Split It out.

content/posts/2025-01-01-hobby-sdk-02.md

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+---
+title: "Split It Out"
+date: 2025-01-01T21:00:00-07:00
+tags:
+ - generics 
+ - go
+ - beam
+categories:
+ - Dev
+ - Talks
+ - Hobby SDK
+---
+
+Since [my last post](/2024/10/my-hobby-beam-go-sdk) I've been quite busy with
+travel and recovery from same, and thus I haven't poked around trying to work on
+my Hobby SDK.
+
+Which also means, I haven't spoken about more widely either. Just after I
+re-built the site to use Github actions too, to make it easier to write more.
+Ah well. Still need to sort out a few CSS things though, since line wrapping is
+weird... I want to have the markdown look good, but not have that affect the
+rendered output beyond formating...
+
+That's for a later post. This is about how to deal with Stateful DoFns!
+
+-----
+
+The next thing to do for the Hobby SDK is add State and Timers, and ultimately,
+stateful transforms. I was stuck on this for a little bit. Specifically how
+to add these to the graph construction mechanisms, while also avoiding writing
+too much code to replicate the type system.
+
+The trick though is that Stateful DoFns, that is, those that make use State and
+Timers in any capacity, have a hard restriction: they must take in KVs as an
+input. This is how the Beam model enables stateful stream processing, without 
+sacrificing parallelism.
+
+The question then, is how do I have the hobby SDK enforce this?
+
+You see, in most Beam SDKs, there is really only a single `ParDo` call as a part
+of graph construction, and it will accept `DoFns` of any kind. The simple light
+weight ones, to having Side Inputs, to Splittable DoFns, and of course,
+stateful DoFns. But then there's a later step that does the enforcement of these
+compatibility requirements.
+
+And that's just a lot of code. Unlike in Java, Go Generics don't erase types,
+nor can I have overloaded implementations to selectively validate different
+constraints. It could be possible to make some way of "detecting" the KV, using
+reflection to enforce validating that the outer type is a KV, but that goes
+against the ethos of this Hobby SDK.
+
+I want to have the Go Compiler do the enforcement, as much as possible.
+
+This means that I'll end up with a separate `StatefulParDo` top level graph
+construction function. There's a benefit to this: It can be clearly documented
+and provides a natural place to put *all* the documentation for stateful DoFns,
+instead of incredibly burdening the standard `ParDo` function.
+
+This `StatefulParDo` will use the same mechanism that `GroupByKey` requires:
+A DoFn that has a KV generic. This way, when folks are writing a pipeline, the
+Go Compiler's type checker will let them know of the problem.
+
+I'll still need to detect stateful DoFns, since those features will still be
+modeled onto Exported fields and field types. That should be fine though.
+The two methods also enable a cleaner split for documenting the incompatibility
+of SplittableDoFns and Stateful DoFns. Sadly, no good way to represten this
+constraint as a type. Fields don't meaningfully participate in the Go type
+system.
+
+We need this because b, a SplittableDoFn can't have State.
+It becomes very difficult to reason about that way due to other transforms that
+happen to SplittableDoFns. Afterall, the key can change while splitting elements.
+
+Sometimes, you just have to have two things that do jobs well separately, instead
+of trying to have a single approach for everything.
+
+--------
+
+Writing the post isn't the same as actually finishing the work, but sometimes
+the best thing to do, is to split them out into separate posts and work. But
+the work does take longer.
+
+Just gotta do it one bit at a time.