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Semantic Conventions for RPC Metrics

Status: Experimental

The conventions described in this section are RPC specific. When RPC operations occur, measurements about those operations are recorded to instruments. The measurements are aggregated and exported as metrics, which provide insight into those operations. By including RPC properties as attributes on measurements, the metrics can be filtered for finer grain analysis.

Warning Existing RPC instrumentations that are using v1.20.0 of this document (or prior):

  • SHOULD NOT change the version of the networking attributes that they emit until the HTTP semantic conventions are marked stable (HTTP stabilization will include stabilization of a core set of networking attributes which are also used in RPC instrumentations).
  • SHOULD introduce an environment variable OTEL_SEMCONV_STABILITY_OPT_IN in the existing major version which is a comma-separated list of values. The only values defined so far are:
    • http - emit the new, stable networking attributes, and stop emitting the old experimental networking attributes that the instrumentation emitted previously.
    • http/dup - emit both the old and the stable networking attributes, allowing for a seamless transition.
    • The default behavior (in the absence of one of these values) is to continue emitting whatever version of the old experimental networking attributes the instrumentation was emitting previously.
  • SHOULD maintain (security patching at a minimum) the existing major version for at least six months after it starts emitting both sets of attributes.
  • SHOULD drop the environment variable in the next major version (stable next major version SHOULD NOT be released prior to October 1, 2023).

Metric instruments

The following metric instruments MUST be used to describe RPC operations. They MUST be of the specified type and units.

Note: RPC server and client metrics are split to allow correlation across client/server boundaries, e.g. Lining up an RPC method latency to determine if the server is responsible for latency the client is seeing.

RPC Server

Below is a table of RPC server metric instruments.

Name Instrument Type (*) Unit Unit (UCUM) Description Status Streaming
rpc.server.duration Histogram milliseconds ms measures duration of inbound RPC Recommended N/A. While streaming RPCs may record this metric as start-of-batch to end-of-batch, it's hard to interpret in practice.
rpc.server.request.size Histogram Bytes By measures size of RPC request messages (uncompressed) Optional Recorded per message in a streaming batch
rpc.server.response.size Histogram Bytes By measures size of RPC response messages (uncompressed) Optional Recorded per response in a streaming batch
rpc.server.requests_per_rpc Histogram count {count} measures the number of messages received per RPC. Should be 1 for all non-streaming RPCs Optional Required
rpc.server.responses_per_rpc Histogram count {count} measures the number of messages sent per RPC. Should be 1 for all non-streaming RPCs Optional Required

RPC Client

Below is a table of RPC client metric instruments. These apply to traditional RPC usage, not streaming RPCs.

Name Instrument Type (*) Unit Unit (UCUM) Description Status Streaming
rpc.client.duration Histogram milliseconds ms measures duration of outbound RPC Recommended N/A. While streaming RPCs may record this metric as start-of-batch to end-of-batch, it's hard to interpret in practice.
rpc.client.request.size Histogram Bytes By measures size of RPC request messages (uncompressed) Optional Recorded per message in a streaming batch
rpc.client.response.size Histogram Bytes By measures size of RPC response messages (uncompressed) Optional Recorded per message in a streaming batch
rpc.client.requests_per_rpc Histogram count {count} measures the number of messages received per RPC. Should be 1 for all non-streaming RPCs Optional Required
rpc.client.responses_per_rpc Histogram count {count} measures the number of messages sent per RPC. Should be 1 for all non-streaming RPCs Optional Required

Attributes

Below is a table of attributes that SHOULD be included on client and server RPC measurements.

Attribute Type Description Examples Requirement Level
rpc.system string A string identifying the remoting system. See below for a list of well-known identifiers. grpc Required
rpc.service string The full (logical) name of the service being called, including its package name, if applicable. [1] myservice.EchoService 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 Recommended
network.transport string OSI Transport Layer or Inter-process Communication method. The value SHOULD be normalized to lowercase. tcp; udp Recommended
network.type string OSI Network Layer or non-OSI equivalent. The value SHOULD be normalized to lowercase. ipv4; ipv6 Recommended
server.address string RPC server host name. [3] example.com Required
server.port int Logical server port number 80; 8080; 443 Conditionally Required: See below
server.socket.address string Physical server IP address or Unix socket address. If set from the client, should simply use the socket's peer address, and not attempt to find any actual server IP (i.e., if set from client, this may represent some proxy server instead of the logical server). 10.5.3.2 See below
server.socket.port int Physical server port. 16456 Recommended: [4]

[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).

[3]: May contain server IP address, DNS name, or local socket name. When host component is an IP address, instrumentations SHOULD NOT do a reverse proxy lookup to obtain DNS name and SHOULD set server.address to the IP address provided in the host component.

[4]: If different than server.port and if server.socket.address is set.

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

rpc.system has the following list of well-known values. If one of them applies, then the respective value MUST be used, otherwise a custom value MAY be used.

Value Description
grpc gRPC
java_rmi Java RMI
dotnet_wcf .NET WCF
apache_dubbo Apache Dubbo
connect_rpc Connect RPC

To avoid high cardinality, implementations should prefer the most stable of server.address or server.socket.address, depending on expected deployment profile. For many cloud applications, this is likely server.address as names can be recycled even across re-instantiation of a server with a different ip.

For client-side metrics server.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 server.port is optional (it describes the port the client is connecting from).

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.

Semantic Conventions for specific RPC technologies

More specific Semantic Conventions are defined for the following RPC technologies:

  • Connect: Semantic Conventions for Connect RPC.
  • gRPC: Semantic Conventions for gRPC.
  • JSON-RPC: Semantic Conventions for JSON-RPC.