Can we do deploys without downtime

[foobacca]

Think about what the potential downsides would be:

• if no migrations, then there may well be no problem
• if there are migrations there might be a few server errors for some pages between updating the code and running the migrations.
• what else?

At the least we could provide an option. Maybe we start by having the default be the current behaviour, with an option to have a no-downtime-deploy, get confident with that, then move to the default being no-downtime but leave an option for when we're a bit scared ...

[foobacca]

If we're careful, we can run migrations without causing server errors. See these guidelines - http://nduthoit.com/blog/2013/09/03/running-out-of-band-migrations/

[foobacca]

The above site is down, but here is the google cache version
https://webcache.googleusercontent.com/search?q=cache:rq3GKmGhHPYJ:nduthoit.com/blog/2013/09/03/running-out-of-band-migrations/+&cd=1&hl=en&ct=clnk&gl=uk

[foobacca]

Sarah raise issue about scenario:

• user starts filling in form using old site. The site is upgraded and then the user hits the submit button and the behaviour might change

But there is no difference between zero downtime deploys and our current deploys in that there is still a difference before and after.

[foobacca]

Sarah idea:

• nodb flag - don't run migrations, still do db dump, but can do it while server is up

[foobacca]

The current things done during downtime is basically just running tasks.py deploy which does:

• update git submodules
• syncdb and migrations
• collectstatic and assets build

and anything else project specific in localtasks.py

In tasks.py we could instead have:

• deploy_before_live - update git submodules, collectstatic, assets build - called by deploy scripts on the copy of the site, while the old copy of the site is still running
• deploy_going_live - syncdb and migrations - called by deploy while the site is down.
• deploy - calls both the above, used on local deploys

Then the nodb flag could skip deploy_going_live and also then skip downtime.