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flask-redis-docker

An example about how to deploy a Flask application inside a Docker able to perform asynchronous requests.

How it works

Though Python allows asynchronous executions through several techniques, forking asynchronous process from a main process in Docker is not trivial (see https://docs.docker.com/config/containers/multi-service_container/). The only way is to define a worker process within the main processes of the container (i.e. handled through supervisor, used as main process defined in the container's CMD), that is able to perform operations in background.

Here, Python Redis Queue is used to define an asynchronous queue of required tasks based on a Redis service, running on a separated (light) container.

How to run it

Once you have docker-compose installed, just create the images and the containers through the command:

$ docker-compose up -d

This will create the two containers, the one with the Flask application listening port 5000, and the one with the Redis service (listening port 6379).

$ docker-compose ps
 Name               Command               State               Ports            
-------------------------------------------------------------------------------
redis    docker-entrypoint.sh redis ...   Up      6379/tcp                     
webapp   /entrypoint.sh /start.sh         Up      443/tcp, 0.0.0.0:5000->80/tcp

Once both services are up and running in your localhost, you can submit a long running request asynchronously as follows:

$ curl -X POST -F "duration=20" http://127.0.0.1:5000/long_task
{
  "data": {
    "task_id": "05aabf85-b34a-4797-8d44-8ad767a3928e"
  }, 
  "status": "success"
}

And retrieve the result with the following one:

$ curl -X GET http://127.0.0.1:5000/tasks/05aabf85-b34a-4797-8d44-8ad767a3928e

If the task is not concluded in the meantime, this is the response of the request:

{
  "data": {
    "task_id": "05aabf85-b34a-4797-8d44-8ad767a3928e", 
    "task_result": null, 
    "task_status": "started"
  }, 
  "status": "success"
}

Otherwise, this is the response with the output stored in the task_result key:

$ curl -X GET http://127.0.0.1:5000/tasks/05aabf85-b34a-4797-8d44-8ad767a3928e
{
  "data": {
    "task_id": "05aabf85-b34a-4797-8d44-8ad767a3928e", 
    "task_result": {
      "task": true
    }, 
    "task_status": "finished"
  }, 
  "status": "success"
}

There's also an example about submitting parallel, asynchronous tasks (here 5 sub-workers are considered):

$ curl -X POST -F "duration=200" http://127.0.0.1:5000/parallel_long_task
{
  "data": {
    "task_id": "3fdf65d2-bd97-477a-888d-bc33a25e750a"
  },
  "status": "success"
}

$ curl -X GET http://127.0.0.1:5000/tasks/3fdf65d2-bd97-477a-888d-bc33a25e750a
{
  "data": {
    "task_id": "3fdf65d2-bd97-477a-888d-bc33a25e750a",
    "task_result": {
      "completed_subtasks": 5,
      "failed_subtasks": 0
    },
    "task_status": "finished"
  },
  "status": "success"
}