Exposing publishAllOutstanding as a public API in PublisherInterface

[elefeint]

In the last release of Spring Cloud GCP, we switched from returning Publisher to returning PublisherInterface in the convenience "Template" layer. This led to an accidental regression (GoogleCloudPlatform/spring-cloud-gcp#790).

Before we proceed with workarounds in Spring Cloud GCP, an API surface question for the client library -- does it make sense to add publishAllOutstanding() to PublisherInterface? Or is it not meant to be part of the public API?

[meredithslota]

@kamalaboulhosn @hannahrogers-google @hongalex @feywind for discussion.

[meredithslota]

Also @anguillanneuf for discussion.

[anguillanneuf]

I have not used publishAllOutstanding() anywhere in samples. Deferring this to the other folks for comment.

[kamalaboulhosn]

The PublisherInterface is used by the Pub/Sub Lite library. It does not implement the publishAllOutstanding() method, though. Therefore, adding the method to the interface would break the Pub/Sub Lite client library unless we added support for this method. I'm not sure how well that fits into Lite. Perhaps @dpcollins-google has some thoughts.

What was the reason for switching from Publisher to PublisherInterface?

[meltsufin]

What was the reason for switching from Publisher to PublisherInterface?

It made it possible to add trace instrumentation here: GoogleCloudPlatform/spring-cloud-gcp#548. We basically needed a TracingPublisher that can be used in place of Publisher.

Note that it's impossible to subclass Publisher because it only has one private constructor.

[elefeint]

Happy New Year! I am going to try switching Spring Cloud GCP to use Publisher.setTransform() instead of a custom implementation of PublisherInterface to add tracing information, since the need to interact with the full set of Publisher functionality came up once again in GoogleCloudPlatform/spring-cloud-gcp#864.

@kamalaboulhosn Could you confirm/clarify a couple of my assumptions --

1. PublisherInterface is not meant to be the way to interact with Publisher functionality, and so is not expected to expose all public API methods of Publisher.
2. Publisher.setTransform() is okay to use. It has @BetaApi on it, so I want to double check. The functionality was added for OpenCensus support, so it fits our usecase perfectly.

[hannahrogers-google]

Hi @elefeint,

The only reason we added the PublisherInterface was because we created a Pub/Sub Lite service, as Kamal mentioned. The interface only exposes methods that are implemented by both the Pub/Sub Lite publisher client and the Cloud Pub/Sub publisher client to allow for seamless transition between Cloud Pub/Sub and Pub/Sub Lite. The other methods exposed by the Cloud Pub/Sub publisher client (like publishAllOutstanding) are not implemented by the Pub/Sub Lite client, which is why these methods are not included in the PublisherInterface.

Publisher.setTransform is okay to use, even though it has the BetaApi tag.

[elefeint]

Thank you!