Semantic conventions for RPC spans

Status: Experimental

This document defines how to describe remote procedure calls (also called “remote method invocations” / “RMI”) with spans.

Common remote procedure call conventions

A remote procedure calls is described by two separate spans, one on the client-side and one on the server-side.

For outgoing requests, the SpanKind MUST be set to CLIENT and for incoming requests to SERVER.

Remote procedure calls can only be represented with these semantic conventions, when the names of the called service and method are known and available.

Span name

The span name MUST be the full RPC method name formatted as:

$package.$service/$method

(where $service MUST NOT contain dots and $method MUST NOT contain slashes)

If there is no package name or if it is unknown, the $package. part (including the period) is omitted.

Examples of span names:

grpc.test.EchoService/Echo
com.example.ExampleRmiService/exampleMethod
MyCalcService.Calculator/Add reported by the server and MyServiceReference.ICalculator/Add reported by the client for .NET WCF calls
MyServiceWithNoPackage/theMethod

Attributes

Attribute	Type	Description	Examples	Required
`rpc.system`	string	A string identifying the remoting system.	`grpc`; `java_rmi`; `wcf`	Yes
`rpc.service`	string	The full (logical) name of the service being called, including its package name, if applicable. [1]	`myservice.EchoService`	No, but recommended
`rpc.method`	string	The name of the (logical) method being called, must be equal to the $method part in the span name. [2]	`exampleMethod`	No, but recommended
`net.peer.ip`	string	Remote address of the peer (dotted decimal for IPv4 or RFC5952 for IPv6)	`127.0.0.1`	See below
`net.peer.name`	string	Remote hostname or similar, see note below.	`example.com`	See below
`net.peer.port`	int	Remote port number.	`80`; `8080`; `443`	See below
`net.transport`	string	Transport protocol used. See note below.	`ip_tcp`	See below

[1]: This is the logical name of the service from the RPC interface perspective, which can be different from the name of any implementing class. The code.namespace attribute may be used to store the latter (despite the attribute name, it may include a class name; e.g., class with method actually executing the call on the server side, RPC client stub class on the client side).

[2]: This is the logical name of the method from the RPC interface perspective, which can be different from the name of any implementing method/function. The code.function attribute may be used to store the latter (e.g., method actually executing the call on the server side, RPC client stub method on the client side).

Additional attribute requirements: At least one of the following sets of attributes is required:

For client-side spans net.peer.port is required if the connection is IP-based and the port is available (it describes the server port they are connecting to). For server-side spans net.peer.port is optional (it describes the port the client is connecting from). Furthermore, setting net.transport is required for non-IP connection like named pipe bindings.

Service name

On the server process receiving and handling the remote procedure call, the service name provided in rpc.service does not necessarily have to match the service.name resource attribute. One process can expose multiple RPC endpoints and thus have multiple RPC service names. From a deployment perspective, as expressed by the service.* resource attributes, it will be treated as one deployed service with one service.name. Likewise, on clients sending RPC requests to a server, the service name provided in rpc.service does not have to match the peer.service span attribute.

As an example, given a process deployed as QuoteService, this would be the name that goes into the service.name resource attribute which applies to the entire process. This process could expose two RPC endpoints, one called CurrencyQuotes (= rpc.service) with a method called getMeanRate (= rpc.method) and the other endpoint called StockQuotes (= rpc.service) with two methods getCurrentBid and getLastClose (= rpc.method). In this example, spans representing client request should have their peer.service attribute set to QuoteService as well to match the server’s service.name resource attribute. Generally, a user SHOULD NOT set peer.service to a fully qualified RPC service name.

Events

In the lifetime of an RPC stream, an event for each message sent/received on client and server spans SHOULD be created. In case of unary calls only one sent and one received message will be recorded for both client and server spans.

The event name MUST be "message".

Attribute	Type	Description	Examples	Required
`message.type`	string	Whether this is a received or sent message.	`SENT`	No
`message.id`	int	MUST be calculated as two different counters starting from `1` one for sent messages and one for received message. [1]		No
`message.compressed_size`	int	Compressed size of the message in bytes.		No
`message.uncompressed_size`	int	Uncompressed size of the message in bytes.		No

[1]: This way we guarantee that the values will be consistent between different implementations.

message.type MUST be one of the following:

Value	Description
`SENT`	sent
`RECEIVED`	received

Distinction from HTTP spans

HTTP calls can generally be represented using just HTTP spans. If they address a particular remote service and method known to the caller, i.e., when it is a remote procedure call transported over HTTP, the rpc.* attributes might be added additionally on that span, or in a separate RPC span that is a parent of the transporting HTTP call. Note that method in this context is about the called remote procedure and not the HTTP verb (GET, POST, etc.).

gRPC

For remote procedure calls via gRPC, additional conventions are described in this section.

rpc.system MUST be set to "grpc".

gRPC Attributes

Attribute	Type	Description	Examples	Required
`rpc.grpc.status_code`	int	The numeric status code of the gRPC request.	`0`	Yes

rpc.grpc.status_code MUST be one of the following:

Value	Description
`0`	OK
`1`	CANCELLED
`2`	UNKNOWN
`3`	INVALID_ARGUMENT
`4`	DEADLINE_EXCEEDED
`5`	NOT_FOUND
`6`	ALREADY_EXISTS
`7`	PERMISSION_DENIED
`8`	RESOURCE_EXHAUSTED
`9`	FAILED_PRECONDITION
`10`	ABORTED
`11`	OUT_OF_RANGE
`12`	UNIMPLEMENTED
`13`	INTERNAL
`14`	UNAVAILABLE
`15`	DATA_LOSS
`16`	UNAUTHENTICATED

gRPC Status

The Span Status MUST be left unset for an OK gRPC status code, and set to Error for all others.

JSON RPC

Conventions specific to JSON RPC.

rpc.system MUST be set to "jsonrpc".

JSON RPC Attributes

Attribute	Type	Description	Examples	Required
`rpc.jsonrpc.version`	string	Protocol version as in `jsonrpc` property of request/response. Since JSON-RPC 1.0 does not specify this, the value can be omitted.	`2.0`; `1.0`	If missing, it is assumed to be “1.0”.
`rpc.jsonrpc.request_id`	string	`id` property of request or response. Since protocol allows id to be int, string, `null` or missing (for notifications), value is expected to be cast to string for simplicity. Use empty string in case of `null` value. Omit entirely if this is a notification.	`10`; `request-7`; ``	No
`rpc.jsonrpc.error_code`	int	`error.code` property of response if it is an error response.	`-32700`; `100`	If missing, response is assumed to be successful.
`rpc.jsonrpc.error_message`	string	`error.message` property of response if it is an error response.	`Parse error`; `User already exists`	No
`rpc.method`	string	The name of the (logical) method being called, must be equal to the $method part in the span name. [1]	`exampleMethod`	Yes

[1]: This is always required for jsonrpc. See the note in the general RPC conventions for more information.