Concurrent task execution in FastAPI

Introduction

Concurrent task execution lets your app do many things at the same time. This helps your app stay fast and responsive.

When your app needs to handle multiple user requests at once.

When you want to run background jobs without stopping the main app.

When you need to call several slow services or APIs together.

When you want to improve app speed by doing tasks in parallel.

Syntax

FastAPI

import asyncio
from fastapi import FastAPI

app = FastAPI()

@app.get("/tasks")
async def run_tasks():
    task1 = asyncio.create_task(some_async_function1())
    task2 = asyncio.create_task(some_async_function2())
    results = await asyncio.gather(task1, task2)
    return {"results": results}

Use async def for functions that run asynchronously.

asyncio.create_task() starts tasks concurrently.

Examples

Two simple async tasks that wait for some time and then return a message.

FastAPI

import asyncio

async def task1():
    await asyncio.sleep(1)
    return "Task 1 done"

async def task2():
    await asyncio.sleep(2)
    return "Task 2 done"

Run both tasks at the same time and wait for both to finish.

FastAPI

import asyncio

async def main():
    t1 = asyncio.create_task(task1())
    t2 = asyncio.create_task(task2())
    results = await asyncio.gather(t1, t2)
    print(results)

FastAPI endpoint that runs two tasks concurrently and returns their results.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/run")
async def run():
    t1 = asyncio.create_task(task1())
    t2 = asyncio.create_task(task2())
    results = await asyncio.gather(t1, t2)
    return {"results": results}

Sample Program

This FastAPI app has an endpoint /tasks that runs two tasks at the same time. Task 1 waits 1 second, Task 2 waits 2 seconds. The endpoint waits for both to finish and returns their messages together.

FastAPI

import asyncio
from fastapi import FastAPI

app = FastAPI()

async def task1():
    await asyncio.sleep(1)
    return "Task 1 done"

async def task2():
    await asyncio.sleep(2)
    return "Task 2 done"

@app.get("/tasks")
async def run_tasks():
    t1 = asyncio.create_task(task1())
    t2 = asyncio.create_task(task2())
    results = await asyncio.gather(t1, t2)
    return {"results": results}

Important Notes

Use asyncio.gather() to wait for many tasks at once.

Concurrent tasks help your app handle slow operations without freezing.

Remember to use async and await keywords properly.

Summary

Concurrent task execution lets your app do many things at once.

Use asyncio.create_task() and asyncio.gather() to run and wait for tasks.

FastAPI supports async endpoints to handle concurrent tasks easily.

Practice

1. What is the main benefit of using asyncio.create_task() in FastAPI?

easy

A. It stops all tasks from running until manually restarted.

B. It makes the code run slower by adding delays.

C. It converts synchronous code into synchronous code.

D. It allows running multiple tasks at the same time without waiting for each to finish.

Solution

1. Step 1: Understand asyncio.create_task() purpose

   This function schedules a coroutine to run concurrently as a separate task without blocking the main program.

2. Step 2: Identify the benefit in FastAPI context

   Using it lets FastAPI handle multiple operations at once, improving responsiveness and efficiency.

3. Final Answer:

   It allows running multiple tasks at the same time without waiting for each to finish. -> Option D

4. Quick Check:

   Concurrent task execution = run tasks simultaneously [OK]

Hint: Remember: create_task starts tasks without waiting [OK]

Common Mistakes:

• Thinking create_task slows code down
• Confusing async with sync code
• Believing tasks stop until restarted

2. Which of the following is the correct way to run two async functions concurrently and wait for both to finish in FastAPI?

easy

A. func1(); func2()

B. await asyncio.create_task(func1()); await asyncio.create_task(func2())

C. await asyncio.gather(func1(), func2())

D. asyncio.run(func1(), func2())

Solution

1. Step 1: Understand how to run and wait for multiple async tasks

   asyncio.gather() runs multiple coroutines concurrently and waits for all to complete.

2. Step 2: Check each option's correctness

   await asyncio.gather(func1(), func2()) correctly uses await asyncio.gather(func1(), func2()). await asyncio.create_task(func1()); await asyncio.create_task(func2()) starts tasks but awaits them separately, not concurrently. func1(); func2() runs functions synchronously. asyncio.run(func1(), func2()) is invalid syntax.

3. Final Answer:

   await asyncio.gather(func1(), func2()) -> Option C

4. Quick Check:

   Use gather to run and wait multiple tasks [OK]

Hint: Use gather to wait all tasks together [OK]

Common Mistakes:

• Awaiting tasks separately losing concurrency
• Calling async functions without await
• Using asyncio.run inside async code

3. Given this FastAPI async endpoint code, what will be the output?

import asyncio
from fastapi import FastAPI
app = FastAPI()

async def task1():
    await asyncio.sleep(1)
    return "Task 1 done"

async def task2():
    await asyncio.sleep(2)
    return "Task 2 done"

@app.get("/run")
async def run_tasks():
    results = await asyncio.gather(task1(), task2())
    return {"results": results}

medium

A. {"results": ["Task 1 done", "Task 2 done"]}

B. {"results": ["Task 2 done", "Task 1 done"]}

C. {"results": ["Task 1 done"]}

D. TimeoutError

Solution

1. Step 1: Analyze the async tasks and their sleep durations

   task1 sleeps 1 second, task2 sleeps 2 seconds. Both run concurrently with gather.

2. Step 2: Understand gather result order

   asyncio.gather returns results in the order of the input tasks, regardless of completion time.

3. Final Answer:

   {"results": ["Task 1 done", "Task 2 done"]} -> Option A

4. Quick Check:

   Gather preserves input order in results [OK]

Hint: Gather returns results in input order, not finish order [OK]

Common Mistakes:

• Assuming results order matches finish order
• Expecting partial results if one task is slower
• Confusing gather with wait

4. What is wrong with this FastAPI code snippet for running tasks concurrently?

import asyncio
from fastapi import FastAPI
app = FastAPI()

async def task():
    await asyncio.sleep(1)
    return "Done"

@app.get("/test")
async def test():
    task1 = asyncio.create_task(task())
    task2 = asyncio.create_task(task())
    return {"result": task1.result() + task2.result()}

medium

A. Calling task1.result() before task completion causes an error.

B. Using create_task is not allowed in FastAPI endpoints.

C. The tasks should not be async functions.

D. The endpoint should not be async.

Solution

1. Step 1: Identify how task results are accessed

   Calling task1.result() immediately after creating the task tries to get the result before the task finishes, causing an error.

2. Step 2: Understand correct way to wait for task completion

   You must await the task or use await asyncio.gather(task1, task2) before accessing results.

3. Final Answer:

   Calling task1.result() before task completion causes an error. -> Option A

4. Quick Check:

   Access task result only after await [OK]

Hint: Always await tasks before accessing results [OK]

Common Mistakes:

• Accessing .result() too early
• Thinking create_task is forbidden
• Making endpoint synchronous

5. You want to run three independent async tasks in a FastAPI endpoint and return their results as soon as all finish. Which approach correctly handles errors in any task without stopping others and returns all results or error messages?

hard

A. Use asyncio.create_task() for each task but do not await them before returning.

B. Use asyncio.gather(task1(), task2(), task3(), return_exceptions=True) and process results for exceptions.

C. Use await task1(); await task2(); await task3() sequentially and catch exceptions globally.

D. Run tasks synchronously in a loop and catch exceptions inside each.

Solution

1. Step 1: Understand error handling in concurrent tasks

   Using asyncio.gather() with return_exceptions=True lets all tasks run concurrently and collects exceptions as results without stopping others.

2. Step 2: Compare options for correct concurrency and error handling

   Use asyncio.gather(task1(), task2(), task3(), return_exceptions=True) and process results for exceptions. correctly runs all tasks concurrently and handles errors individually. Use await task1(); await task2(); await task3() sequentially and catch exceptions globally. runs tasks sequentially, losing concurrency. Use asyncio.create_task() for each task but do not await them before returning. does not await tasks, so results are unavailable. Run tasks synchronously in a loop and catch exceptions inside each. runs synchronously, blocking concurrency.

3. Final Answer:

   Use asyncio.gather(task1(), task2(), task3(), return_exceptions=True) and process results for exceptions. -> Option B

4. Quick Check:

   Gather with return_exceptions handles errors concurrently [OK]

Hint: Use gather with return_exceptions=True for safe concurrency [OK]

Common Mistakes:

• Awaiting tasks sequentially losing concurrency
• Not awaiting tasks before returning
• Ignoring exceptions stopping all tasks