DeanKelly751 Concurrency
Purpose
Decision frameworks for concurrency model selection and safe concurrent code.
Concurrency Models
| Model | Best for | Trade-off |
|---|---|---|
| Thread-per-request | CPU-bound, simple mental model | Memory overhead, context switches |
| Event loop (async) | I/O-bound, many connections | Can't do CPU work, callback complexity |
| Actor model | Independent stateful entities | Message overhead, debugging complexity |
| CSP (channels) | Pipeline-style processing | Channel capacity management |
| Fork-join | Parallelizable computation | Overhead for small tasks |
Decision Framework
- Is the work I/O-bound or CPU-bound?
- I/O → async/event loop
- CPU → thread pool / parallel
- Do workers need shared state?
- No → embarrassingly parallel, simplest model
- Yes → choose coordination carefully
- What's the concurrency level?
- Thousands of connections → async (not thread-per-request)
- Dozens of workers → threads are fine
Race Condition Prevention
Hierarchy of preference (best to worst):
- Eliminate shared state — immutable data, message passing
- Confine state — each thread owns its data exclusively
- Synchronize access — locks, atomics (last resort)
Common Concurrency Bugs
| Bug | Cause | Prevention |
|---|---|---|
| Race condition | Unsynchronized shared state | Immutability, locks, atomics |
| Deadlock | Circular lock dependency | Lock ordering, timeouts |
| Starvation | Unfair scheduling | Fairness policies, bounded queues |
| Thundering herd | All waiters wake simultaneously | Jitter, leader election |
Safe Patterns
- Immutable shared data: Safe to read from any thread
- Thread-local storage: No sharing, no races
- Producer-consumer queues: Bounded, backpressure-aware
- Compare-and-swap (CAS): Lock-free updates for simple values
Anti-Patterns
- Sharing mutable state without synchronization
- Holding locks during I/O operations
- Unbounded queues (eventual OOM)
sleep()as synchronization
Related Skills
backend/execution-background-jobs— concurrent job processingperformance/knowledge-optimization-principles— concurrency for performance