Attribute Requirement Levels for Semantic Conventions

Status: Experimental

This section applies to Log, Metric, Resource, and Span, and describes requirement levels for attributes defined in semantic conventions.

The following attribute requirement levels are specified:

The requirement level for attribute is defined by semantic conventions depending on attribute availability across instrumented entities, performance, security, and other factors. When defining requirement levels, semantic conventions MUST take into account signal-specific requirements.

For example, Metric attributes that may have high cardinality can only be defined with Optional level.

Semantic convention that refers to an attribute from another semantic convention MAY modify the requirement level within its own scope. Otherwise, requirement level from the referred semantic convention applies.

For example, Database semantic convention references net.transport attribute defined in General attributes with Conditionally Required level on it.

Required

All instrumentations MUST populate the attribute. Semantic convention defining a Required attribute expects that an absolute majority of instrumentation libraries and applications are able to efficiently retrieve and populate it, can ensure cardinality, security, and other requirements specific to signal defined by the convention. http.method is an example of a Required attribute.

Note: Consumers of telemetry can detect if telemetry item follows a specific semantic convention by checking the presence of a Required attribute defined by such convention. For example, the presence of db.system attribute on a span can be used as an indication that the span follows database semantics.

Conditionally Required

All instrumentations MUST add the attribute when given condition is satisfied. Semantic convention of a Conditionally Required level of an attribute MUST clarify the condition under which the attribute is expected to be populated.

http.route is an example of a conditionally required attribute to be populated when instrumented HTTP framework provides route information for the instrumented request. Some low-level HTTP server implementations do not support routing and corresponding instrumentations can’t populate the attribute.

When the condition on Conditionally Required attribute is not satisfied and there is no requirement to populate attribute, semantic conventions MAY provide special instructions on how to handle it. If no instructions are given and if instrumentation can populate the attribute, instrumentation SHOULD use the Optional requirement level on the attribute.

For example, net.peer.name is Conditionally Required by Database convention when available. When only net.peer.ip is available, instrumentation can do a DNS lookup, cache and populate net.peer.name but only if user explicitly enables instrumentation to do so, considering performance issues the DNS lookup introduces.

Instrumentations SHOULD add the attribute by default if it’s readily available and can be efficiently populated. Instrumentations MAY offer a configuration option to disable Recommended attributes.

Instrumentations that decide not to populate Recommended attributes due to performance, security, privacy, or other consideration by default, SHOULD use the Optional requirement level on them if the attributes are logically applicable.

Optional

Instrumentations SHOULD populate the attribute if and only if the user configures the instrumentation to do so. Instrumentation that doesn’t support configuration MUST NOT populate Optional attributes.

Performance suggestions

Here are several examples of expensive operations to be avoided by default:

  • DNS lookup to populate net.peer.name if only IP address is available to the instrumentation. Caching lookup results does not solve the issue for all possible cases and should be avoided by default too.
  • forcing http.route calculation before HTTP framework calculates it
  • reading response stream to find http.response_content_length when Content-Length header is not available