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 conventions that they emit until the HTTP semantic conventions are marked stable (HTTP stabilization will include stabilization of a core set of networking conventions which are also used in RPC instrumentations). Conventions include, but are not limited to, attributes, metric and span names, and unit of measure.
• 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 conventions, and stop emitting the old experimental networking conventions that the instrumentation emitted previously.
  • http/dup - emit both the old and the stable networking conventions, 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 conventions the instrumentation was emitting previously.
  • Note: http/dup has higher precedence than http in case both values are present
• SHOULD maintain (security patching at a minimum) the existing major version for at least six months after it starts emitting both sets of conventions.
• 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 list of RPC server metric instruments.

Metric: rpc.server.duration

This metric is recommended.

Name	Instrument Type	Unit (UCUM)	Description
rpc.server.duration	Histogram	ms	Measures the duration of inbound RPC. [1]

[1]: While streaming RPCs may record this metric as start-of-batch to end-of-batch, it’s hard to interpret in practice.

Streaming: N/A.

Metric: rpc.server.request.size

This metric is recommended.

Name	Instrument Type	Unit (UCUM)	Description
rpc.server.request.size	Histogram	By	Measures the size of RPC request messages (uncompressed). [1]

[1]: Streaming: Recorded per message in a streaming batch

Metric: rpc.server.response.size

This metric is recommended.

Name	Instrument Type	Unit (UCUM)	Description
rpc.server.response.size	Histogram	By	Measures the size of RPC response messages (uncompressed). [1]

[1]: Streaming: Recorded per response in a streaming batch

Metric: rpc.server.requests_per_rpc

This metric is recommended.

Name	Instrument Type	Unit (UCUM)	Description
rpc.server.requests_per_rpc	Histogram	{count}	Measures the number of messages received per RPC. [1]

[1]: Should be 1 for all non-streaming RPCs.

Streaming : This metric is required for server and client streaming RPCs

Metric: rpc.server.responses_per_rpc

This metric is recommended.

Name	Instrument Type	Unit (UCUM)	Description
rpc.server.responses_per_rpc	Histogram	{count}	Measures the number of messages sent per RPC. [1]

[1]: Should be 1 for all non-streaming RPCs.

Streaming: This metric is required for server and client streaming RPCs

RPC Client

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

Metric: rpc.client.duration

This metric is recommended.

Name	Instrument Type	Unit (UCUM)	Description
rpc.client.duration	Histogram	ms	Measures the duration of outbound RPC. [1]

[1]: While streaming RPCs may record this metric as start-of-batch to end-of-batch, it’s hard to interpret in practice.

Streaming: N/A.

Metric: rpc.client.request.size

This metric is recommended.

Name	Instrument Type	Unit (UCUM)	Description
rpc.client.request.size	Histogram	By	Measures the size of RPC request messages (uncompressed). [1]

[1]: Streaming: Recorded per message in a streaming batch

Metric: rpc.client.response.size

This metric is recommended.

Name	Instrument Type	Unit (UCUM)	Description
rpc.client.response.size	Histogram	By	Measures the size of RPC response messages (uncompressed). [1]

[1]: Streaming: Recorded per response in a streaming batch

Metric: rpc.client.requests_per_rpc

This metric is recommended.

Name	Instrument Type	Unit (UCUM)	Description
rpc.client.requests_per_rpc	Histogram	{count}	Measures the number of messages received per RPC. [1]

[1]: Should be 1 for all non-streaming RPCs.

Streaming: This metric is required for server and client streaming RPCs

Metric: rpc.client.responses_per_rpc

This metric is recommended.

Name	Instrument Type	Unit (UCUM)	Description
rpc.client.responses_per_rpc	Histogram	{count}	Measures the number of messages sent per RPC. [1]

[1]: Should be 1 for all non-streaming RPCs.

Streaming: This metric is required for server and client streaming RPCs

Attributes

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

Attribute	Type	Description	Examples	Requirement Level
network.transport	string	OSI transport layer or inter-process communication method. [1]	tcp; udp	Recommended
network.type	string	OSI network layer or non-OSI equivalent. [2]	ipv4; ipv6	Recommended
rpc.method	string	The name of the (logical) method being called, must be equal to the $method part in the span name. [3]	exampleMethod	Recommended
rpc.service	string	The full (logical) name of the service being called, including its package name, if applicable. [4]	myservice.EchoService	Recommended
rpc.system	string	A string identifying the remoting system. See below for a list of well-known identifiers.	grpc	Required
server.address	string	Server address - domain name if available without reverse DNS lookup, otherwise IP address or Unix domain socket name. [5]	example.com; 10.1.2.80; /tmp/my.sock	Recommended
server.port	int	Server port number. [6]	80; 8080; 443	Recommended

[1]: The value SHOULD be normalized to lowercase.

Consider always setting the transport when setting a port number, since a port number is ambiguous without knowing the transport, for example different processes could be listening on TCP port 12345 and UDP port 12345.

[2]: The value SHOULD be normalized to lowercase.

[3]: 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).

[4]: 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).

[5]: When observed from the client side, and when communicating through an intermediary, server.address SHOULD represent the server address behind any intermediaries (e.g. proxies) if it’s available.

[6]: When observed from the client side, and when communicating through an intermediary, server.port SHOULD represent the server port behind any intermediaries (e.g. proxies) if it’s available.

network.transport 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
tcp	TCP
udp	UDP
pipe	Named or anonymous pipe. See note below.
unix	Unix domain socket

network.type 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
ipv4	IPv4
ipv6	IPv6

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.