Simple retry until with WGet

Romain Manni-BucauJanuary 5th, 2023Last Updated: January 2nd, 2023

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Ensure some commands are executed only when a service is up is a common need of
docker-compose or even
Kubernetes when using some container to setup some state. If you can use
wget, it has almost this feature built-in.

For a HTTP backend, awaiting it is ready – most of the time its healthcheck too – is often testing a resource returns a HTTP 200 (can be a version endpoint, an asset or whatever simple and not writing anything).

There are a ton of curl based script doing some while loops to solve that, even some custom go, node scripts but if you have wget if you base image, it is actually quite trivial since wget supports retries:

wget \
  --no-check-certificate \ (1)
  --read-timeout=30 --timeout=30 \ (2)
  --retry-connrefused \ (3)
  --waitretry=5 \ (4)
  --tries=60 \ (5)
  "https://$MY_SERVICE_HOST:$MY_SERVICE_PORT/some/resource" (6)

1	(optional) ignore SSL errors, depends your environment,
2	the common request timeouts, just ensure to fail if it hangs,
3	consider a connect refused as a retryable case,
4	wait 5s before retrying again – depends how fast/slow is your backend,
5	retry max 60 times (so 5 minutes max with the wait configuration),
6	the test endpoint configured using Kubernetes service environment variables (for a `my` service in this sample).

Adding this kind of wget at the beginning of a script or custom entrypoint.sh enables to ensure some dependency are available or at least await for them a few before giving up. The big advantage compared to what we find on the net is that it does not need any shell knowledge with loops or whatever to implement it which is always a concern in a team but the assumption you have wget in the base image is required.

curl has similar option if you have a recent enough version:
curl \
  --connect-timeout 30 --max-time 30 \
  --retry-connrefused \
  --retry-all-errors \
  --retry 60 \
  --retry-delay 5 \
  --retry-max-time 300 \
  "https://$MY_SERVICE_HOST:$MY_SERVICE_PORT/some/resource"
if you need a compatible debian based image you can use rmannibucau/curl:bookworm-slim@sha256:208599dbcf1d2e673a8f6f463093b9264e52c02ee05e99ed35ba3bdbdf416704.

Published on Java Code Geeks with permission by Romain Manni, partner at our JCG program. See the original article here: Simple retry until with WGet

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