# Hook Queue NATS JetStream Migration Guide

## Overview

The git hook queue now supports both Redis Lists and NATS JetStream as backend message queues. This allows gradual migration from Redis to NATS without downtime.

## Architecture

### Producer (`ReceiveSyncService`)

The producer tries NATS first (if configured), then falls back to Redis:

```rust
pub struct ReceiveSyncService {
    pool: deadpool_redis::cluster::Pool,
    redis_prefix: String,
    nats_publish: Option<Arc<dyn Fn(String, Vec<u8>) -> Pin<Box<dyn Future<Output = Result<u64>> + Send>> + Send + Sync>>,
}
```

### Consumer (`RedisConsumer`)

The consumer uses NATS if configured, otherwise falls back to Redis:

```rust
pub struct RedisConsumer {
    pool: deadpool_redis::cluster::Pool,
    prefix: String,
    block_timeout_secs: u64,
    nats_consume: Option<NatsHookConsumeFn>,
}
```

## Integration with AppTransport

### Producer Integration

```rust
use git::ssh::ReceiveSyncService;
use transport::AppTransport;

let transport = Arc::new(AppTransport::new(/* ... */));

// Create NATS publish function
let nats_publish = {
    let transport = transport.clone();
    Arc::new(move |subject: String, payload: Vec<u8>| {
        let transport = transport.clone();
        Box::pin(async move {
            let ack = transport.publish(&subject, payload).await?;
            Ok(ack.sequence)
        }) as Pin<Box<dyn Future<Output = anyhow::Result<u64>> + Send>>
    })
};

// Create service with NATS support
let sync_service = ReceiveSyncService::with_nats(redis_pool, nats_publish);

// Or use Redis-only mode
let sync_service = ReceiveSyncService::new(redis_pool);
```

### Consumer Integration

```rust
use git::hook::pool::redis::{RedisConsumer, NatsHookConsumeFn};

// Create NATS consume function
let nats_consume: NatsHookConsumeFn = {
    let transport = transport.clone();
    Arc::new(move |subject: String, batch_size: usize| {
        let transport = transport.clone();
        Box::pin(async move {
            let mut results = Vec::new();
            
            // Pull messages from JetStream consumer
            for _ in 0..batch_size {
                match transport.pull_one(&subject).await {
                    Ok(Some(msg)) => {
                        let data = msg.payload.to_vec();
                        let msg_clone = msg.clone();
                        let ack_fn = Box::new(move || {
                            let msg = msg_clone.clone();
                            Box::pin(async move {
                                msg.ack().await?;
                                Ok(())
                            }) as Pin<Box<dyn Future<Output = anyhow::Result<()>> + Send>>
                        });
                        results.push((data, ack_fn));
                    }
                    Ok(None) => break,
                    Err(e) => return Err(e),
                }
            }
            
            Ok(results)
        }) as Pin<Box<dyn Future<Output = anyhow::Result<Vec<(Vec<u8>, Box<dyn Fn() -> Pin<Box<dyn Future<Output = anyhow::Result<()>> + Send>> + Send>)>>> + Send>>
    })
};

// Create consumer with NATS support
let consumer = RedisConsumer::with_nats(
    redis_pool,
    "{hook}".to_string(),
    5, // block_timeout_secs
    nats_consume,
);

// Or use Redis-only mode
let consumer = RedisConsumer::new(redis_pool, "{hook}".to_string(), 5);
```

## Queue Subjects

The hook queue uses the following NATS subjects:

- `queue.hook.sync` - Repository sync tasks (git push/pull operations)

Additional task types can be added by extending the subject pattern:
- `queue.hook.{task_type}` - Generic pattern for any hook task type

## Migration Strategy

### Phase 1: Dual Write (Current)
- Producer writes to both NATS and Redis
- Consumer reads from Redis only
- Zero risk, full rollback capability

### Phase 2: Dual Read
- Producer writes to both NATS and Redis
- Consumer reads from NATS, falls back to Redis on error
- Validates NATS consumer stability

### Phase 3: NATS Primary
- Producer writes to NATS only (Redis disabled)
- Consumer reads from NATS only
- Redis queue deprecated

### Phase 4: Redis Removal
- Remove Redis Lists code
- Remove `pool` parameter
- Simplify to NATS-only implementation

## NATS JetStream Setup

### Stream Configuration

```bash
nats stream add HOOK_QUEUE \
  --subjects "queue.hook.>" \
  --retention limits \
  --max-msgs=-1 \
  --max-age=7d \
  --storage file \
  --replicas 3
```

### Consumer Configuration

```bash
nats consumer add HOOK_QUEUE hook-sync-worker \
  --filter "queue.hook.sync" \
  --ack explicit \
  --pull \
  --deliver all \
  --max-deliver 3 \
  --max-pending 100
```

## Differences from Email Queue

### Redis Backend
- **Email Queue**: Uses Redis Streams (XADD/XREADGROUP)
- **Hook Queue**: Uses Redis Lists (LPUSH/BLMOVE)

### Atomicity
- **Email Queue**: Consumer group provides at-least-once delivery
- **Hook Queue**: BLMOVE provides atomic move-to-work-queue pattern

### Work Queue Pattern
- **Email Queue**: No work queue, relies on consumer group
- **Hook Queue**: Uses separate work queue (`{hook}:sync:work`) for in-flight tracking

### Acknowledgment
- **Email Queue**: XACK removes from pending entries list
- **Hook Queue**: LREM removes from work queue

### Retry Logic
- **Email Queue**: Automatic via consumer group pending entries
- **Hook Queue**: Manual via Lua script (LREM + LPUSH)

## Monitoring

### Logs

- NATS publish: `"hook task queued to NATS"`
- Redis publish: `"hook task queued to Redis"`
- NATS consume: `"task dequeued from NATS"`
- Redis consume: `"task dequeued"`

### Metrics

Add these metrics to track hook queue performance:

```rust
counter!("hook_task_queued_total", "backend" => "nats").increment(1);
counter!("hook_task_queued_total", "backend" => "redis").increment(1);
counter!("hook_task_consumed_total", "backend" => "nats").increment(1);
counter!("hook_task_consumed_total", "backend" => "redis").increment(1);
```

## Rollback

To disable NATS and return to Redis-only:

```rust
// Producer
let sync_service = ReceiveSyncService::new(redis_pool);

// Consumer
let consumer = RedisConsumer::new(redis_pool, "{hook}".to_string(), 5);
```

No code changes required, just use the `new()` constructor instead of `with_nats()`.

## Benefits

1. **Zero Downtime**: Gradual migration with fallback
2. **No Circular Dependency**: Uses function pointers instead of crate dependencies
3. **Backward Compatible**: Existing code works without changes
4. **Type Safe**: Compile-time guarantees for integration
5. **Observable**: Consistent logging for both backends

## Known Limitations

### NATS Acknowledgment Timing

The current implementation acks NATS messages immediately after deserialization, not after successful processing. This is different from the Redis pattern where:

- Redis: Task moves to work queue → processes → acks (removes from work queue)
- NATS: Task received → acks immediately → processes

**Future Enhancement**: Store ack functions in a map keyed by task ID, then call them after successful processing. This requires refactoring the worker loop to track pending acks.

### Work Queue Pattern

NATS JetStream doesn't have a direct equivalent to Redis's work queue pattern. The current implementation relies on JetStream's built-in redelivery mechanism instead of a separate work queue.

## Next Steps

1. Add NATS integration to `apps/git-hook/src/main.rs`
2. Add configuration flags for queue backend selection
3. Test dual-write mode in staging
4. Monitor NATS consumer stability
5. Implement proper ack-after-processing pattern
6. Add metrics for queue depth and processing latency