Dispatching channels with multiple message types

[limx0]

Hi @jcrist

I'm using msgspec to decode some JSON from a channel that sends multiple message types, and am wondering how best to dispatch these messages to msgspec (please close if this is out of scope or you don't have any opinion).

I observe (roughly) the following times for a couple of methods I've tried, and they're pretty much ordered from fastest to most correct:

raw = b'{"Type":"Trade", "Symbol":"AAPL", "LastTradedPrice":100}'

# startswith - fastest, but requires Type to be at the beginning of JSON
%timeit raw.startswith(b'{"Type":"Trade"')
178 ns ± 0.0241 ns per loop (mean ± std. dev. of 7 runs, 10,000,000 loops each)

# in - slightly slower but more correct
%timeit b'"Type":"Trade"' in raw
430 ns ± 2.78 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)

# msgspec - most correct, parsing explicit Literal
class MessageType(msgspec.Struct):
    MessageType: Literal["Trade", "Quote", "Heartbeat"]

%timeit _ = msgspec.json.decode(raw, type=MessageType)
1.2 µs ± 0.707 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)

# JSON decoding - Another option?
decoded = msgspec.json.decode(raw)
2.89 µs ± 15.7 ns per loop (mean ± std. dev. of 7 runs, 100,000 loops each)

Obviously theres standard performance trade offs here; but I wonder if you've thought about handling this in msgspec or have any other more correct but also performant suggestions?

Thank you again for this wonderful library!

[limx0]

I should also note I'm willing to PR / test something if you have any suggestions. I'm not comfortable in C, but am reasonably comfortable writing some Cython.

[jcrist]

Hi @limx0, if I understand correctly you're looking to dispatch the type of the message to one of several types based on the value of a Type key in the message? If so, have you seen https://jcristharif.com/msgspec/structs.html#tagged-unions? This is exactly what that feature is for.

[limx0]

Yes thats correct; and this looks exactly like what I am after. Apologies for not reading the docs correctly!

[jcrist]

No worries! Is the documentation clear enough? Is there a different place you looked for information like this? There might be a better way to organize the docs.

[limx0]

I think the docs are clear now that they're been pointed out to me. Probably what I was looking for was something along the lines of "processing/handling multiple types in a stream" or something to that effect; Tagged Unions didn't jump out at me as what I was after.