Delivering a technical talk has a lot in common with running a half-marathon or biking a 40k time trial. You’re excited and maybe a little nervous, you’re prepared to go relatively hard for a relatively long time, and you’re acutely aware of the clock. In both situations, you might be tempted to take off right from the gun, diving into your hardest effort (or most technical material), but this is a bad strategy.
By going out too hard in the half-marathon, you’ll be running on adrenaline instead of on your aerobic metabolism, will burn matches by working hard before warming up fully, and ultimately won’t be able to maintain your best possible pace because you’ll be spent by the second half of the race. Similarly, in the talk, your impulse might be to get right to the most elegant and intricate parts of your work immediately after introducing yourself, but if you get there without warming up the audience first, you’ll lose most of them along the way. In both cases, your perception of what you’re doing is warped by energy and nerves; the right pace will feel sluggish and awkward; and starting too fast will put you in a hole that will be nearly impossible to recover from.
Delivering a technical talk successfully has a lot in common with choosing an appropriate pacing strategy for an endurance event: by starting out slower than you think you need to, you’ll be able to go faster at the end. Most runners1 will be able to maintain a higher average pace by doing negative splits. In a race, this means you start out slower than your desired average pace and gradually ramp up over the course of the race so that by the end, you’re going faster than your desired average pace. By starting out easy, your cardiovascular system will warm up, your connective tissue will get used to the stress of pounding on the pavement, and your muscles will start buffering lactic acid; this will reduce muscle fatigue and save your anaerobic energy for the final sprint.
You can apply the general strategy of negative splits to a talk as well. Instead of warming up cold muscles and your aerobic energy systems before making them work, you’re preparing a group of smart people to learn why they should care about your topic before making them think about it too much. Start off slow: provide background, context, and examples. Unless you’re a very experienced speaker, this will feel agonizingly slow at first.
It’s understandable that it might feel remedial and boring to you to explain why your work is relevant. After all, you’re deep in your topic and have probably long since forgotten what it was like to learn about it for the first time. Examples and visual explanations might seem like a waste of time before you get to your clever implementation, elegant proof, or sophisticated model. You have some serious detail to cover, after all! Your audience, however, isn’t prepared for that detail yet. If you skip the warm-up and go straight to that detail, you’ll lose audience engagement, and it’s nearly impossible to recover from that; it’ll certainly prevent you from covering as much as you might have otherwise wanted to.
Remember that your audience is made up of smart people who chose to attend your talk instead of sitting out in the hall. They’d probably rather be learning something from you than halfheartedly reading email. But they also almost certainly don’t know as much about your topic as you do. Ease them in to it, warm them up, and give them plenty of context first. You’ll be able to cover more ground that way.
Pacing in cycling time trials can be a little more complicated depending on the terrain and wind but in general being able to finish stronger than you started is still desirable.↩
I was in Berlin last week for Flink Forward, the inaugural Apache Flink conference. I’m still learning about Flink, and Flink Forward was a great place to learn more. In this post, I’ll share some of what I consider its coolest features and highlight some of the talks I especially enjoyed. Videos of the talks should all be online soon, so you’ll be able to check them out as well.
Apache Flink is a data processing framework for the JVM that is most popular for streaming workloads with high throughput and low latency, although it is also general enough to support batch processing. Flink has a pleasant collection-style API, offers stateful elementwise transformations on streams (think of a
fold function), can be configured to support fault-tolerance with exactly-once delivery, and does all of this while achieving extremely high performance. Flink is especially attractive for use in contemporary multitenant environments because it manages its own memory and thus Flink jobs can run well in containers on overprovisioned systems (where CPU cycles may be relatively abundant but memory may be strictly constrained).
Keynotes and lightning talks
Kostas Tzoumas and Stephan Ewan (both of data Artisans) shared a keynote in which they presented the advancements in Flink 0.10 (to be released soon) and shared the roadmap for the next release, which will be Flink 1.0. The most interesting parts of this keynote for me were the philosophical arguments for the generality and importance of stream processing in contemporary event-driven data applications. Many users of batch-processing systems simulate streaming workflows by explicitly encoding windows in the structure of their input data (e.g., by using one physical file or directory to correspond to a day, month, or year worth of records) or by using various workarounds inspired by technical limitations (e.g., the “lambda architecture” or bespoke but narrowly-applicable stream processors). However, mature stream processing frameworks not only enable a wide range of applications that process live events, but they also are general enough to handle batch workloads as a special case (i.e., by processing a stream with only one window).1
Of course, the workarounds that data engineers have had to adopt to handle streaming data in batch systems are only necessary given an absence of mature stream processing frameworks. The streaming space has improved a great deal recently, and this talk gave a clear argument that Flink was mature enough for demanding and complex applications. Flink offers a flexible treatment of time: events can be processed immediately (one at a time), in windows based on when the events arrived at the processor, or in windows based on when the events were actually generated (even if they arrived out of order). Flink supports failure recovery with exactly-once delivery but also offers extremely high throughput and low latency: a basic Flink stream processing application offers two orders of magnitude more throughput than an equivalent Storm application. Flink also provides a batch-oriented API with a collection-style interface and an optimizing query planner.
After the keynote, there were several lightning talks. Lightning talks at many events are self-contained (and often speculative, provocative, or describing promising work in progress). However, these lightning talks were abbreviated versions of talks on the regular program. In essence, they were ads for talks to see later (think of how academic CS conference talks are really ads for papers to read later). This was a cool idea and definitely helped me navigate a two-track program that was full of interesting abstracts.
Michael Häusler of ResearchGate gave a talk in which he talked about the process of evaluating new data processing frameworks, focusing in particular on determining whether a framework makes simple tasks simple. (Another step, following Alan Kay’s famous formulation, is determining whether or not a framework makes complex tasks possible.) The “simple task” that Häusler set out to solve was finding the top 5 coauthors for every author in a database of publications; he implemented this task in Hive (running on Tez), Hadoop MapReduce, and Flink. Careful readers will note that this is not really a fair fight: SQL and HiveQL do not admit straightforward implementations of top-k queries and MapReduce applications are not known for elegant and terse codebases; indeed, Häusler acknowledged as much. However, it was still impressive to see how little code was necessary to solve this problem with Flink, especially when contrasted with the boilerplate of MapReduce or all of the machinery to implement a user-defined aggregate function to support top-k in Hive. The Flink solution was also twice as fast as the custom MapReduce implementation, which was in turn faster than Hive on Tez.
Declarative Machine Learning with the Samsara DSL
Sebastian Schelter introduced Samsara, a DSL for machine learning and linear algebra. Samsara supports in-memory vectors (both dense and sparse), in-memory matrices, and distributed row matrices, and provides an R-like syntax embedded in Scala for operations. The distributed row matrices are a unique feature of Samsara; they support only a subset of matrix operations (i.e., ones that admit efficient distributed implementations) and go through a process of algebraic optimization (including generating logical and physical plans) to minimize communication during execution. Samsara can target Flink, Spark, and H2O.
Streaming and parallel decision trees in Flink
Training decision trees in batch frameworks requires a view of the entire learning set (and sufficient training data to generate a useful tree). In streaming applications, each event is seen only once, the classifier must be available immediately (even if there is little data to train on) and the classifier should take feedback into account in real time. In this talk, Anwar Rizal of Amadeus presented a technique for training decision trees on streaming data by building and aggregating approximate histograms for incoming features and labels.
Juggling with bits and bytes — how Apache Flink operates on binary data
Applications using the Java heap often exhibit appalling memory efficiency; the heap footprint of Java library data structures can be 75% overhead or more. Since data processing applications frequently create, manipulate, and serialize many objects — some of which may be quite short-lived — there are potentially significant performance pitfalls to using the JVM directly for memory allocation. In this talk, Fabian Hueske of data Artisans presented Flink’s approach: combining a custom memory-management and serialization stack with algorithms that operate directly on compressed data. Flink jobs are thus more memory-efficient than programs that use the Java heap directly, exhibit predictable space usage, and handle running out of memory gracefully by spilling intermediate results to disk. In addition, Flink’s use of database-style algorithms to sort, filter, and join compressed data reduces computation and communication costs.
Stateful Stream Processing
Data processing frameworks like Flink and Spark support collection-style APIs where distributed collections or streams can be processed with operations like
filter, and so on. In addition to these, it is useful to support transformations that include state, analogously to the
fold function on local collections. Of course,
fold by itself is fairly straightforward, but a reliable
fold-style operation that can recover in the face of worker failures is more interesting. In this talk, Márton Balassi and Gábor Hermann presented an overview of several different approaches to supporting reliable stream processing with state: the approaches used by Flink (both versions 0.9.1 and 0.10), Spark Streaming, Samza, and Trident. As one might imagine, Spark Streaming and Samza get a lot of mileage out of delegating to underlying models (immutable RDDs in Spark’s case and a reliable unified log in Samza’s). Flink’s approach of using distributed snapshots exhibits good performance and enables exactly-once semantics, but it also seems simpler to use than alternatives. This has become a recurring theme in my investigation of Flink: technical decisions that are advertised as improving performance (latency, throughput, etc.) also, by happy coincidence, admit a more elegant programming model.
Fault-tolerance and job recovery in Apache Flink
This talk was an apt chaser for the Stateful Stream Processing talk. Till Rohrmann presented Flink’s approaches to checkpointing and recovery, showing how Flink can be configured to support at-most-once delivery (the weakest guarantee), at-least-once delivery, or exactly-once delivery (the strongest guarantee). The basic approach Flink uses for checkpointing operator state is the Chandy-Lamport snapshot algorithm, which enables consistent distributed snapshots in a way that is transparent to the application programmer. This approach also enables configurable tradeoffs between throughput and snapshot interval, but it’s far faster (and nicer to use) than Storm’s approach in any case. Recovering operator state is only part of the fault-tolerance picture, though; Till’s talk also introduced Flink’s approach for supporting a highly-available Job Manager.
Other talks worth checking out
Here are a few talks that I’d like to briefly call out as worth watching:
- “Automatic detection of web trackers”, presented by Vasia Kalavri, was a cool application of graph processing in Flink.
- “Applying Kappa architecture in the telecom industry”, presented by Ignacio Mulas Viela, showed how to put a realistic streaming topology into production.
- In “A tale of squirrels and storms”, Matthias Sax introduced the Storm compatibility layer for Flink, enabling users to run Storm topologies on Flink with minimal code changes.
- Aljoscha Krettek’s talk covered the different approaches Flink supports for defining windows over streams.
- My colleague Suneel Marthi presented on a Flink port of the BigPetStore big data application blueprints.
The data Artisans team and the Flink community clearly put a lot of hard work towards making this a really successful conference. The venue (pictured above) was unique and cool, the overall vibe was friendly and technical, and I didn’t see a single talk that I regretted attending. (This is high praise indeed for a technical conference; I may have been lucky, but I suspect it’s more likely that the committee picked a good program.) I especially appreciated the depth of technical detail in the talks by Flink contributors on the second afternoon, covering both design tradeoffs and implementation decisions. I’m hoping to be back for a future iteration.
I’m speaking at Spark Summit today about using Spark to analyze operational data from the Fedora project. Here are some links to further resources related to my talk:
- My talk slides are online; the online deck includes some extra slides that I skipped in the talk as delivered
- An earlier post provides some background on fedmsg and Spark
- You may also be interested in a higher-level discussion of issues with schema inference from the perspective of type theory
- Here’s the annotated source code for the ML pipeline transformer I discussed in my talk
You should also check out my team’s Silex library, which contains useful code factored out of real Spark applications we’ve built in Red Hat’s Emerging Technology group. It includes a lot of cool functionality, but the part I mentioned in the talk is this handy interface for preprocessing JSON data before inferring a schema.
In this post, we’ll see how to make a simple transformer for Spark ML Pipelines. The transformer we’ll design will generate a sparse binary feature vector from an array-valued field representing a set.
The first thing we’ll need to do is expose Spark’s user-defined type for vectors. This will enable us to write a user-defined data frame function that returns a Spark vector. (We could also implement our own user-defined type, but reusing Spark’s, which is currently private to Spark, will save us some time. By the time you read this, the type may be part of Spark’s public API — be sure to double-check!)
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Here are the imports we’ll need for the transformer and support code. I’ll use
VEC for Spark vectors to avoid confusion with Scala’s
Vector type. We’ll assume that the
VectorType code from above is available on your project’s classpath.
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Transformer and support code
Most of the ML pipeline classes distributed with Spark follow the convention of putting groups of related pipeline stage parameters in a trait. We’ll do this as well, declaring a trait for the three parameters that our transformer will use: the name of the input column, the name of the output column, and the maximum number of elements our sparse vector can hold. We’ll also define a convenience method to return a triple of the parameter values we care about.
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Note that Spark 1.4 supports calling
getOrElse directly on a
ParamMap instance, so you can slightly simplify the code in
pvals if you don’t care about source compatibility with Spark 1.3.
Here’s what the actual transformer implementation looks like:
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The first thing we do in the transformer class is declare an instance of
VectorType to use in other data frame type declarations later in the class. The
transformSchema method returns the schema after applying this transformer to a given data frame; it creates a new data frame schema that includes all of the fields from the original frame as well as a
Vector-valued field whose name is the parameter specified in the
outputCol parameter. Finally, the
transform method creates a new data frame with an additional column (again, named with the value of the
outputCol parameter); its values result of applying a user-defined function to each row in the data frame, taking arguments from the input column. The function itself simply creates a sparse binary vector from an array-backed set, so that the array-backed set
Array(1,2,4,8) would become a sparse vector with the first, second, fourth, and eighth elements set to 1 and everything else set to 0.
The code above is a reasonable starting point for your own transformers, but you’ll want to add error checking to code you use in production: at a minimum, you’d need to validate the schema of the input data frame (to ensure that expected columns exist and are of the correct type), verify that the output column name doesn’t already exist in the data frame, and make sure no input array has more than
vecSize elements. I hope this code is helpful as you develop your own pipeline stages!
This post will show you an extremely simple way to make quick-and-dirty Bokeh plots from data you’ve generated in Spark, but the basic technique is generally applicable to any data that you’re generating in some application that doesn’t necessarily link in the Bokeh libraries.
We’ll need to have a recent version of Bokeh installed and some place to put our data. If you don’t already have Bokeh installed, use
virtualenv to get it set up:
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This will download and build a nontrivial percentage of the Internet. Consider getting some coffee or catching up with a colleague while it does its thing.
The next thing we’ll need is some place to stash data we’ve generated. I’ll use a very simple service that I developed just for this kind of application, but you can use whatever you want as long as it will let you store and retrieve JSON data from a given URL.
With all that in place, we’re ready to make a simple plot.
A basic static-HTML plot
We’re going to use Bokeh to construct a basic line graph in a static HTML file. The catch is that instead of specifying the data statically, we’re going to specify it as a URL, which the page will poll so it can update the plot when it changes. Here’s what a script to construct that kind of plot looks like:
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Note that the above example assumes you’re using the Firkin service as your data store. If you’re using something else, you will probably need to change the
data_url parameter to the
Publishing and updating data
Now we’ll fire up the Firkin server and post some data. From a checkout of the Firkin code, run
sbt server/console and then create a connection to the server using the client library:
You can now supply some values for
y. We’ll start simple:
If you don’t already have
json.html from the previous step loaded in your browser, fire it up. You should see a plot that looks like this:
If we update the data stored at this URL, the plot will update automatically. Try it out; publish a new data set:
The plot will refresh, reflecting the updated
(Again, if you’re using some other JSON object cache to store your data sets, the principles will be the same but the syntax won’t be.)
Connecting Bokeh to Spark
Once we have a sink for JSON data, it’s straightforward to publish data to it from Spark code. Here’s a simple example, using
json4s to serialize the RDDs:
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After we do this, the data will be available at
The approach described so far in this post was designed to provide the simplest possible way to make a basic plot backed by a relatively small amount of dynamic data. (If we have data to plot that we can’t store in memory on a single node, we’ll need something more sophisticated.) It is suitable for prototyping or internal use. Other approaches, like the two below, might make more sense for other situations:
- Bokeh offers several alternate language bindings, including one for Scala. This means that you can define plots and supply data for them from within applications that link these libraries in. This is the approach that spark-notebook takes, for example.
- Another possibility is setting up the Bokeh server and publishing plots and data to it. The Bokeh server offers several additional capabilities, including support for scalable visualizations (e.g., via sampling large datasets) and support for multiple authenticated users.
Most people set personal and professional goals. If you work in software, your near-term professional goals might sound like this:
- Ship the next version of our team’s product without any undiscovered crashing bugs
- Improve the performance of the caching layer by 10%
- Become a committer on some particular open-source project
Those probably sound pretty reasonable, right? They aren’t vague wishes, they’re goals. They probably aren’t totally unrealistic, and you’ll know you’ve achieved them when some objective, measurable thing happens. (They’re SMART, in other words.)
We’ll come back to these goals. First, though, I’d like digress and talk about bike racing, and specifically about the kinds of goals an amateur racer might set for a year. Maybe those would look like this:
- Stay healthy and avoid injuries all season
- Improve my one-minute power output by 0.5 W/kg
- Upgrade my racing category
These are more or less analogous to the professional goals. They’re objective, measurable, and reasonable. However, they’re also at least partially out of your control. You can’t totally prevent yourself from getting sick or crashing. Your one-minute power might be right at the limits of your genetic capabilities (or maybe your season shapes up so that it makes more sense to focus on improving other aspects of your fitness). And the category upgrade is up to an official in your local cycling association — you can get an automatic upgrade by winning races, but whether or not you win is also largely out of your control.
I first read about the idea of process and outcome goals in Joe Friel’s The Cyclist’s Training Bible, but I’ve found them extremely helpful in talking about professional goals as well. The idea is that there are two kinds of goals: outcome goals, which are results, and process goals, which relate to how you prepare and execute. Of course, you’re ultimately interested in outcomes (winning races, shipping great products, becoming recognized as an authority in your field), but outcomes are always to some extent out of your control.
Instead of focusing on outcomes for goals, it makes more sense to focus on what you can control: the processes that support those outcomes. So, in the bike case, you could recast the outcome goals as:
- Make sure I get at least 7 hours of sleep a night and avoid sketchy racers in corners or sprints
- Do plyometric exercises at least twice a week in the offseason
- Identify key races that I can do well in and make sure to show up for them well-rested and with adequate nutrition; then focus on avoiding particular tactical mistakes I’ve made in past races
The software goals I mentioned at the beginning of the article are also outcome goals. You can’t guarantee that your product won’t have any undiscovered crashing bugs, but you can sure put processes in place so that you’re more likely to discover crashing bugs before you ship. Similarly, a 10% performance improvement sounds great, but it might require unjustifiable effort or be unachievable (conversely, it might turn out that 10% improvement is insufficient to improve the behavior of the whole system)! Community leadership positions always involve political as well as meritocratic considerations; while you can make a case for yourself as a good candidate, you can’t force people to vote for you.
Recast as process goals, the software goals might look like this:
- Ensure that no new commit to our product introduces untested code, use property-based tests for all internal data structures, and incorporate fuzzing of every public interface into the QA process.
- Profile the caching layer under a range of conditions and document the hot spots, identifying algorithmic and implementation-level avenues for performance improvement. Add performance testing to CI.
- Participate in the project community, answering questions on the mailing list every week and submitting at least one nontrivial patch a week.
It’s easy to imagine the rest of your career and think about outcomes you want to see: earning a prestigious position, achieving acknowledged impact in your department or company, or writing a popular framework (or book).1 But those goals are often pretty far off from what you need to do to get there. Setting process goals supports the outcomes that you want to achieve. But the really remarkable thing is that they do so much more than that. By focusing on improving your preparation and execution, you’re making yourself a better engineer (or bike racer) even if you don’t get the outcomes you want right away — and making it more likely you’ll see great outcomes in the future.
Outcome goals are a particularly dangerous trap if you’re coming from an environment like academia or open-source development, where outcome-related metrics like tenure, citation count, and number of GitHub “stars” are never far from your evaluation of your own career. (The frictionless recognitions afforded by social media “likes” and “favorites” bring this headache to a much broader audience.)↩
Elasticsearch has offered Hadoop
OutputFormat implementations for quite some time. These made it possible to process Elasticsearch indices with Spark just as you would any other Hadoop data source. Here’s an example of this in action, taken from Elastic’s documentation:
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However, the latest versions of the Elasticsearch client offers more idiomatic Spark support to generate RDDs (and Spark data frames) directly from Elasticsearch indices without explicitly dealing with the Hadoop formatting machinery. This support is extremely cool, but the documentation is still a bit sparse and I hit a few snags using it. In this post, I’ll cover the problems I encountered and explain how to make it work.
Native Spark support is available in
elasticsearch-hadoop version 2.1.0. If you’d tried to set this up last week, you’d have needed to run against a snapshot build to get support for Spark data frames. Fortunately, the most recent beta release of
elasticsearch-hadoop includes this support. Add the following library dependency to your project:
Since we won’t be using the lower-level Hadoop support, that’s the only library we’ll need.
We need to supply some configuration in order to create RDDs directly from indices.
elasticsearch-spark expects this configuration to be available as parameters on our Spark context object. Here’s an example of how you’d do that in a vanilla Spark application:
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Note that since the default value for
localhost, you don’t need to set it at all if you’re running against a local Elasticsearch instance. If you were running Elasticsearch behind a reverse proxy running on
proxyhost on port 80, you might specify
proxyhost:80 instead (and you would probably also want to set
false so your client wouldn’t discover nodes that weren’t reachable outside of the private network). There are other options you can set, but this is the most critical.
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elasticsearch-spark offers implicit conversions to make RDDs directly from Elasticsearch resources. (Careful readers may have already seen one in action!) The following code example assumes that you’ve initialized
spark as a
SparkContext with whatever Elasticsearch configuration data you need to set:
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This is far more convenient than using the Hadoop
InputFormat, but the interface still leaves a little to be desired. Our data may have a sensible schema, but the result we’re getting back is as general as possible: just a collection of key-value pairs, where the keys are strings and the values are maps from strings to arbitrary (and arbitrarily-typed) values. If we want more explicit structure in our data, we’ll need to bring it in to Spark in a different way.
Generating data frames
In order to benefit from the structure in our data, we can generate data frames that store schema information. The most recent release of
elasticsearch-spark supports doing this directly. Here’s what that looks like, again assuming that
spark is an appropriately-configured
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Once we have a data frame for our ES resource, we can run queries against it in Spark:
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Elasticsearch’s Spark support offers many additional capabilities, including storing Spark RDDs to Elasticsearch and generating RDDs from queries. Check out the upstream documentation for more!
Silex is a small library of helper code intended to make it easier to build real-world Spark applications;1 most of it is factored out from applications we’ve developed internally at Red Hat. We have a couple of long-term goals for the Silex project:
- We want to make it very easy for us to spin up on new data-processing problems without spending a lot of time dealing with accidental complexity, and
- we want to have a generally-useful standard library atop Spark that provides primitives and solutions to simplify common tasks and reflects best practices as we discover them.
Part of having a library that people will want to use is having a quality test suite and continuous integration. There are some tricks to developing automated tests for Spark applications, but even keeping those in mind may not be sufficient to let you test your Spark apps in a hosted CI environment. This post will show you how to bridge the gap.
Continuous integration challenges
Since we want to make it easy to review contributions to Silex, I set up Travis CI to watch branches and pull requests. Travis CI is a great service, but if you’re used to running tests locally, you might have some problems running Spark-based tests under hosted CI environments. Here’s what we learned:
- When running tests locally, creating a Spark context with
local[*](in order to use all of the available cores on your machine) might be a good idea. However, a hosted CI environment may offer you a lot of cores but relatively little memory, so your tests might be killed because each Spark executor has nontrivial memory overhead. Instead, consider limiting your code to use fewer cores.
- If you’re using Kryo serialization and have set the
spark.kryoserializer.buffer.mbproperty to something large (perhaps because you often have serialization buffer crashes in production), don’t be surprised if you run out of memory while running in CI. Spark doesn’t share serializers between threads, so you could be allocating a huge buffer for each thread even if your test code doesn’t need to serialize anything all that large.2
- Spark SQL (at least as of version 1.3.1) defaults to creating 200 partitions for shuffles. This is probably a good starting place for real-world data, but it’s overkill for test cases running on a single machine. Furthermore, since each partition has some extra memory overhead, it’s another possible cause of OOMEs.
Fixing the problems
Silex provides an application skeleton trait, and we use a class extending this trait to package up Spark and SQL contexts for test cases. That class isn’t currently part of the public Silex API, but you can see what it looks like here:
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As you can see, we allow users to specify a Spark master URL but default to using two cores locally. Furthermore, we use the
addConfig function from
AppCommon — which takes a
SparkConf and returns a modified
SparkConf — to ensure that our Kryo buffer size is the Spark default of 2 mb, rather than the larger Silex default.
If you’re used to writing test code that exercises Spark, you probably already have boilerplate (using something like ScalaTest’s
BeforeAndAfterEach) to set up and tear down a Spark context for each test case. We set the Spark SQL property to control parallelism in data frame and SQL shuffles in the test setup code itself:
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Good CI is one of those things that makes it fun to develop software, but the accidental differences between a hosted environment and your local environment can be totally frustrating. By taking into account the constraints you’re likely to see when running in a resource-limited container on a CI server, you can help to ensure that your tests only fail when your code is actually broken.
Natural join is a useful special case of the relational join operation (and is extremely common when denormalizing data pulled in from a relational database). Spark’s DataFrame API provides an expressive way to specify arbitrary joins, but it would be nice to have some machinery to make the simple case of natural join as easy as possible. Here’s what a natural join needs to do:
- For relations R and S, identify the columns they have in common, say c1 and c2;
- join R and S on the condition that R.c1 == S.c1 and R.c2 == S.c2; and
- project away the duplicated columns.
so, in Spark, a natural join would look like this:
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We can generalize this as follows (note that joining two frames with no columns in common will produce an empty frame):
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Furthermore, we can make this operation available to any
DataFrame via implicit conversions:
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One of the great things about Apache Spark is that you can experiment with new analyses interactively. In the past, I’ve used the
sbt console to try out new data transformations and models; the console is especially convenient since you can set it up as a custom Scala REPL with your libraries loaded and some test fixtures already created.
However, some of Spark’s coolest new functionality depends on some aspects of Scala reflection that aren’t compatible with how
sbt uses classloaders for tasks, so you’re liable to see
MissingRequirementError exceptions when you try and run code that exercises parts of Spark SQL or the DataFrame API from the
You can certainly run a regular Scala REPL or the
spark-shell, but doing so sacrifices a lot of the flexibility of running from
sbt: every time your code or dependencies change, you’ll need to package your application and set your classpath, ensuring that all of your application classes and dependencies are available to the REPL application.
Fortunately, there’s an easier way: you can make a small application that runs a Scala REPL set up the way you like and ask
sbt how to set its classpath. First, write up a simple custom Scala REPL, like this one:
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ReplApp application sets up a Scala REPL with imports for some common Spark classes and bindings to a
ConsoleApp object is just a simple wrapper for Spark context and configuration; see the Silex project, where my team is collecting and generalizing infrastructure code from Spark applications, for more details — or just change this code to set up a
SparkContext as you see fit.)
In order to run this application, you’ll need to set its classpath, and
sbt gives you a way to do find out exactly what environment it would be using so you can run the application manually.2 First, make sure you have a copy of
sbt-extras either in your repository or somewhere pointed to by
SBT in your environment. Then, create a shell script that looks like this:
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You can then run
repl.sh and get a Scala REPL that has all of your app’s dependencies and classes loaded and will let you experiment with structured data manipulation in Spark.
Frustratingly, apps that use these features will work, since the classes Scala reflection depends on will be loaded by the bootstrap classloader, and test cases will work as long as you have
sbtfork a new JVM to execute them! Unfortunately,
sbtcannot currently fork a new JVM to run a console.↩
run-maintask is the right way to run most applications from
sbt, but it seems to be somewhat flaky when launching interactive console applications.↩