Curl retry mechanism

I have a script I run to deploy 2 web services and a front-end application. The script calls a method that checks to see if the 2 back-end services are up and running. If so, deem the deploy successful, otherwise revert.

The 2 services sometimes take longer than the front-end to start up. Currently I put in a sleep call to delay the web service check. This allows them time to start up.

I want to remove this sleep and add in a retry mechanism so that if a service is down, just retry the check repeatedly until I get a response.

Tp check if the device is up, I use curl. I've read that curl has a retry mechanism but I've never used it.

Have any of you solved this problem before? I want to understand the things I must consider when solving it, e.g. do I retry until i get a HTTP 200 from my service?

Anyone any suggestions how I'd test this? I'd need to find a service that was down.

EDIT: I see that -retry only reacts to transient errors 'Transient error means either: a timeout, an FTP 4xx response code or an HTTP 5xx response code'. My service can return a 404 therefore curl's retry is not my solution.

The following statement will retry 5 times or a maximum of 40 seconds with a connection timeout of 5 seconds, and no exponential backoff policy

curl --connect-timeout 5 \
    --max-time 10 \
    --retry 5 \
    --retry-delay 0 \
    --retry-max-time 40 \
    'http://your_url'


--max-time 10     (how long each retry will wait)
--retry 5         (it will retry 5 times)
--retry-delay 0   (an exponential backoff algorithm)
--retry-max-time  (total time before it's considered failed)

Note that there is also a --retry-connrefused (since curl 7.52.0) that retries even when the connection is refused and --retry-all-errors (since curl 7.71.0) which "is the sledgehammer of retrying".

Just to clear up any possible confusion for anyone new...

Please see: https://curl.haxx.se/docs/manpage.html

> --retry > > If a transient error is returned when curl tries to perform a > transfer, it will retry this number of times before giving up. Setting > the number to 0 makes curl do no retries (which is the default). > Transient error means either: a timeout, an FTP 4xx response code or > an HTTP 5xx response code. > > When curl is about to retry a transfer, it will first wait one second > and then for all forthcoming retries it will double the waiting time > until it reaches 10 minutes which then will be the delay between the > rest of the retries. By using --retry-delay you disable this > exponential backoff algorithm. See also --retry-max-time to limit the > total time allowed for retries. > > If this option is used several times, the last one will be used. > > Added in 7.12.3.

If you use --max-time 10 this will kill your connection before 10 seconds so its not useful when downloading.

Following will retry 300 times with 5sec delay and 30sec connection timeout

curl \
	--connect-timeout 30 \
	--retry 300 \
	--retry-delay 5 \
    URL