Allow writing to existing keys mapped to a slot in migrating state.

[priyanjgupta]

Feature request type

enhancement

Is your feature request related to a problem? Please describe

Currently if a slot is in migrating state and we try to write to an existing key mapped to that slot, we encounter a -MIGRATING error, resulting in write failures.

Describe the solution you'd like

As per my understanding

1. The existing keys which have not yet been migrated should be served by the source node, whether it's a read or a write.
2. And for the keys which have been migrated to some other node, the write requests should return a -ASK redirect to guide the client to the target node

With this solution, the write availability improves for the existing keys in the migrating slot.

Describe alternatives you've considered

No response

Additional context

With redis following is the read/write request behaviour for keys mapped to the migrating slot

For context: