APIv2: Changes in the next version of the API

[edenhill]

This is a summary of changes that should go into the next version of the API (SONAME version 2).

C

• Replace all error codes (rd_kafka_resp_err_t) with error objects (rd_kafka_error_t *).
• Rename error_cb to event_cb - usable both for events and errors.
• Provide a severity level in event_cb to let the application know the severity of the error.
• Move log_cb to event_cb.
• Logs, errors and events are all events and shall be treated equally through the event_cb.
• Decomission all uses of errno. Consistently return error objects from all relevant functions.
• Remove dr_cb.
• Add flags field to rd_kafka_message_t, see issue Add producing batched messages #112
• Add opaque to rd_kafka_message_t
• Add errstr to rd_kafka_message_t, cant reuse payload in dr_cb.
• Make rd_kafka_message_t a private type
• Remove legacy consumer.
• Add a richer error type that contains severity. ✅

C++

• ABI safe design

General

• Make murmur2_random the default partitioner to be consistent with Java producer.
• Remove legacy configuration properties, such as the topic-level auto.commit.enable that is often confused with the new consumer enable.auto.commit

[winbatch]

As part of #1, would you consider converging/moving the logging callback into event_cb? If you think about it, a log represents information about an event. It would be good to:
- be able to subscribe to event callbacks of a certain type and/or minimum severity
- when the user receives the callback, they can choose to either ignore it, log it or do act on it.

[edenhill]

Sounds like a good idea. Logging is a bit weird today because it is decoupled from the config object where everything else is set.

And as you say, all these things are really the same thing: errors, logs and events,