60-trace-id-format.md

Considerations for trace-id field generation

This section suggests some best practices to consider when platform or tracing vendor implement trace-id generation and propagation algorithms. These practices will ensure better interoperability of different systems.

Uniqueness of trace-id

The value of trace-id SHOULD be globally unique. This field is typically used for unique identification of a distributed trace. It is common for distributed traces to span various components, including, for example, cloud services. Cloud services tend to serve variety of clients and have a very high throughput of requests. So global uniqueness of trace-id is important, even when local uniqueness might seem like a good solution.

Randomness of trace-id

Randomly generated value of trace-id SHOULD be preferred over other algorithms of generating a globally unique identifiers. Randomness of trace-id addresses some security and privacy concerns of exposing unwanted information. Randomness also allows tracing vendors to base sampling decisions on trace-id field value and avoid propagating an additional sampling context.

As shown in the next section, it is important for trace-id to carry "uniqueness" and "randomness" in the right part of the trace-id, for better inter-operability with some existing systems.

Handling trace-id for compliant platforms with shorter internal identifiers

There are tracing systems which use a trace-id that is shorter than 16 bytes, which are still willing to adopt this specification.

If such a system is capable of propagating a fully compliant trace-id, even while still requiring a shorter, non-compliant identifier for internal purposes, the system is encouraged to utilize the tracestate header to propagate the additional internal identifier. However, if a system would instead prefer to use the internal identifier as the basis for a fully compliant trace-id, it SHOULD be incorporated at the as rightmost part of a trace-id. For example, tracing system may receive 234a5bcd543ef3fa53ce929d0e0e4736 as a trace-id, hovewer internally it will use 53ce929d0e0e4736 as an identifier.

Interoperating with existing systems which use shorter identifiers

There are tracing systems which are not capable of propagating the entire 16 bytes of a trace-id. For better interoperability between a fully compliant systems with these existing systems, the following practices are recommended:

1. When a system creates an outbound message and needs to generate a fully compliant 16 bytes trace-id from a shorter identifier, it SHOULD left pad the original identifier with zeroes. For example, the identifier 53ce929d0e0e4736, SHOULD be converted to trace-id value 000000000000000053ce929d0e0e4736.
2. When a system receives an inbound message and needs to convert the 16 bytes trace-id to a shorter identifier, the rightmost part of trace-id SHOULD be used as this identifier. For instance, if the value of trace-id was 234a5bcd543ef3fa53ce929d0e0e4736 on an incoming request, tracing system SHOULD use identifier with the value of 53ce929d0e0e4736.

Similar transformations are expected when tracing system converts other distributed trace context propagation formats to W3C Trace Context. Shorter identifiers SHOULD be left padded with zeros when converted to 16 bytes trace-id and rightmost part of trace-id SHOULD be used as a shorter identifier.

Note, many existing systems that are not capable of propagating the whole trace-id will not propagate tracestate header either. However, such system can still use tracestate header to propagate additional data that is known by this system. For example, some systems use two flags indicating whether distributed trace needs to be recorded or not. In this case one flag can be send as sampled flag of traceparent header and tracestate can be used to send and receive an additional flag. Compliant systems will propagate this flag along all other key/value pairs. Existing systems which are not capable of tracestate propagation will truncate all additional values from tracestate and only pass along that flag.