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The fetch epoch keeps the state on the leader and the follower synchronized. It ensures that if a message is duplicated or lost, the server will always notice. It is also used to associate requests and responses in the logs. Other numbers, such as IP addresses and ports, or NetworkClient sequence numbers, can also be helpful for this purpose-- but they are less likely to be unique.
FetchRequest Metadata meaning
Request SessionId | Request SessionEpoch | Meaning |
|---|---|---|
| 0 | -1 | Make a full FetchRequest that does not use or create a session. This is the session ID used by pre-KIP-227 FetchRequests. |
| 0 | 0 | Make a full FetchRequest. Create a new incremental fetch session if possible. If a new fetch session is created, it will start at epoch 1. |
| $ID | 0 | Close the incremental fetch session identified by $ID. Make a full FetchRequest. Create a new incremental fetch session if possible. If a new fetch session is created, it will start at epoch 1. |
| $ID | $EPOCH | If the ID and EPOCH are correct, make an incremental fetch request. |
| $ID | -1 | Close the incremental fetch session identified by $ID. Make a full FetchRequest. |
Incremental Fetch Requests
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When the top-level error code is set, the caller should assume that all the partitions in the fetch received the given error.
Fetch Session ID
The FetchResponse now contains a 32-bit fetch session ID.
FetchResponse Metadata meaning
Request SessionId | Meaning |
|---|---|
| 0 | No fetch session was created. |
| $ID | The next request can be an incremental fetch request with the given $ID. |
Incremental Fetch Responses
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