[ADDED] PushConsumer implementation in jetstream package

[piotrpio]

This PR introduces a PushConsumer implementation in jetstream API. For now it only exposes callback-based Consume() implementation.
Due to interface differences and breaking change concerns, a completely new set of methods were added to enable operating on push consumers.

Signed-off-by: Piotr Piotrowski [email protected]

[coveralls]

coverage: 85.095% (+0.2%) from 84.911%
when pulling 05b9311 on js-simplification-push-consumer
into f7dfee9 on main.

[alexbozhenko]

@piotrpio
https://docs.nats.io/using-nats/developer/develop_jetstream/consumers#pull-consumers says:

We recommend using pull consumers for new projects. In particular when scalability, detailed flow control or error handling are a design focus. Most client API have been updated to provide convenient interfaces for consuming messages through callback handler or iterators without the need to manage message retrieval.

Since we are adding push consumers to the new package, I guess the statement above is not true, and should be updated?

[Jarema]

No @alexbozhenko , the statement still holds.

We still recommend using pull consumers.
We're just adding support for push so people can migrate to the new api without migrating every consumer, plus to support some niche cases where it can still make sense.

[piotrpio]

@alexbozhenko I don't think so - pull consumers are still the recommended way to go, we only want to provide an option to use push consumers as:
a) users may want to switch to jetstream package but are already using push consumers
b) in some scenarios, using push consumers can be a good idea

[Jarema]

Do we want to provide the Messages variant with Next? Was it available in old JS? Can't recall.

[piotrpio]

It kind of was, via ChanSubscribe and yes, I think we should add that API, but I would do it in a separate PR.

[Jarema]

e.g. fetch messages is not a best example here ;)

[piotrpio]

Right, changed to "allowing to consume messages"