Hacker News Reader

Comment by phamilton | Hacker News Reader

Is there a mechanism to migrate existing data to the new code?

If you have external state stored in say a database, I think you have to control your deployments to migrate the schema before the model objects can be deployed.

So no.

One of the beauties of Erlang's hot code loading is the ability to migrate internal application state. An example:

I'm storing a lookup table going from key to value as a tuple {key, value}. With a change going out, I'm going to start doing {key, {value, other-value}}. In Erlang there's an easy pattern in place to do that. Just provide a function that goes from {key, value} to {key, {value, sensible-default}} and as part of the code reloading process that function will be run before the new code starts executing.

You may have misunderstood. A file can have multiple, say 'deployment units'. You can have a 'migrate deployment unit' which depend on the 'model deployment unit' which forces the app server to deploy the migrate unit first. You can have code there that migrates the tables and fill with default values before your model entities become available for use. From your comment this seems to be the same as what Erlang does for your example, unless you are claiming that Erlang's implementation is superior which could be the true, since I have never written any non-trivial programs in it.

I think the difference here is that the migration of internal state happens the moment the module is swapped out in memory. Erlang doesn't have mutable state, so state is instead passed around within a module. When a fix needs to be deployed, the two versions (old and new) run simultaneously, any new processes immediately run the new code, old processes call a `code_change` function which takes the current state within that process, performs any transformations required for the new code, and then the old module code is swapped out for the new.

That's not at all a perfect explanation of the process, but it's more or less useful enough to describe the difference between the two platforms.

Reply on news.ycombinator.com