docker-compose generic readiness condition

docker-compose.yml

# Example : waiting for DB and Elasticsearch to be ready before launching a service
# The svc docker image must contain the start-when-ready.sh script

services:

  svc:
    image: svc
    environment:
      READINESS_CONDITION: '[ $$(curl --write-out %{http_code} --silent --output /dev/null http://elasticsearch:9200/_cat/health?h=st) = 200 ] && nc -w 1 -z $$DB_HOST $$DB_PORT 2>/dev/null'
      READINESS_SLEEP: 5
      READINESS_TIMEOUT: 1000
    command: start-when-ready.sh

start-when-ready.sh

# Flag to exit if trying to use an uninitialised variable
set -u
# Flag to exit if any statement returns a non-true return value
set -e

# ------------------------ Wait for READINESS_CONDITION : begin

# Generic wait condition using READINESS_CONDITION, READINESS_TIMEOUT and READINESS_SLEEP env var given in docker-compose.yml
[[ ! -z READINESS_CONDITION ]] && echo "Waiting condition : ${READINESS_CONDITION}"

# Loop until READINESS_CONDITION is true or WAIT_TIMEOUT is reached
timeout ${READINESS_TIMEOUT:-10} bash -c "until ${READINESS_CONDITION:-true}; do echo ... waiting to be ready...; sleep ${READINESS_SLEEP:-5}; done"

# If READINESS_CONDITION times out, exit with an error message
[[ $? -eq 124 ]] && { echo "Timeout reached and waiting condition still false. Check READINESS_CONDITION or increase READINESS_TIMEOUT."; exit 1; }

# ------------------------ Wait for READINESS_CONDITION : end

echo "Service ready"

# Start the service...