Comments on: Doing it Live

By: Geoff

Geoff — Wed, 04 Feb 2009 22:27:10 +0000

You can also use the gen_(server|fsm) code change stuff via the ‘sys’ module. sys:suspend/1 the gen_servers running the module to be upgraded, load the new code, sys:change_code/4 and then sys:resume/1 them all. It’s handy if you don’t want to go to the hassle of a full release upgrade.

By: Bryan

Bryan — Wed, 04 Feb 2009 20:47:38 +0000

Thanks, Nick – glad you liked it. I agree with implementing reload/0. Among other conveniences, it shortens the downtime. No typing delay, no typo penalty, no chance of botching it bad enough that you have to spend time fixing something else!

By: Nick Gerakines

Nick Gerakines — Wed, 04 Feb 2009 19:02:18 +0000

That’s great. Very cool write up. I’m quite the fan of rolling your own reload/0 function that iterates through your app’s modules with a quick code:purge/1 and code:load_file/1.

By: Bryan

Bryan — Wed, 04 Feb 2009 18:22:14 +0000

A very good point, Harish! My usual way of dealing with this? Let the process fail. If the computation was necessary, it has been registered with a supervisor, and will be restarted, ready to grab the fresh data.

Also, I generally try to write long-running processes such that they don’t hang onto data representations from other modules any longer than necessary. For example, a process may keep a list of ids and names from a #person, but won’t hold onto the whole record. So, that process won’t be affected by a change in the person record shape. This doesn’t account for those short periods of time when a process *is* holding onto an external data format, but in those cases, I just revert back to “let it fail.”

I haven’t done an “appup” or “rel” yet, but you’re right: even if gen_server:code_change does get hairy, it’s still better than not being able to do it at all.

Thanks for the note.

By: Harish Mallipeddi

Harish Mallipeddi — Wed, 04 Feb 2009 16:10:40 +0000

This is assuming you don’t have any #person records stored anywhere in memory – for instance in the State of some gen_server? If yes then you’ve to write code to migrate via gen_server:code_change, add “appup” file, add “rel” files, etc and very soon it becomes complicated (I guess it’s better than those other languages which don’t support hot code loading out of the box). But still very cool example of hot code loading in production!