based on Spark 2.3.
Our evaluations examined three different conditions: PySpark code running on Flint, PySpark code running on the Spark cluster, and equivalent Scala Spark code running on the Spark cluster. For the Spark cluster, we only measure query execution time (derived from the cluster logs) and do not include startup costs of the cluster (around five minutes). This puts Spark performance in the best possible light. We had separately examined Amazon EMR, which initializes clusters automatically per job—but for reasons unknown from available documentation, its performance (even excluding startup costs) was significantly worse than a Spark cluster we provisioned ourselves.
For evaluation, we considered a typical exploratory data analysis task described in a popular blog post by Todd Schneider . The New York City Taxi & Limousine Commission has released a detailed historical dataset covering approximately 1.3 billion taxi trips in the city from January 2009 through June 2016. The entire dataset is stored on S3 and is around 215 GB. Each record contains information about pick-up and drop-off date/time, trip distance, payment type, tip amount, etc. Inspired by Schneider’s blog post, we evaluated the following queries, some of which replicate his analyses:
Q0. Line count. In this query, we simply counts the number of lines in the dataset. This evaluates the raw I/O performance of S3 under our experimental conditions.
Q1. Taxi drop-offs at Goldman Sachs headquarters, 200 West St. This query filters by geo coordinates and aggregates by hour, as follows:
[fontsize=] arr = src.map(lambda x: x.split(’,’)) .filter(lambda x: inside(x, goldman)) .map(lambda x: (get_hour(x), 1)) .reduceByKey(add, 30) .collect()
This is exactly the query issued in PySpark to both Flint and the Spark cluster. The Scala Spark condition evaluates exactly the same query, except in Scala. Note that Flint is able to support UDFs transparently.
For brevity, we omit code for the following queries and instead provide a concise verbal description.
Q2. Same query as above, but for Citigroup headquarters, at 388 Greenwich St.
Q3. Goldman Sachs taxi drop-offs with tips greater than $10. Who are the generous tippers?
Q4. Cash vs. credit card payments. This query computes the proportion of rides paid for using credit cards, aggregated monthly across the dataset.
Q5. Yellow taxi vs. green taxi. This query computes the number of different taxi rides, aggregated by month.
Q6. Effect of precipitation on taxi trips, i.e., do people take the taxi more when it rains? This query aggregates rides for different amounts of precipitation.
reports latency (in seconds) and estimated cost of each query (in USD) under the three different experimental conditions discussed above. For Flint, we report averages over five trials (after warm-up) and show 95% confidence intervals in brackets. The latency of PySpark and Spark exhibit little variance, and thus we omit confidence intervals (over three trials) for brevity. Estimated costs for Spark and PySpark are computed as the query latency multiplied by the per-second cost of the cluster. For Flint, we used logging information to compute the execution duration of the AWS Lambdas and the associated SQS costs.
We find that latency is roughly the same for all queries on Spark and appears to be dominated by the cost of reading from S3. This is perhaps not surprising since none of our test queries are shuffle intensive, as the number of intermediate groups is modest. Interestingly, for some queries, Flint is actually faster than Spark. The explanation for this can be found in Q0, which simply counts lines in the dataset and represents a test of read throughput from S3. Evidently, the Python library that we use (boto) achieves much better throughput than the library that Spark uses to read from S3. This is confirmed via microbenchmarks that isolate read throughput from a single EC2 instance. In our queries, the performance of Flint appears to be dependent on the number of intermediate groups, and this variability makes sense as we are offloading data movement to SQS. PySpark is much slower than Flint because every input record passes from the JVM to the Python interpreter, which adds a significant amount of overhead. In terms of query costs, Flint is in general more expensive than Spark, even for queries with similar running times (Flint has additional SQS costs). Although a direct cost conversion between Lambda and dedicated EC2 instances is difficult (the actual instance type and the multiplexing mechanism are both unknown), it makes sense that Lambda has a higher per-unit resource cost, which corresponds to the price we pay for on-demand invocation, elasticity, etc.
For the above reasons, it is difficult to obtain a truly fair comparison between Flint and Spark. Nevertheless, our experiments show that serverless analytics is feasible, though a broader range of queries is needed to tease apart performance and cost differences—for example, large complex joins, iterative algorithms, etc. However, results do suggest that data shuffling is a potential area for future improvement.
V Related Work
The origins of Flint can be traced back to a course project at the University of Waterloo in the Fall of 2016. Since then, there have been other attempts at exploring serverless architectures for data analytics. In June 2017, Databricks announced a serverless product , best described as a more flexible resource manager: administrators define a “serverless pool” that elastically scales up and down. This can be viewed as more convenient tooling around traditional Spark clusters, and is not serverless in the sense that we mean here.
In November 2017, Qubole announced an implementation of Spark on AWS Lambda . This effort shares the same goals as Flint, but with several important differences. Qubole attempted to “port” the existing Spark executor infrastructure onto AWS Lambda, whereas Flint is a from-scratch implementation. As a result, we are better able to optimize for the unique execution environment of Lambda. For example, Qubole reports executor startup time to be around two minutes in the cold start case. In addition, Qubole’s implementation uses S3 directly for shuffling intermediate data, which differs from our SQS-based shuffle. Using S3 allows Qubole’s executors to remain more faithful to Spark, but we believe that the I/O patterns are not a good fit for S3.
Amazon provides two data analytics services that are worth discussing: Amazon Athena (announced November 2016) and Amazon Redshift Spectrum (announced July 2017). Both are targeted at more traditional data warehousing applications and only support SQL queries, as opposed to a general-purpose computing platform like Spark. Athena offers a pay-as-you-go, per-query pricing with zero idle costs, similar to Flint, but under the covers it uses the Presto distributed SQL engine for query execution, so architecturally it is not serverless. Redshift Spectrum is best described as a connector that supports querying over S3 data; the customer still pays for the cost of running the instances that comprise the Redshift cluster itself (i.e., per hour charge, even when idle).
PyWren  is another project advocating a serverless execution model for analytics tasks, although unlike our effort PyWren does not attempt to target Spark or any specific analytics framework. Since Flint is a Spark execution engine, it supports arbitrary RDD transformations; in contrast, PyWren examines only three classes of dataflow patterns: map-only, map + monolithic reduce, and MapReduce using either S3 or Redis for shuffling (the latter is not pay as you go).
Serverless computing in general is an emerging computing paradigm and previous work has mostly focused on examining system-level issues in building serverless infrastructure  as opposed to designing applications. Indeed, as Baldini et al.  write, the advantages of serverless architectures are most apparent for bursty, compute-intensive workloads and event-based processing pipelines. Data analytics represents a completely different workload and Flint opens up exploration in a completely different part of the application space.
Vi Future Work and Conclusions
There are a number of future directions that we are actively exploring. We have not been able to conduct an experimental evaluation between Qubole’s implementation and Flint, but the design choice of using S3 vs. SQS for data shuffling should be examined in detail. Each service has its strengths and weaknesses, and we can imagine hybrid techniques that exploit the strengths of both.
Robustness is an issue that we have not explored at all in this work, although to some extent the point of serverless designs is to offload these problems to the cloud provider. Executor failures can be overcome by retries, but another issue is the at-least-once message semantics of SQS. Under typical operating circumstances, SQS messages are only delivered once, but AWS documentation explicitly acknowledges the possibility of duplicate messages. We believe that this issue can be overcome with sequence ids to deduplicate message batches, as the exact physical plan is known ahead of time.
Another ongoing effort is to ensure that higher-level Spark libraries (e.g., MLlib, SparkSQL, etc.) work with Flint. To the extent that SchedulerBackend provides a clean abstraction, in theory everything should work transparently. However, as every developer knows, abstractions are always leaky, with hidden dependencies. We are pushing the limits of our current implementation by iteratively expanding the scope of Spark libraries and features that we use.
To conclude, we note that Flint is interesting in two different ways: First, we show that big data analytics is feasible using a serverless architecture, and that we can coordinate the data shuffling associated with analytical queries in a restrictive execution environment. Second, there are compelling reasons to prefer using our execution engine over Spark’s default, particularly for ad hoc analytics and exploratory data analysis: the tradeoff is a bit of performance for elasticity in a pure pay-as-you-go cost model. Thus, Flint appears to be both architecturally interesting as well as potentially useful.
Flint traces back to a serverless analytics prototype called Iris developed as part of a course project at the University of Waterloo in Fall 2016 with Jonathan Ma, Ronak Patel, and Pranjal Shah. Although Flint does not share any code with Iris, we’d like to acknowledge these individuals for their contributions to early developments of the serverless concept.
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