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Erlang APIs are designed around the availability of dynamic pattern matching on messages, but static message typing is definitely feasible. The only feature of

by jasone 6y ago

Erlang APIs are designed around the availability of dynamic pattern matching on messages, but static message typing is definitely feasible. The only feature of Erlang that stands out me as fundamentally incompatible with static typing is "live upgrade". And although this feature might be critical in the context for which it was designed (Ericcson telecom switches), it appears that very few Erlang-based deployments use live upgrade; the most common practice is to restart services.

toast0 6y ago
I'm not that up on static typing, but it would seem to me that static typing would be difficult with different versioned nodes on dist as well as hot upgrade. While a lot of users avoid hot load and restart/replace nodes instead, I don't think many restart/replace the whole cluster.
- dnautics 6y ago
  > different versioned nodes Exactly. Blue-green deploys could get seriously messed up if the static typing system doesn't strongly think about this.
  - carlmr 6y ago
    I'm not too familiar with Erlang. Doesn't dynamic typing just mean you have to strongly think about it?
    - macintux 6y ago
      You're absolutely right: with Erlang you have to spend a great deal of effort planning for, implementing, and testing code that can work correctly across nodes with different versions of the code and/or versions of Erlang itself. I think the open question is whether strong, _static_ type system can handle it at all.
      - dnautics 6y ago
        you actually don't. You just have to use pattern matching correctly.
        macintux 6y ago
        Well, yes, that's the primary tool, but from my now-fuzzy memories working on Riak at Basho, I recall there being subtle and not-so-subtle risks and complexities to manage.
    - toast0 6y ago
      If you follow the philosophy of Erlang, you can take shortcuts to strongly thinking about it. On the server side: what happens if a process geta a message it doesn't handle? Either it will crash (and be restarted by supervision, hopefully), it will receive and discard it (possibly logging it), or it will ignore it and leave it in the message queue (not great for long term memory use). On the client side: what happens if the process sends a message that is ignored or crashes the receiver? The client will timeout, and won't know if the work was done or not; best to bubble up the error, perhaps by crashing (and maybe restarting by supervision). If you're ok with these 'worst cases', you don't have to proactively deep think. The proper deployment strategy is clear: deploy servers that handle new messages first, then deploy clients that send new messages, but if you mess the order, you'll be within your failure model. It gets more tricky when there's a difference in data types and a piece of data could be used by old code, then new code, then old code, of course. Of course, the actual worst cases are more fun, but less likely to happen. You could find a bug that crashes BEAM or something; I don't think that would be easily found by sending a unexpected message, but maybe (depends on what you do with unexpected messages). But if your static typing just says yeah I dunno, it could crash, I don't think you've gained much from the exercise. I'm probably biased against static typing, but if it can model the system and provide useful insights that are accurate on the system, that sounds helpful; if it can't model the system though...
- bjz_ 6y ago
  Yeah, would really love to see a static type system that tackled this directly - ie. handling versioned nodes in a cluster and ensuring deployments happen safety. I think it would be possible, but it would require some careful thought and design.

by jasone 6y ago

toast0 6y ago

I'm not that up on static typing, but it would seem to me that static typing would be difficult with different versioned nodes on dist as well as hot upgrade. While a lot of users avoid hot load and restart/replace nodes instead, I don't think many restart/replace the whole cluster.

dnautics 6y ago
> different versioned nodes Exactly. Blue-green deploys could get seriously messed up if the static typing system doesn't strongly think about this.
- carlmr 6y ago
  I'm not too familiar with Erlang. Doesn't dynamic typing just mean you have to strongly think about it?
  - macintux 6y ago
    You're absolutely right: with Erlang you have to spend a great deal of effort planning for, implementing, and testing code that can work correctly across nodes with different versions of the code and/or versions of Erlang itself. I think the open question is whether strong, _static_ type system can handle it at all.
    - dnautics 6y ago
      you actually don't. You just have to use pattern matching correctly.
      - macintux 6y ago
        Well, yes, that's the primary tool, but from my now-fuzzy memories working on Riak at Basho, I recall there being subtle and not-so-subtle risks and complexities to manage.
  - toast0 6y ago
    If you follow the philosophy of Erlang, you can take shortcuts to strongly thinking about it. On the server side: what happens if a process geta a message it doesn't handle? Either it will crash (and be restarted by supervision, hopefully), it will receive and discard it (possibly logging it), or it will ignore it and leave it in the message queue (not great for long term memory use). On the client side: what happens if the process sends a message that is ignored or crashes the receiver? The client will timeout, and won't know if the work was done or not; best to bubble up the error, perhaps by crashing (and maybe restarting by supervision). If you're ok with these 'worst cases', you don't have to proactively deep think. The proper deployment strategy is clear: deploy servers that handle new messages first, then deploy clients that send new messages, but if you mess the order, you'll be within your failure model. It gets more tricky when there's a difference in data types and a piece of data could be used by old code, then new code, then old code, of course. Of course, the actual worst cases are more fun, but less likely to happen. You could find a bug that crashes BEAM or something; I don't think that would be easily found by sending a unexpected message, but maybe (depends on what you do with unexpected messages). But if your static typing just says yeah I dunno, it could crash, I don't think you've gained much from the exercise. I'm probably biased against static typing, but if it can model the system and provide useful insights that are accurate on the system, that sounds helpful; if it can't model the system though...
bjz_ 6y ago
Yeah, would really love to see a static type system that tackled this directly - ie. handling versioned nodes in a cluster and ensuring deployments happen safety. I think it would be possible, but it would require some careful thought and design.