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Pravega Streaming Service Wire Protocol¶

This page describes the proposed Wire Protocol for the Streaming Service. See Pravega Concepts for more information.

Protocol¶

Data is sent over the wire in self-contained "messages" that are either "requests" (messages sent from the client to the server) or "replies" (responses sent from the server to the client).

All the requests and replies have 8 byte headers with two fields (all data is written in BigEndian format).

Message Type: An Integer (4 bytes) identifies the message type and determines the subsequent fields. (Note that the protocol can be extended by adding new types.)
Length: An Integer (4 bytes) (Messages should be less than 2²⁴, but the upper bits remain zero). Payload size of the message (possibly zero, indicating there is no data). The remainder of the fields are specific to the type of the message. A few important messages are listed below.

Protocol Primitive Types¶

The protocol is built out of the following primitive types.

Type	Description
BOOLEAN (1 byte)	Values 0 and 1 are used to represent False and True respectively. When reading a boolean value, any non-zero value is considered true.
STRING (2 bytes)	A sequence of characters. The first 2 bytes are used to indicate the byte length of the UTF-8 encoded character sequence, which is non-negative. This is followed by the UTF-8 encoding of the string.
VARLONG (8 bytes)	An Integer between -2⁶³ and 2⁶³-1 inclusive. Encoding follows the variable-length zig-zag encoding from Google Protocol Buffers.
INT (4 bytes)	An Integer between -2³¹ and 2³¹-1 inclusive.
UUID (16 bytes)	Universally Unique Identifiers (UUID) as defined by RFC 4122, ISO/IEC 9834-8:2005, and related standards. It can be used as a global unique 128-bit identifier.

Reading¶

Read Segment - Request¶

Field	Datatype	Description
`Segment`	String	The Stream Segment that was read.
`Offset`	Long	The `Offset` in the Stream Segment to read from.
`suggestedLength` of Reply	Integer	The clients can request for the required length to the server (but the server may allot a different number of bytes.
`delegationToken`	String	This was added to perform auth. It is an opaque-to-the-client token provided by the Controller that says it's allowed to make this call.
`RequestId`	Long	The client-generated ID that identifies a client request.

More information on Segment Request messages like MergeSegment, SealSegment, TruncateSegment and DeleteSegment, can be found here.

Segment Read - Reply¶

Field	Datatype	Description
`Segment`	String	This Segment indicates the Stream Segment that was read.
`Offset`	Long	The `Offset` in the Stream Segment to read from.
`Tail`	Boolean	If the read reached the tail of the Stream Segment.
`EndOfSegment`	Boolean	If the read reached the end of the Stream Segment.
`Data`	Binary	Remaining length in the message.
`RequestId`	Long	The client-generated ID that identifies a client request.

The client requests to read from a particular Segment at a particular Offset. It then receives one or more replies in the form of SegmentRead messages. These contain the data they requested (assuming it exists). The server may decide transferring to the client more or less data than it was asked for, splitting that data in a suitable number of reply messages.

More information on Segment Reply messages like SegmentIsSealed,SegmentIsTruncated, SegmentAlreadyExists,NoSuchSegment and TableSegmentNotEmpty, can be found here.

Appending¶

Setup Append - Request¶

Field	Datatype	Description
`RequestId`	Long	The client-generated ID that identifies a client request.
`writerId`	UUID	Identifies the requesting appender.
`Segment`	String	This Segment indicates the Stream Segment that was read.
`delegationToken`	String	This was added to perform auth. It is an opaque-to-the-client token provided by the Controller that says it's allowed to make this call.

Append Setup - Reply¶

Field	Datatype	Description
`RequestId`	Long	The client-generated ID that identifies a client request.
`Segment`	String	This Segment indicates the Stream Segment to append to.
`writerId`	UUID	Identifies the requesting appender. This ID is used to identify the Segment for which an AppendBlock is destined.
`lastEventNumber`	Long	Specifies the last event number in the Stream.

AppendBlock - Request¶

Field	Datatype	Description
`writerId`	UUID	Identifies the requesting appender.
`Data`	Binary	This holds the contents of the block.
`RequestId`	Long	The client-generated ID that identifies a client request.

AppendBlockEnd - Request¶

Field	Datatype	Description
`writerId`	UUID	Identifies the requesting appender.
`sizeOfWholeEvents`	Integer	The total number of bytes in this block (starting from the beginning) that is composed of whole (meaning non-partial) events.
`Data`	Binary	This holds the contents of the block.
`numEvents`	Integer	Specifies the current number of events.
`lastEventNumber`	Long	Specifies the value of last event number in the Stream.
`RequestId`	Long	The client-generated ID that identifies a client request.

The ApppendBlockEnd has a sizeOfWholeEvents to allow the append block to be less than full. This allows the client to begin writing a block before it has a large number of events. This avoids the need to buffer up events in the client and allows for lower latency.

Partial Event - Request/Reply¶

Data: A Partial Event is an Event at the end of an Append block that did not fully fit in the Append block. The remainder of the Event will be available in the AppendBlockEnd.

Event - Request¶

Field	Description
`Data`	Specifies the Event's data (only valid inside the block).

Data Appended - Reply¶

Field	Datatype	Description
`writerId`	UUID	Identifies the requesting appender.
`eventNumber`	Long	This matches the `lastEventNumber` in the append block.
`previousEventNumber`	Long	This is the previous value of `eventNumber` that was returned in the last `DataAppeneded`.
`RequestId`	Long	The client-generated ID that identifies a client request.

When appending a client:

Establishes a connection to the host chosen by it.
Sends a "Setup Append" request.
Waits for the "Append Setup" reply.

After receiving the "Append Setup" reply, it performs the following:

Send a AppendBlock request.
Send as many Events that can fit in the block.
Send an AppendBlockEnd request.

While this is happening, the server will be periodically sending it DataAppended replies acking messages. Note that there can be multiple "Appends Setup" for a given TCP connection. This allows a client to share a connection when producing to multiple Segments.

A client can optimize its appending by specifying a large value in it's AppendBlock message, as the events inside of the block do not need to be processed individually.

Segment Attribute¶

GetSegmentAttribute - Request¶

Field	Datatype	Description
`RequestId`	Long	The client-generated ID that identifies a client request.
`SegmentName`	String	The Segment to retrieve the attribute from.
`attributeId`	UUID	The attribute to retrieve.
`delegationToken`	String	This was added to perform auth. It is an opaque-to-the-client token provided by the Controller that says it's allowed to make this call.

SegmentAtrribute - Reply¶

Field	Datatype	Description
`RequestId`	Long	The client-generated ID that identifies a client request.
`Value`	Long	The value of the attribute.

More information on SegmentAttribute Request message like updateSegmentAttribute and Reply message like SegmentAttributeUpdate can be found here.

TableSegment¶

ReadTable - Request¶

Field	Datatype	Description
`RequestId`	Long	The client-generated ID that identifies a client request.
`Segment`	String	The Stream Segment that was read.
`delegationToken`	String	This was added to perform auth. It is an opaque-to-the-client token provided by the Controller that says it's allowed to make this call.
`keys`	List	The version of the key is always set to `io.pravega.segmentstore.contracts.tables.TableKey.NO_VERSION`.

More information on TableSegments Request messages like MergeTableSegments, SealTableSegment, DeleteTableSegment, UpdateTableEntries, RemoveTableKeys, ReadTableKeys and ReadTableEntries can be found here.

TableRead - Reply¶

Field	Datatype	Description
`RequestId`	Long	The client-generated ID that identifies a client request.
`Segment`	String	The Stream Segment that was read.
`Entries`	TableEntries	The entries of the Table that was read.

More information on TableSegments Reply messages like TableEntriesUpdated, TableKeysRemoved, TableKeysRead and TableEntriesRead can be found here.