Make user aware when sdk.batch.size is specified but sdk.batch.delay isn't

[hariso]

When sdk.batch.size is specified but sdk.batch.delay isn't, the SDK will wait indefinitely for the batch size to fill up.

We should make the user aware of that (docs, and a warning /failed validation).

Tasks:

• Update docs (done as part of Explain batching, document best practices conduit-site#298)
• Log a warning Warn if one of sdk.batch.size or sdk.batch.delay is set to 0 #309
• [ ] Remove 0 as default value.

[nickchomey]

I've run into this issue various times.

Might it be possible or desirable to make batch.delay conditionally required - optional by default, along with batch.size, but required if you have set batch.size?

[lovromazgon]

We have a batching documentation page, but this behavior is not called out.

Making it required to specify both could be a good way of preventing this pitfall, although we should still allow the user to set the delay to 0s if they want to. In a sufficiently fast stream, the delay would just add unnecessary overhead, so skipping it might be preferable in that case. That said, we can at least print a warning when the connector starts, if we notice such a configuration.

So my proposal:

• Document it on the documentation page (make it stand out, a warning admonition).
• Output a warning in the middleware in Open if only one of the batch parameters is greater than 0.
• Make sdk.batch.size and sdk.batch.delay required together (either the user sets both or none). Note that the connector needs to call the Validate method properly if they override it (related to Automatically execute middleware validations when configuration struct uses custom validation #264).