How to get Kafka offsets for structured query for manual and reliable offset management?

Question

Spark 2.2 introduced a Kafka's structured streaming source. As I understand, it's relying on HDFS checkpoint directory to store offsets and guarantee an "exactly-once" message delivery.

But old docks (like https://blog.cloudera.com/blog/2017/06/offset-management-for-apache-kafka-with-apache-spark-streaming/) says that Spark Streaming checkpoints are not recoverable across applications or Spark upgrades and hence not very reliable. As a solution, there is a practice to support storing offsets in external storage that supports transactions like MySQL or RedshiftDB.

If I want to store offsets from Kafka source to a transactional DB, how can I obtain offset from a structured stream batch?

Previously, it can be done by casting RDD to HasOffsetRanges:

val offsetRanges = rdd.asInstanceOf[HasOffsetRanges].offsetRanges    

But with new Streaming API, I have an Dataset of InternalRow and I can't find an easy way to fetch offsets. The Sink API has only addBatch(batchId: Long, data: DataFrame) method and how can I suppose to get an offset for given batch id?

Solution 1

Relevant Spark DEV mailing list discussion thread is here.

Summary from it:

Spark Streaming will support getting offsets in future versions (> 2.2.0). JIRA ticket to follow - https://issues-test.apache.org/jira/browse/SPARK-18258

For Spark <= 2.2.0, you can get offsets for the given batch by reading a json from checkpoint directory (the API is not stable, so be cautious):

val checkpointRoot = // read 'checkpointLocation' from custom sink params
val checkpointDir = new Path(new Path(checkpointRoot), "offsets").toUri.toString
val offsetSeqLog = new OffsetSeqLog(sparkSession, checkpointDir)

val endOffset: Map[TopicPartition, Long] = offsetSeqLog.get(batchId).map { endOffset =>
  endOffset.offsets.filter(_.isDefined).map { str =>
    JsonUtilsWrapper.jsonToOffsets(str.get.json)
  }
}


/**
  * Hack to access private API
  * Put this class into org.apache.spark.sql.kafka010 package
  */
object JsonUtilsWrapper {
  def offsetsToJson(partitionOffsets: Map[TopicPartition, Long]): String = {
    JsonUtils.partitionOffsets(partitionOffsets)
  }

  def jsonToOffsets(str: String): Map[TopicPartition, Long] = {
    JsonUtils.partitionOffsets(str)
  }
}

This endOffset will contain the until offset for each topic/partition. Getting the start offsets is problematic, cause you have to read the 'commit' checkpoint dir. But usually, you don't care about start offsets, because storing end offsets is enough for reliable Spark job re-start.

Please, note that you have to store the processed batch id in your storage as well. Spark can re-run failed batch with the same batch id in some cases, so make sure to initialize a Custom Sink with latest processed batch id (which you should read from external storage) and ignore any batch with id < latestProcessedBatchId. Btw, batch id is not unique across queries, so you have to store batch id for each query separately.

Solution 2

Spark 2.2 introduced a Kafka's structured streaming source. As I understand, it's relying on HDFS checkpoint dir to store offsets and guarantee an "exactly-once" message delivery.

Correct.

Every trigger Spark Structured Streaming will save offsets to offset directory in the checkpoint location (defined using checkpointLocation option or spark.sql.streaming.checkpointLocation Spark property or randomly assigned) that is supposed to guarantee that offsets are processed at most once. The feature is called Write Ahead Logs.

The other directory in the checkpoint location is commits directory for completed streaming batches with a single file per batch (with a file name being the batch id).

Quoting the official documentation in Fault Tolerance Semantics:

To achieve that, we have designed the Structured Streaming sources, the sinks and the execution engine to reliably track the exact progress of the processing so that it can handle any kind of failure by restarting and/or reprocessing. Every streaming source is assumed to have offsets (similar to Kafka offsets, or Kinesis sequence numbers) to track the read position in the stream. The engine uses checkpointing and write ahead logs to record the offset range of the data being processed in each trigger. The streaming sinks are designed to be idempotent for handling reprocessing. Together, using replayable sources and idempotent sinks, Structured Streaming can ensure end-to-end exactly-once semantics under any failure.

Every time a trigger is executed StreamExecution checks the directories and "computes" what offsets have been processed already. That gives you at least once semantics and exactly once in total.

But old docs (...) says that Spark Streaming checkpoints are not recoverable across applications or Spark upgrades and hence not very reliable.

There was a reason why you called them "old", wasn't there?

They refer to the old and (in my opinion) dead Spark Streaming that kept not only offsets but the entire query code that led to situations where the checkpointing were almost unusable, e.g. when you change the code.

The times are over now and Structured Streaming is more cautious what and when is checkpointed.

If I want to store offsets from Kafka source to a transactional DB, how can I obtain offset from a structured stream batch?

A solution could be to implement or somehow use MetadataLog interface that is used to deal with offset checkpointing. That could work.

how can I suppose to get an offset for given batch id?

It is not currently possible.

My understanding is that you will not be able to do it as the semantics of streaming are hidden from you. You simply should not be dealing with this low-level "thing" called offsets that Spark Structured Streaming uses to offer exactly once guarantees.

Quoting Michael Armbrust from his talk at Spark Summit Easy, Scalable, Fault Tolerant Stream Processing with Structured Streaming in Apache Spark:

you should not have to reason about streaming

and further in the talk (on the next slide):

you should write simple queries & Spark should continuously update the answer

---

There is a way to get offsets (from any source, Kafka including) using StreamingQueryProgress that you can intercept using StreamingQueryListener and onQueryProgress callback.

onQueryProgress(event: QueryProgressEvent): Unit Called when there is some status update (ingestion rate updated, etc.)

With StreamingQueryProgress you can access sources property with SourceProgress that gives you what you want.

Solution 3

Streaming Dataset with Kafka source has offset as one of the field. You can simply query for all offsets in query and save them into JDBC Sink