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Scala eXchange: Building robust data pipelines in Scala

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Over the past couple of years, Scala has become a go-to language for building data processing applications, as evidenced by the emerging ecosystem of frameworks and tools including LinkedIn's Kafka, Twitter's Scalding and our own Snowplow project (

In this talk, Alex will draw on his experiences at Snowplow to explore how to build rock-sold data pipelines in Scala, highlighting a range of techniques including:

* Translating the Unix stdin/out/err pattern to stream processing
* "Railway oriented" programming using the Scalaz Validation
* Validating data structures with JSON Schema
* Visualizing event stream processing errors in ElasticSearch

Alex's talk draws on his experiences working with event streams in Scala over the last two and a half years at Snowplow, and by Alex's recent work penning Unified Log Processing, a Manning book.

Publicado en: Software
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Scala eXchange: Building robust data pipelines in Scala

  1. 1. Building robust data pipelines in Scala: the Snowplow experience
  2. 2. Introducing myself • Alex Dean • Co-founder and technical lead at Snowplow, the open-source event analytics platform based here in London [1] • Weekend writer of Unified Log Processing, available on the Manning Early Access Program [2] [1] [2]
  3. 3. Snowplow – what is it?
  4. 4. Snowplow is an open source event analytics platform 1a. Trackers 2. Collectors 3. Enrich 4. Storage 5. AnalyticsB C D A D Standardised data protocols 1b. Webhooks A • Your granular, event-level and customer-level data, in your own data warehouse • Connect any analytics tool to your data • Join your event data with any other data set
  5. 5. Today almost all users/customers are running a batch-based Snowplow configuration Hadoop- based enrichment Snowplow event tracking SDK Amazon Redshift Amazon S3 HTTP-based event collector • Batch-based • Normally run overnight; sometimes every 4-6 hours
  6. 6. We also have a real-time pipeline for Snowplow in beta, built on Amazon Kinesis (Apache Kafka support coming next year) scala- stream- collector scala- kinesis- enrich S3 Redshift S3 sink Kinesis app Redshift sink Kinesis app Snowplow Trackers = not yet released kinesis- elasticsearch- sink DynamoDB Elastic- search Event aggregator Kinesis app Analytics on Read for agile exploration of events, machine learning, auditing, re- processing… Analytics on Write for operational reporting, real-time dashboards, audience segmentation, personalization… Raw event stream Bad raw event stream Enriched event stream
  7. 7. Snowplow and Scala
  8. 8. Today, Snowplow is primarily developed in Scala Data modelling scripts • Used for Snowplow orchestration • No event-level processing occurs in Ruby • Used for event validation, enrichment and other processing • Increasingly used for event storage • Starting to be used for event collection too
  9. 9. Our initial skunkworks version of Snowplow had no Scala  Website / webapp Snowplow data pipeline v1 CloudFront- based pixel collector HiveQL + Java UDF “ETL” Amazon S3 JavaScript event tracker
  10. 10. But our schema-first, loosely coupled approach made it possible to start swapping out existing components… Website / webapp Snowplow data pipeline v2 CloudFront- based event collector Scalding- based enrichment JavaScript event tracker HiveQL + Java UDF “ETL” Amazon Redshift / PostgreSQL Amazon S3 or Clojure- based event collector
  11. 11. What is Scalding? • Scalding is a Scala API over Cascading, the Java framework for building data processing pipelines on Hadoop: Hadoop DFS Hadoop MapReduce Cascading Hive Pig Java Scalding Cascalog PyCascading cascading. jruby
  12. 12. We chose Cascading because we liked their “plumbing” abstraction over vanilla MapReduce
  13. 13. Why did we choose Scalding instead of one of the other Cascading DSLs/APIs? • Lots of internal experience with Scala – could hit the ground running (only very basic awareness of Clojure when we started the project) • Scalding created and supported by Twitter, who use it throughout their organization – so we knew it was a safe long-term bet • More controversial opinion (although maybe not at a Scala conference): we believe that data pipelines should be as strongly typed as possible – all the other DSLs/APIs on top of Cascading encourage dynamic typing
  14. 14. Robust data pipelines
  15. 15. Robust data pipelines means strongly typed data pipelines – why? • Catch errors as soon as possible – and report them in a strongly typed way too • Define the inputs and outputs of each of your data processing steps in an unambiguous way • Forces you to formerly address the data types flowing through your system • Lets you write code like this:
  16. 16. Robust data processing is a state of mind: failures will happen, don’t panic, but don’t sweep them under the carpet either • Our basic processing model for Snowplow looks like this: • Looks familiar? stdin, stdout, stderr Raw events Snowplow enrichment process “Bad” raw events + reasons why they are bad “Good” enriched events
  17. 17. This pattern is extremely composable, especially with Kinesis or Kafka streams/topics as the core building block
  18. 18. Validation, the “gateway drug” to Scalaz
  19. 19. Inside and across our components, we use the Validation applicative functor from the Scalaz project extensively • Scalaz Validation lets us perform a variety of different event validations and enrichments, and then compose (i.e. collate) the failures • This is really powerful! • The Scalaz codebase calls |@| a “DSL for constructing Applicative expressions” – I think of it as “the Scream operator” • Individual components of the enrichment process can themselves collate their own internal failures
  20. 20. There is a great F# article by Scott Wlaschin which describes this approach as “railway-oriented programming” [1] The Happy Path • If everything succeeds, then this path outputs an enriched event • Any individual failure along the path could switch us onto the failure path • We never get back onto the happy path once we leave it The Failure Path • Any failure can take us onto the failure path • We can choose whether to switch straight to the failure path (“fail fast”), or collate failures from multiple independent tests [1]
  21. 21. Putting it all together, the Snowplow enrichment process boils down to one big type transformation • Types abstracting over simpler types • No mutable state • Railway-oriented programming • Collate failures inside a processing stage, fail fast between processing stages
  22. 22. • Using Scott Wlaschin’s “fruit as cargo” metaphor: • Currently Snowplow uses a Non-Empty List of Strings to collect our failures: • We are working on a ProcessingMessage case class, to capture much richer and more structured failures than we can using Strings The only limitation is that the Failure Path restricts us to a single type
  23. 23. A brief aside on testing
  24. 24. On the testing side: we love Specs2 data tables… • They let us test a variety of inputs and expected outputs without making the mistake of just duplicating the data processing functionality in the test:
  25. 25. … and are starting to do more with ScalaCheck • ScalaCheck is a property-based testing framework, originally inspired by Haskell’s QuickCheck • We use it in a few places – including to generate unpredictable bad data and also to validate our new Thrift schema for raw Snowplow events:
  26. 26. Robustness in the face of user-defined types
  27. 27. Snowplow is evolving from a fixed-schema platform to a platform supporting user-defined JSONs • Where other analytics tools depend on schema-less JSONs or custom variables, we use JSON Schema • Snowplow users send in events as “self-describing JSONs” which have to include the schema URI which validates the event’s JSON body:
  28. 28. To support JSON Schema, we have open-sourced Iglu, a new schema repository system in Scala/Spray/Swagger/Jackson
  29. 29. Our Scala client library for Iglu lets us work with JSONs in a safe way from within Snowplow • If a JSON passes its JSON Schema validation, we should be able to deserialize it and work with it safely in Scala in a strongly-typed way: • We use json4s with the Jackson bindings, as JSON Schema support in Java/Scala is Jackson-based • We still wrap our JSON deserialization in Scalaz Validations in case of any mismatch between the Scala deserialization code and the JSON schema
  30. 30. Questions? @snowplowdata To meet up or chat, @alexcrdean on Twitter or Discount code: ulogprugcf (43% off Unified Log Processing eBook)