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Java
ExecutorService — Thread Pools
`ExecutorService` is the modern way to manage threads. Submit Callables/Runnables, get Futures back. Always shutdown the executor — and prefer try-with-resources on Java 19+ where ExecutorService is AutoCloseable.
@admin
JavaScript
Queue (Async Task Queue)
A serialised async task queue that executes queued async functions one at a time, in order. Useful for sequencing operations that must not run concurrently (file writes, ordered API mutations, step-by-step wizards). add() returns a Promise that resolves/rejects when that specific task completes.
JavaScript
Promise All with Concurrency Limit
Runs an array of async tasks with a maximum concurrency cap, preventing you from firing hundreds of requests simultaneously. Processes items in batches, moving to the next as slots free up. Essential when hitting rate-limited APIs or processing large datasets without overwhelming the server.
JavaScript
Worker Pool (Web Workers)
Manages a pool of Web Workers for CPU-intensive tasks (image processing, crypto, parsing). Distributes tasks across available workers using a round-robin strategy and returns a Promise per task. Falls back gracefully to inline execution in environments where workers are unavailable.
Go
sync.Mutex vs sync.RWMutex
`Mutex` allows one goroutine at a time — for both reads and writes. `RWMutex` allows multiple concurrent readers OR one writer — much better when reads dominate.
Go
select — Multiplex Channel Operations
`select` is like a switch for channels — runs whichever case is ready. Use `default` for non-blocking sends/receives; `case <-time.After(d)` for timeouts.
Go
Worker Pool with Channels
Spawn N worker goroutines, feed them tasks over a jobs channel, collect results on a results channel. Idiomatic Go pattern for bounded concurrency.
Go
Channels — Basics
Channels are typed pipes between goroutines. Unbuffered = synchronous handoff; buffered = up to N values queued. Close a channel to signal "no more values" — receivers see ok=false.
Go
Fan-Out / Fan-In
Distribute work across N workers (fan-out), then merge their results back into one channel (fan-in). Used when you can't process items strictly in order but want to retain output as one stream.
Rust
Atomic Counter (lock-free)
For shared counters and flags, `AtomicU64` / `AtomicBool` etc. are much faster than `Mutex`. Operations like `fetch_add` are single CPU instructions on most architectures.
Java
Semaphore — Bounded Concurrency
Limit how many threads can enter a critical section. `acquire()` blocks until a permit is available; `release()` returns one. Use for rate limits, connection pools, or any "max N at a time" pattern.
Go
errgroup — Fan-Out with Cancellation
`golang.org/x/sync/errgroup` runs N goroutines, returns the first error, and cancels the others via a shared context. The right pattern for parallel I/O where any failure should abort the rest.
TypeScript
Concurrency Limiter (semaphore)
Run an array of async tasks but limit concurrency to N at a time — like p-limit, in 30 lines. Preserves input order in the output array.
Go
context.Context — Cancellation & Deadlines
`context.Context` propagates deadlines, cancellation, and request-scoped values down a call chain. EVERY blocking / long-running function should take a `ctx context.Context` as its first parameter.
Go
Goroutine Leak Prevention
A goroutine that's blocked forever on a channel send/receive leaks — it's never garbage-collected. Always pair channels with `select { case <-ctx.Done(): return }` to give them an exit path.
Go
Pipeline (channel chain)
A pipeline is a chain of stages connected by channels — each stage runs in its own goroutine. Classic Go pattern for streaming transformations with backpressure built in.
Go
sync.Pool — Reuse Allocations
`sync.Pool` keeps a pool of reusable objects to reduce allocator pressure in hot paths. Good for byte buffers, JSON encoders, or anything you allocate frequently and only need briefly.