tidaldb/docs/planning/milestone-8/phase-2/task-07-replication-integration-tests.md

# Task 07: Replication Integration Tests

## Delivers

Integration tests in `tidal/tests/m8p2_replication.rs` covering the full replication stack: leader->follower segment delivery, decay score equivalence to 6 decimal places, follower read-only enforcement, lag convergence, and segment corruption rejection.

## Complexity: M

## Dependencies

- Tasks 01-06 complete

## Technical Design

```rust
// tidal/tests/m8p2_replication.rs

use tidaldb::{TidalDb, TidalDbBuilder, NodeRole, ShardId, RegionId, NodeConfig};
use tidaldb::replication::{InProcessTransportFactory, ReplicationLagGauge};

fn leader_config(data_dir: &Path) -> Config {
    Config {
        cluster: NodeConfig {
            role: NodeRole::Leader,
            shard_id: ShardId(0),
            ..Default::default()
        },
        ..Config::with_data_dir(data_dir)
    }
}

fn follower_config(data_dir: &Path) -> Config {
    Config {
        cluster: NodeConfig {
            role: NodeRole::Follower,
            shard_id: ShardId(0),
            ..Default::default()
        },
        ..Config::with_data_dir(data_dir)
    }
}

#[tokio::test]
async fn replication_decay_scores_match() {
    // Leader writes 1,000 signals.
    // Follower replays all segments.
    // Verify: read_decay_score on follower matches leader to 6 decimal places.
}

#[tokio::test]
async fn follower_rejects_writes() {
    // Open follower. Attempt signal() write.
    // Verify: returns TidalError::ReadOnly.
}

#[tokio::test]
async fn follower_serves_retrieve_queries() {
    // Leader writes items + signals.
    // Follower applies.
    // Follower.retrieve() returns ranked results.
}

#[tokio::test]
async fn replication_lag_converges_to_zero() {
    // Leader writes 500 segments.
    // Wait for follower to apply all.
    // Assert: lag_seqno(follower) == 0 within 5 seconds.
}

#[tokio::test]
async fn corrupted_segment_is_rejected() {
    // Manually corrupt BLAKE3 checksum in segment bytes.
    // Send to follower via transport.
    // Verify: segment is not applied (decay scores unchanged).
}

#[tokio::test]
async fn leader_restart_follower_continues() {
    // Leader writes 100 signals.
    // Leader shuts down.
    // Follower serves read queries from replayed state.
    // Leader restarts; ships remaining segments.
    // Follower catches up.
}

#[tokio::test]
async fn idempotent_segment_replay() {
    // Ship same segment twice to follower.
    // Verify: signal counts NOT doubled (seqno idempotency).
}
```

## Acceptance Criteria

- [ ] All 7 integration tests pass under `cargo test --test m8p2_replication`
- [ ] Test `replication_decay_scores_match`: leader 1K signals -> follower matches to 6 decimal places
- [ ] Test `follower_rejects_writes`: `TidalError::ReadOnly` on all write methods
- [ ] Test `follower_serves_retrieve_queries`: follower returns correct ranked results
- [ ] Test `replication_lag_converges_to_zero`: lag = 0 within 5 seconds of leader quiesce
- [ ] Test `corrupted_segment_is_rejected`: corrupt checksums rejected, no state change
- [ ] Test `leader_restart_follower_continues`: follower serves reads after leader crash
- [ ] Test `idempotent_segment_replay`: no double-counting on duplicate segments
- [ ] `cargo clippy -D warnings` and `cargo fmt` pass