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Connect Protocol Reference

This document specifies the Connect protocol for making RPCs over HTTP. The protocol does not depend on framing details specific to a particular HTTP version.

Rules use ABNF syntax, but the design goals and summary are approachable for casual readers. There are also examples of unary and streaming RPCs.

Design Goals​

This protocol aims to:

  • Be human-readable and debuggable with general-purpose HTTP tools, especially for unary RPCs.
  • Remain conceptually close to gRPC's HTTP/2 protocol, so Connect implementations can support both protocols.
  • Depend only on widely-implemented HTTP features and specify behavior in terms of high-level semantics, so that Connect implementations can easily use off-the-shelf networking libraries.

Summary​

When used with Protocol Buffer schemas, the Connect protocol supports unary, client streaming, server streaming, and bidirectional streaming RPCs, with either binary Protobuf or JSON payloads. Bidirectional streaming requires HTTP/2, but the other RPC types also support HTTP/1.1. The protocol doesn't use HTTP trailers at all, so it works with any networking infrastructure.

Unary RPCs use the application/proto and application/json Content-Types and look similar to a stripped-down REST dialect. Most requests are POSTs, paths are derived from the Protobuf schema, request and response bodies are valid Protobuf or JSON (without gRPC-style binary framing), and responses have meaningful HTTP status codes. For example:

> POST /connectrpc.greet.v1.GreetService/Greet HTTP/1.1
> Host: demo.connectrpc.com
> Content-Type: application/json
>
> {"name": "Buf"}

< HTTP/1.1 200 OK
< Content-Type: application/json
<
< {"greeting": "Hello, Buf!"}

For RPCs that have no side effects, it is possible to use GET requests instead:

> GET /connectrpc.greet.v1.GreetService/Greet?encoding=json&message=%7B%22name%22%3A%22Buf%22%7D HTTP/1.1
> Host: demo.connectrpc.com

< HTTP/1.1 200 OK
< Content-Type: application/json
<
< {"greeting": "Hello, Buf!"}

A request is considered to have no side effects if the associated RPC is annotated as such with the IdempotencyLevel option:

service ElizaService {
  rpc Say(stream SayRequest) returns (SayResponse) {
    option idempotency_level = NO_SIDE_EFFECTS;
  }
}

The unary ABNF rules describe how metadata (like timeouts and compression schemes) are encoded into HTTP headers and explain the details of the error model. The examples below also show a wider variety of scenarios.

Streaming RPCs are naturally a bit more complex: they use the application/connect+proto and application/connect+json Content-Types and look similar to gRPC-Web. Requests are still POSTs and paths are still derived from the Protobuf schema, but each request and response message is wrapped in a few bytes of binary framing data. Responses always have an HTTP status of 200 OK, with any errors sent in the last portion of the body. For example, a client streaming call might look like this:

> POST /connectrpc.greet.v1.GreetService/GreetGroup HTTP/1.1
> Host: demo.connectrpc.com
> Content-Type: application/connect+json
>
> <binary framing: standard message>{"name": "Buf"}
> <binary framing: standard message>{"name": "Connect"}

< HTTP/1.1 200 OK
< Content-Type: application/connect+json
<
< <binary framing: standard message>{"greeting": "Hello, Buf and Connect!"}
< <binary framing: error and trailers>{}

Again, the ABNF rules describe the streaming HTTP headers, error model, and framing data. The examples below show both successes and errors.

Outline​

Clients send HTTP requests to servers. Unary requests contain exactly one message, while streaming requests contain zero or more messages.

  • Response → Unary-Response / Streaming-Response
  • Unary-Response → Unary-Response-Headers Bare-Message
  • Streaming-Response → Streaming-Response-Headers 1*Enveloped-Message

Servers return HTTP responses to clients. Unary responses contain exactly one message, while streaming responses contain one or more messages.

The rules in this document use HTTP/2-style notation (for example, ":method POST" and ":path /foo/bar" instead of "POST /foo/bar HTTP/1.1"). On the wire, Connect implementations must represent these semantics appropriately for the HTTP version in use.

Unary (Request-Response) RPCs​

Most RPCs are unary (or request-response). Structurally, unary RPC is similar to the resource-oriented model of the web. The Connect protocol takes special care to make unary RPCs easy to work with using web browsers, cURL, and other general-purpose HTTP tools.

Unary-Request​

  • Unary-Request → Unary-Request-Headers Bare-Message
  • Unary-Request-Headers → Unary-Call-Specification *Leading-Metadata
  • Unary-Call-Specification → Method-Post Path Unary-Content-Type [Connect-Protocol-Version] [Timeout] [Content-Encoding] [Accept-Encoding]
  • Method-Post → ":method POST"
  • Path → ":path" "/" [Routing-Prefix "/"] Procedure-Name ; case-sensitive
  • Routing-Prefix → {arbitrary prefix}
  • Procedure-Name → {IDL-specific service & method name} ; see Protocol Buffers
  • Message-Codec → ("proto" / "json" / {custom})
  • Unary-Content-Type → "content-type" "application/" Message-Codec
  • Connect-Protocol-Version → "connect-protocol-version" "1"
  • Timeout → "connect-timeout-ms" Timeout-Milliseconds
  • Timeout-Milliseconds → {positive integer as ASCII string of at most 10 digits}
  • Content-Encoding → "content-encoding" Content-Coding
  • Content-Coding → "identity" / "gzip" / "br" / "zstd" / {custom}
  • Accept-Encoding → "accept-encoding" Content-Coding *("," [" "] Content-Coding) ; subset of HTTP quality value syntax
  • Leading-Metadata → Custom-Metadata
  • Custom-Metadata → ASCII-Metadata / Binary-Metadata
  • ASCII-Metadata → Header-Name ASCII-Value
  • Binary-Metadata → {Header-Name "-bin"} {base64-encoded value}
  • Header-Name → 1*( %x30-39 / %x61-7A / "_" / "-" / ".") ; 0-9 a-z _ - .
  • ASCII-Value → 1*( %x20-%x7E ) ; space & printable ASCII
  • Bare-Message → *{binary octet}

Unary-Request-Headers are sent as — and have the same semantics as — HTTP headers. Servers may respond with an error if the client sends too many headers.

If the server doesn't support the specified Message-Codec, it must respond with an HTTP status code of 415 Unsupported Media Type.

The Connect-Protocol-Version header distinguishes unary Connect RPC traffic from other requests that may use the same Content-Type. (In the future, it may also be used to support revisions to this protocol.) Clients, especially generated clients, should send this header. Servers and proxies may reject traffic without this header with an HTTP status code of 400.

Following standard HTTP semantics, servers must assume "identity" if the client omits Content-Encoding. If the client omits Accept-Encoding, servers must assume that the client accepts the Content-Encoding used for the request. Servers must assume that all clients accept "identity" as their least preferred encoding. Server implementations may choose to accept the full HTTP quality value syntax for Accept-Encoding, but client implementations must restrict themselves to sending the easy-to-parse subset outlined here. Servers should treat Accept-Encoding as an ordered list, with the client's most preferred encoding first and least preferred encoding last. If the client uses an unsupported Content-Encoding, servers should return an error with code "unimplemented" and a message listing the supported encodings.

If Timeout is omitted, the server should assume an infinite timeout. The protocol accommodates timeouts of more than 100 days. Client implementations may set a default timeout for all RPCs, and server implementations may clamp timeouts to an appropriate maximum.

HTTP doesn't allow header values to be arbitrary binary blobs, so Connect differentiates between ASCII-Metadata and Binary-Metadata. Binary headers must use keys ending in "-bin", and implementations should emit unpadded base64-encoded values. Implementations must accept both padded and unpadded values. Because binary and non-ASCII headers are relatively uncommon, implementations may represent HTTP headers using an off-the-shelf type rather than reifying these rules in a custom type. Implementations using an off-the-shelf type should prominently document these rules. For both ASCII and binary metadata, keys beginning with "connect-" are reserved for use by the Connect protocol.

Bare-Message is the RPC request payload, serialized using the codec indicated by Unary-Content-Type and possibly compressed using Content-Encoding. It's sent on the wire as the HTTP request content (often called the body). Servers must not attempt to decompress zero-length HTTP request content.

Unary-Get-Request​

  • Unary-Get-Request → Unary-Get-Request-Headers
  • Unary-Get-Request-Headers → Unary-Get-Call-Specification *Leading-Metadata
  • Unary-Get-Call-Specification → Method-Get Path "?" Query-Get [Timeout] [Accept-Encoding]
  • Method-Get → ":method GET"
  • Query-Get → Message-Query Encoding-Query [Base64-Query] [Compression-Query] [Connect-Version-Query]
  • Message-Query → "message=" (*{percent-encoded octet})
  • Base64-Query → "&base64=1"
  • Encoding-Query → "&encoding=" Message-Codec
  • Compression-Query → "&compression=" Content-Coding
  • Connect-Version-Query → "&connect=v1"

Unary-Get-Request is a special variant of the unary request designed to utilize only ordinary HTTP GET requests, for side-effect-free RPCs such as simple queries. These requests do not require any special HTTP headers, and can be viewed in a browser tab. They are designed to be easy to make and easy to cache in browsers, proxies, and CDNs.

Unary-Get-Request-Headers are sent as — and have the same semantics as — HTTP headers. Servers may respond with an error if the client sends too many headers.

Query-Get is sent as the query part of the URI for the request and has the same semantics as HTTP URI query parameters. Servers should accept query parameters in any order and allow unknown query parameters alongside the Connect query parameters. Servers may respond with an error if the query parameter is too long to fit in a header or results in a path or URL that is too long.

Message payloads in Get requests should be encoded in a deterministic fashion. Not all codecs can provide a single "canonical" encoding for any given semantically-identical message; however, when a codec is used for an HTTP GET request, it should at least use deterministic encoding such that a given domain object will encode to the same exact message where possible. This is important for the use case of cached HTTP GET requests, where multiple distinct message encodings could lead to sub-optimal cache behavior.

Message-Query is the RPC request payload, serialized using the codec indicated by Encoding-Query and possibly compressed using the encoding specified in Compression-Query. If Base64-Query is specified, the raw binary octets will be encoded with the RFC 4648 §5 "URL-safe" base64 data encoding, otherwise a text payload can be encoded as UTF-8 using the percent-encoding scheme specified in RFC 1738. Optionally, the base64-encoded message may contain padding characters, but these characters must be percent-encoded if present.

Note that base64 encoding must be used if the message payload contains binary data. Payloads using percent-encoding without base64 are only permitted to have bytes that form valid UTF-8 codepoints. Clients should not send requests with percent-encoded binary data, and servers may reject requests with percent-encoded binary data. Therefore, requests with any compression other than identity and any encoding that uses binary data (such as proto) should use base64 encoding. Also, note that clients are still required to explicitly specify Base64-Query even when base64 encoding is required.

If Base64-Query is present, the value of the parameter should be one. Other values for the "base64" parameter will simply be ignored. Generally, a request with a payload that is not base64-encoded shouldn't need a "base64" parameter.

If the server doesn't support the specified Message-Codec, it must respond with an HTTP status code of 415 Unsupported Media Type.

Servers must assume "identity" if the client omits Compression-Query. If the client omits Accept-Encoding, servers must assume that the client accepts the Compression-Query value used for the request. Servers must assume that all clients accept "identity" as their least preferred encoding. Server implementations may choose to accept the full HTTP quality value syntax for Accept-Encoding, but client implementations must restrict themselves to sending the easy-to-parse subset outlined above. Servers should treat Accept-Encoding as an ordered list, with the client's most preferred encoding first and least preferred encoding last. If the client uses an unsupported Compression-Query value, servers should return an error with code "unimplemented" and a message listing the supported encodings. Servers must not attempt to decompress zero-length Message-Query.

If Timeout is omitted, the server should assume an infinite timeout. The protocol accommodates timeouts of more than 100 days. Client implementations may set a default timeout for all RPCs, and server implementations may clamp timeouts to an appropriate maximum. Note that timeouts, if specified, remain specified using HTTP headers rather than query parameters.

The Connect-Version-Query parameter distinguishes Connect Get requests from other HTTP GET requests. (In the future, it may also be used to support revisions to this protocol.) Clients, especially generated clients, should send this header. Servers and proxies may reject traffic without this parameter with an HTTP status code of 400.

Unary-Response​

  • Unary-Response → Unary-Response-Headers Bare-Message
  • Unary-Response-Headers → HTTP-Status Unary-Content-Type [Content-Encoding] [Accept-Encoding] *Leading-Metadata *Prefixed-Trailing-Metadata
  • HTTP-Status → ":status" ("200" / {error code translated to HTTP})
  • Prefixed-Trailing-Metadata → Prefixed-ASCII-Metadata / Prefixed-Binary-Metadata
  • Prefixed-ASCII-Metadata → Prefixed-Header-Name ASCII-Value
  • Prefixed-Binary-Metadata → {Prefixed-Header-Name "-bin"} {base64-encoded value}
  • Prefixed-Header-Name → "trailer-" Header-Name

Unary-Response-Headers are sent as — and have the same semantics as — HTTP headers. This includes Prefixed-Trailing-Metadata: though it's sent on the wire alongside Leading-Metadata, support for trailing metadata lets Connect implementations use common interfaces for streaming and unary RPC. Implementations must transparently prefix trailing metadata keys with "trailer-" when writing data to the wire and strip the prefix when reading data from the wire. As noted above, Leading-Metadata keys beginning with "connect-" and Prefixed-Trailing-Metadata keys beginning with "trailer-connect-" are reserved for use by the Connect protocol.

If Content-Encoding is omitted, clients must assume "identity". Servers must either respond with an error or use a Content-Encoding supported by the client.

Successful responses have an HTTP-Status of 200. In those cases, Unary-Content-Type is the same as the request's Unary-Content-Type. Bare-Message is the RPC response payload, serialized using the codec indicated by Unary-Content-Type and possibly compressed using Content-Encoding. It's sent on the wire as the HTTP response content (often called the body). Clients must not attempt to decompress zero-length HTTP response content.

Errors are sent with a non-200 HTTP-Status. In those cases, Unary-Content-Type must be "application/json". Bare-Message is either omitted or a JSON-serialized Error, possibly compressed using Content-Encoding and sent on the wire as the HTTP response content. Clients must not attempt to decompress zero-length HTTP response content. If Bare-Message is an Error, HTTP-Status should match Error.code as specified in the table below. When reading data from the wire, client implementations must use the HTTP-to-Connect mapping to infer a Connect error code if Bare-Message is missing or malformed.

Examples​

Using HTTP/1.1 notation, a simple request and successful response:

> POST /connectrpc.greet.v1.GreetService/Greet HTTP/1.1
> Host: demo.connectrpc.com
> Content-Type: application/json
>
> {"name": "Buf"}

< HTTP/1.1 200 OK
< Content-Type: application/json
<
< {"greeting": "Hello, Buf!"}

The same RPC, but sent as a unary GET request instead:

> GET /connectrpc.greet.v1.GreetService/Greet?message=%7B%22name%22%3A%22Buf%22%7D&encoding=json&connect=v1 HTTP/1.1
> Host: demo.connectrpc.com

< HTTP/1.1 200 OK
< Content-Type: application/json
<
< {"greeting": "Hello, Buf!"}

(Note that when typing into a browser URL bar, most characters will automatically be encoded for you.)

The same RPC, but with a 5s timeout, asymmetric compression, and some custom leading and trailing metadata.

> POST /connectrpc.greet.v1.GreetService/Greet HTTP/1.1
> Host: demo.connectrpc.com
> Content-Type: application/json
> Accept-Encoding: gzip, br
> Connect-Timeout-Ms: 5000
> Acme-Shard-Id: 42
>
> {"name": "Buf"}

< HTTP/1.1 200 OK
< Content-Type: application/json
< Content-Encoding: gzip
< Trailer-Acme-Operation-Cost: 237
<
< <gzipped JSON>

The same RPC again, but with a Protobuf-encoded request and an error response:

> POST /connectrpc.greet.v1.GreetService/Greet HTTP/1.1
> Host: demo.connectrpc.com
> Content-Type: application/proto
>
> <uncompressed binary Protobuf>

< HTTP/1.1 404 Not Found
< Content-Type: application/json
<
< {
<   "code": "unimplemented",
<   "message": "connectrpc.greet.v1.GreetService/Greet is not implemented"
< }

Streaming RPCs​

Streaming RPCs may be half- or full-duplex. In server streaming RPCs, the client sends a single message and the server responds with a stream of messages. In client streaming RPCs, the client sends a stream of messages and the server responds with a single message. In bidirectional streaming RPCs, both the client and server send a stream of messages. Depending on the Connect implementation, IDL, and HTTP version in use, some or all of these streaming RPC types may be unavailable.

Streaming-Request​

  • Streaming-Request → Streaming-Request-Headers *Enveloped-Message
  • Streaming-Request-Headers → Streaming-Call-Specification *Leading-Metadata
  • Streaming-Call-Specification → Method-Post Path Streaming-Content-Type [Connect-Protocol-Version] [Timeout] [Streaming-Content-Encoding] [Streaming-Accept-Encoding]
  • Streaming-Content-Type → "content-type" "application/connect+" ("proto" / "json" / {custom})
  • Streaming-Content-Encoding → "connect-content-encoding" Content-Coding
  • Streaming-Accept-Encoding → "connect-accept-encoding" Content-Coding *("," [" "] Content-Coding)
  • Enveloped-Message → Envelope-Flags Message-Length Message
  • Envelope-Flags → %d0-255 ; 8 bitwise flags encoded as 1 byte unsigned integer
  • Message-Length → {length of Message} ; encoded as 4 byte unsigned integer, big-endian
  • Message → *{binary octet}

If Streaming-Content-Type does not begin with "application/connect+", servers should respond with an HTTP status of 415 Unsupported Media Type. This prevents HTTP clients unaware of Connect's semantics from interpreting a streaming error response, which uses an HTTP Status of 200 OK, as successful.

Servers must interpret Streaming-Content-Encoding and Streaming-Accept-Encoding using the same inference and error-reporting rules as Content-Encoding and Accept-Encoding.

The HTTP request content is a sequence of zero or more Enveloped-Message. The first byte is Envelope-Flags, a set of 8 bitwise flags.

  • If the least significant bit is 1, the Message is compressed using the algorithm specified in Streaming-Content-Encoding. If Streaming-Content-Encoding is omitted or "identity", this bit must be 0. Compression contexts are not maintained over message boundaries.
  • If the next least significant bit is 1, the Message is an EndStreamResponse rather than the response type defined in the service's IDL. Response streams must set this bit on the final Enveloped-Message in the stream, and must leave this bit unset on all other messages in the stream. Request streams must always leave this bit unset. (Note that this is not the same as the gRPC-Web protocol, which uses the most significant bit to mark trailers.)
  • The six most significant bits are reserved for future protocol extensions.

Streaming-Response​

  • Streaming-Response → Streaming-Response-Headers 1*Enveloped-Message
  • Streaming-Response-Headers → ":status 200" Streaming-Content-Type [Streaming-Content-Encoding] [Streaming-Accept-Encoding] *Leading-Metadata

Streaming responses always have an HTTP status of 200 OK. As noted above, server implementations must send an EndStreamResponse as the final message in the stream, and must not send an EndStreamResponse earlier in the stream.

Examples​

Using HTTP/1.1 notation and putting each Enveloped-Message on a separate line for readability, a successful client streaming RPC:

> POST /connectrpc.greet.v1.GreetService/GreetGroup HTTP/1.1
> Host: demo.connectrpc.com
> Content-Type: application/connect+json
>
> <flags: 0><length: 15>{"name": "Buf"}
> <flags: 0><length: 19>{"name": "Connect"}

< HTTP/1.1 200 OK
< Content-Type: application/connect+json
<
< <flags: 0><length: 39>{"greeting": "Hello, Buf and Connect!"}
< <flags: 2><length: 2>{}

A failed server streaming RPC:

> POST /connectrpc.greet.v1.GreetService/GreetIndividuals HTTP/1.1
> Host: demo.connectrpc.com
> Content-Type: application/connect+proto
>
> <flags: 0><length: 8><binary proto>

< HTTP/1.1 200 OK
< Content-Type: application/connect+proto
<
< <flags: 2><length: 58>{"error": {"code": "unavailable", "message": "overloaded"}}

Error Codes​

Connect represents categories of errors as codes, and each code maps to a specific HTTP status code. The codes and their semantics were chosen to match gRPC. Only the codes below are valid — there are no user-defined codes.

CodeHTTP StatusDescription
canceled499 Client Closed RequestRPC canceled, usually by the caller.
unknown500 Internal Server ErrorCatch-all for errors of unclear origin and errors without a more appropriate code.
invalid_argument400 Bad RequestRequest is invalid, regardless of system state.
deadline_exceeded504 Gateway TimeoutDeadline expired before RPC could complete or before the client received the response.
not_found404 Not FoundUser requested a resource (for example, a file or directory) that can't be found.
already_exists409 ConflictCaller attempted to create a resource that already exists.
permission_denied403 ForbiddenCaller isn't authorized to perform the operation.
resource_exhausted429 Too Many RequestsOperation can't be completed because some resource is exhausted. Use unavailable if the server is temporarily overloaded and the caller should retry later.
failed_precondition400 Bad RequestOperation can't be completed because the system isn't in the required state.
aborted409 ConflictThe operation was aborted, often because of concurrency issues like a database transaction abort.
out_of_range400 Bad RequestThe operation was attempted past the valid range.
unimplemented501 Not ImplementedThe operation isn't implemented, supported, or enabled.
internal500 Internal Server ErrorAn invariant expected by the underlying system has been broken. Reserved for serious errors.
unavailable503 Service UnavailableThe service is currently unavailable, usually transiently. Clients should back off and retry idempotent operations.
data_loss500 Internal Server ErrorUnrecoverable data loss or corruption.
unauthenticated401 UnauthorizedCaller doesn't have valid authentication credentials for the operation.

When choosing between invalid_argument, failed_precondition, and out_of_range, use invalid_argument if the failure is independent of the system state. For example, attempting to seek to a file offset larger than 2^32-1 on a 32-bit system should return invalid_argument. Use out_of_range as a common sub-category of failed_precondition, so clients iterating through a space can detect when they're done. For example, attempting to seek past the end of a particular file should return out_of_range.

When choosing between unauthenticated, permission_denied, resource_exhausted, and not_found, use unauthenticated if the user can't be identified or presents invalid credentials. Use resource_exhausted if a per-user quota (for example, a rate limit) is exhausted. Use not_found if an operation is denied for a class of users (for example, because of a gradual rollout or an undocumented allowlist). Use permission_denied for other authorization-based rejections.

When choosing between failed_precondition, aborted, and unavailable, use unavailable if the client can back off and retry the operation. Use aborted if the client should retry at a higher level (for example, restarting a read-modify-write cycle). Use failed_precondition if the client must explicitly fix the system state before retrying (for example, by emptying a directory before attempting to remove it).

Clients must exercise judgment when deciding which errors to retry — there's no set of error codes which are safe to retry for all applications. Typically, services explicitly identify idempotent methods in their IDL.

If clients receive a response with non-200 HTTP status codes and no explicit Connect error code, they should infer a Connect code using the following table. This mapping is a superset of gRPC's: in all cases where gRPC maps an HTTP status code to a specific gRPC status code (that is, something other than unknown or internal), Connect maps to the semantically-equivalent error code.

HTTP to Error Code​

HTTP StatusInferred Code
400 Bad Requestinternal
401 Unauthorizedunauthenticated
403 Forbiddenpermission_denied
404 Not Foundunimplemented
429 Too Many Requestsunavailable
502 Bad Gatewayunavailable
503 Service Unavailableunavailable
504 Gateway Timeoutunavailable
all othersunknown

Error and EndStreamResponse​

Connect serializes errors and the block of data at the end of each response stream using JSON. This keeps errors human-readable and easy to debug.

An Error is a code, an optional message, and an optional array of details. Code and message (if present) are UTF-8 strings. When message is omitted or the empty string, clients may synthesize a user-facing message (and thereby avoid representing the message as an optional or nullable type, if such types exist in the implementation language). {"code": null} and {} are invalid. The simplest form of Error contains just a code:

{
  "code": "unavailable"
}

Details are an optional mechanism for servers to attach strongly-typed messages to errors. Each detail is an object with "type" and "value" properties and any number of other properties. The "type" field contains the fully-qualified Protobuf message name as a UTF-8 string, and the "value" field contains unpadded, base64-encoded binary Protobuf data. For readability on the wire, server implementations may also serialize the detail to JSON and include the resulting value under the "debug" key. Clients must not depend on data in the "debug" key when deserializing details.

{
  "code": "unavailable",
  "message": "overloaded: back off and retry",
  "details": [
    {
      "type": "google.rpc.RetryInfo",
      "value": "CgIIPA",
      "debug": {"retryDelay": "30s"}
    }
  ]
}

An EndStreamMessage is the final message in streaming response. It conveys whether or not the RPC succeeded and any trailing metadata. Trailing metadata is modeled as an object: keys follow Header-Name, and values are arrays of UTF-8 strings. Each string is either an ASCII-Value or a base64-encoded binary value. Keys beginning with "connect-" are reserved for use by the Connect protocol. Semantically, trailing metadata should be treated as HTTP headers: keys are case-insensitive, values for the same key can be joined with commas, and so on. (In practice, Connect implementations typically deserialize trailing metadata into the same data structure used for HTTP headers.)

{
  "error": {
    "code": "unavailable"
  },
  "metadata": {
    "acme-operation-cost": ["237"]
  }
}

Failed RPCs must provide an Error in the "error" property, and successful RPCs must omit the property. {"error": null}, {"error": {}}, and {"error": {"code": null}} are invalid. The "metadata" property is optional. After a successful RPC, EndStreamResponse can be as simple as {}.

Protocol Buffers​

When used with Protocol Buffer IDL,

  • Procedure-Name → ?( {proto package name} "." ) {service name} "/" {method name}
  • Unary-Content-Type → "content-type application/" ("proto" / "json")
  • Streaming-Content-Type → "content-type application/connect+" ("proto" / "json")

Choose the "proto" content types for binary serialization and the "json" types to use the canonical JSON mapping.

Protocol Buffers support unary RPCs and all three types of streaming.