Request: adding Docker health check to `postgres/README.md`

[jamesbraza]

It would be good to add a healthcheck to the postgres/README.md.

There are some possible starting points posed here: https://stackoverflow.com/questions/65115627/safe-ways-to-specify-postgres-parameters-for-healthchecks-in-docker-compose

---

Aside: running the below:

---
version: "3.8"
services:
    postgres:
        image: postgres:latest
        environment:
            POSTGRES_USER: user
            POSTGRES_PASSWORD: pass
            POSTGRES_DB: db

docker compose up postgres --detach 
docker compose run postgres pg_isready --username user --dbname db

I can never get pg_isready to output something besides /var/run/postgresql:5432 - no response

[LaurentGoderre]

The following works for me:

services:
    postgres:
        image: postgres:latest
        environment:
            POSTGRES_USER: user
            POSTGRES_PASSWORD: pass
            POSTGRES_DB: db
        healthcheck:
            test: pg_isready
            interval: 60s
            retries: 3
            start_period: 10s
            timeout: 10s

CONTAINER ID   IMAGE             COMMAND                  CREATED              STATUS                        PORTS      NAMES
c1242e57029b   postgres:latest   "docker-entrypoint.s…"   About a minute ago   Up About a minute (healthy)   5432/tcp   postgres-health-postgres-1

[jamesbraza]

@LaurentGoderre can you tell me what this outputs for you?

docker compose run postgres pg_isready
# or
docker compose run postgres pg_isready --username user --dbname db

And also thanks for sharing your config, appreciate your help!

[LaurentGoderre]

Running that command doesn't give you real result because you are overwriting the command so the postgres doesn't start.

A better way would be

docker run --name my-db  -e POSTGRES_PASSWORD=password -d postgres
sleep 2
docker exec my-db pg_isready

820871225dadb64bf79b128613e48a84224f2f53504164be989ce9cd770754fc
/var/run/postgresql:5432 - accepting connections

[jamesbraza]

Ah I see, thank you!! One follow up question, is the healthcheck seems to be conservatively slow. With the healthcheck:

---
version: "3.8"
services:
    postgres:
        image: postgres:16.1-bookworm
        ports:
            - "5432:5432"
        healthcheck:
            test: pg_isready
            start_period: 0s
        environment:
            POSTGRES_HOST_AUTH_METHOD: trust

 > time docker compose up --detach --wait postgres
...
docker compose up --detach --wait postgres  0.20s user 0.16s system 1% cpu 31.287 total

We see it takes about 31 seconds. Now removing the healthcheck, it is way faster:

> docker compose up --detach postgres && sleep 1 && docker compose exec postgres pg_isready
...
/var/run/postgresql:5432 - accepting connections

We see it worked within about 1 second.

Why is the healthcheck using pg_isready taking over 30 seconds?

[jamesbraza]

Update, I have realized from https://docs.docker.com/engine/reference/builder/#healthcheck the default interval is 30-seconds.

So moving to:

        healthcheck:
            test: pg_isready
            interval: 1s

This solved the issue:

>  time docker compose up --detach --wait postgres
...
docker compose up --detach --wait postgres  0.10s user 0.06s system 7% cpu 2.053 total

[tianon]

#2393 (comment) 🙈

(see also https://github.com/docker-library/faq#healthcheck, many of which applies to trying to make "generic" documentation for health checks too, and https://github.com/docker-library/healthcheck/blob/40afbf64d69cf933af0da4df6383958a29113601/postgres/docker-healthcheck which is likely more than we'd want in the documentation but the main point there being explicitly using TCP to ensure we don't connect to the temporary server)