Concurrent

Concurrent -- Concurrency & Parallelism

Activate When

• User invokes /godmode:concurrent
• User says "thread safety", "race condition", "deadlock", "lock-free"
• User says "async/await", "actor model", "parallelism", "concurrent"
• User says "mutex", "semaphore", "channel", "goroutine", "tokio"
• When building multi-threaded or async systems
• When /godmode:review flags potential race conditions or deadlocks
• When /godmode:test reveals non-deterministic test failures

Workflow

Step 1: Concurrency Context Assessment

Understand the concurrency requirements before designing solutions:

CONCURRENCY CONTEXT:
Project: <name and purpose>
Language/Runtime: <Go | Rust | Node.js | Python | Java | Erlang/Elixir>
Concurrency Model: Threads | Green Threads | Event Loop | Actor Model | CSP
Current Issues: <race conditions, deadlocks, performance bottlenecks, none>
Shared State: <what mutable state is shared across concurrent units>
I/O Profile: CPU-bound | I/O-bound | Mixed
Scale: <concurrent connections/operations expected>
  ...

If the user has not provided context, ask: "What language/runtime are you using, and is the workload primarily CPU-bound or I/O-bound? This determines the right concurrency model."

Step 2: Thread Safety Analysis

Identify shared mutable state and classify access patterns:

SHARED STATE INVENTORY:
| State | Access Pattern | Protection | Risk |
|--|--|--|--|
| <variable/resource> | Read-Write | <mutex/none> | HIGH |
| <variable/resource> | Read-Only | None needed | LOW |
| <variable/resource> | Write-Only | <channel> | MED |

CRITICAL SECTIONS:
  ...

Rules:

• Identify every piece of shared mutable state before writing any concurrent code
• Prefer immutable data and message passing over shared mutable state
• If shared mutable state is unavoidable, protect it with the narrowest possible lock scope
• Document every critical section and its invariants

Step 3: Race Condition Detection

Systematically check for race conditions:

RACE CONDITION ANALYSIS:
| Pattern | Found | Location | Fix |
|--|--|--|--|
| Check-then-act | Y/N | <file:line> | |
| Read-modify-write | Y/N | <file:line> | |
| Compound actions | Y/N | <file:line> | |
| Lazy initialization | Y/N | <file:line> | |
| Iterator invalidation | Y/N | <file:line> | |
  ...

Common Race Condition Patterns and Fixes

Check-then-act (TOCTOU)

UNSAFE:
 if (map.has(key)) { // Check
 return map.get(key); // Act -- another thread may have removed key
 }

SAFE (atomic operation):
 return map.getOrDefault(key, fallback); // Single atomic operation

  ...

Read-modify-write

UNSAFE:
 counter = counter + 1; // Read, modify, write -- not atomic

SAFE (atomic):
 counter.incrementAndGet(); // Java AtomicInteger
 counter.fetch_add(1, Ordering::SeqCst); // Rust AtomicUsize
 atomic.AddInt64(&counter, 1); // Go sync/atomic

  ...

Step 4: Async/Await Patterns

Design correct async code for the target runtime:

Node.js (Event Loop)

PATTERNS:
1. Sequential async -- await one at a time (when order matters)
 const user = await getUser(id);
 const orders = await getOrders(user.id);

2. Concurrent async -- Promise.all (when independent)
 const [user, products, config] = await Promise.all([
 getUser(id),
  ...

Python (asyncio)

PATTERNS:
1. Concurrent tasks -- asyncio.gather
 results = await asyncio.gather(
 fetch_user(user_id),
 fetch_orders(user_id),
 fetch_preferences(user_id)
 )

  ...

Go (Goroutines + Channels)

PATTERNS:
1. Fan-out/fan-in
 results := make(chan Result, len(jobs))
 for _, job := range jobs {
 go func(j Job) {
 results <- process(j)
 }(job)
 }
  ...

Rust (Tokio / async-std)

PATTERNS:
1. Concurrent futures -- tokio::join!
 let (user, orders) = tokio::join!(
 get_user(id),
 get_orders(id)
 );

2. Spawned tasks with handles
  ...

Step 5: Lock-Free Data Structures

Design or select lock-free alternatives when locks are a bottleneck:

LOCK-FREE DATA STRUCTURES:
| Structure | Use Case | Implementation |
|--|--|--|
| Atomic counter | Counters, flags | AtomicI64, atomic|
| CAS loop | Compare-and-swap ops | compare_exchange |
| Lock-free queue | Producer-consumer | crossbeam, ConcurrentLinkedQueue |
| Lock-free stack | LIFO work stealing | Treiber stack |
| Lock-free hash map | Concurrent KV | dashmap (Rust), ConcurrentHashMap (Java) |
  ...

Step 6: Actor Model Design

Design actor-based systems for message-driven concurrency:

ACTOR MODEL PRINCIPLES:
1. Each actor has private state -- no shared mutable state
2. Actors communicate only through asynchronous messages
3. Each actor processes one message at a time -- sequential within actor
4. Actors can create child actors, send messages, change behavior

ACTOR SYSTEM DESIGN:
| Actor | Messages Received | State | Children|
  ...

Step 7: Deadlock Detection and Prevention

Systematically prevent and detect deadlocks:

DEADLOCK CONDITIONS (all four must hold):
1. Mutual exclusion -- resource held exclusively
2. Hold and wait -- holding one resource, waiting for another
3. No preemption -- resources cannot be forcibly taken
4. Circular wait -- A waits for B, B waits for A

PREVENTION STRATEGIES:
| Strategy | Breaks Condition | How |
  ...

Step 8: Concurrent Testing Strategies

Verify correctness of concurrent code:

CONCURRENT TESTING APPROACH:
| Technique | What It Catches | Tools |
|--|--|--|
| Race detector | Data races | go -race, TSan |
| Stress testing | Intermittent bugs | Run N times |
| Loom (Rust) | All interleavings | loom crate |
| Property testing | Invariant violations | QuickCheck, Hypothesis |
| Linearizability | Correctness of | Jepsen, elle |
  ...

Step 9: Concurrency Model Selection

Choose the right concurrency model for the workload:

CONCURRENCY MODEL DECISION:
| Workload | Recommended Model | Language/Runtime |
|--|--|--|
| High I/O, many | Event loop / async | Node.js, Python |
| connections | | asyncio |
| CPU-bound parallel | Thread pool / rayon | Rust, Go, Java |
| Message-driven | Actor model | Erlang, Akka |
| distributed | | |
  ...

Step 10: Validation & Artifacts

Validate the concurrency design:

CONCURRENCY VALIDATION:
| Check | Status |
|--|--|
| All shared mutable state identified | PASS | FAIL |
| Protection strategy for each shared state| PASS | FAIL |
| Race condition analysis complete | PASS | FAIL |
| Deadlock prevention strategy in place | PASS | FAIL |
| Cancellation/shutdown paths verified | PASS | FAIL |
  ...

Generate deliverables:

CONCURRENCY DESIGN COMPLETE:

Artifacts:
- Thread safety analysis: docs/concurrency/<feature>-thread-safety.md
- Race condition report: docs/concurrency/<feature>-race-analysis.md
- Concurrency tests: tests/concurrent/<feature>-concurrent.test.<ext>
- Validation: <SAFE | NEEDS WORK>

  ...

Commit: "concurrent: <feature> -- <model>, <N> shared states protected, <verdict>"

Quality Targets

• Lock contention: <10ms per critical section
• CPU utilization: >80% under load
• Data race count: <1 detected by race detector

Key Behaviors

Never ask to continue. Loop autonomously until done.

# Run concurrency checks
go test -race ./...
cargo test -- --test-threads=1
python -m pytest tests/ -x --timeout=30

IF race detector reports > 0 races: fix before merging. WHEN stress test (1000 iterations) shows any failure: investigate. IF mutex hold time > 100ms: refactor to reduce critical section.

1. Identify shared mutable state first. Enumerate before coding.
2. Prefer message passing. Channels > mutexes.
3. Always use race detector. Run on every test. No exceptions.
4. Lock ordering prevents deadlocks. Document global order.

Flags & Options

Auto-Detection

Detect language and concurrency model:
Go: goroutines+channels. Rust: tokio/async-std. Node: event loop.
Python: asyncio/threading. Java: threads/virtual threads.
## Keep/Discard Discipline
Each concurrency fix either passes the race detector or gets reverted.
- **KEEP**: Race detector clean, stress test (1000 iterations) passes, no new deadlock risk introduced.
- **DISCARD**: Fix introduces a new race, deadlock, or performance regression. Revert immediately.
- **CRASH**: Stress test reveals intermittent failure. Increase iterations to 10,000 to reproduce reliably, then fix.
- Log every analysis to `.godmode/concurrent-results.tsv`.

## Stop Conditions