34 lines
1.2 KiB
Markdown
34 lines
1.2 KiB
Markdown
# SQ-021: Write Replication
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**Status:** `[ ] TODO`
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**Blocked by:** SQ-020, SQ-010
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**Priority:** High
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## Description
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Writes are replicated to N peers before ack to client. Simple quorum approach: coordinator writes locally, sends to peers, waits for majority ack.
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## Files to Create/Modify
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- `crates/sq-cluster/src/replication.rs` - Replicator with quorum logic
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- `crates/sq-server/src/grpc/cluster.rs` - ReplicateEntries RPC impl
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- `crates/sq-server/src/grpc/data_plane.rs` - update Publish to use Replicator
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## Replication Flow
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1. Coordinator receives Publish request
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2. Coordinator writes to local WAL, assigns offset
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3. Coordinator sends ReplicateEntries to all known alive peers
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4. Coordinator waits for W acks (W = floor(N/2) + 1, where N = replication factor)
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5. On quorum reached: ack to client
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6. On quorum timeout: return error to client
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## Acceptance Criteria
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- [ ] 3-node cluster: publish message, verify all 3 nodes have it in WAL
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- [ ] 3-node cluster, 1 node down: publish succeeds (2/3 quorum)
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- [ ] 3-node cluster, 2 nodes down: publish fails (no quorum)
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- [ ] ACK_MODE_LOCAL: ack after local WAL only (skip replication)
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- [ ] ACK_MODE_NONE: return immediately, replicate async
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- [ ] Replication timeout: configurable, default 5 seconds
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