Feedback desired: non-disruptive deployment strategies for production Lisp webapps

Question

I am interested in hearing how people do their Lisp webapp deployments and updates (especially updates) in production.

In Ruby many, myself included, use Capistrano for deployments. It provides some nice indirection and the ability to execute commands remotely and most importantly (in my mind) the ability to rollback to a working code base.

I know that the idea of a long running Lisp process being connected to via Swank through an SSH tunnel and modified in place is a popular idea that's knocked around, but I haven't drunk that Koolaid, mostly because of the issue of updating a stateful process (which seems like asking for trouble if something goes wrong - like unforeseen impedance mismatches between current state in memory and new object definitions that will soon be in memory).

Given that you can create nearly stateless (or completely) webapps using hunchentoot (or insert your favorite Lisp app server here), it seems like using something like Capistrano could be used for non-disruptive updates to Lisp code too if the Lisp process(es) hide behind nginx in its upstream channel and if you can correctly choreograph taking down the hunchentoot processes and spin them back up after an update to code, e.g., bring them back up all the while leaving at least one hunchentoot process running in the cluster at any given moment (CGI or mod_lisp could be used, but I am not particularly interested in that approach - though if you really like that approach, please at least say something about it, I want to learn). For instance, using Passenger (which is comparing oranges to apples since it spins up processes on demand), you touch tmp/restart.txt and the app server restarts this time with freshly updated code - no interruptions from the users perspective.

Well, this is a bit of a ramble, and actually I am about to try all this out, but I'd like to get some feedback on these ideas from others. Maybe you have a better idea.

Thanks

Answer 1

You can accomplish non-disruptive (zero downtime) deployments by writing capistrano scripts for an intelligent front-end/load balancer like HAProxy that pulls app servers out of rotation, restarts them with the newly deployed code, and puts them back in the mix.

By incrementally rolling your appservers while they are out of live rotation in production you can achieve smooth deployments.

This doesn't touch on having persistent app server loops with specific state, that seems scary for exactly the reasons you mentioned. REPLs are cool for debugging and tweaking, but your instincts to run the code on disk seem well founded.