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Table of Contents

Overview

Kafka Streams allows for stateful stream processing, i.e. operators that have an internal state. This internal state is managed in so-called state stores. A state store can be ephemeral (lost on failure) or fault-tolerant (restored after the failure). The default implementation used by Kafka Streams DSL is a fault-tolerant state store using 1. an internally created and compacted changelog topic (for fault-tolerance) and 2. one (or multiple) RocksDB instances (for cached key-value lookups). Thus, in case of starting/stopping applications and rewinding/reprocessing, this internal data needs to get managed correctly.

Current State

We first want to give an overview about the current implementation details of Kafka Streams with regard to (internally) created topics and the usage of RocksDB. We can categorize available transformations for KStream and KTable as shown below. All operators within a category use the same internal state management mechanism. Therefore, we get an overview of the state management strategy for each transformation.

  • single stream tuple-by-tuple: those transformations do not use an internal state — however, if they change the key ("new key") data re-partitioning might be required after the transformation (i.e., writing to and reading from a topic  KAFKA-3561 - Getting issue details... STATUS   KAFKA-3576 - Getting issue details... STATUS )
  • aggregation and joins: those transformations do use internal state (RocksDB plus changelog topic)
  • operations marked with "+ state" allows the usage of user defined state

State management details are given below.

KStream API

KStream currently offers the following methods which do have different implication with regard to (internally) created topics and RocksDB usage.

 

single stream

multiple streams 

data transformation

other

data transformation

tuple-by-tuple
(KStream -> KStream)

aggregation

tuple-by-tuple (i.e., joins) 

same key

new key

non-windowed
(KStream -> KTable)

windowed

(KTable -> KTable<<W,K> V>

non-windowed

KStream-KTable

windowed

KStream-KStream 

filter

(1:[0,1])

 

aggregateByKey

aggregateByKey

print

join (as of 0.10.2)join

filterNot

(1:[0,1])

 

reduceByKey

reduceByKey

writeAsText (deprecated as of 1.0)

leftJoinleftJoin
 

selectKey

(1:1)

countByKey

countByKey

foreach

 outerJoin

mapValues

(1:1)

map

(1:1)

  

through

merge (as of 1.0) 

flatMapValues

(1:n)

flatMap

(1:n)

  

to

  

transformValues

(1:1 + state)

transform

(1:n + state)

  

branch

  
 

process

(1:0 + state)

  peek (as of 0.11.0)  

 

KTable API

KTable currently offers the following methods which do have different implication with regard to (internally) created topics and RocksDB usage.  KAFKA-3576 - Getting issue details... STATUS

 

single stream

multiple streams

data transformation

other

data transformation

tuple-by-tuple
(KTable -> KTable/KGroupedTable)

aggregation

(KGroupedTable -> KTable)

tuple-by-tuple

(KTable-KTable joins)
 

 

same key

(-> KTable)

new key

(-> KGroupedTable)

filter

(1:[0,1])

 

aggregate

print (deprecated as of 1.0)

join

filterNot

(1:[0,1])

 

reduce

writeAsText (deprecated as of 1.0)

leftJoin

mapValues

(1:1)

 

count

foreach (deprecated as of 1.0)

outerJoin

 

groupBy

(1:1) [internally simple map]

 

through (deprecated as of 1.0)

 
   

to (deprecated as of 1.0)

 
   

toStream

 

 

 

Data Management

Overview

There are four methods to explicitly deal with user topics:

  1. KStreamBuilder#stream() for consuming
  2. KStreamBuilder#table() for consuming
  3. KStream/KTable#to() for writing

  4. KStream/KTable#through() for writing and reading again.

User topics are required to be created by the user before Kafka Streams application is started. Furthermore, an interal topic is created each time re-partitioning is required (via an ingested through()  KAFKA-3561 - Getting issue details... STATUS  ); this happens when the key is changed before an aggregation is performed (not necessarily directly after each other):

// Pseudo code
KStream source = builder.stream(...)
source.someTransformation().newKey().someTransformation().someAgg(); // newKey() refers to any transformation that changes the key

Here's an illustration of the above pseudo-code topology:

 


This implies that after “new key” there was no to()/through() performed. The aggregation itself uses a RocksDB instance as key-value state store that also persists to local disk. Flushing to disk happens asynchronously. Furthermore, an internal compacted changelog topic is created. The state store sends changes to the changelog topic in a batch, either when a default batch size has been reached or when the commit interval (see "Commits" below) is reached.

 

 

RocksDB is just used as an internal lookup table (that is able to flush to disk if the state does not fit into memory KAFKA-3776 - Getting issue details... STATUS ) and the internal changelog topic is created for fault-tolerance reasons. Thus, the changelog topic is the source of truth for the state (= the log of the state), while RocksDB is used as (non-fault tolerant) cache. RocksDB cannot be used for fault-tolerance because flushing happens to local disk, and it cannot be controlled when flushing happens. RocksDB flushing is only required because state could be larger than available main-memory. Thus, the internal changelog topic is used for fault-tolerance: If a task crashes and get restarted on different machine, this internal changelog topic is used to recover the state store. Currently, the default replication factor of internal topics is 1.

There are two main differences between non-windowed and windowed aggregation with regard to key-design. For window aggregation the key is <K,W>, i.e., for each window a new key is used. Thus, the memory usage grows over time ( KAFKA-4015 - Getting issue details... STATUS ), even if the key-space is bounded (i.e., the number of unique keys). This implies that log-compaction cannot purge any old data. The second difference is about RocksDB instances: instead of using a single instance, Streams uses multiple instances (called “segments”) for different time periods. After the window retention time has passed old segments can be dropped. Thus, RocksDB memory requirement does not grow infinitely (in contrast to changelog topic). (KAFKA-4015 was fixed in 0.10.1 release, and windowed changelog topics don't grow unbounded as they apply an additional retention time parameter).

For KTable a similar behavior applies. Using groupBy().someAgg() results in internal topic and RocksDB creation.

For stateful KStream transformation (transformtransformValue, and process) an explicit state store is used. Depending on the use state store, a changelog topic might get created.

For joins, one or two internal state stores (RocksDB plus internal changelog topic) are used. Behavior is same as for aggregates. Joins can also be windowed (see window aggregates).


Related Work to state stores:  KAFKA-3909 - Getting issue details... STATUS

Commits

Kafka Streams commit the current processing progress in regular intervals (parameter commit.interval.ms). If a commit is triggered, all state stores need to flush data to disk, i.e., all internal topics needs to get flushed to Kafka. Furthermore, all user topics get flushed, too. Finally, all current topic offsets are committed to Kafka. In case of failure and restart, the application can resume processing from its last commit point (providing at-least-once processing guarantees).

Internal Topics and State Store Names

Currently in the Streams DSL we are trying to abstract the auto generated internal topics and state store names as "KTable names" and "window names"; however, in future release all state store name will be exposed to the user.  KAFKA-3870 - Getting issue details... STATUS Internal topics follow the naming convention <application.id>-<operatorName>-<suffix>; this naming convention might change any time in.


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