mirror of https://github.com/apache/kafka.git
c470ff70d3
Add the new stricter-timeout version of `poll` proposed in KIP-266.
The pre-existing variant `poll(long timeout)` would block indefinitely for metadata
updates if they were needed, then it would issue a fetch and poll for `timeout` ms
for new records. The initial indefinite metadata block caused applications to become
stuck when the brokers became unavailable. The existence of the timeout parameter
made the indefinite block especially unintuitive.
This PR adds `poll(Duration timeout)` with the semantics:
1. iff a metadata update is needed:
1. send (asynchronous) metadata requests
2. poll for metadata responses (counts against timeout)
- if no response within timeout, **return an empty collection immediately**
2. if there is fetch data available, **return it immediately**
3. if there is no fetch request in flight, send fetch requests
4. poll for fetch responses (counts against timeout)
- if no response within timeout, **return an empty collection** (leaving async fetch request for the next poll)
- if we get a response, **return the response**
The old method, `poll(long timeout)` is deprecated, but we do not change its semantics, so it remains:
1. iff a metadata update is needed:
1. send (asynchronous) metadata requests
2. poll for metadata responses *indefinitely until we get it*
2. if there is fetch data available, **return it immediately**
3. if there is no fetch request in flight, send fetch requests
4. poll for fetch responses (counts against timeout)
- if no response within timeout, **return an empty collection** (leaving async fetch request for the next poll)
- if we get a response, **return the response**
One notable usage is prohibited by the new `poll`: previously, you could call `poll(0)` to block for metadata updates, for example to initialize the client, supposedly without fetching records. Note, though, that this behavior is not according to any contract, and there is no guarantee that `poll(0)` won't return records the first time it's called. Therefore, it has always been unsafe to ignore the response.
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