Andy Doan andy.doan@linaro.org writes:
On 07/02/2012 12:31 AM, Michael Hudson-Doyle wrote:
Multi-machine code
This is easy IMHO: all machines should have the same code installed. With the appropriate ssh keys scattered around, it should be easy to write a fabric job or just plain bash script to run ldt update on each machine.
+1
Multi-machine data
Accessing postgres from another machine is a solved problem, to put it mildly :-)
I don't have a good idea on how to access media files across the network. In an ideal world, we would have a Django storage backend that talked to something like http://ceph.com/ceph-storage/object-storage/ or http://hadoop.apache.org/hdfs/ -- we don't need anything like full file system semantics -- but for now, maybe just mounting the media files over NFS might be the quickest way to get things going.
This feels like the messiest part of the problem to me. I keep trying to think of how we can avoid solving it, but if our fastmodel dispatcher conversation is any indicator - we'll have to use some hacks to work around things until its solved.
So let's think about our requirements here a bit. We have two kinds of storage requirements really:
* bundles -- these come in via XML-RPC (i.e. the web front end) currently, although it's possible that we'll stop sending results from the dispatcher via XML-RPC in favour of some more direct insertion at some point.
They need to be read when being deserialized, which currently is done in the web front end but maybe we should stop doing that. They arrive in one lump and I can't really see that changing.
They only need to be read by the web front end in the "view bundle" tab, and I can't really see that changing.
* dispatcher log files. These are created where the dispatcher runs, although we already have the output handled by a specific process -- that could easily send the output via an API rather than just writing it to a file as it does now. They need to be accessible by the web front end for display and we really want to keep the incremental output we currently have.
Oh, there are also attachments -- but they are like bundles really.
If we get to rework how these files are stored, we should really figure out how to do backups sensibly. I'm sure things like Ceph support snapshots which would be much better than tarballing all the files every day...
That said, it feels like we could get away with keeping the media files on the web front end machine and shipping them from wherever the dispatcher runs to the web front end via celery or something.
Multi-machine configuration
I think by and large the configuration of each instance should be the same. This means we need a mechanism to distribute changes to the instances. One way would be to store the configuration in a branch, and have ldt update upgrade this branch too (I think it would even be fairly easy to have multiple copies of the configuration on disk, similar to the way we have multiple copies of the buildouts, and have each buildout point to a specific revision of the config).
We could also have the revision of the config branch to use be specified in the lava-manifest branch but that doesn't sound friendly to third party deployments -- I think the config branch should be specified as a parameter of the instance set and updating an instance set should update each instance to the latest version of the config branch. This will require a certain discipline in making changes to the branch!
A thought: what if the "master" server had some sort of API where it listed what code/config level it was at. Then the worker nodes could periodically pull from that and update themselves as needed? This might make upgrades easier. However, maybe this is the wrong idea and we should get to a point where puppet can handle this.
So, the stuff I'm reading suggests that a "push" mode using something like fabric is better for deployments than a "pull" oriented tool like puppet.
Setup issues
There will be a requirement to make sure ssh keys etc are set up on the various machines involved. Ideally this would be done via puppet but for now I think we can just do it by hand...
yeah- we are only scaling to like 2 nodes, but puppet does seem like the most sane way to manage this long term.
My impression is that we'd use something like puppet to do the sort of thing ldt setup does, and also maybe some of the unix-user and ssh key management, and stuff built on fabric or similar to run ldt install/update.
Cheers, mwh