Wait for linked MySQL server to accept TCP connection before database operation....

[Aagat]

... Solves issues with fig due to slow MySQL startup. Retries every 5 seconds until MySQL server exposes its TCP port.

[Aagat]

Related to #17

[yosifkit]

This could be a useful feature, but I would want a timeout so that the container will stop if there is no db coming (maybe it died). This could possibly be configurable (MYSQL_WAIT_TIMEOUT or something). Either way, this will definitely need to be documented.

[tianon]

I'm not sure on this one. This is a fairly general problem that every single application is going to have, and has had since the introduction of networking (Docker just makes it a lot more obvious since it's now trivial to start both of these services at exactly the same moment). At the very least, this feels like something we ought to have some kind of general solution for, possibly in Docker itself. Anything else feels a little bit hacky, IMO.

[yosifkit]

Currently the only other solution is to add --restart=always in the docker run to the dependent service (like wordpress), which seems worse than a retrying nice timeout. I agree that it would be better to be solved in docker since controlling a group of dependent services should definitely be in the future of Docker. Services need a way to tell Docker that they are ready to receive connections on the exposed port or, alternatively, have docker do the checking with something like --ensure-port 3000.

[Aagat]

@tianon You're right about having a general solution for this problem as it's fairly common, would be very helpful if docker itself had some option for this. But unfortunately there isn't anything available right now in docker as @yosifkit mentioned and since WP container is useless without MySQL container attached to it, shouldn't ensure MySQL is indeed available before doing anything?Also since we are already ensuring we have MySQL container is attached in our entrypoint script, shouldn't we also ensure that service is actually running before attempting to perform database operation?

[christoph-jerolimov]

But in my opinion you should switch from while not .. sleep n to if not .. sleep n. The current change request works only if the MySQL database is running on the same host (/dev/... usage). I expect this generic WordPress Docker image should also work the the MySQL instance is running on another host (independent if the database is running in or not in a Docker scenario).

[tianon]

@jerolimov /dev/tcp/... is actually a bash construction: http://www.linuxjournal.com/content/more-using-bashs-built-devtcp-file-tcpip

[christoph-jerolimov]

@tianon Oh that is funny. Thanks for the link, really interesting. But keep in mind that a startup process should timeout, or? But in most cases I expect now this would work.

[schmunk42]

Here's another workaround with a while do bash construct.

[darwingr]

Are there plans to merge this?
Having a fix via docker would be nice but this fix does seem to work well right now.

[yosifkit]

While I am not opposed to the idea of Wordpress automatically waiting for mysql to come up, I definitely want it to fail at some timeout instead of trying forever.