How do you perform a HEALTHCHECK in the Redis Docker image?

Question

Recently, we had an outage due to Redis being unable to write to a file system (not sure why it's Amazon EFS) anyway I noted that there was no actual HEALTHCHECK set up for the Docker service to make sure it is running correctly, Redis is up so I can't simply use nc -z to check if the port is open.

Is there a command I can execute in the redis:6-alpine (or non-alpine) image that I can put in the healthcheck block of the docker-compose.yml file.

Note I am looking for command that is available internally in the image. Not an external healthcheck.

Answer 1

If I remember correctly that image includes redis-cli so, maybe, something along these lines:

...
healthcheck:
   test: ["CMD", "redis-cli","ping"]

Answer 2

Although the ping operation from @nitrin0 answer generally works. It does not handle the case where the write operation will actually fail. So instead I perform a change that will just increment a value to a key I don't plan to use.

image: redis:6
healthcheck:
  test: [ "CMD", "redis-cli", "--raw", "incr", "ping" ]

Answer 3

I've just noticed that there is a phase in which redis is still starting up and loading data. In this phase, redis-cli ping shows the error

LOADING Redis is loading the dataset in memory

but stills returns the exit code 0, which would make redis already report has healthy.

Also redis-cli --raw incr ping returns 0 in this phase without actually incrementing this key successfully.

As a workaround, I'm checking whether the redis-cli ping actually prints a PONG, which it only does after the LOADING has been finished.

services:
  redis:
    healthcheck:
      test: ["CMD-SHELL", "redis-cli ping | grep PONG"]
      interval: 1s
      timeout: 3s
      retries: 5

This works because grep returns only 0 when the string ("PONG") is found.