Use Docker ENTRYPOINT

[SuperQ]

Proposal

Use Docker ENTRYPOINT0 rather than CMD for setting the default command line flag values.

When CMD is used, any additional flags cause the default flags to be dropped.

This makes starting up Prometheus with a simple additional flag like --storage.tsdb.retention=15d to need the full list of flags added.

Environment

Docker

[DrakeW]

@simonpasquier I would like to help with this issue :)

[simonpasquier]

@DrakeW sure, feel free to submit a PR. IIUC @SuperQ's suggestion is to "merge" the CMD flags with ENTRYPOINT command.

prometheus/Dockerfile

Lines 18 to 22 in fe0f0da

	ENTRYPOINT [ "/bin/prometheus" ]
	CMD [ "--config.file=/etc/prometheus/prometheus.yml", \
	"--storage.tsdb.path=/prometheus", \
	"--web.console.libraries=/usr/share/prometheus/console_libraries", \
	"--web.console.templates=/usr/share/prometheus/consoles" ]

[DrakeW]

@simonpasquier PR submitted, let me know if there's anything else that you would like me to add

[AndorCS]

Guys, this change brokes deployment which was works for years:

Error parsing commandline arguments: flag 'config.file' cannot be repeated

and

Error parsing commandline arguments: flag 'storage.tsdb.path' cannot be repeated

I have this in my deployment manifest:

          args: # copy-pasted from official Dockerfile and adjusted to current flags
            - '--config.file=/etc/prometheus-config/prometheus.yml'
            - '--storage.tsdb.path=/data'
            - '--storage.tsdb.retention=21d'
            - '--web.console.libraries=/etc/prometheus/console_libraries'
            - '--web.console.templates=/etc/prometheus/consoles'
            - '--web.external-url=http://{prometheus.hostname}'
            - '--web.enable-lifecycle'

[SuperQ]

@AndorCS You will have to update your configuration.

[AndorCS]

@SuperQ sure I can do it, but I really don't see any real reason for breaking what works for years.

[SuperQ]

This was done to make deployment easier for everyone. You don't have to copy-and-paste all flags to add one additional flag change.

[AndorCS]

But I want to change the path to the config file, and now I should change the whole entrypoint because of that.

[SuperQ]

You change the path of the config file via -v/--volume. The path inside the container doesn't need to be changed.

[AndorCS]

In this case, it's better for me to override entrypoint in case you will break something else in the future releases.

I'm not against this change, but I don't like you kinda breaking what works for a long time even it was not the best way to run the application. Especially not for major version change.