Common specification concepts

Status: Stable, Feature-freeze


Attributes are a list of zero or more key-value pairs. An Attribute MUST have the following properties:

  • The attribute key, which MUST be a non-null and non-empty string.
  • The attribute value, which 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 array values such values SHOULD be represented as JSON strings.

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.

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 attributes 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. 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.