Hi folks,
For some reason, the server component of the apache http-components [1]
library doesn't get any love from the Clojure community. This is
particularly strange to me, given the early/broad adoption of the client
component (i.e. clj-http). In any case, I took version 5 for a spin, and
in particular the async-server aspect of it, as I wanted to see whether
it would integrate cleanly/nicely with core.async channels.
The ring model of async handlers taking 3 arguments, albeit necessary,
is hard to work with. Not only does it feel kind of magic - it is
actually non-trivial to (correctly) propagate the `respond` and `raise`
callbacks across the middleware stack (which can lead to bugs). The way
I see it, it would be much nicer if the response's body was enough to
distinguish whether this is a sync or async response. In other words,
returning a core.async channel as the body should suffice to treat this
as an async response.
I spent a few hours today, and came up with this server wrapper [2]. If
you need convincing that this works, evaluate the two expressions at the
bottom of the file (inside a `comment` expression), and cURL into it
from your terminal (per the comments). The actual construct which
enables a channel to act as body can be seen here
(`ChannelContentProducer`) [3].
The convention (for handlers) is simple. Use `promise-chan` for single
response, but regular `chan` for streaming responses. That is it! No
respond/raise indirection - just putting into the channel (and closing
it when streaming). Other than that, honors the ring-spec (wrt requests).
So there you have it - async from the ground-up, HTTP2 supporting,
ring-friendly web-server prototype (in less than 250 lines).
Kind regards,
Dimitrios
[1]: https://hc.apache.org/httpcomponents-core-5.0.x/index.html
[2]:
https://github.com/jimpil/asynctopia/blob/master/src/asynctopia/server/embedded.clj
[3]:
https://github.com/jimpil/asynctopia/blob/master/src/asynctopia/server/channel_body.clj
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