SignalR Hub Protocol

The SignalR Protocol is a protocol for two-way RPC over any Message-based transport. Either party in the connection may invoke procedures on the other party, and procedures can return zero or more results or an error.

Terms

Caller - The node that is issuing an Invocation, StreamInvocation, CancelInvocation, Ping messages and receiving Completion, StreamItem and Ping messages (a node can be both Caller and Callee for different invocations simultaneously)
Callee - The node that is receiving an Invocation, StreamInvocation, CancelInvocation, Ping messages and issuing Completion, StreamItem and Ping messages (a node can be both Callee and Caller for different invocations simultaneously)
Binder - The component on each node that handles mapping Invocation and StreamInvocation messages to method calls and return values to Completion and StreamItem messages

Transport Requirements

The SignalR Protocol requires the following attributes from the underlying transport.

Reliable, in-order, delivery of messages - Specifically, the SignalR protocol provides no facility for retransmission or reordering of messages. If that is important to an application scenario, the application must either use a transport that guarantees it (i.e. TCP) or provide their own system for managing message order.

Overview

This document describes two encodings of the SignalR protocol: JSON and MessagePack. Only one format can be used for the duration of a connection, and the format must be agreed on by both sides after opening the connection and before sending any other messages. However, each format shares a similar overall structure.

In the SignalR protocol, the following types of messages can be sent:

Message Name	Sender	Description
`HandshakeRequest`	Client	Sent by the client to agree on the message format.
`HandshakeResponse`	Server	Sent by the server as an acknowledgment of the previous `HandshakeRequest` message. Contains an error if the handshake failed.
`Close`	Callee, Caller	Sent by the server when a connection is closed. Contains an error if the connection was closed because of an error. Sent by the client when it's closing the connection, unlikely to contain an error.
`Invocation`	Caller	Indicates a request to invoke a particular method (the Target) with provided Arguments on the remote endpoint.
`StreamInvocation`	Caller	Indicates a request to invoke a streaming method (the Target) with provided Arguments on the remote endpoint.
`StreamItem`	Callee, Caller	Indicates individual items of streamed response data from a previous `StreamInvocation` message or streamed uploads from an invocation with streamIds.
`Completion`	Callee, Caller	Indicates a previous `Invocation` or `StreamInvocation` has completed or a stream in an `Invocation` or `StreamInvocation` has completed. Contains an error if the invocation concluded with an error or the result of a non-streaming method invocation. The result will be absent for `void` methods. In case of streaming invocations no further `StreamItem` messages will be received.
`CancelInvocation`	Caller	Sent by the client to cancel a streaming invocation on the server.
`Ping`	Caller, Callee	Sent by either party to check if the connection is active.
`Ack`	Caller, Callee	Sent by either party to acknowledge that messages have been received up to the provided sequence ID.
`Sequence`	Caller, Callee	Sent by either party as the first message when a connection reconnects. Specifies what sequence ID they will start sending messages starting at. Duplicate messages are possible to receive and should be ignored.

After opening a connection to the server the client must send a HandshakeRequest message to the server as its first message. The handshake message is always a JSON message and contains the name of the format (protocol) as well as the version of the protocol that will be used for the duration of the connection. The server will reply with a HandshakeResponse, also always JSON, containing an error if the server does not support the protocol. If the server does not support the protocol requested by the client or the first message received from the client is not a HandshakeRequest message the server must close the connection. Both the HandshakeRequest and HandshakeResponse messages must be terminated by the ASCII character 0x1E (record separator).

The HandshakeRequest message contains the following properties:

protocol - the name of the protocol to be used for messages exchanged between the server and the client
version - the value must always be 1, for both MessagePack and Json protocols

Example:

{
    "protocol": "messagepack",
    "version": 1
}

The HandshakeResponse message contains the following properties:

error - the optional error message if the server does not support the requested protocol

Example:

{
    "error": "Requested protocol 'messagepack' is not available."
}

Communication between the Caller and the Callee

There are three kinds of interactions between the Caller and the Callee:

Invocations - the Caller sends a message to the Callee and expects a message indicating that the invocation has been completed and optionally a result of the invocation
Non-Blocking Invocations - the Caller sends a message to the Callee and does not expect any further messages for this invocation
Streaming Invocations - the Caller sends a message to the Callee and expects one or more results returned by the Callee followed by a message indicating the end of invocation

Invocations

In order to perform a single invocation, the Caller follows the following basic flow:

Allocate a unique (per connection) Invocation ID value (arbitrary string, chosen by the Caller) to represent the invocation
Send an Invocation or StreamingInvocation message containing the Invocation ID, the name of the Target being invoked, and the Arguments to provide to the method.
If the Invocation is marked as non-blocking (see "Non-Blocking Invocations" below), stop here and immediately yield back to the application.
Wait for a StreamItem or Completion message with a matching Invocation ID
If a Completion message arrives, go to 8
If the StreamItem message has a payload, dispatch the payload to the application (i.e. by yielding a result to an IObservable, or by collecting the result for dispatching in step 8)
Go to 4
Complete the invocation, dispatching the final payload item (if any) or the error (if any) to the application

The Target of an Invocation message must refer to a specific method, overloading is not permitted. In the .NET Binder, the Target value for a method is defined as the simple name of the Method (i.e. without qualifying type name, since a SignalR endpoint is specific to a single Hub class). Target is case-sensitive

NOTE: Invocation IDs are arbitrarily chosen by the Caller and the Callee is expected to use the same string in all response messages. Callees may establish reasonable limits on Invocation ID lengths and terminate the connection when an Invocation ID that is too long is received.

Message Headers

All messages, except the Ping message, can carry additional headers. Headers are transmitted as a dictionary with string keys and string values. Clients and servers should disregard headers they do not understand. Since there are no headers defined in this spec, a client or server is never expected to interpret headers. However, clients and servers are expected to be able to process messages containing headers and disregard the headers.

Non-Blocking Invocations

Invocations can be sent without an Invocation ID value. This indicates that the invocation is "non-blocking", and thus the caller does not expect a response. When a Callee receives an invocation without an Invocation ID value, it must not send any response to that invocation.

Streaming

The SignalR protocol allows for multiple StreamItem messages to be transmitted in response to a StreamingInvocation message, and allows the receiver to dispatch these results as they arrive, to allow for streaming data from one endpoint to another.

On the Callee side, it is up to the Callee's Binder to determine if a method call will yield multiple results. For example, in .NET certain return types may indicate multiple results, while others may indicate a single result. Even then, applications may wish for multiple results to be buffered and returned in a single Completion frame. It is up to the Binder to decide how to map this. The Callee's Binder must encode each result in separate StreamItem messages, indicating the end of results by sending a Completion message.

On the Caller side, the user code which performs the invocation indicates how it would like to receive the results and it is up the Caller's Binder to handle the result. If the Caller expects only a single result, but multiple results are returned, or if the caller expects multiple results but only one result is returned, the Caller's Binder should yield an error. If the Caller wants to stop receiving StreamItem messages before the Callee sends a Completion message, the Caller can send a CancelInvocation message with the same Invocation ID used for the StreamInvocation message that started the stream. When the Callee receives a CancelInvocation message it will stop sending StreamItem messages and will send a Completion message. The Caller is free to ignore any StreamItem messages as well as the Completion message after sending CancelInvocation.

Upload streaming

The Caller can send streaming data to the Callee, they can begin such a process by making an Invocation or StreamInvocation and adding a "StreamIds" property with an array of IDs that will represent the stream(s) associated with the invocation. The IDs must be unique from any other stream IDs used by the same Caller. The Caller then sends StreamItem messages with the "InvocationId" property set to the ID for the stream they are sending over. The Caller can end the stream by sending a Completion message with the ID of the stream they are completing. If the Callee sends a Completion the Caller should stop sending StreamItem and Completion messages, and the Callee is free to ignore any StreamItem and Completion messages that are sent after the invocation has completed.

Completion and results

An Invocation is only considered completed when the Completion message is received. Receiving any message using the same Invocation ID after a Completion message has been received for that invocation is considered a protocol error and the recipient may immediately terminate the connection.

If a Callee is going to stream results, it MUST send each individual result in a separate StreamItem message, and complete the invocation with a Completion. If the Callee is going to return a single result, it MUST not send any StreamItem messages, and MUST send the single result in a Completion message. If the Callee receives an Invocation message for a method that would yield multiple results or the Callee receives a StreamInvocation message for a method that would return a single result it MUST complete the invocation with a Completion message containing an error.

Errors

Errors are indicated by the presence of the error field in a Completion message. Errors always indicate the immediate end of the invocation. In the case of streamed responses, the arrival of a Completion message indicating an error should not stop the dispatching of previously-received results. The error is only yielded after the previously-received results have been dispatched.

If either endpoint commits a Protocol Error (see examples below), the other endpoint may immediately terminate the underlying connection.

It is a protocol error for any message to be missing a required field, or to have an unrecognized field.
It is a protocol error for a Caller to send a StreamItem or Completion message with an Invocation ID that has not been received in an Invocation message from the Callee
It is a protocol error for a Caller to send a StreamItem or Completion message in response to a Non-Blocking Invocation (see "Non-Blocking Invocations" above)
It is a protocol error for a Caller to send a Completion message with a result when a StreamItem message has previously been sent for the same Invocation ID.
It is a protocol error for a Caller to send a Completion message carrying both a result and an error.
It is a protocol error for an Invocation or StreamInvocation message to have an Invocation ID that has already been used by that endpoint. However, it is not an error for one endpoint to use an Invocation ID that was previously used by the other endpoint (allowing each endpoint to track it's own IDs).

Ping (aka "Keep Alive")

The SignalR Hub protocol supports "Keep Alive" messages used to ensure that the underlying transport connection remains active. These messages help ensure:

Proxies don't close the underlying connection during idle times (when few messages are being sent)
If the underlying connection is dropped without being terminated gracefully, the application is informed as quickly as possible.

Keep alive behavior is achieved via the Ping message type. Either endpoint may send a Ping message at any time. The receiving endpoint may choose to ignore the message, it has no obligation to respond in anyway. Most implementations will want to reset a timeout used to determine if the other party is present.

Ping messages do not have any payload, they are completely empty messages (aside from the encoding necessary to identify the message as a Ping message).

The default ASP.NET Core implementation automatically pings both directions on active connections. These pings are at regular intervals, and allow detection of unexpected disconnects (for example, unplugging a server). If the client detects that the server has stopped pinging, the client will close the connection, and vice versa. If there's other traffic through the connection, keep-alive pings aren't needed. A Ping is only sent if the interval has elapsed without a message being sent.

Example

Consider the following C# methods

public int Add(int x, int y)
{
    return x + y;
}

public int SingleResultFailure(int x, int y)
{
    throw new Exception("It didn't work!");
}

public IEnumerable<int> Batched(int count)
{
    for (var i = 0; i < count; i++)
    {
        yield return i;
    }
}

public async IAsyncEnumerable<int> Stream(int count)
{
    for (var i = 0; i < count; i++)
    {
        await Task.Delay(10);
        yield return i;
    }
}

public async IAsyncEnumerable<int> StreamFailure(int count)
{
    for (var i = 0; i < count; i++)
    {
        await Task.Delay(10);
        yield return i;
    }
    throw new Exception("Ran out of data!");
}

private List<string> _callers = new List<string>();
public void NonBlocking(string caller)
{
    _callers.Add(caller);
}

public async Task<int> AddStream(IAsyncEnumerable<int> stream)
{
    int sum = 0;
    await foreach(var item in stream)
    {
        sum += item;
    }
    return sum;
}

In each of the below examples, lines starting C->S indicate messages sent from the Caller ("Client") to the Callee ("Server"), and lines starting S->C indicate messages sent from the Callee ("Server") back to the Caller ("Client"). Message syntax is just a pseudo-code and is not intended to match any particular encoding.

Single Result (`Add` example above)

C->S: Invocation { Id = 42, Target = "Add", Arguments = [ 40, 2 ] }
S->C: Completion { Id = 42, Result = 42 }

NOTE: The following is NOT an acceptable encoding of this invocation:

C->S: Invocation { Id = 42, Target = "Add", Arguments = [ 40, 2 ] }
S->C: StreamItem { Id = 42, Item = 42 }
S->C: Completion { Id = 42 }

Single Result with Error (`SingleResultFailure` example above)

C->S: Invocation { Id = 42, Target = "SingleResultFailure", Arguments = [ 40, 2 ] }
S->C: Completion { Id = 42, Error = "It didn't work!" }

Batched Result (`Batched` example above)

C->S: Invocation { Id = 42, Target = "Batched", Arguments = [ 5 ] }
S->C: Completion { Id = 42, Result = [ 0, 1, 2, 3, 4 ] }

Streamed Result (`Stream` example above)

C->S: StreamInvocation { Id = 42, Target = "Stream", Arguments = [ 5 ] }
S->C: StreamItem { Id = 42, Item = 0 }
S->C: StreamItem { Id = 42, Item = 1 }
S->C: StreamItem { Id = 42, Item = 2 }
S->C: StreamItem { Id = 42, Item = 3 }
S->C: StreamItem { Id = 42, Item = 4 }
S->C: Completion { Id = 42 }

NOTE: The following is NOT an acceptable encoding of this invocation:

C->S: StreamInvocation { Id = 42, Target = "Stream", Arguments = [ 5 ] }
S->C: StreamItem { Id = 42, Item = 0 }
S->C: StreamItem { Id = 42, Item = 1 }
S->C: StreamItem { Id = 42, Item = 2 }
S->C: StreamItem { Id = 42, Item = 3 }
S->C: Completion { Id = 42, Result = 4 }

This is invalid because the Completion message for streaming invocations must not contain any result.

Streamed Result with Error (`StreamFailure` example above)

C->S: StreamInvocation { Id = 42, Target = "Stream", Arguments = [ 5 ] }
S->C: StreamItem { Id = 42, Item = 0 }
S->C: StreamItem { Id = 42, Item = 1 }
S->C: StreamItem { Id = 42, Item = 2 }
S->C: StreamItem { Id = 42, Item = 3 }
S->C: StreamItem { Id = 42, Item = 4 }
S->C: Completion { Id = 42, Error = "Ran out of data!" }

This should manifest to the Calling code as a sequence which emits 0, 1, 2, 3, 4, but then fails with the error Ran out of data!.

Streamed Result closed early (`Stream` example above)

C->S: StreamInvocation { Id = 42, Target = "Stream", Arguments = [ 5 ] }
S->C: StreamItem { Id = 42, Item = 0 }
S->C: StreamItem { Id = 42, Item = 1 }
C->S: CancelInvocation { Id = 42 }
S->C: StreamItem { Id = 42, Item = 2} // This can be ignored
S->C: Completion { Id = 42 } // This can be ignored

Non-Blocking Call (`NonBlocking` example above)

C->S: Invocation { Target = "NonBlocking", Arguments = [ "foo" ] }

Stream from Client to Server (`AddStream` example above)

C->S: Invocation { Id = 42, Target = "AddStream", Arguments = [ ], StreamIds = [ 1 ] }
C->S: StreamItem { Id = 1, Item = 1 }
C->S: StreamItem { Id = 1, Item = 2 }
C->S: StreamItem { Id = 1, Item = 3 }
C->S: Completion { Id = 1 }
S->C: Completion { Id = 42, Result = 6 }

Ping

C->S: Ping

JSON Encoding

In the JSON Encoding of the SignalR Protocol, each Message is represented as a single JSON object, which should be the only content of the underlying message from the Transport. All property names are case-sensitive. The underlying protocol is expected to handle encoding and decoding of the text, so the JSON string should be encoded in whatever form is expected by the underlying transport. For example, when using the ASP.NET Sockets transports, UTF-8 encoding is always used for text.

All JSON messages must be terminated by the ASCII character 0x1E (record separator).

Invocation Message Encoding

An Invocation message is a JSON object with the following properties:

type - A Number with the literal value 1, indicating that this message is an Invocation.
invocationId - An optional String encoding the Invocation ID for a message.
target - A String encoding the Target name, as expected by the Callee's Binder
arguments - An Array containing arguments to apply to the method referred to in Target. This is a sequence of JSON Tokens, encoded as indicated below in the "JSON Payload Encoding" section
streamIds - An optional Array of strings representing unique ids for streams coming from the Caller to the Callee and being consumed by the method referred to in Target.

Example:

{
    "type": 1,
    "invocationId": "123",
    "target": "Send",
    "arguments": [
        42,
        "Test Message"
    ]
}

Example (Non-Blocking):

{
    "type": 1,
    "target": "Send",
    "arguments": [
        42,
        "Test Message"
    ]
}

Example (Invocation with stream from Caller):

{
    "type": 1,
    "invocationId": "123",
    "target": "Send",
    "arguments": [
        42
    ],
    "streamIds": [
        "1"
    ]
}

StreamInvocation Message Encoding

A StreamInvocation message is a JSON object with the following properties:

type - A Number with the literal value 4, indicating that this message is a StreamInvocation.
invocationId - A String encoding the Invocation ID for a message.
target - A String encoding the Target name, as expected by the Callee's Binder.
arguments - An Array containing arguments to apply to the method referred to in Target. This is a sequence of JSON Tokens, encoded as indicated below in the "JSON Payload Encoding" section.
streamIds - An optional Array of strings representing unique ids for streams coming from the Caller to the Callee and being consumed by the method referred to in Target.

Example:

{
    "type": 4,
    "invocationId": "123",
    "target": "Send",
    "arguments": [
        42,
        "Test Message"
    ]
}

StreamItem Message Encoding

A StreamItem message is a JSON object with the following properties:

type - A Number with the literal value 2, indicating that this message is a StreamItem.
invocationId - A String encoding the Invocation ID for a message.
item - A Token encoding the stream item (see "JSON Payload Encoding" for details).

Example

{
    "type": 2,
    "invocationId": "123",
    "item": 42
}

Completion Message Encoding

A Completion message is a JSON object with the following properties

type - A Number with the literal value 3, indicating that this message is a Completion.
invocationId - A String encoding the Invocation ID for a message.
result - A Token encoding the result value (see "JSON Payload Encoding" for details). This field is ignored if error is present.
error - A String encoding the error message.

It is a protocol error to include both a result and an error property in the Completion message. A conforming endpoint may immediately terminate the connection upon receiving such a message.

Example - A Completion message with no result or error

{
    "type": 3,
    "invocationId": "123"
}

Example - A Completion message with a result

{
    "type": 3,
    "invocationId": "123",
    "result": 42
}

Example - A Completion message with an error

{
    "type": 3,
    "invocationId": "123",
    "error": "It didn't work!"
}

Example - The following Completion message is a protocol error because it has both of result and error

{
    "type": 3,
    "invocationId": "123",
    "result": 42,
    "error": "It didn't work!"
}

CancelInvocation Message Encoding

A CancelInvocation message is a JSON object with the following properties

type - A Number with the literal value 5, indicating that this message is a CancelInvocation.
invocationId - A String encoding the Invocation ID for a message.

Example

{
    "type": 5,
    "invocationId": "123"
}

Ping Message Encoding

A Ping message is a JSON object with the following properties:

type - A Number with the literal value 6, indicating that this message is a Ping.

Example

{
    "type": 6
}

Close Message Encoding

A Close message is a JSON object with the following properties

type - A Number with the literal value 7, indicating that this message is a Close.
error - An optional String encoding the error message.
allowReconnect - An optional Boolean indicating to clients with automatic reconnects enabled that they should attempt to reconnect after receiving the message.

Example - A Close message without an error

{
    "type": 7
}

Example - A Close message with an error

{
    "type": 7,
    "error": "Connection closed because of an error!"
}

Example - A Close message with an error that allows automatic client reconnects.

{
    "type": 7,
    "error": "Connection closed because of an error!",
    "allowReconnect": true
}

Ack Message Encoding

An Ack message is a JSON object with the following properties

type - A Number with the literal value 8, indicating that this message is an Ack.
sequenceId - A Number specifying how many trackable messages have been received.

Example:

{
    "type": 8,
    "sequenceId": 1394
}

Sequence Message Encoding

A Seqeunce message is a JSON object with the following properties

type - A Number with the literal value 9, indicating that this message is a Sequence.
sequenceId - A Number specifying what the new starting message number will be. Only sent on reconnects.

Example:

{
    "type": 9,
    "sequenceId": 1234
}

JSON Header Encoding

Message headers are encoded into a JSON object, with string values, that are stored in the headers property. For example:

{
    "type": 1,
    "headers": {
        "Foo": "Bar"
    },
    "invocationId": "123",
    "target": "Send",
    "arguments": [
        42,
        "Test Message"
    ]
}

JSON Payload Encoding

Items in the arguments array within the Invocation message type, as well as the item value of the StreamItem message and the result value of the Completion message, encode values which have meaning to each particular Binder. A general guideline for encoding/decoding these values is provided in the "Type Mapping" section at the end of this document, but Binders should provide configuration to applications to allow them to customize these mappings. These mappings need not be self-describing, because when decoding the value, the Binder is expected to know the destination type (by looking up the definition of the method indicated by the Target).

MessagePack (MsgPack) encoding

In the MsgPack Encoding of the SignalR Protocol, each Message is represented as a single MsgPack array containing items that correspond to properties of the given hub protocol message. The array items may be primitive values, arrays (e.g. method arguments) or objects (e.g. argument value). The first item in the array is the message type.

MessagePack uses different formats to encode values. Refer to the MsgPack format spec for format definitions.

Invocation Message Encoding

Invocation messages have the following structure:

[1, Headers, InvocationId, Target, [Arguments], [StreamIds]]

1 - Message Type - 1 indicates this is an Invocation message.
Headers - A MsgPack Map containing the headers, with string keys and string values (see MessagePack Headers Encoding below)
InvocationId - One of:
- A Nil, indicating that there is no Invocation ID, OR
- A String encoding the Invocation ID for the message.
Target - A String encoding the Target name, as expected by the Callee's Binder.
Arguments - An Array containing arguments to apply to the method referred to in Target.
StreamIds - An Array of strings representing unique ids for streams coming from the Caller to the Callee and being consumed by the method referred to in Target.

Example:

The following payload

0x96 0x01 0x80 0xa3 0x78 0x79 0x7a 0xa6 0x6d 0x65 0x74 0x68 0x6f 0x64 0x91 0x2a 0x90

is decoded as follows:

0x96 - 6-element array
0x01 - 1 (Message Type - Invocation message)
0x80 - Map of length 0 (Headers)
0xa3 - string of length 3 (InvocationId)
0x78 - x
0x79 - y
0x7a - z
0xa6 - string of length 6 (Target)
0x6d - m
0x65 - e
0x74 - t
0x68 - h
0x6f - o
0x64 - d
0x91 - 1-element array (Arguments)
0x2a - 42 (Argument value)
0x90 - 0-element array (StreamIds)

Non-Blocking Example:

The following payload

0x96 0x01 0x80 0xc0 0xa6 0x6d 0x65 0x74 0x68 0x6f 0x64 0x91 0x2a 0x90

is decoded as follows:

0x96 - 6-element array
0x01 - 1 (Message Type - Invocation message)
0x80 - Map of length 0 (Headers)
0xc0 - nil (Invocation ID)
0xa6 - string of length 6 (Target)
0x6d - m
0x65 - e
0x74 - t
0x68 - h
0x6f - o
0x64 - d
0x91 - 1-element array (Arguments)
0x2a - 42 (Argument value)
0x90 - 0-element array (StreamIds)

StreamInvocation Message Encoding

StreamInvocation messages have the following structure:

[4, Headers, InvocationId, Target, [Arguments], [StreamIds]]

4 - Message Type - 4 indicates this is a StreamInvocation message.
Headers - A MsgPack Map containing the headers, with string keys and string values (see MessagePack Headers Encoding below)
InvocationId - A String encoding the Invocation ID for the message.
Target - A String encoding the Target name, as expected by the Callee's Binder.
Arguments - An Array containing arguments to apply to the method referred to in Target.
StreamIds - An Array of strings representing unique ids for streams coming from the Caller to the Callee and being consumed by the method referred to in Target.

Example:

The following payload

0x96 0x04 0x80 0xa3 0x78 0x79 0x7a 0xa6 0x6d 0x65 0x74 0x68 0x6f 0x64 0x91 0x2a 0x90

is decoded as follows:

0x96 - 6-element array
0x04 - 4 (Message Type - StreamInvocation message)
0x80 - Map of length 0 (Headers)
0xa3 - string of length 3 (InvocationId)
0x78 - x
0x79 - y
0x7a - z
0xa6 - string of length 6 (Target)
0x6d - m
0x65 - e
0x74 - t
0x68 - h
0x6f - o
0x64 - d
0x91 - 1-element array (Arguments)
0x2a - 42 (Argument value)
0x90 - 0-element array (StreamIds)

StreamItem Message Encoding

StreamItem messages have the following structure:

[2, Headers, InvocationId, Item]

2 - Message Type - 2 indicates this is a StreamItem message
Headers - A MsgPack Map containing the headers, with string keys and string values (see MessagePack Headers Encoding below)
InvocationId - A String encoding the Invocation ID for the message
Item - the value of the stream item

Example:

The following payload:

0x94 0x02 0x80 0xa3 0x78 0x79 0x7a 0x2a

is decoded as follows:

0x94 - 4-element array
0x02 - 2 (Message Type - StreamItem message)
0x80 - Map of length 0 (Headers)
0xa3 - string of length 3 (InvocationId)
0x78 - x
0x79 - y
0x7a - z
0x2a - 42 (Item)

Completion Message Encoding

Completion messages have the following structure

[3, Headers, InvocationId, ResultKind, Result?]

3 - Message Type - 3 indicates this is a Completion message
Headers - A MsgPack Map containing the headers, with string keys and string values (see MessagePack Headers Encoding below)
InvocationId - A String encoding the Invocation ID for the message
ResultKind - A flag indicating the invocation result kind:
- 1 - Error result - Result contains a String with the error message
- 2 - Void result - Result is absent
- 3 - Non-Void result - Result contains the value returned by the server
Result - An optional item containing the result of invocation. Absent if the server did not return any value (void methods)

Examples:

Error Result:

The following payload:

0x95 0x03 0x80 0xa3 0x78 0x79 0x7a 0x01 0xa5 0x45 0x72 0x72 0x6f 0x72

is decoded as follows:

0x94 - 4-element array
0x03 - 3 (Message Type - Result message)
0x80 - Map of length 0 (Headers)
0xa3 - string of length 3 (InvocationId)
0x78 - x
0x79 - y
0x7a - z
0x01 - 1 (ResultKind - Error result)
0xa5 - string of length 5
0x45 - E
0x72 - r
0x72 - r
0x6f - o
0x72 - r

Void Result:

The following payload:

0x94 0x03 0x80 0xa3 0x78 0x79 0x7a 0x02

is decoded as follows:

0x94 - 4-element array
0x03 - 3 (Message Type - Result message)
0x80 - Map of length 0 (Headers)
0xa3 - string of length 3 (InvocationId)
0x78 - x
0x79 - y
0x7a - z
0x02 - 2 (ResultKind - Void result)

Non-Void Result:

The following payload:

0x95 0x03 0x80 0xa3 0x78 0x79 0x7a 0x03 0x2a

is decoded as follows:

0x95 - 5-element array
0x03 - 3 (Message Type - Result message)
0x80 - Map of length 0 (Headers)
0xa3 - string of length 3 (InvocationId)
0x78 - x
0x79 - y
0x7a - z
0x03 - 3 (ResultKind - Non-Void result)
0x2a - 42 (Result)

CancelInvocation Message Encoding

CancelInvocation messages have the following structure

[5, Headers, InvocationId]

5 - Message Type - 5 indicates this is a CancelInvocation message
Headers - A MsgPack Map containing the headers, with string keys and string values (see MessagePack Headers Encoding below)
InvocationId - A String encoding the Invocation ID for the message

Example:

The following payload:

0x93 0x05 0x80 0xa3 0x78 0x79 0x7a

is decoded as follows:

0x93 - 3-element array
0x05 - 5 (Message Type CancelInvocation message)
0x80 - Map of length 0 (Headers)
0xa3 - string of length 3 (InvocationId)
0x78 - x
0x79 - y
0x7a - z

Ping Message Encoding

Ping messages have the following structure

[6]

6 - Message Type - 6 indicates this is a Ping message.

Examples:

Ping message

The following payload:

0x91 0x06

is decoded as follows:

0x91 - 1-element array
0x06 - 6 (Message Type - Ping message)

Close Message Encoding

Close messages have the following structure

[7, Error, AllowReconnect?]

7 - Message Type - 7 indicates this is a Close message.
Error - Error - A String encoding the error for the message.
AllowReconnect - An optional Boolean indicating to clients with automatic reconnects enabled that they should attempt to reconnect after receiving the message.

Examples:

Close message

The following payload:

0x92 0x07 0xa3 0x78 0x79 0x7a

is decoded as follows:

0x92 - 2-element array
0x07 - 7 (Message Type - Close message)
0xa3 - string of length 3 (Error)
0x78 - x
0x79 - y
0x7a - z

Close message that allows automatic client reconnects

The following payload:

0x93 0x07 0xa3 0x78 0x79 0x7a 0xc3

is decoded as follows:

0x93 - 3-element array
0x07 - 7 (Message Type - Close message)
0xa3 - string of length 3 (Error)
0x78 - x
0x79 - y
0x7a - z
0xc3 - True (AllowReconnect)

Ack Message Encoding

Ack messages have the following structure

[8, SequenceId]

8 - Message Type - 8 indicates this is an Ack message.

Examples:

Ack message

The following payload:

0x92 0x08 0xcc 0x24

is decoded as follows:

0x92 - 2-element array
0x08 - 8 (Message Type - Ack message)
0xcc - 8-bit unsigned int (any unsigned int size is fine)
0x24 - 36

Sequence Message Encoding

Sequence messages have the following structure

[9, SequenceId]

9 - Message Type - 9 indicates this is an Sequence message.

Examples:

Sequence message

The following payload:

0x92 0x09 0xcc 0x13

is decoded as follows:

0x92 - 2-element array
0x09 - 9 (Message Type - Sequence message)
0xcc - 8-bit unsigned int (any unsigned int size is fine)
0x13 - 19

MessagePack Headers Encoding

Headers are encoded in MessagePack messages as a Map that immediately follows the type value. The Map can be empty, in which case it is represented by the byte 0x80. If there are items in the map, both the keys and values must be String values.

Headers are not valid in a Ping message. The Ping message is always exactly encoded as 0x91 0x06

Below shows an example encoding of a message containing headers:

0x96 0x01 0x82 0xa1 0x78 0xa1 0x79 0xa1 0x7a 0xa1 0x7a 0xa3 0x78 0x79 0x7a 0xa6 0x6d 0x65 0x74 0x68 0x6f 0x64 0x91 0x2a 0x90

and is decoded as follows:

0x96 - 6-element array
0x01 - 1 (Message Type - Invocation message)
0x82 - Map of length 2
0xa1 - string of length 1 (Key)
0x78 - x
0xa1 - string of length 1 (Value)
0x79 - y
0xa1 - string of length 1 (Key)
0x7a - z
0xa1 - string of length 1 (Value)
0x7a - z
0xa3 - string of length 3 (InvocationId)
0x78 - x
0x79 - y
0x7a - z
0xa6 - string of length 6 (Target)
0x6d - m
0x65 - e
0x74 - t
0x68 - h
0x6f - o
0x64 - d
0x91 - 1-element array (Arguments)
0x2a - 42 (Argument value)
0x90 - 0-element array (StreamIds)

and interpreted as an Invocation message with headers: 'x' = 'y' and 'z' = 'z'.

Type Mappings

Below are some sample type mappings between JSON types and the .NET client. This is not an exhaustive or authoritative list, just informative guidance. Official clients will provide ways for users to override the default mapping behavior for a particular method, parameter, or parameter type

.NET Type	JSON Type	MsgPack format family
`System.Byte`, `System.UInt16`, `System.UInt32`	`Number`	`positive fixint`, `uint`
`System.SByte`, `System.Int16`, `System.Int32`	`Number`	`fixint`, `int`
`System.UInt64`	`Number`	`positive fixint`, `uint`
`System.Int64`	`Number`	`fixint`, `int`
`System.Single`	`Number`	`float`
`System.Double`	`Number`	`float`
`System.Boolean`	`true` or `false`	`true`, `false`
`System.String`	`String`	`fixstr`, `str`
`System.Byte`[]	`String` (Base64-encoded)	`bin`
`IEnumerable<T>`	`Array`	`bin`
custom `enum`	`Number`	`fixint`, `int`
custom `struct` or `class`	`Object`	`fixmap`, `map`

MessagePack payloads are wrapped in an outer message framing described below.

Binary encoding

([Length][Body])([Length][Body])... continues until end of the connection ...

[Length] - A 32-bit unsigned integer encoded as VarInt. Variable size - 1-5 bytes.
[Body] - The body of the message, exactly [Length] bytes in length.

VarInt

VarInt encodes the most significant bit as a marker indicating whether the byte is the last byte of the VarInt or if it spans to the next byte. Bytes appear in the reverse order - i.e. the first byte contains the least significant bits of the value.

Examples:

VarInt: 0x35 (%00110101) - the most significant bit is 0 so the value is %x0110101 i.e. 0x35 (53)
VarInt: 0x80 0x29 (%10000000 %00101001) - the most significant bit of the first byte is 1 so the remaining bits (%x0000000) are the lowest bits of the value. The most significant bit of the second byte is 0 meaning this is last byte of the VarInt. The actual value bits (%x0101001) need to be prepended to the bits we already read so the values is %01010010000000 i.e. 0x1480 (5248)

The biggest supported payloads are 2GB in size so the biggest number we need to support is 0x7fffffff which when encoded as VarInt is 0xFF 0xFF 0xFF 0xFF 0x07 - hence the maximum size of the length prefix is 5 bytes.

For example, when sending the following frames (\n indicates the actual Line Feed character, not an escape sequence):

"Hello\nWorld"
0x01 0x02

The encoding will be as follows, as a list of binary digits in hex (text in parentheses () are comments). Whitespace and newlines are irrelevant and for illustration only.

0x0B                                                   (start of frame; VarInt value: 11)
0x68 0x65 0x6C 0x6C 0x6F 0x0A 0x77 0x6F 0x72 0x6C 0x64 (UTF-8 encoding of 'Hello\nWorld')
0x02                                                   (start of frame; VarInt value: 2)
0x01 0x02                                              (body)

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SignalR Hub Protocol

Terms

Transport Requirements

Overview

Communication between the Caller and the Callee

Invocations

Message Headers

Non-Blocking Invocations

Streaming

Upload streaming

Completion and results

Errors

Ping (aka "Keep Alive")

Example

Single Result (Add example above)

Single Result with Error (SingleResultFailure example above)

Batched Result (Batched example above)

Streamed Result (Stream example above)

Streamed Result with Error (StreamFailure example above)

Streamed Result closed early (Stream example above)

Non-Blocking Call (NonBlocking example above)

Stream from Client to Server (AddStream example above)

Ping

JSON Encoding

Invocation Message Encoding

StreamInvocation Message Encoding

StreamItem Message Encoding

Completion Message Encoding

CancelInvocation Message Encoding

Ping Message Encoding

Close Message Encoding

Ack Message Encoding

Sequence Message Encoding

JSON Header Encoding

JSON Payload Encoding

MessagePack (MsgPack) encoding

Invocation Message Encoding

Example:

Non-Blocking Example:

StreamInvocation Message Encoding

StreamItem Message Encoding

Completion Message Encoding

Error Result:

Void Result:

Non-Void Result:

CancelInvocation Message Encoding

Ping Message Encoding

Ping message

Close Message Encoding

Close message

Close message that allows automatic client reconnects

Ack Message Encoding

Ack message

Sequence Message Encoding

Sequence message

MessagePack Headers Encoding

Type Mappings

Binary encoding

VarInt

Single Result (`Add` example above)

Single Result with Error (`SingleResultFailure` example above)

Batched Result (`Batched` example above)

Streamed Result (`Stream` example above)

Streamed Result with Error (`StreamFailure` example above)

Streamed Result closed early (`Stream` example above)

Non-Blocking Call (`NonBlocking` example above)

Stream from Client to Server (`AddStream` example above)