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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 name of the service being called, including its package name, if applicable. myservice.EchoService No, but recommended
rpc.method string The name of the method being called, must be equal to the $method part in the span name. 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 number Remote port number. 80; 8080; 443 See below
net.transport string Transport protocol used. See note below. IP.TCP See below

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.

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 number The numeric status code of the gRPC request. 0; 1; 16 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.

Events

In the lifetime of a gRPC stream, an event for each message sent/received on client and server spans SHOULD be created with the following attributes:

-> [time],
    "name" = "message",
    "message.type" = "SENT",
    "message.id" = id
    "message.compressed_size" = <compressed size in bytes>,
    "message.uncompressed_size" = <uncompressed size in bytes>

-> [time],
    "name" = "message",
    "message.type" = "RECEIVED",
    "message.id" = id
    "message.compressed_size" = <compressed size in bytes>,
    "message.uncompressed_size" = <uncompressed size in bytes>

The message.id MUST be calculated as two different counters starting from 1 one for sent messages and one for received message. This way we guarantee that the values will be consistent between different implementations. In case of unary calls only one sent and one received message will be recorded for both client and server spans.