Common specification concepts

Status: Stable, Feature-freeze


An Attribute is a key-value pair, which MUST have the following properties:

  • The attribute key MUST be a non-null and non-empty string.
    • Case sensitivity of keys is preserved. Keys that differ in casing are treated as distinct keys.
  • The attribute value is either:
    • A primitive type: string, boolean, double precision floating point (IEEE 754-1985) or signed 64 bit integer.
    • An array of primitive type values. The array MUST be homogeneous, i.e., it MUST NOT contain values of different types.

For protocols that do not natively support non-string values, non-string values SHOULD be represented as JSON-encoded strings. For example, the expression int64(100) will be encoded as 100, float64(1.5) will be encoded as 1.5, and an empty array of any type will be encoded as [].

Attribute values expressing a numerical value of zero, an empty string, or an empty array are considered meaningful and MUST be stored and passed on to processors / exporters.

Attribute values of null are not valid and attempting to set a null value is undefined behavior.

null values SHOULD NOT be allowed in arrays. However, if it is impossible to make sure that no null values are accepted (e.g. in languages that do not have appropriate compile-time type checking), null values within arrays MUST be preserved as-is (i.e., passed on to span processors / exporters as null). If exporters do not support exporting null values, they MAY replace those values by 0, false, or empty strings. This is required for map/dictionary structures represented as two arrays with indices that are kept in sync (e.g., two attributes header_keys and header_values, both containing an array of strings to represent a mapping header_keys[i] -> header_values[i]).

See Attribute Naming for naming guidelines.

See Requirement Level for requirement levels guidelines.

See this document to find out how to map values obtained outside OpenTelemetry into OpenTelemetry attribute values.

Standard Attribute

Attributes are used in various places throughout the OpenTelemetry data model. We designate the previous attribute section as the standard attribute definition, in order to facilitate more intuitive and consistent API / SDK design.

The standard attribute definition SHOULD be used to represent attributes in data modeling unless there is a strong justification to diverge. For example, the Log Data Model has an extended attributes definition allowing values of type Any. This reflects that LogRecord attributes are expected to model data produced from external log APIs, which do not necessarily have the same value type restrictions as the standard attribute definition.

Note: Extending the set of standard attribute value types is a breaking change. This was decided after extensive debate, with arguments as follows:

  • Limiting the types of attribute values to a set which has proved sufficient during several years of OpenTelemetry’s development is a useful guardrail for design. In taking additional value types off the table, we narrow the solution space and have more productive design conversations.
  • Upon proposing to extend support for complex value types, we received significant pushback. Limiting attribute value types to primitives and arrays of primitives simplifies data consumers’ efforts to create search indexes and perform statistical analysis.
  • To address concerns over restricting standard attributes to primitive types, it was called out that complex types can be encoded as existing primitive types, such as representing datetime as a string or 64 bit integer.

Attribute Limits

Execution of erroneous code can result in unintended attributes. If there are no limits placed on attributes, they can quickly exhaust available memory, resulting in crashes that are difficult to recover from safely.

By default an SDK SHOULD apply truncation as per the list of configurable parameters below.

If an SDK provides a way to:

  • set an attribute value length limit such that for each attribute value:
    • if it is a string, if it exceeds that limit (counting any character in it as 1), SDKs MUST truncate that value, so that its length is at most equal to the limit,
    • if it is an array of strings, then apply the above rule to each of the values separately,
    • otherwise a value MUST NOT be truncated;
  • set a limit of unique attribute keys such that:
    • for each unique attribute key, addition of which would result in exceeding the limit, SDK MUST discard that key/value pair.

There MAY be a log emitted to indicate to the user that an attribute was truncated or discarded. To prevent excessive logging, the log MUST NOT be emitted more than once per record on which an attribute is set.

If the SDK implements the limits above, it MUST provide a way to change these limits programmatically. Names of the configuration options SHOULD be the same as in the list below.

An SDK MAY implement model-specific limits, for example SpanAttributeCountLimit or LogRecordAttributeCountLimit. If both a general and a model-specific limit are implemented, then the SDK MUST first attempt to use the model-specific limit, if it isn’t set, then the SDK MUST attempt to use the general limit. If neither are defined, then the SDK MUST try to use the model-specific limit default value, followed by the global limit default value.

Configurable Parameters

  • AttributeCountLimit (Default=128) - Maximum allowed attribute count per record;
  • AttributeValueLengthLimit (Default=Infinity) - Maximum allowed attribute value length;

Exempt Entities

Resource attributes SHOULD be exempt from the limits described above as resources are not susceptible to the scenarios (auto-instrumentation) that result in excessive attributes count or size. Resources are also sent only once per batch instead of per span so it is relatively cheaper to have more/larger attributes on them. Resources are also immutable by design and they are generally passed down to TracerProvider along with limits. This makes it awkward to implement attribute limits for Resources.

Attributes, which belong to Metrics, are exempt from the limits described above at this time, as discussed in Metrics Attribute Limits.

Attribute Collections

Resources, Instrumentation Scopes, Metric points, Spans, Span Events, Span Links and Log Records may contain a collection of attributes. The keys in each such collection are unique, i.e. there MUST NOT exist more than one key-value pair with the same key. The enforcement of uniqueness may be performed in a variety of ways as it best fits the limitations of the particular implementation.

Normally for the telemetry generated using OpenTelemetry SDKs the attribute key-value pairs are set via an API that either accepts a single key-value pair or a collection of key-value pairs. Setting an attribute with the same key as an existing attribute SHOULD overwrite the existing attribute’s value. See for example Span’s SetAttribute API.

A typical implementation of SetAttribute API will enforce the uniqueness by overwriting any existing attribute values pending to be exported, so that when the Span is eventually exported the exporters see only unique attributes. The OTLP format in particular requires that exported Resources, Spans, Metric data points and Log Records contain only unique attributes.

Some other implementations may use a streaming approach where every SetAttribute API call immediately results in that individual attribute value being exported using a streaming wire protocol. In such cases the enforcement of uniqueness will likely be the responsibility of the recipient of this data.