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Often making errors helps to progress. It was my case with spark-submit and local/remote JAR pair. They helped me to understand the role of driver, closures, serialization and some configuration properties.
Even if a lot of Docker containers exist for Apache Spark, it's always a good exercise to make one in your own. It can help to understand some new concepts as well as improve skills of building Docker images.
Broadcast variables send object to executors only once and can be easily used to reduce network transfer and thus are precious in terms of distributed computing.
updateStateByKey function, explained in the post about Stateful transformations in Spark Streaming, is not the single solution provided by Spark Streaming to deal with state. Another one, much more optimized, is mapWithState.
Some time ago I was wondering why an object created once in the driver is recreated every time with new stage on executors - even if this object is sent through a broadcast variable. After some code digging, the response related to Java serialization appeared.
Some of previous posts (Serialization issues - part 1) presented some of solutions for serialization problems. This post is its continuation.
Issues with not serializable objects are maybe the most painful when we start to work with Spark. But hopefully there are several solutions to them.
Spark has 2 deployment modes that can be controlled in fine-grained way thanks to master URL property.
One of previous posts talked about checkpoint types in Spark Streaming. This one focuses more on one type of them - metadata checkpoint.
Even if it's always better to explicit things, in programming we have often the possibility to let the computer to guess. Spark SQL also has this level of intelligence, for example during schema resolving.
Keeping in mind which parts of Spark code are executed on driver and which ones on workers is important and can help to avoid some of annoying errors, as the ones related to serialization.
As every library, Spark has methods than are used more often than the others. As often used methods we could certainly define map or filter. In the other side of less popular transformations we could place, among others, tree-like methods that will be presented in this post.
In general Spark's actions reflects logic implemented in a lot of equivalent methods in programming languages. As an example we can consider isEmpty() that in Spark checks the existence of only 1 element and similarly in Java's List. But it can often lead to troubles, especially when more than 1 action is invoked.
After writing a post about testing Spark applications, I decided to take a look at Spark project tests and see which patterns they use to verify framework features.
It's difficult to contest the importance of testing in programming. Tests help to avoid regressions (a lot of regressions) and also to better understand developed code. Spark (and other data processing frameworks by the way) is not an exception of this rule. But, obviously, testing applications working in distributed mode is more tricky than in the case of standalone programs.
Metadata checkpoint is useful in quickly restoring failing jobs. However, it won't work if the context creation and processing parts aren't declared correctly.
Project Tungsten, explained in one of previous posts, brought a lot of optimizations - especially in terms of memory use. Until now it was essentially used by Spark SQL and Spark MLib projects. However, since 2.0.0, some work was done to integrate DataFrame/Dataset in streaming processing (Spark Streaming).
Every distributed computation is divided in small parts called jobs, stages and tasks. It's useful to know them especially during monitoring because it helps to detect bottlenecks.
Spark SQL schema is very flexible. It supports global data types, as booleans, integers, strings, but it also supports custom data types called User Defined Type (UDT).
Spark SQL - even if the SQL suffix makes automatically think about RDBMS - works well with other data sources, as even plain CSVs or JSON files. This relation would be difficult to achieve without the concept of schema.
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