Status
Current state: ACCEPTED
Discussion thread: N/A
JIRA:
Released: Samza 1.0
Purpose
This document outlines a proposal for extending Samza’s Execution Planner to verify agreement in partition count among the stream(s) behind Tables and other streams participating in Stream-Table Joins in applications written using Samza High-Level APIs.
Background
Motivating Example: Stream-Stream Join
In a typical application using Samza high-level API, the user defines a sequence of operations to be performed on a stream of messages using a set of methods defined on the org.apache.samza.operators.MessageStream interface.
For instance, to perform the operations illustrated in Fig. 1 on a stream of messages, a user can write the Samza app in listing 1 using Samza high-level API:
Fig. 1 — A logical workflow of stream processing operations |
public class StreamStreamJoinApp implements StreamApplication { @Override public void init(StreamGraph graph, Config config) { MessageStream s1 = graph .getInputStream("S1") .filter(/* Omitted for brevity */); MessageStream s2 = graph .getInputStream("S2"); OutputStream s3 = graph.getOutputStream("S3"); s1.join(s2, /* Omitted for brevity */) .sendTo(s3); } } |
Listing 1 — Sample application using Samza high-level API to perform Stream-Stream Join |
This code gets translated by Samza’s core libraries into an internal representation where every operation/transformation is represented by an OperatorSpec
object, a logical abstraction that describes the operation specified by the app author.
Fig. 2 — An illustration of the OperatorSpec graph of objects generated by Samza for the application in listing 1. OperatorSpecs associated with input/output streams are highlighted in yellow. |
Some of these OperatorSpecs
represent message streams which can be thought of as channels propagating data into and out of the application, e.g.
InputOperatorSpecs
represent input data streams, e.g. S1 and S2, from which input messages are read.OutputOperatorSpecs
represent output data streams, e.g. S3, to which processed data is produced.
A common example of such message streams is Kafka topics which can be used to carry data consumed or produced by Samza applications. But the fact that Kafka topics are typically sharded into different partitions presents an interesting challenge to some stream operations. In the application above, for instance, it is specified that a Join operation is to be performed between a filtered version of data stream S1 and data stream S2. But in the case where these data streams are Kafka topics, Samza has to ensure that these Kafka topics are — at the very least — sharded into the same number of partitions, or else the Join operation would be invalid.
Responsibilities of Samza’s Execution Planner
Verifying Compatibility of Joined Input Streams
The Execution Planner is the core Samza module responsible for verifying that all streams participating in any given Join operation agree in partition count. To achieve this, it traverses the graph of OperatorSpec
s produced by Samza High-Level API to verify compliance to this requirement among all such sets of streams.
Fig. 3 — 2 examples cases of Stream-Stream Joins. After considering the partition counts of the joined input streams, Samza’s Execution Planner accepts the one to the left but rejects the one to the right. |
Inferring Partition Counts of Intermediate Streams
Another closely-related responsibility of Samza’s Execution Planner is inferring partition counts of all intermediate streams present in the OperatorSpec
graph. Such streams are introduced into the OperatorSpec
graph whenever the Partition-By operation is used, and are represented by the same type of OperatorSpec
s used to represent input streams, i.e. InputOperatorSpec
. Unlike input streams however, intermediate streams have no defined partition counts by default. As we said, it is the Execution Planner that decides the partition count of every intermediate stream after traversing the OperatorSpec
graph, according to the following rules:
Any intermediate stream joined with an input stream gets assigned the same partition count as that input stream.
Any intermediate stream not covered by the first rule is assigned the partition count value specified by the Samza config property
job.intermediate.stream.partitions
.If no value is specified for
job.intermediate.stream.partitions
, the Execution Planner falls back to using the maximum partition count among all input and output streams, capped at a maximum hard-coded value of 256.
Fig. 4 — The OperatorSpec graph of an example high-level Samza application that employs the Partition-By operation. The Execution Planner decides to assign the partition count value 16 to intermediate stream S2′, the same value of input stream S1, since they are joined together. |
It is important to realize there are situations where it is not possible to enforce agreement between an intermediate stream and the input streams it is joined with, a scenario that would cause the Execution Planner to signal an error and reject the whole application. Fig. 5 illustrates one such case.
Fig. 5 — The |
Tables and Stream-Table Joins
A recent addition to Samza is the introduction of Table, a key-value abstraction that facilitates accessing remotely stored data. And with this addition, it was also made possible to perform Join operations between Tables and streams.
The code sample below demonstrates how this can be achieved using Samza High-Level API.
public class StreamTableJoinApp implements StreamApplication { @Override public void init(StreamGraph graph, Config config) { MessageStream s1 = graph.getInputStream("S1"); MessageStream s1Prime = s1.partitionBy(/* Omitted for brevity */); // Assume local table Table t = graph.getTable(/* Omitted for brevity */); s1Prime.sendTo(t); MessageStream s2 = graph.getInputStream("S2"); OutputStream s3 = graph.getOutputStream("S3"); s2.join(t, /* Omitted for brevity */) .sendTo(s3); } } |
Listing 2 — Sample application using Samza high-level API to perform Stream-Table Join |
Fig. 6 — A diagram illustrating the logical data flow in the example Samza application in listing 2. Stream S1 is partitioned then sent to table T which is then joined with stream S2. |
But since Tables can be populated with data flowing from input streams (aka local tables), it is still important to ensure that the stream used to populate the table has the same number of partitions as the stream the table is joined with. Failing to do so exposes Stream-Table Joins to the same class of problems Stream-Stream Joins could run into if Samza were to allow joining 2 streams with different partition counts, i.e. invalid Joins.
Side-Input Streams
Another recent addition to Samza that is related to Tables is Side-Input Streams. Simply put, this feature allows Samza application authors to specify that a table should be populated — and constantly updated — with data from one or more input streams. Such streams have been given the name Side-Input Streams or Side-Inputs for short.
As far as Samza’s Execution Planner is concerned, Tables with side-input Streams are not much different from other tables. They can be treated like other tables populated with data from a non-side-input stream. However, there is one key difference that requires treating tables with side-inputs differently. This difference is that, unlike all other streams, side-input streams are not represented in the OperatorSpec
graph. At the time of writing, no InputOperatorSpecs
— or any OperatorSpec
for that matter — are created to represent side-input streams in the OperatorSpec
graph. Only the names of side-input streams are specified in the configuration of the tables they populate. And since Samza’s Execution Planner currently relies on traversing the OperatorSpec
graph to identify groups of input streams related through Joins, accounting for tables with side-inputs will require some special handling.
Problem Definition
At the time of writing, Samza’s Execution Planner does not account for Tables, which means it is simply incapable of fulfilling its responsibilities with Samza High-Level applications performing Stream-Table Joins.
In particular, assuming that we have:
A table T
An input stream S1, with partition count P1, used to populate table T
An input stream S2, with partition count P2, joined with table T
The table below enumerates a number of cases in which Samza’s current Execution Planner does not enforce the necessary constraints on P1 and P2 to ensure the validity of the Stream-Table Join between table T and stream S2.
# | S1 | S2 | Required Constraint |
---|---|---|---|
1 | Input stream | Input stream | P1 must be equal to P2 |
2 | Input stream | Intermediate stream | P2 must be set to P1 |
3 | Intermediate stream | Input stream | P1 must be set to P2 |
4 | Intermediate stream | Intermediate stream | If the result of joining S1 and S2 is subsequently joined with an input stream S3, P1 and P2 must be set to P3. |
Cases #1 and #2 apply equally well if S1 is a side-input stream.
In all these cases, Samza application authors have no defense against essentially invalid Stream-Table Joins.
Problem Analysis
As explained in the Responsibilities of Samza’s Execution Planner section, Samza’s Execution Planner employs traversal of the OperatorSpec
graph to identify groups of input and/or intermediate streams involved in Join operations and verify agreement among their partition counts. To understand the reason why this does not work in the case of Stream-Table Joins, we examine the OperatorSpec
graph generated for the Samza application in the code sample below.
public class StreamTableJoinApp implements StreamApplication { @Override public void init(StreamGraph graph, Config config) { Table<KV<Integer, String>> t = graph .getTable(/* Omitted for brevity */); MessageStream s1 = graph .getInputStream("S1") .filter(/* Omitted for brevity*/); s1.sendTo(t); MessageStream s2 = graph.getInputStream("S2"); OutputStream s3 = graph.getOutputStream("S3"); s2.join(t, /* Omitted for brevity*/) .sendTo(s3); } } |
Listing 3 — Sample application using Samza high-level API to perform Stream-Table Join. |
Fig. 7 — A graph representing the OperatorSpec graph generated by Samza for the application in listing 3. As usual, OperatorSpecs associated with input/output streams are highlighted in yellow. |
It is important to observe the following key differences between this graph and the graph representing the Stream-Stream Join application in Fig. 2:
A different
OperatorSpec
,StreamTableJoinOperator
, is used to represent Stream-Table Join operations.A new terminal
SendToTableOperatorSpec
is used to represent the operation of producing data to a table.The
StreamTableJoinOperatorSpec
is not connected to theSendToTableOperatorSpec
. It only has a reference to the table (TableSpec
) participating in the Stream-Table Join operation.
The disconnect between the SendToTable
and StreamTableJoin
OperatorSpec
s is attributed to the way the OperatorSpec
graph gets constructed, whereby it is modeled exactly after the StreamApplication
defined by the user, i.e.
Chaining
StreamGraph
’sgetInputStream()
,filter()
, andSendTo(Table)
results in anInputOperatorSpec
connected to aFilterOperatorSpec
which is in turn connected to aSendToTableOperatorSpec
.Chaining
join(Table)
andsendTo(Stream)
results in aStreamTableJoinOperatorSpec
connected to anOutputStreamSpec
.
And since no further operations can be chained to sendTo(Table)
according to the current Samza High-Level APIs, a SendToTableOperatorSpec
is always a terminal vertex in the OperatorSpec
graph, and all subsequent operations on a table will have their corresponding OperatorSpec
s in disconnected components of the graph.
Contrasting the OperatorSpec
graph generated in this scenario with that of the Stream-Stream Join application in listing 1 confirms this causation.
Proposed Changes
To extend Samza’s ExecutionPlanner to support Tables, we need to address the disconnect between a SendToTableOperatorSpec
and all relevant StreamTableJoinOperatorSpec
s. One possibility that does not require changing Samza’s High-Level APIs is to modify the OperatorSpec
graph traversal such that virtual connections are assumed between every SendToTableOperatorSpec
and all the StreamTableJoinOperatorSpec
s that reference the same table (TableSpec
) in the entire OperatorSpec
graph.
Fig. 8 — A graph representing the |
As it stands at the time of writing, the implementation of Samza’s Execution Planner requires significant restructuring so it can be easily extended to support Tables. A separate document that explains the current state of the Execution Planner as well as the proposed changes is Modularizing Join Validation in Samza's Execution Planner.