# Task 03: UAT Scenario Tests (Steps 1–5)

## Delivers

Integration test suite in `tidal/tests/m8_uat.rs` covering all 5 UAT scenario steps. Uses `SimulatedCluster` and fault injection from Tasks 01–02. This is the gate for M8 completion.

## Complexity: M

## Dependencies

- Tasks 01–02 complete (SimulatedCluster, fault injection)
- All phases 8.1–8.5 complete

## Technical Design

```rust
// tidal/tests/m8_uat.rs

use tidaldb::{
    EntityId, Timestamp, Window,
    query::{retrieve::Retrieve, search::Search},
    replication::{RegionId, ShardId, NodeRole},
};
use tidaldb::testing::{
    cluster::{SimulatedCluster, ClusterConfig},
    faults::{NetworkPartition, ShardCrash},
};

fn m8_schema() -> Schema {
    SchemaBuilder::new()
        .signal("view", EntityKind::Item,
            DecaySpec::Exponential { half_life: Duration::from_secs(7 * 24 * 3600) })
        .windows(&[Window::OneHour, Window::TwentyFourHours])
        .add()
        .signal("like", EntityKind::Item,
            DecaySpec::Exponential { half_life: Duration::from_secs(24 * 3600) })
        .add()
        .build()
        .unwrap()
}

fn three_region_config() -> ClusterConfig {
    ClusterConfig {
        regions: vec![RegionId(0), RegionId(1), RegionId(2)],
        shards_per_region: 1,
        leader: (RegionId(0), ShardId(0)),
        schema: m8_schema(),
    }
}

/// UAT Step 1: Cross-region signal replication < 2 seconds.
///
/// Write signals for a user in us-east (region 0), read in eu-west (region 1)
/// after < 2 seconds. Verified by ReplicationLagGauge assertion and
/// read_decay_score equivalence.
#[tokio::test]
async fn uat_step1_cross_region_replication() {
    let cluster = SimulatedCluster::build(three_region_config()).await;

    let item = EntityId::new(1);
    let t = Timestamp::now();

    // Write 25 signals in us-east (region 0 leader).
    for _ in 0..25 {
        cluster.write_signal("view", item, 1.0);
    }

    // Wait for convergence (< 2 seconds on in-process transport).
    cluster.await_full_convergence(Duration::from_secs(2)).await;

    // Read in eu-west (region 1) and ap-south (region 2).
    let score_east = cluster.read_decay_score(RegionId(0), item, "view").unwrap();
    let score_west = cluster.read_decay_score(RegionId(1), item, "view").unwrap();
    let score_south = cluster.read_decay_score(RegionId(2), item, "view").unwrap();

    // All regions should report the same score (within floating point epsilon).
    let epsilon = 1e-6;
    assert!((score_east - score_west).abs() < epsilon,
        "eu-west score {} diverges from us-east score {} by > {}", score_west, score_east, epsilon);
    assert!((score_east - score_south).abs() < epsilon,
        "ap-south score {} diverges from us-east score {} by > {}", score_south, score_east, epsilon);

    // Verify via replication lag gauge.
    let lag_1 = cluster.control_plane().lag_seqno(RegionId(1));
    let lag_2 = cluster.control_plane().lag_seqno(RegionId(2));
    assert_eq!(lag_1, 0, "eu-west should have no replication lag");
    assert_eq!(lag_2, 0, "ap-south should have no replication lag");
}

/// UAT Step 2: Shard crash and follower promotion.
///
/// Crash an entire shard primary. Follower is promoted within 10 seconds.
/// All acknowledged signals are present on the promoted follower. No data loss.
#[tokio::test]
async fn uat_step2_shard_crash_and_failover() {
    let cluster = Arc::new(SimulatedCluster::build(three_region_config()).await);

    let item = EntityId::new(2);

    // Write 100 signals (all acknowledged by leader before crash).
    for _ in 0..100 {
        cluster.write_signal("view", item, 1.0);
    }

    // Wait for eu-west follower to receive all events.
    cluster.await_full_convergence(Duration::from_secs(2)).await;

    // Record the pre-crash seqno on eu-west.
    let pre_crash_seqno = cluster.applied_seqno(RegionId(1));

    // Crash the us-east primary.
    let crash = ShardCrash::crash(ShardId(0), cluster.clone(), false).await;

    // Follower promotion should complete within 10 seconds.
    let deadline = Instant::now() + Duration::from_secs(10);
    loop {
        if Instant::now() > deadline {
            panic!("failover timeout: no new leader elected within 10 seconds");
        }
        if cluster.has_leader() { break; }
        tokio::time::sleep(Duration::from_millis(100)).await;
    }

    // New leader (eu-west promoted follower) must have all 100 signals.
    let new_leader_seqno = cluster.leader().db.applied_seqno();
    assert!(new_leader_seqno >= pre_crash_seqno,
        "promoted leader must have at least {} events (had {})", pre_crash_seqno, new_leader_seqno);

    let score_on_promoted = cluster.read_decay_score(RegionId(1), item, "view").unwrap();
    assert!(score_on_promoted > 0.0, "all 100 signals must be present on the promoted leader");
}

/// UAT Step 3: Degraded query during partition.
///
/// Execute RETRIEVE while ap-south (region 2) is partitioned.
/// Query succeeds using available shards. Degradation flag is set in QueryStats.
#[tokio::test]
async fn uat_step3_degraded_query_during_partition() {
    let cluster = SimulatedCluster::build(three_region_config()).await;

    let item = EntityId::new(3);

    // Seed some data.
    for _ in 0..10 {
        cluster.write_signal("view", item, 1.0);
    }
    cluster.await_full_convergence(Duration::from_secs(1)).await;

    // Inject partition: ap-south (region 2) is isolated.
    let _partition = NetworkPartition::symmetric(
        RegionId(0), RegionId(2),
        cluster.transport_factory(),
    );

    // Write more signals during the partition.
    for _ in 0..5 {
        cluster.write_signal("view", item, 1.0);
    }

    // Query should still succeed from us-east or eu-west.
    let results = cluster.leader().db.retrieve(&Retrieve::builder()
        .candidates(vec![item])
        .build()
        .unwrap()
    ).unwrap();

    assert!(!results.items.is_empty(), "query must succeed with 2 of 3 regions available");

    // QueryStats should indicate degradation.
    // (Exact API for degradation flag verified in m7p4 visibility tests -- same pattern)
    let stats = results.stats;
    // degraded = true is set when < all shards participated
    // (exact field name TBD during implementation; verified in UAT step 3 acceptance)
}

/// UAT Step 4: Partition heal and reconciliation.
///
/// Heal the partition from Step 3. ReconciliationEngine runs. After reconciliation:
/// no duplicate signal counts, hard negatives never leaked, decay scores on all
/// shards match analytical formula to 6 decimal places.
#[tokio::test]
async fn uat_step4_partition_heal_reconciliation() {
    let cluster = SimulatedCluster::build(three_region_config()).await;

    let item = EntityId::new(4);
    let user = EntityId::new(100);

    // Phase 1: write events on both sides of partition.
    let partition = NetworkPartition::symmetric(
        RegionId(0), RegionId(2),
        cluster.transport_factory(),
    );

    // Write to leader (us-east, region 0) during partition.
    for _ in 0..50 {
        cluster.write_signal("view", item, 1.0);
    }

    // Write to ap-south (region 2) directly during partition.
    // (ap-south is isolated, so it accumulates its own events)
    for _ in 0..30 {
        cluster.node(RegionId(2)).db
            .signal("view", item, 1.0, Timestamp::now())
            .unwrap();
    }

    // Apply hard negative on ap-south during partition.
    let ts_hide = HlcTimestamp { wall_ns: 200, logical: 0, node_id: 2 };
    cluster.node(RegionId(2)).db.hide_item_with_ts(user, item, ts_hide).unwrap();

    // Phase 2: heal partition.
    drop(partition);

    // Run reconciliation.
    cluster.reconcile_all().await;
    cluster.await_full_convergence(Duration::from_secs(5)).await;

    // Verify: total signal count = 50 + 30 = 80 (no double-counting).
    let score_east = cluster.read_decay_score(RegionId(0), item, "view").unwrap();
    let score_west = cluster.read_decay_score(RegionId(1), item, "view").unwrap();
    let score_south = cluster.read_decay_score(RegionId(2), item, "view").unwrap();

    // Analytical formula: 80 events × weight=1.0, all at approximately t=now.
    // Decay score = sum of decayed events; with very short elapsed time, ≈ 80.0.
    let epsilon = 1e-6;
    assert!((score_east - score_west).abs() < epsilon,
        "post-reconciliation scores diverge between us-east and eu-west");
    assert!((score_east - score_south).abs() < epsilon,
        "post-reconciliation scores diverge between us-east and ap-south");

    // Verify: hard negative applied on ap-south is propagated to all regions.
    // Item must not appear in query results for the user on any region.
    for &region in &[RegionId(0), RegionId(1), RegionId(2)] {
        let results = cluster.node(region).db.retrieve(&Retrieve::builder()
            .for_user(user)
            .candidates(vec![item])
            .build()
            .unwrap()
        ).unwrap();
        assert!(results.items.is_empty(),
            "hard negative must suppress item in region {:?} after reconciliation", region);
    }
}

/// UAT Step 5: Tenant migration with zero downtime.
///
/// Move a tenant to a new region by changing routing config.
/// During migration: zero downtime, all queries succeed.
/// After migration: tenant's data is on new region only; old region's copy is GC'd.
#[tokio::test]
async fn uat_step5_tenant_migration() {
    let cluster = SimulatedCluster::build(three_region_config()).await;

    let tenant = TenantId(42);
    let item = EntityId::new(5);

    // Register tenant on shard 0, region 0.
    cluster.register_tenant(TenantConfig {
        tenant_id: tenant,
        max_signals_per_sec: None,
        max_entities: None,
        max_storage_bytes: None,
        required_regions: vec![RegionId(0)],
        label: "migrating-tenant".into(),
    });

    // Write 100 signals before migration.
    for _ in 0..100 {
        cluster.leader().db
            .signal_for_tenant(tenant, "view", item, 1.0, Timestamp::now())
            .unwrap();
    }

    cluster.await_full_convergence(Duration::from_secs(1)).await;

    // Begin migration: move tenant 42 from shard 0 (region 0) to shard 0 (region 2).
    let migration = cluster.begin_tenant_migration(tenant, ShardId(0), ShardId(0), RegionId(2));
    migration.prepare_target().await.unwrap();
    migration.enter_dual_write().await.unwrap();

    // Write 50 more signals during dual-write window.
    for _ in 0..50 {
        cluster.leader().db
            .signal_for_tenant(tenant, "view", item, 1.0, Timestamp::now())
            .unwrap();
    }

    tokio::time::sleep(Duration::from_millis(200)).await;
    migration.finalize().await.unwrap();

    // All 150 signals must be present on the new region.
    let score_new = cluster.read_decay_score(RegionId(2), item, "view").unwrap();
    assert!(score_new > 0.0, "all signals must be on new region after migration");

    // Queries during migration must have succeeded (no error returned during dual-write).
    // (Verified by the fact that all writes above returned Ok)

    // GC old region.
    migration.gc_source(0).unwrap();

    // Old region should have no data for this tenant.
    let score_old = cluster.read_score_for_tenant(RegionId(0), tenant, item, "view").unwrap_or(0.0);
    assert_eq!(score_old, 0.0, "source region must have no tenant data after GC");
}
```

## Acceptance Criteria

- [ ] `uat_step1_cross_region_replication`: scores in all 3 regions equal within 6 decimal places after < 2s; replication lag = 0
- [ ] `uat_step2_shard_crash_and_failover`: failover completes within 10 seconds; no data loss on promoted follower
- [ ] `uat_step3_degraded_query_during_partition`: query succeeds with 2/3 regions; `QueryStats` degradation flag set
- [ ] `uat_step4_partition_heal_reconciliation`: no duplicate signal counts after reconciliation (50 + 30 = 80 distinct events); hard negatives propagated to all regions; scores match analytical formula to 6 decimal places
- [ ] `uat_step5_tenant_migration`: 150 signals present on target region after migration; old region has 0; zero errors during dual-write window
- [ ] All 5 tests pass in `cargo test --test m8_uat`
- [ ] Total test suite runtime < 60 seconds (InProcessTransport keeps this fast)
- [ ] `cargo clippy -D warnings` and `cargo fmt` pass