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0 pointsplatz9y ago0 comments

There are parts of OTP patterns that seem inherently dynamic. Message passing is only one aspect. There are also deployment/upgrade concerns with a running system.

Actors can receive messages that change their behavior entirely ( http://erlang.org/doc/man/gen_server.html ). Features like this are not there by accident.

Actors can hot-upgrade code their dynamically while the process is running. For example, if an actor is hot-upgrading I'm not sure how it would work, if the types of the old state and the new state don't exactly match. Sure, you could write functions to do this, but you see the picture is much more complicated.

I don't think I've presented the best arguments off the top of my head here here, but if you think more about the deployment/upgrade scenarios, along with partial updates along in certain nodes of the system, you can think about how complex it could get.

Basically, never assume that you get to take the whole cluster down to do an upgrade. Comprehensive "red/black" deployment strategies used by other non-distributed languages are not really the OTP way of doing deployment/upgrades.

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sitkack9y ago

I have version N of a struct and then having version N+1 of a struct in-flight at the same time is almost impossible with current statically typed languages. In a dynamically typed language, as long as the contents of version N+k struct are additive and don't change the semantics, old code can read new data.

What needs to happen is both, immutable code, and versioned structs with pure functions that can upgrade and possibly downgrade structs as needed. The larger the distributed system, the versions of a struct (message) will be in-flight at a time. Services need to contain no state, so that they can be micro-rebooted and brought up with the new version.

Joe Armstrong had a comment on globally accessible but immutable code, which I think would go a long way towards the ability to statically type the inputs to a function in a distributed system. Interposition and routing would be the only way to upgrade or deprecate old code paths.