Common specification concepts
Status: Stable, Feature-freeze
Attribute is a key-value pair, which MUST have the following properties:
- The attribute key MUST be a non-
nulland non-empty string.
- 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
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
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
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
both containing an array of strings to represent a mapping
header_keys[i] -> header_values[i]).
See Attribute Naming for naming guidelines.
See this document to find out how to map values obtained outside OpenTelemetry into OpenTelemetry attribute values.
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
- 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. 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.
AttributeCountLimit(Default=128) - Maximum allowed attribute count per record;
AttributeValueLengthLimit(Default=Infinity) - Maximum allowed attribute value length;
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.
Resources, Metrics data 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.