Is a MetricReader supposed to implement ForceFlush?

[pichlermarc]

What are you trying to achieve?

The specification for ForceFlush on the MeterProvider mentions that

ForceFlush MUST invoke ForceFlush on all registered MetricReader and Push Metric Exporter instances.

However, the specification for MetricReader's operations does not specify ForceFlush.

What did you expect to see?

It seems like the spec hints that MetricReader should implement ForceFlush. If this is the intention, it would be nice to have this clarified by explicitly specifying it in the MetricReader operations spec. 🙂

The goal of this issue is to determine if MetricReader should implement ForceFlush, and - if yes - clarify the spec accordingly.

Additional context.

This came up during the spec review of the JavaScript Metrics SDK, see open-telemetry/community#1204 (comment)

Aside from the JavaScript SDK at least 2 other language SDKs do already implement a ForceFlush on the MetricReader, see:

• Python
• Java

[srikanthccv]

IIRC, when we discussed the same issue for Python implementation (issue here open-telemetry/opentelemetry-python#2554), I believe it was @jack-berg who pointed us that there is a text in MetricReader spec that says.

The SDK SHOULD provide a way to allow MetricReader to respond to MeterProvider.ForceFlush and MeterProvider.Shutdown. OpenTelemetry SDK authors MAY decide the language idiomatic approach, for example, as OnForceFlush and OnShutdown callback functions.

I think it could have been clearer if there was a ForceFlush defined in the list of MetricReader operations, but an implementation with ForceFlush is still a spec complaint.

[jsuereth]

@srikanthccv Answered this well. You can use different method names, appropriate to the paradigm you use for MetricReader.

However, the key is that you implement a working ForceFlush for appropriate readers. (E.g. PeriodicMetricReader should perform a flush when this is called, while a Pull-based reader/exporter may be unable to).

[reyang]

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#forceflush mentioned:

This method provides a way for provider to notify the registered MetricReader and MetricExporter instances, so they can do as much as they could to consume or send the metrics. Note: unlike Push Metric Exporter which can send data on its own schedule, Pull Metric Exporter can only send the data when it is being asked by the scraper, so ForceFlush would not make much sense.

[reyang]

@jsuereth @jmacd I feel it will probably help if we define MetricReader.ForceFlush in the spec, what do you think?