Concurrency Policies Overview

When building interactive applications, user interactions often happen faster than backend APIs can respond. ngx-task solves this with explicit concurrency policies.

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The 5 Concurrency Policies

┌───────────┬────────────────────────────────────────────────────────────────────────┐
│ Policy    │ Action on New Invocation while Active                                  │
├───────────┼────────────────────────────────────────────────────────────────────────┤
│ drop      │ Ignore new invocation completely                                       │
│ restart   │ Cancel active execution immediately and start new invocation           │
│ enqueue   │ Queue new invocation and run sequentially in FIFO order                │
│ latest    │ Finish active execution, but keep only the newest invocation in queue  │
│ parallel  │ Run up to N invocations simultaneously, queueing excess                │
└───────────┴────────────────────────────────────────────────────────────────────────┘

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Detailed Policy Matrix

Policy	Active Execution	New Invocation	Visual / UI Use Case
drop	Continues uninterrupted	Dropped / Ignored	Form submission, Login, Payment checkout
restart	Cancelled immediately	Executes immediately	Live search autocomplete, Filter inputs, Tab switching
enqueue	Continues	Queued in FIFO order	Audit log items, Sequential file processing
latest	Continues	Queued (Supersedes previous queued)	Document autosave, Slider/Canvas state sync
parallel	Continues up to limit	Runs in parallel (or queues excess)	Bulk image uploads, Parallel data preloading

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How to Set a Concurrency Policy

Set the policy via the concurrency option in createTask():

// 1. Drop
const saveTask = createTask(saveHandler, { concurrency: 'drop' });

// 2. Restart
const searchTask = createTask(searchHandler, { concurrency: 'restart' });

// 3. Enqueue
const logTask = createTask(logHandler, { concurrency: 'enqueue' });

// 4. Latest
const autoSaveTask = createTask(autoSaveHandler, { concurrency: 'latest' });

// 5. Parallel (with optional limit)
const uploadTask = createTask(uploadHandler, {
  concurrency: { mode: 'parallel', limit: 3 },
});