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+# RFC: SQL-Based Distributed Queue
+
+## Metadata
+
+| Field | Value                            |
+|-------|----------------------------------|
+| **Author** | Preetam Dwived<preetam@uber.com> |
+| **Status** | In Review                        |
+| **Created** | 2026-02-16                       |
+| **Updated** | 2026-02-16                       |
+
+## Summary
+
+MySQL-based distributed message queue with partition leasing, visibility timeout, and at-least-once delivery. Workers coordinate via database-native primitives without external systems.
+
+## Background
+
+### Motivation
+
+SubmitQueue needs a reliable message queue for coordinating asynchronous workflows:
+- **Orchestrator** publishes merge jobs to workers
+- **Speculator** publishes speculative build requests
+- **Workers** need distributed coordination without duplicate processing
+- **Crash recovery** must preserve exactly where processing stopped
+
+### Existing Solutions
+
+We evaluated several approaches:
+
+1. **External Message Brokers** (Kafka, RabbitMQ)
+   - ❌ Additional operational overhead and infrastructure
+   - ❌ Network hops increase latency
+   - ✅ Battle-tested and highly scalable
+
+2. **Watermill Library** (github.com/ThreeDotsLabs/watermill)
+   - ✅ Database-backed queue with mature abstractions
+   - ✅ Built-in middleware (retry, poison queue, metrics)
+   - ❌ Generic interface hides database-specific optimizations
+   - ❌ Additional dependency and learning curve
+   - ❌ Less control over exact SQL queries and behavior
+
+3. **dbqueue-go** (github.com/yunussandikci/dbqueue-go)
+   - ✅ Lightweight, simple FIFO queue over SQL (MySQL, PostgreSQL, SQLite)
+   - ✅ Basic features: priority, deduplication, visibility timeout
+   - ❌ No distributed worker coordination or partition leasing
+   - ❌ No built-in retry mechanism or DLQ
+   - ❌ Designed for single-worker scenarios, not multi-worker distribution
+
+4. **Database-Backed Queue** (Custom implementation)
+   - ✅ Reuses existing MySQL infrastructure
+   - ✅ Full control over queries and behavior
+   - ✅ No additional services or dependencies
+   - ❌ More code to maintain
+
+### Decision
+
+We chose **custom database-backed queue** because:
+- Full control over SQL queries for optimal performance
+- No additional libraries - direct use of database/sql
+- Simpler to understand and debug (no abstraction layers)
+- Can optimize for our specific use case (partition ordering, visibility timeout)
+- Watermill adds valuable abstractions but we need fine-grained control
+
+## Requirements
+
+### Functional Requirements
+
+1. **Publish/Subscribe** - Standard pub/sub with topics
+2. **Partitioning** - Messages with same key processed in order by single worker
+3. **At-Least-Once Delivery** - Guaranteed delivery, duplicates possible
+4. **Crash Recovery** - Workers resume from last committed offset
+5. **Distributed Workers** - Multiple workers coordinate without duplicate processing
+6. **Dead Letter Queue** - Failed messages isolated after max retries
+7. **Visibility Timeout** - Messages invisible during processing, visible if worker crashes
+
+### Non-Functional Requirements
+
+1. **Operational Simplicity** - No additional infrastructure
+2. **Observability** - Metrics and logging for debugging
+3. **Testability** - In-memory testing without external MySQL
+4. **Performance** - Sub-second latency for typical workloads
+5. **Scalability** - Handle hundreds of workers, thousands of partitions
+
+### Non-Goals
+
+1. **Exactly-Once Delivery** - Application must handle duplicates
+2. **Kafka-Scale Throughput** - Not optimizing for millions of messages/sec
+3. **Cross-Datacenter Replication** - Single MySQL instance only
+4. **Message Ordering Across Partitions** - Only within partition
+5. **Real-Time Streaming** - Polling introduces configurable latency
+
+## Design Overview
+
+### High-Level Architecture
+
+```
+┌─────────────┐
+│  Publisher  │───┐
+└─────────────┘   │
+                  ▼
+┌─────────────────────────────────────┐
+│          MySQL Database             │
+│  ┌───────────────────────────────┐  │
+│  │     queue_messages            │  │
+│  │  - topic, partition_key       │  │
+│  │  - offset, invisible_until    │  │
+│  │  - retry_count, payload       │  │
+│  └───────────────────────────────┘  │
+│  ┌───────────────────────────────┐  │
+│  │  queue_partition_leases       │  │
+│  │  - consumer_group, topic      │  │
+│  │  - partition_key, leased_by   │  │
+│  └───────────────────────────────┘  │
+│  ┌───────────────────────────────┐  │
+│  │     queue_offsets             │  │
+│  │  - consumer_group, topic      │  │
+│  │  - partition_key, offset      │  │
+│  └───────────────────────────────┘  │
+└─────────────────────────────────────┘
+                  │
+                  ▼
+┌─────────────────────────────────────┐
+│         Subscriber Workers          │
+│  ┌──────────┐  ┌──────────┐         │
+│  │ Worker-1 │  │ Worker-2 │  ...    │
+│  │(part-A,B)│  │(part-C,D)│         │
+│  └──────────┘  └──────────┘         │
+│  ┌──────────┐  ┌──────────┐         │
+│  │ Worker-3 │  │ Worker-N │         │
+│  │(part-E,F)│  │(part-X,Y)│         │
+│  └──────────┘  └──────────┘         │
+└─────────────────────────────────────┘
+```
+
+### Core Concepts
+
+**Partition Leasing:** Workers coordinate using database-native leases. Each partition leased by exactly one worker. Stale leases automatically stolen on crash.
+
+**Visibility Timeout:** Messages invisible during processing. Auto-retry on crash when timeout expires.
+
+**Persistent Retry Tracking:** `retry_count` incremented atomically on fetch, survives crashes, triggers DLQ.
+
+**Offset Tracking:** Per-partition offsets enable crash recovery from last acked message.
+
+## Database Schema
+
+### Messages Table
+
+**Key Fields:**
+- `offset` (PK): Auto-incrementing global offset for message ordering
+- `topic`, `partition_key`: Message routing and partitioning
+- `id`: Unique message identifier
+- `payload`, `metadata`: Message content
+- `retry_count`: Persistent retry tracking (survives worker crashes)
+- `invisible_until`: Visibility timeout (epoch ms) for crash recovery
+- `created_at`, `published_at`: Timestamps
+
+**Indexes:**
+- `(topic, partition_key, invisible_until, offset)`: Core fetch query - find visible messages in partition ordered by offset
+- `(topic, partition_key, id)`: Unique constraint and fast lookup for Ack/Nack
+
+See `extensions/queue/sql/schema/queue_messages.sql` for full schema.
+
+### Partition Leases Table
+
+**Key Fields:**
+- `consumer_group`, `topic`, `partition_key` (PK): Identifies which partition is leased
+- `leased_by`: Worker that owns the lease
+- `leased_at`, `lease_renewed_at`: Lease timestamps for staleness detection
+
+**Indexes:**
+- `(leased_by)`: Find all partitions owned by a worker
+- `(lease_renewed_at)`: Detect stale leases across workers
+
+See `extensions/queue/sql/schema/queue_partition_leases.sql` for full schema.
+
+### Consumer Offsets Table
+
+**Key Fields:**
+- `consumer_group`, `topic`, `partition_key` (PK): Identifies offset position
+- `offset_acked`: Last successfully processed offset
+- `updated_at`: Last update timestamp
+
+**Indexes:**
+- `(consumer_group)`: Monitor all offsets for a consumer group
+- `(topic)`: Find all consumers for a topic
+
+See `extensions/queue/sql/schema/queue_offsets.sql` for full schema.
+
+### Dead Letter Queue Table
+
+**Key Fields:**
+- `offset` (PK): Auto-incrementing offset for DLQ ordering
+- `topic`, `partition_key`, `id`: Original message identification
+- `payload`, `metadata`: Original message content
+- `failed_at`: When message moved to DLQ
+- `failure_count`, `last_error`: Failure diagnostics
+
+**Indexes:**
+- `(topic, partition_key, failed_at)`: Query DLQ by topic/partition, ordered by failure time
+- `(failed_at)`: Time-based queries and cleanup
+- `(topic, partition_key, id)`: Unique constraint, prevents duplicates
+
+See `extensions/queue/sql/schema/queue_dlq.sql` for full schema.
+
+## Message Flow
+
+**1. Publish** - Insert messages with AUTO_INCREMENT offset
+
+**2. Lease Acquisition** - `INSERT ... ON DUPLICATE KEY UPDATE` with stale lease detection
+
+**3. Fetch** - Atomic UPDATE sets `invisible_until` and increments `retry_count`
+
+**4. Ack** - Transaction: DELETE message + UPDATE offset_acked
+
+**5. Nack** - UPDATE `invisible_until` for retry after delay
+
+**6. DLQ** - If `retry_count >= MaxAttempts`: DELETE from messages + INSERT into dlq
+
+## Crash Recovery
+
+**Scenario:** Worker crashes while processing message
+
+**What happens:**
+1. Message has `invisible_until = crash_time + VisibilityTimeout`
+2. After timeout expires, message becomes visible
+3. Another worker detects stale lease and steals partition
+4. Message redelivered (at-least-once guarantee)
+5. `retry_count` incremented prevents infinite retries
+
+**Key properties:** Automatic failover, no data loss, configurable retry delay
+
+## Distributed Processing
+
+**Same Consumer Group:** Workers distribute partitions via leasing. Each partition processed by one worker.
+
+**Different Consumer Groups:** Independent consumption with separate offsets. Same messages delivered to all groups.
+
+## Alternatives Considered
+
+### Watermill Library
+
+**Evaluation:** We prototyped a full implementation using `github.com/ThreeDotsLabs/watermill-sql`
+
+**Pros:**
+- Mature abstractions for pub/sub
+- Built-in middleware (poison queue, retry, metrics)
+- Multi-backend support (MySQL, PostgreSQL, Kafka)
+- Well-tested and documented
+
+**Cons:**
+- **No partition key support** - can't guarantee ordering within partitions
+- **Requires table per topic** - schema migration for every new topic, not friendly for dynamic topics
+- Generic interface hides database-specific optimizations
+- Less control over exact SQL queries (e.g., can't optimize visibility timeout logic)
+- Additional dependency to maintain and version
+- More infrastructure to maintain (separate tables, schema management)
+- Learning curve for team (new library semantics)
+
+**Example of Watermill's limitations:**
+
+For our use case, we need ordering per repository. With Watermill:
+- Creates one table per topic (table explosion for dynamic partitions)
+- To get per-repo ordering: either one table per repo OR multiple offset tables per consumer group
+- Schema migrations required for each new repository
+
+With our custom implementation:
+- Single `queue_messages` table for all topics and partitions
+- Rows like `('merge_events', 'repo-123', offset, ...)` provide ordering within partition
+- No schema migrations for new repos or topics
+- Ordering guaranteed within `(topic, partition_key)`
+
+**Decision:** Custom implementation gives us partition ordering guarantees and single-table design. Watermill is valuable for complex multi-backend scenarios but doesn't fit our partition-based ordering requirements.
+
+### Single-Table Per Topic
+
+**Pros:** Better isolation, easier to drop topics
+
+**Cons:** Schema migration per topic. Not friendly for dynamic topic creation.
+
+**Decision:** Single-table design for operational simplicity.
+
+## Trade-offs
+
+**Polling vs Push**
+- ✅ Simpler (no connection management), natural backpressure
+- ❌ Higher latency (configurable via PollInterval)
+- Mitigation: Tune PollInterval (default 100ms, tests 20ms)
+
+**Visibility Timeout vs Heartbeat**
+- ✅ No heartbeat protocol, automatic retry
+- ❌ Full timeout delay even on immediate crash
+- Mitigation: ExtendVisibilityTimeout() for long tasks
+
+**Database Leasing vs External Coordinator**
+- ✅ No ZooKeeper/etcd, transactional consistency
+- ❌ Lease renewal overhead
+- Mitigation: Tunable renewal interval (default 10s)
+
+**At-Least-Once vs Exactly-Once**
+- ✅ Simpler, better performance
+- ❌ Applications must handle duplicates
+- Mitigation: Idempotency keys (e.g., merge request ID)
+
+
+## Observability
+
+**Metrics (via tally):**
+- Publisher: `messages_published`, `publish_errors`
+- Subscriber: `messages_acked`, `messages_nacked`, `messages_moved_to_dlq`, `message_age`, `leases_acquired`
+- Stores: `insert.latency`, `fetch.latency`, `ack_message.latency`, `renew_lease.latency`
+
+**Logging (via zap):**
+- Debug: Message fetch, lease operations
+- Info: Publish success, DLQ moves, partition acquisition
+- Error: Database errors, unrecoverable failures
+- Structured fields: `topic`, `partition_key`, `message_id`, `offset`, `retry_count`
+
+## Performance
+
+- **Throughput:** ~1k-5k msg/sec publish, ~500-2k msg/sec consume (single MySQL)
+- **Latency:** Best case = PollInterval (100ms), Retry after crash = VisibilityTimeout (60s)
+- **Bottlenecks:** MySQL write throughput, lease renewal overhead, polling overhead
+
+## Appendix
+
+### References
+
+**Database & Locking:**
+- [MySQL InnoDB Locking](https://dev.mysql.com/doc/refman/8.0/en/innodb-locking.html) - Understanding MySQL transaction isolation and row-level locking
+- [MySQL AUTO_INCREMENT](https://dev.mysql.com/doc/refman/8.0/en/example-auto-increment.html) - AUTO_INCREMENT behavior for offset generation
+
+**Queue Patterns:**
+- [Amazon SQS Visibility Timeout](https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-visibility-timeout.html) - Inspiration for visibility timeout mechanism
+- [Kafka Consumer Groups](https://kafka.apache.org/documentation/#consumergroups) - Consumer group and partition assignment patterns
+- [RabbitMQ Dead Letter Exchanges](https://www.rabbitmq.com/dlx.html) - Dead letter queue concepts
+
+**Alternative Implementations:**
+- [Watermill Documentation](https://watermill.io/) - Go library for message streaming (evaluated alternative)
+- [PostgreSQL SKIP LOCKED](https://www.postgresql.org/docs/current/sql-select.html#SQL-FOR-UPDATE-SHARE) - Alternative database queue pattern
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