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Couple of points, please correct me if I've misunderstood anything :)
Can you not just do a `docker run .. {image} ..` and let docker take care of
pulling the image if needed? By default, docker will pull the image if one with
the same registry/repo/tag combo doesn't exist.
The assumption here is that an image (comprised of {registry + repository +
tag}) is never going to change. For example, the default tag used by docker is
`latest`, which suggests to me that you can push new versions of your image to
a registry, and update the `latest` tag to point to the new image. After this
change in mesos, I would need to log in to every mesos slave that had ever
downloaded this image, and run a `docker pull`.
The alternative is of course to use new tags for every new image (e.g. git
hashes). Though this means I need to update every framework that has been
configured with docker image names and change them to the new tag. I can see
the appeal of this approach when thinking soley about service schedulers,
because it could be problematic to control a rolling release if any new task
will automatically run the new image (as it takes the latest image from the
registry).
I've actually raised this issue several times with various people in the docker
community and never managed to get a concrete answer other than just run
`docker pull` every time (which is what we've been doing outside of mesos). I
think the difference between these use cases needs to be given some serious
thought, as it's caused us pain in various ways, hence why we ended up running
`docker pull` before every task to avoid the problem.
A working example would be the redis repository
(https://registry.hub.docker.com/v1/repositories/redis/tags), you'll see that
the `latest` tag is pointing at version 2.8. This tag is updated every time a
new image is published, and if I were to use the `latest` tag (or not specify a
tag, since it's the default) I would need to either explicity change my
deployment of redis to use a strict version, or manually `docker pull` on all
slaves and restart all the tasks using this container image.
It's also important to take into consideration long running frameworks like
Hadoop on Mesos, if this change were to be merged, and to avoid logging into
every slave and running `docker pull` we would need to restart the JobTracker
and change the image to a newer (never previously used) tag. As opposed to new
TaskTrackers automatically being launched inside the new image.
I guess a fair amount of this depends on what you're expecting to get from
using Docker. Software deployment or just dependency management and isolation?
I'm not against running `docker inspect && docker pull` on every slave in the
cluster, but I'd like the requirement to do that to be chosen. Perhaps you guys
have already had this discussion... I'm very interested to see what others have
been doing to solve this problem.
- Tom Arnfeld
On Sept. 1, 2014, 7:16 p.m., Timothy Chen wrote:
>
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> This is an automatically generated e-mail. To reply, visit:
> https://reviews.apache.org/r/25237/
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>
> (Updated Sept. 1, 2014, 7:16 p.m.)
>
>
> Review request for mesos, Benjamin Hindman and Jie Yu.
>
>
> Repository: mesos-git
>
>
> Description
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>
> Avoid Docker pull on each run.
>
> Currently each Docker run will run a docker pull which calls the docker
> registry each time.
> To avoid this this patch adds a docker inspect <image> and skip calling pull
> if it already exists.
>
>
> Diffs
> -----
>
> src/slave/containerizer/docker.cpp 0febbac5df4126f6c8d9a06dd0ba1668d041b34a
>
> Diff: https://reviews.apache.org/r/25237/diff/
>
>
> Testing
> -------
>
> make check
>
>
> Thanks,
>
> Timothy Chen
>
>
