Timestamp granularity and log ordering

[lmolkova]

OTel logs timestamp support up to nanosecond precision.

In practice, timestamp precision is lower than 1ns and also platform and runtime-specific. E.g. Python time_ns precision on Windows measures in milliseconds. It's possible to make it more precise, but the granularity may still be limited.

Backends may (and do) truncate timestamps (e.g. Azure Monitor truncates to microseconds, there are at least a few backends that truncate to milliseconds).

Milli- or microsecond granularity, in general case, is not enough to preserve the order in which logs were emitted.

Imagine code like in this:

logger.warn("Connection dropped");
logger.warn("Request cancelled");

This is an oversimplified snippet, the reality is probably a bit more complicated - e.g. connection is terminated, we log it, the request cancellation handler is executed synchronously which logs cancellation.

These log records are generated extremely fast. If they are emitted during the same thread cycle they can easily be emitted within one microsecond and de-facto share the same timestamp on the backend.

Now when you query those, you can't establish the original order:

2024-12-19T21:00:00.015Z - WARN - Request cancelled
2024-12-19T21:00:00.015Z - WARN - Connection dropped

Unless there is some information in the log body, the original order is lost and it's now even harder to distinguish the cause from the consequence.

Note

It might affect spans too, but 1) related spans have context to record causality 2) when sibling spans run in parallel, they run on different threads and the order is random/does not usually matter 3) spans are intended to be long and not so verbose

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Proposal:

Introduce an attribute like log.sequence.number - monotonically incremented number within the service instance lifetime. The absolute value of this counter does not mean anything. Within one service instance id (process id) it reflects the order in which logs were emitted by the SDK. It may be implemented as a custom LogRecordProcessor, but has to be implemented in-process to avoid reordering caused by network retries or similar side-effects.

It'd be opt-in: OTel SDKs may provide this feature on demand. Vendors that truncate timestamps are encouraged to enable it in their distros.

[lmolkova]

Some context:

#1621 (comment)

If the timestamps can not be produced with sufficient precision to ensure an absolute ordering, consider artificially adding a monotonically increasing counter to the event as early as possible in the pipeline to break the tie between “simultaneous” events.

https://www.elastic.co/blog/considerations-for-timestamps-in-centralized-logging-platforms

It can also be useful in detecting duplicates and/or lost records.