KafkaSource

Note
Kafka topics are checked for new records every trigger and so there is some noticeable delay between when the records have arrived to Kafka topics and when a Spark application processes them.

KafkaSource uses the streaming metadata log directory to persist offsets. The directory is the source ID under the sources directory in the checkpointRoot (of the StreamExecution).

Note

The checkpointRoot directory is one of the following:

KafkaSource is created for kafka format (that is registered by KafkaSourceProvider).

KafkaSource creating instance.png
Figure 1. KafkaSource Is Created for kafka Format by KafkaSourceProvider

KafkaSource uses a predefined (fixed) schema (that cannot be changed).

KafkaSource also supports batch Datasets.

Table 1. KafkaSource’s Internal Registries and Counters
Name Description

currentPartitionOffsets

Current partition offsets (as Map[TopicPartition, Long])

Initially NONE and set when KafkaSource is requested to get the maximum available offsets or generate a DataFrame with records from Kafka for a batch.

Tip

Enable INFO or DEBUG logging levels for org.apache.spark.sql.kafka010.KafkaSource to see what happens inside.

Add the following line to conf/log4j.properties:

log4j.logger.org.apache.spark.sql.kafka010.KafkaSource=DEBUG

Refer to Logging.

rateLimit Internal Method

rateLimit(
  limit: Long,
  from: Map[TopicPartition, Long],
  until: Map[TopicPartition, Long]): Map[TopicPartition, Long]

rateLimit requests KafkaOffsetReader to fetchEarliestOffsets.

Caution
FIXME
Note
rateLimit is used exclusively when KafkaSource gets available offsets (when maxOffsetsPerTrigger option is specified).

getSortedExecutorList Method

Caution
FIXME

reportDataLoss Internal Method

Caution
FIXME
Note

reportDataLoss is used when KafkaSource does the following:

Generating DataFrame with Records From Kafka for Streaming Batch — getBatch Method

getBatch(start: Option[Offset], end: Offset): DataFrame
Note
getBatch is a part of Source Contract.

getBatch initializes initial partition offsets (unless initialized already).

You should see the following INFO message in the logs:

INFO KafkaSource: GetBatch called with start = [start], end = [end]

getBatch requests KafkaSourceOffset for end partition offsets for the input end offset (known as untilPartitionOffsets).

getBatch requests KafkaSourceOffset for start partition offsets for the input start offset (if defined) or uses initial partition offsets (known as fromPartitionOffsets).

getBatch finds the new partitions (as the difference between the topic partitions in untilPartitionOffsets and fromPartitionOffsets) and requests KafkaOffsetReader to fetch their earliest offsets.

getBatch reports a data loss if the new partitions don’t match to what KafkaOffsetReader fetched.

Cannot find earliest offsets of [partitions]. Some data may have been missed

You should see the following INFO message in the logs:

INFO KafkaSource: Partitions added: [partitionOffsets]

getBatch reports a data loss if the new partitions don’t have their offsets 0.

Added partition [partition] starts from [offset] instead of 0. Some data may have been missed

getBatch reports a data loss if the fromPartitionOffsets partitions differ from untilPartitionOffsets partitions.

[partitions] are gone. Some data may have been missed

You should see the following DEBUG message in the logs:

DEBUG KafkaSource: TopicPartitions: [comma-separated topicPartitions]

getBatch gets the executors (sorted by executorId and host of the registered block managers).

Important
That is when getBatch goes very low-level to allow for cached KafkaConsumers in the executors to be re-used to read the same partition in every batch (aka location preference).

You should see the following DEBUG message in the logs:

DEBUG KafkaSource: Sorted executors: [comma-separated sortedExecutors]

getBatch creates a KafkaSourceRDDOffsetRange per TopicPartition.

getBatch filters out KafkaSourceRDDOffsetRanges for which until offsets are smaller than from offsets. getBatch reports a data loss if they are found.

Partition [topicPartition]'s offset was changed from [fromOffset] to [untilOffset], some data may have been missed

getBatch creates a KafkaSourceRDD (with executorKafkaParams, pollTimeoutMs and reuseKafkaConsumer flag enabled) and maps it to an RDD of InternalRow.

Important
getBatch creates a KafkaSourceRDD with reuseKafkaConsumer flag enabled.

You should see the following INFO message in the logs:

INFO KafkaSource: GetBatch generating RDD of offset range: [comma-separated offsetRanges sorted by topicPartition]

getBatch sets currentPartitionOffsets if it was empty (which is when…​FIXME)

In the end, getBatch creates a DataFrame from the RDD of InternalRow and schema.

Fetching Offsets (From Metadata Log or Kafka Directly) — getOffset Method

getOffset: Option[Offset]
Note
getOffset is a part of the Source Contract.

Internally, getOffset fetches the initial partition offsets (from the metadata log or Kafka directly).

KafkaSource initialPartitionOffsets.png
Figure 2. KafkaSource Initializing initialPartitionOffsets While Fetching Initial Offsets
Note
initialPartitionOffsets is a lazy value and is initialized the very first time getOffset is called (which is when StreamExecution constructs a streaming batch).
scala> spark.version
res0: String = 2.3.0-SNAPSHOT

// Case 1: Checkpoint directory undefined
// initialPartitionOffsets read from Kafka directly
val records = spark.
  readStream.
  format("kafka").
  option("subscribe", "topic1").
  option("kafka.bootstrap.servers", "localhost:9092").
  load
// Start the streaming query
// dump records to the console every 10 seconds
import org.apache.spark.sql.streaming.{OutputMode, Trigger}
import scala.concurrent.duration._
val q = records.
  writeStream.
  format("console").
  option("truncate", false).
  trigger(Trigger.ProcessingTime(10.seconds)).
  outputMode(OutputMode.Update).
  start
// Note the temporary checkpoint directory
17/08/07 11:09:29 INFO StreamExecution: Starting [id = 75dd261d-6b62-40fc-a368-9d95d3cb6f5f, runId = f18a5eb5-ccab-4d9d-8a81-befed41a72bd] with file:///private/var/folders/0w/kb0d3rqn4zb9fcc91pxhgn8w0000gn/T/temporary-d0055630-24e4-4d9a-8f36-7a12a0f11bc0 to store the query checkpoint.
...
INFO KafkaSource: Initial offsets: {"topic1":{"0":1}}

// Stop the streaming query
q.stop

// Case 2: Checkpoint directory defined
// initialPartitionOffsets read from Kafka directly
// since the checkpoint directory is not available yet
// it will be the next time the query is started
val records = spark.
  readStream.
  format("kafka").
  option("subscribe", "topic1").
  option("kafka.bootstrap.servers", "localhost:9092").
  load.
  select($"value" cast "string", $"topic", $"partition", $"offset")
import org.apache.spark.sql.streaming.{OutputMode, Trigger}
import scala.concurrent.duration._
val q = records.
  writeStream.
  format("console").
  option("truncate", false).
  option("checkpointLocation", "/tmp/checkpoint"). // <-- checkpoint directory
  trigger(Trigger.ProcessingTime(10.seconds)).
  outputMode(OutputMode.Update).
  start
// Note the checkpoint directory in use
17/08/07 11:21:25 INFO StreamExecution: Starting [id = b8f59854-61c1-4c2f-931d-62bbaf90ee3b, runId = 70d06a3b-f2b1-4fa8-a518-15df4cf59130] with file:///tmp/checkpoint to store the query checkpoint.
...
INFO KafkaSource: Initial offsets: {"topic1":{"0":1}}
...
INFO StreamExecution: Stored offsets for batch 0. Metadata OffsetSeqMetadata(0,1502098526848,Map(spark.sql.shuffle.partitions -> 200, spark.sql.streaming.stateStore.providerClass -> org.apache.spark.sql.execution.streaming.state.HDFSBackedStateStoreProvider))

// Review the checkpoint location
// $ ls -ltr /tmp/checkpoint/offsets
// total 8
// -rw-r--r--  1 jacek  wheel  248  7 sie 11:21 0
// $ tail -2 /tmp/checkpoint/offsets/0 | jq

// Produce messages to Kafka so the latest offset changes
// And more importanly the offset gets stored to checkpoint location
-------------------------------------------
Batch: 1
-------------------------------------------
+---------------------------+------+---------+------+
|value                      |topic |partition|offset|
+---------------------------+------+---------+------+
|testing checkpoint location|topic1|0        |2     |
+---------------------------+------+---------+------+

// and one more
// Note the offset
-------------------------------------------
Batch: 2
-------------------------------------------
+------------+------+---------+------+
|value       |topic |partition|offset|
+------------+------+---------+------+
|another test|topic1|0        |3     |
+------------+------+---------+------+

// See what was checkpointed
// $ ls -ltr /tmp/checkpoint/offsets
// total 24
// -rw-r--r--  1 jacek  wheel  248  7 sie 11:35 0
// -rw-r--r--  1 jacek  wheel  248  7 sie 11:37 1
// -rw-r--r--  1 jacek  wheel  248  7 sie 11:38 2
// $ tail -2 /tmp/checkpoint/offsets/2 | jq

// Stop the streaming query
q.stop

// And start over to see what offset the query starts from
// Checkpoint location should have the offsets
val q = records.
  writeStream.
  format("console").
  option("truncate", false).
  option("checkpointLocation", "/tmp/checkpoint"). // <-- checkpoint directory
  trigger(Trigger.ProcessingTime(10.seconds)).
  outputMode(OutputMode.Update).
  start
// Whoops...console format does not support recovery (!)
// Reported as https://issues.apache.org/jira/browse/SPARK-21667
org.apache.spark.sql.AnalysisException: This query does not support recovering from checkpoint location. Delete /tmp/checkpoint/offsets to start over.;
  at org.apache.spark.sql.streaming.StreamingQueryManager.createQuery(StreamingQueryManager.scala:222)
  at org.apache.spark.sql.streaming.StreamingQueryManager.startQuery(StreamingQueryManager.scala:278)
  at org.apache.spark.sql.streaming.DataStreamWriter.start(DataStreamWriter.scala:284)
  ... 61 elided

// Change the sink (= output format) to JSON
val q = records.
  writeStream.
  format("json").
  option("path", "/tmp/json-sink").
  option("checkpointLocation", "/tmp/checkpoint"). // <-- checkpoint directory
  trigger(Trigger.ProcessingTime(10.seconds)).
  start
// Note the checkpoint directory in use
17/08/07 12:09:02 INFO StreamExecution: Starting [id = 02e00924-5f0d-4501-bcb8-80be8a8be385, runId = 5eba2576-dad6-4f95-9031-e72514475edc] with file:///tmp/checkpoint to store the query checkpoint.
...
17/08/07 12:09:02 INFO KafkaSource: GetBatch called with start = Some({"topic1":{"0":3}}), end = {"topic1":{"0":4}}
17/08/07 12:09:02 INFO KafkaSource: Partitions added: Map()
17/08/07 12:09:02 DEBUG KafkaSource: TopicPartitions: topic1-0
17/08/07 12:09:02 DEBUG KafkaSource: Sorted executors:
17/08/07 12:09:02 INFO KafkaSource: GetBatch generating RDD of offset range: KafkaSourceRDDOffsetRange(topic1-0,3,4,None)
17/08/07 12:09:03 DEBUG KafkaOffsetReader: Partitions assigned to consumer: [topic1-0]. Seeking to the end.
17/08/07 12:09:03 DEBUG KafkaOffsetReader: Got latest offsets for partition : Map(topic1-0 -> 4)
17/08/07 12:09:03 DEBUG KafkaSource: GetOffset: ArrayBuffer((topic1-0,4))
17/08/07 12:09:03 DEBUG StreamExecution: getOffset took 122 ms
17/08/07 12:09:03 DEBUG StreamExecution: Resuming at batch 3 with committed offsets {KafkaSource[Subscribe[topic1]]: {"topic1":{"0":4}}} and available offsets {KafkaSource[Subscribe[topic1]]: {"topic1":{"0":4}}}
17/08/07 12:09:03 DEBUG StreamExecution: Stream running from {KafkaSource[Subscribe[topic1]]: {"topic1":{"0":4}}} to {KafkaSource[Subscribe[topic1]]: {"topic1":{"0":4}}}

getOffset requests KafkaOffsetReader to fetchLatestOffsets (known later as latest).

Note
(Possible performance degradation?) It is possible that getOffset will request the latest offsets from Kafka twice, i.e. while initializing initialPartitionOffsets (when no metadata log is available and KafkaSource’s KafkaOffsetRangeLimit is LatestOffsetRangeLimit) and always as part of getOffset itself.

getOffset then calculates currentPartitionOffsets based on the maxOffsetsPerTrigger option.

Table 2. getOffset’s Offset Calculation per maxOffsetsPerTrigger
maxOffsetsPerTrigger Offsets

Unspecified (i.e. None)

latest

Defined (but currentPartitionOffsets is empty)

rateLimit with limit limit, initialPartitionOffsets as from, until as latest

Defined (and currentPartitionOffsets contains partitions and offsets)

rateLimit with limit limit, currentPartitionOffsets as from, until as latest

You should see the following DEBUG message in the logs:

DEBUG KafkaSource: GetOffset: [offsets]

In the end, getOffset creates a KafkaSourceOffset with offsets (as Map[TopicPartition, Long]).

Creating KafkaSource Instance

KafkaSource takes the following when created:

KafkaSource initializes the internal registries and counters.

Fetching and Verifying Specific Offsets — fetchAndVerify Internal Method

fetchAndVerify(specificOffsets: Map[TopicPartition, Long]): KafkaSourceOffset

fetchAndVerify requests KafkaOffsetReader to fetchSpecificOffsets for the given specificOffsets.

fetchAndVerify makes sure that the starting offsets in specificOffsets are the same as in Kafka and reports a data loss otherwise.

startingOffsets for [tp] was [off] but consumer reset to [result(tp)]

In the end, fetchAndVerify creates a KafkaSourceOffset (with the result of KafkaOffsetReader).

Note
fetchAndVerify is used exclusively when KafkaSource initializes initial partition offsets.

Initial Partition Offsets (of 0th Batch) — initialPartitionOffsets Internal Lazy Property

initialPartitionOffsets: Map[TopicPartition, Long]

initialPartitionOffsets is the initial partition offsets for the batch 0 that were already persisted in the streaming metadata log directory or persisted on demand.

As the very first step, initialPartitionOffsets creates a custom HDFSMetadataLog (of KafkaSourceOffsets metadata) in the streaming metadata log directory.

initialPartitionOffsets requests the HDFSMetadataLog for the metadata of the 0th batch (as KafkaSourceOffset).

If the metadata is available, initialPartitionOffsets requests the metadata for the collection of TopicPartitions and their offsets.

If the metadata could not be found, initialPartitionOffsets creates a new KafkaSourceOffset per KafkaOffsetRangeLimit:

initialPartitionOffsets requests the custom HDFSMetadataLog to add the offsets to the metadata log (as the metadata of the 0th batch).

initialPartitionOffsets prints out the following INFO message to the logs:

Initial offsets: [offsets]
Note

initialPartitionOffsets is used when KafkaSource is requested for the following:

HDFSMetadataLog.serialize

serialize(metadata: KafkaSourceOffset, out: OutputStream): Unit
Note
serialize is part of the HDFSMetadataLog Contract to…​FIXME.

serialize requests the OutputStream to write a zero byte (to support Spark 2.1.0 as per SPARK-19517).

serialize creates a BufferedWriter over a OutputStreamWriter over the OutputStream (with UTF_8 charset encoding).

serialize requests the BufferedWriter to write the v1 version indicator followed by a new line.

serialize then requests the KafkaSourceOffset for a JSON-serialized representation and the BufferedWriter to write it out.

In the end, serialize requests the BufferedWriter to flush (the underlying stream).

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