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TraceState Handling

Status: Development

Table of Contents

In alignment to the TraceContext specification, this section uses the Augmented Backus-Naur Form (ABNF) notation of RFC5234, including the DIGIT rule in that document.

When setting TraceState values that are part of the OTel ecosystem, they MUST all be contained in a single entry using the ot key, with the value being a semicolon separated list of key-value pairs such as:

  • ot=p:8;r:62
  • ot=foo:bar;k1:13

The TraceContext specification defines support for multiple "tenants" each to use their own tracestate entry by prefixing tenant@ to tenant-specific values in a mixed tracing environment. OpenTelemetry recognizes this syntax but does not specify an interpretation for multi-tenant tracestate. The list can be formally defined as:

list        = list-member *( ";" list-member )
list-member = key ":" value

The complete list length MUST NOT exceed 256 characters, as defined by the TraceState value section, and the used keys MUST be unique.

Instrumentation libraries and clients MUST NOT use this entry, and they MUST instead use their own entry.

Key

The key is an identifier that describes an OTel concern. Simple examples are p, ts, or s1.

The key can be formally defined as:

key        = lcalpha *(lcalpha / DIGIT )
lcalpha    = %x61-7A ; a-z

Specific keys used by OTel concerns MUST be defined as part of the Specification, and hence it is forbidden to use any key that has not been defined in the Specification itself.

Value

The value is an opaque string. Although it has no maximum allowed length, it is recommended to use short values, as the entire list of key-values MUST NOT exceed 256 characters.

The value can be formally defined as:

value      = *(chr)
chr        = ucalpha / lcalpha / DIGIT / "." / "_" / "-"
ucalpha    = %x41-5A ; A-Z
lcalpha    = %x61-7A ; a-z

Setting values

Set values MUST be either updated or added to the ot entry in TraceState, in order to preserve existing values belonging to other OTel concerns. For example, if a given concern K wants to set k1:13:

  • ot=p:8;r:62 will become ot=p:8;r:62;k1:13.
  • ot=p:8;k1:7;r:62 will become ot=p:8;r:62;k1:13. Preserving the order is not required.

If setting a value ends up making the entire ot entry exceed the 256 characters limit, SDKs are advised to abort the operation and signal the user about the error, e.g.

traceState, ok := SetTraceStateValue(traceState, value)
if ok {
  // Successfully set the specified value, traceState was updated.
} else {
  // traceState was not updated.
}

Pre-defined OpenTelemetry sub-keys

The following values have been defined by OpenTelemetry.

Sampling threshold value th

The OpenTelemetry TraceState th sub-key defines a sampling threshold, which conveys effective sampling probability. Valid values of the th sub-fields include between 1 and 14 lowercase hexadecimal digits.

hexdigit = DIGIT ; a-f

To decode the threshold from the OpenTelemetry TraceState th value, first extend the value with trailing zeros to make 14 digits. Then, parse the 14-digit value as a 56-bit unsigned number, yielding a rejection threshold.

OpenTelemetry defines consistent sampling in terms of a 56-bit trace randomness value compared with the 56-bit rejection threshold. When the randomness value is less than the rejection threshold, the span is not sampled.

The threshold value 0 indicates that no spans are being rejected, corresponding with 100% sampling. For example, the following TraceState value identifies a trace with 100% sampling:

tracestate: ot=th:0

To calculate sampling probability from the rejection threshold, define a constant MaxAdjustedCount equal to 2^56, the number of distinct 56-bit values. The sampling probability is defined:

Probability = (MaxAdjustedCount - Threshold) / MaxAdjustedCount

Threshold can be calculated from Probability:

Threshold = MaxAdjustedCount * (1 - Probability)

In sampling, the term adjusted count refers to the effective number of items represented by a sampled item of telemetry. The adjusted count of a span is the inverse of its sampling probability and can be derived from the threshold as follows.

AdjustedCount = MaxAdjustedCount / (MaxAdjustedCount - Threshold)

As an example, 25% probability sampling corresponds with adjusted count 4 and threshold c.
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