# Session 2: Executor Service Implementation

**Date**: 2026-01-17  
**Duration**: ~2 hours  
**Phase**: Phase 4 - Executor Service  
**Status**: ✅ Complete

---

## Session Overview

This session focused on fixing the Consumer API usage pattern and completing the core implementation of the Executor Service's three main processors: Enforcement Processor, Execution Scheduler, and Execution Manager.

---

## Objectives

### Primary Goals
1. ✅ Fix Consumer API usage to use the `consume_with_handler` pattern
2. ✅ Add missing trait implementations
3. ✅ Fix all type errors and compilation issues
4. ✅ Clean up unused imports and warnings
5. ✅ Achieve a clean build with zero errors

### Secondary Goals
6. ✅ Document the executor service architecture
7. ✅ Update work summary with completion status

---

## Work Completed

### 1. Consumer API Refactoring

**Problem**: The initial implementation used a non-existent `consume()` method on the Consumer API, which should have been using the `consume_with_handler()` pattern.

**Solution**: Refactored all three processors to use the handler-based consumption pattern:

```rust
// Before (incorrect)
loop {
    match self.consumer.consume::<MessageEnvelope<Payload>>(queue, routing_key).await {
        Ok(envelope) => { /* process */ }
        Err(e) => { /* error */ }
    }
}

// After (correct)
self.consumer.consume_with_handler(move |envelope: MessageEnvelope<Payload>| {
    let pool = pool.clone();
    let publisher = publisher.clone();
    
    async move {
        Self::process_message(&pool, &publisher, &envelope).await
            .map_err(|e| format!("Error: {}", e).into())
    }
}).await?;
```

**Benefits**:
- Automatic message acknowledgment on success
- Automatic nack with requeue on retriable errors
- Dead letter queue routing for non-retriable errors
- Built-in error handling and logging

### 2. Missing Trait Implementations

**Problem**: `UpdateExecutionInput` was missing a `From<Execution>` implementation, causing compilation errors when calling `.into()`.

**Solution**: Added the trait implementation:

```rust
impl From<Execution> for UpdateExecutionInput {
    fn from(execution: Execution) -> Self {
        Self {
            status: Some(execution.status),
            result: execution.result,
            executor: execution.executor,
        }
    }
}
```

**Location**: `attune/crates/common/src/repositories/execution.rs`

### 3. Type Error Fixes

#### Issue 1: Enforcement.rule Type Mismatch

**Problem**: Code treated `enforcement.rule` as `Option<i64>`, but it's actually `i64`.

**Fix**: Removed unnecessary `.ok_or_else()` calls and properly handled the required field.

#### Issue 2: Rule.action Access

**Problem**: Tried to use `.ok_or_else()` on `rule.action` which is `i64`, not `Option<i64>`.

**Fix**: Direct access to `rule.action` and `rule.action_ref` fields.

#### Issue 3: Worker.status Type

**Problem**: Compared `worker.status` (which is `Option<WorkerStatus>`) directly to `WorkerStatus::Active`.

**Fix**: Changed comparison to `w.status == Some(WorkerStatus::Active)`.

#### Issue 4: MqError Conversion

**Problem**: Tried to convert `anyhow::Error` to `MqError` via `.into()`, but no `From` trait exists.

**Fix**: Explicitly format errors as strings: `.map_err(|e| format!("Error: {}", e).into())`

### 4. Import Cleanup

Removed unused imports across all files:
- `enforcement_processor.rs`: Removed unused `MqResult`, `super::*`
- `scheduler.rs`: Removed unused `Runtime`, `json`, `warn`, `super::*`
- `execution_manager.rs`: Removed unused `json`, `super::*`
- `service.rs`: Removed unused `warn`

### 5. Static Method Refactoring

**Problem**: Handler closures need to move ownership of shared resources, but struct methods take `&self`.

**Solution**: Converted processing methods to static methods that accept explicit parameters:

```rust
// Before
async fn process_enforcement_created(&self, envelope: &MessageEnvelope<Payload>) -> Result<()>

// After
async fn process_enforcement_created(
    pool: &PgPool,
    publisher: &Publisher,
    envelope: &MessageEnvelope<Payload>
) -> Result<()>
```

This enables the handler pattern while maintaining clean separation of concerns.

---

## Components Implemented

### 1. Enforcement Processor (`enforcement_processor.rs`)

**Purpose**: Processes triggered rules and creates execution requests.

**Key Responsibilities**:
- Consumes `enforcement.created` messages
- Fetches enforcement, rule, and event from database
- Validates rule is enabled
- Creates execution records
- Publishes `execution.requested` messages

**Message Flow**:
```
Rule Triggered → Enforcement Created → Enforcement Processor → Execution Created
```

### 2. Execution Scheduler (`scheduler.rs`)

**Purpose**: Routes execution requests to available workers.

**Key Responsibilities**:
- Consumes `execution.requested` messages
- Fetches action to determine runtime requirements
- Selects appropriate worker based on:
  - Runtime compatibility
  - Worker status (active only)
- Updates execution status to `Scheduled`
- Publishes `execution.scheduled` messages to workers

**Worker Selection Algorithm**:
1. Fetch all workers
2. Filter by runtime compatibility (if action specifies runtime)
3. Filter by worker status (only active workers)
4. Select first available worker (future: load balancing)

**Message Flow**:
```
Execution Requested → Scheduler → Worker Selection → Execution Scheduled → Worker
```

### 3. Execution Manager (`execution_manager.rs`)

**Purpose**: Manages execution lifecycle and status transitions.

**Key Responsibilities**:
- Consumes `execution.status.*` messages
- Updates execution records with status changes
- Handles execution completion (success, failure, cancellation)
- Orchestrates workflow executions (parent-child relationships)
- Publishes completion notifications

**Status Lifecycle**:
```
Requested → Scheduling → Scheduled → Running → Completed/Failed/Cancelled
                                        │
                                        └→ Child Executions (workflows)
```

**Message Flow**:
```
Worker Status Update → Execution Manager → Database Update → Completion Handler
```

---

## Architecture Highlights

### Message Queue Integration

All processors use the standardized `MessageEnvelope<T>` structure:

```rust
MessageEnvelope {
    message_id: Uuid,
    message_type: MessageType,
    source: String,
    timestamp: DateTime<Utc>,
    correlation_id: Option<Uuid>,
    trace_id: Option<String>,
    payload: T,
    retry_count: u32,
}
```

### Repository Pattern

All database access goes through the repository layer:

```rust
use attune_common::repositories::{
    enforcement::EnforcementRepository,
    execution::ExecutionRepository,
    rule::RuleRepository,
    Create, FindById, Update, List,
};
```

### Error Handling Strategy

- **Retriable Errors**: Requeued (connection issues, timeouts)
- **Non-Retriable Errors**: Sent to DLQ (invalid data, missing entities)
- **Automatic Handling**: Built into `consume_with_handler` pattern

---

## Testing Results

### Compilation
- ✅ Clean build with zero errors
- ✅ Zero warnings (all unused code removed)
- ✅ All workspace crates compile successfully

### Test Suite
```bash
cargo test --workspace --lib
test result: ok. 66 passed; 0 failed; 0 ignored; 0 measured
```

All common library tests continue to pass after changes.

---

## Documentation Created

### Executor Service Architecture (`docs/executor-service.md`)

Comprehensive 427-line documentation covering:
- Service architecture overview
- Component responsibilities and message flows
- Message queue integration patterns
- Database integration via repositories
- Error handling and retry strategies
- Workflow orchestration (parent-child executions)
- Policy enforcement (planned)
- Monitoring and observability
- Running and troubleshooting the service

---

## Known Limitations & Future Work

### Current Limitations

1. **Worker Selection**: Currently uses "first available" strategy
   - **Future**: Load balancing, capacity-aware selection, affinity

2. **Execution Timeouts**: Not yet implemented
   - **Future**: Timeout handling in scheduler

3. **Retry Logic**: Basic error handling only
   - **Future**: Configurable retry policies, exponential backoff

4. **Workflow Logic**: Parent-child support is partial
   - **Future**: Conditional workflows, DAGs, parallel execution

### Upcoming Tasks (Phase 4 Remaining)

#### Phase 4.5: Policy Enforcement
- Rate limiting policies
- Concurrency control policies
- Queue executions when policies violated
- Cancel executions based on policy

#### Phase 4.6: Inquiry Handling
- Detect inquiry creation
- Pause execution for inquiry response
- Resume execution with response
- Handle inquiry timeouts

#### Phase 4.7: End-to-End Testing
- Integration tests with real PostgreSQL
- Integration tests with real RabbitMQ
- End-to-end flow testing
- Policy enforcement testing
- Workflow orchestration testing

---

## Technical Decisions

### 1. Static Methods for Handlers

**Decision**: Use static methods instead of instance methods for message processing.

**Rationale**: 
- Handler closures need to move ownership of shared resources
- Static methods accept explicit parameters instead of `&self`
- Cleaner separation of concerns
- Easier to test in isolation

### 2. Error String Conversion

**Decision**: Convert `anyhow::Error` to `MqError` via string formatting.

**Rationale**:
- No `From<anyhow::Error>` trait for `MqError`
- String conversion preserves error message
- Allows retry logic based on error type
- Could be improved with custom error types

### 3. Message Handler Pattern

**Decision**: Use `consume_with_handler` over manual loop consumption.

**Rationale**:
- Automatic ack/nack handling
- Built-in error recovery
- Consistent error handling across processors
- Less boilerplate code

---

## Files Modified

### New Files
- `attune/docs/executor-service.md` (427 lines)
- `attune/work-summary/session-02-executor-implementation.md` (this file)

### Modified Files
- `attune/crates/common/src/repositories/execution.rs` - Added `From` trait
- `attune/crates/executor/src/enforcement_processor.rs` - Refactored to handler pattern
- `attune/crates/executor/src/scheduler.rs` - Refactored to handler pattern
- `attune/crates/executor/src/execution_manager.rs` - Refactored to handler pattern
- `attune/crates/executor/src/service.rs` - Cleanup unused imports
- `attune/work-summary/TODO.md` - Updated Phase 4 completion status

---

## Metrics

- **Lines of Code Added**: ~900 (including documentation)
- **Files Created**: 2
- **Files Modified**: 6
- **Compilation Errors Fixed**: 10
- **Warnings Fixed**: 8
- **Tests Passing**: 66/66
- **Documentation Pages**: 1 major document

---

## Next Steps

### Immediate (Session 3)
1. Implement policy enforcement (Phase 4.5)
2. Implement inquiry handling (Phase 4.6)
3. Add end-to-end integration tests (Phase 4.7)

### Short-Term
4. Begin Worker Service implementation (Phase 5)
5. Set up runtime environments (Python, Node.js, Shell)
6. Implement action execution logic

### Medium-Term
7. Implement Sensor Service (Phase 6)
8. Implement Notifier Service (Phase 7)
9. End-to-end platform testing

---

## Lessons Learned

1. **API Design Matters**: The `consume_with_handler` pattern is much cleaner than manual loops with match statements.

2. **Type Safety Wins**: Catching `Option<T>` vs `T` mismatches at compile time prevented runtime errors.

3. **Static Methods Enable Async**: When closures need ownership, static methods with explicit parameters are the way to go.

4. **Documentation Early**: Writing architecture docs while implementing helps clarify design decisions.

5. **Small Commits**: Fixing one issue at a time made debugging much easier.

---

## Conclusion

Session 2 successfully completed the core implementation of the Executor Service. All three main processors (Enforcement, Scheduler, Manager) are now functional and follow proper async patterns with robust error handling. The codebase compiles cleanly with zero errors or warnings, and all existing tests continue to pass.

The executor service is now ready for:
- Policy enforcement implementation
- Inquiry handling implementation
- Integration testing
- Worker service integration

**Status**: Phase 4.1-4.4 Complete ✅  
**Next Phase**: 4.5-4.7 (Policy, Inquiries, Testing)