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Working with Pravega: StreamCuts

This section describes StreamCuts and its usage with streaming clients and batch clients.

Pre-requisites

Familiarity with Pravega Concepts.

Definition

A Pravega Stream is formed by one or multiple parallel Stream Segments for storing/reading events. A Pravega Stream is elastic, as it handles the changes in the number of parallel Stream Segments along time to accommodate fluctuating workloads. A StreamCut represents a consistent position in the stream. It contains a set of Stream Segments and offset pairs for a single stream which represents the complete keyspace at a given point in time. The offset always points to the event boundary and hence there will be no offset pointing to an incomplete event.

The StreamCut representing the tail of the stream (with the newest event) is an ever changing one since events can be continuously added to the stream and the StreamCut pointing to the tail of the stream with newer events would have a different value. Similarly the StreamCut representing the head of the stream (with the oldest event) is an ever changing one as the stream retention policy could truncate the stream and the StreamCut pointing to the head of the stream post truncation would have a different value. StreamCut.UNBOUNDED is used to represent such a position in the stream and the user can use it to specify this ever changing stream position (both head and tail of the stream).

It should be noted that StreamCuts obtained using the streaming client and batch client can be used interchangeably.

StreamCut with Reader

A Reader Group is a named collection of Readers that together, in parallel, read Events from a given stream. Every Reader is always associated with a Reader Group. StreamCut(s) can be obtained from a Reader Group using the following APIs:

  • getStreamCuts(): The API io.pravega.client.stream.ReaderGroup.getStreamCuts returns a Map<Stream, StreamCut> which represents the last known Position of the Readers for all the streams managed by the Reader Group.

  • generateStreamCuts(): The API io.pravega.client.stream.ReaderGroup.generateStreamCuts, generates a StreamCut after co-ordinating with all the Readers using io.pravega.client.state.StateSynchronizer. A StreamCut is generated by using the latest Stream Segment read offsets returned by the Readers along with unassigned segments (if any). The configuration ReaderGroupConfig.getGroupRefreshTimeMillis() decides the maximum delay by which the Readers return the latest read offsets of their assigned segments. The StreamCut generated by this API can be used by the application as a reference to a Position in the stream. This is guaranteed to be greater than or equal to the position of the Readers at the point of invocation of the API.

A StreamCut can be used to configure a Reader Group to enable bounded processing of a Stream. The start and/or end StreamCut of a Stream can be passed as part of the Reader Group configuration. The below example shows the different ways to use StreamCuts as part of the Reader Group configuration.

/*
 * The below ReaderGroup configuration ensures that the readers belonging to
 * the ReaderGroup read events from
 *   - Stream "s1" from startStreamCut1 (representing the oldest event) upto
          endStreamCut1 (representing the newest event)
 *   - Stream "s2" from startStreamCut2 upto the tail of the stream, this is similar to using StreamCut.UNBOUNDED
 *        for endStreamCut.
 *   - Stream "s3" from the current head of the stream upto endStreamCut2
 *   - Stream "s4" from the current head of the stream upto the tail of the stream.
 */
ReaderGroupConfig.builder()
        .stream("scope/s1", startStreamCut1, endStreamCut1)
        .stream("scope/s2", startStreamCut2)
        .stream("scope/s3", StreamCut.UNBOUNDED, endStreamCut2)
        .stream("scope/s4")
        .build();

The below API can be used to reset an existing Reader Group with a new Reader Group configuration instead creating a Reader Group.

/*
 * ReaderGroup API used to reset a ReaderGroup to a newer ReaderGroup configuration.
 */
io.pravega.client.stream.ReaderGroup.resetReaderGroup(ReaderGroupConfig config)

StreamCut with Stream Manager

StreamCut representing the current head and current tail of a stream can be obtained using the StreamManager API getStreamInfo(String scopeName, String streamName).

/**
 * Get information about a given Stream, {@link StreamInfo}.
 * This includes {@link StreamCut}s pointing to the current HEAD and TAIL of the Stream.
 *
 * @param scopeName The scope of the stream.
 * @param streamName The stream name.
 * @return stream information.
 */
StreamInfo getStreamInfo(String scopeName, String streamName);

StreamCut with BatchClient

BatchClient can be used to perform bounded processing of the stream given the start and end StreamCuts. BatchClient API io.pravega.client.batch.BatchClient.getSegments(stream, startStreamCut, endStreamCut) is used to fetch segments which reside between the given startStreamCut and endStreamCut. With the retrieved segment information, the user can consume all the events in parallel without adhering to time ordering of events.

It must be noted that passing StreamCut.UNBOUNDED to startStreamCut and endStreamCut will result in using the current head of stream and the current tail of the stream, respectively.

We have provided a simple yet illustrative example of using StreamCut here.