[erlang-questions] Feedback on webmachine route

Jesper Louis Andersen jesper.louis.andersen@REDACTED
Sat Sep 26 13:07:42 CEST 2015

On Fri, Sep 25, 2015 at 6:03 PM, Damien Krotkine <damien@REDACTED>

> I'm sure this code is horrible from the point of view of a seasoned
> Erlang developer, and I welcome any feedback. Especially regarding speed
> and robustness against faults.

The key two questions to think about:

* What happens if your spawned children die?
* If a child locks up, does you system exhibit liveness/progress?

Since you spawn children with spawn_link, they all link to the parent
process. So if one child dies, so does the parent. This gives you
all-or-nothing semantics: either all keys are returned, or the system
fails. An alternative semantics, based on spawn_monitor for instance, could
have you handle each key individually and return an error for keys which

Your receive expression never times out. So if a key fetch blocks, so does
your call. In some situations, when you can guarantee children have
progress, this is no problem, but it is often clever to add some kind of
timeout. However, when doing so, you must handle the case where a blob
arrives late to the process. This usually means you need to make sure the
mailbox flushes appropriately (which is something you need to peruse the
webmachine documentation for).

It is often impossible to a-priori declare what kind of semantics you want
for these things. It is a weakness of "frameworks" that tries to automate
these kinds of things. You get a predetermined semantics, but then you
can't switch it up when you need a different semantics. So you have to make
the analysis of how you want your system to behave if it starts failing.

Finally, binary() values form a monoid under concatenation, with <<>> as
the neutral element. Thus you can write your binary join as

binary_join(Parts) ->
    lists:foldl(fun(P, Acc) -> <<Acc/binary, P/binary>> end, <<>>, Parts).

The special case [] then returns <<>> and the case [X] forms <<
(<<>>)/binary, X/binary>> which evaluates to X as expected. With these
changes, binary_join/1 is so simple it is probably worth just folding into
the call site directly.

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