How to use complex health checks in docker?

JRichardsz.java

2 min readJan 7, 2023

I will show you how to use complex check health validations in docke.

The following post was my answer to https://stackoverflow.com/a/75044887/3957754

image source: https://youtu.be/PSO_xKhSKEM

Introduction

The validation of health of our services is a common practice and docker give us a feature for that.

Here an example of mysql health check

Basically using bash commands, you could query to your service and if the result is an exit code with value 0, docker will flag your container as healthy. Any other value will cause that your container be flagged as unhealthy

Approaches

Commonly I use the following tools or techniques to perform validations

curl

test: curl --fail http://localhost || exit 1

Search text in logs with GREP

test: 'cat /tmp/mysql-general-log.log | grep "root@localhost on using Socket"'

internal tool

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

Complex validations with a script

For complex health check validations you could use a script in the test parameter of docker-compose.yml

version: '3.7'

services:
  rosariosis_db:
    image: mysql:5.7
    volumes:
     - ./healthcheck.sh:/tmp/healthcheck.sh
    healthcheck:
      test: 'bash /tmp/healthcheck.sh'
      timeout: 5s
      retries: 1

healthcheck.sh

#!/bin/bash
exit 0;

You could put any logic , just make sure to return exit 0 for success and exit 1 for error

#!/bin/bash

# https://superuser.com/a/442395
http_validation=$(curl -s -o /dev/null -w "%{http_code}" http://www.example.org)

if [[ $http_validation == 200 && $foo == "foo" ]]
then 
  exit 0 
else 
  exit 1
fi

inline complex validation

According to this you could use a complex conditional in the healthcheck:

test: ["CMD-SHELL", "if [ \"`echo \\\"SELECT ACCOUNT_STATUS FROM DBA_USERS WHERE USERNAME = 'ANONYMOUS' AND ACCOUNT_STATUS = 'EXPIRED';\\\"|/u01/app/oracle/product/12.1.0/xe/bin/sqlplus -S sys/oracle as sysdba|grep ACCOUNT_STATUS`\" = \"ACCOUNT_STATUS\" ];then true;else false;fi"]

docker don’t support $?

If docker health check supported $? we could use

Basically I store the exit status in a file and then I compare the values.

But since docker don’t support that, I got this error:

Error: No such container: rosariosis_db ERROR: Invalid interpolation format for "healthcheck" option in service "rosariosis_db": "mysqladmin ping -h localhost ; echo "$?""

Lectures

Until the next,
JRichardsz

Originally published at https://jrichardsz.github.io on January 7, 2023.