[erlang-questions] Process Dictionary vs Proplist in Web Frameworks

Geoff Cant <>
Thu Oct 29 12:39:59 CET 2009


Ngoc Dao <> writes:

> Hi,
>
> Web frameworks are normally designed as layers (web server ->
> middleware -> front controller -> controller -> action -> view ->
> etc.). Data needs to be passed from one layer to another. There are 2
> ways to pass:
> 1. Proplist (environment variables)
> 2. Process dictionary
>
> The 2nd way:
> * Is Simple and natural in Erlang because normally one HTTP request is
> processed by one process.
> * Makes application code which uses the framework appear to be clean,
> because application developer does not have to manually pass an ugly
> proplist arround and arround.
> I want to ask about the (memory, CPU etc.) overhead of process
> dictionary, compared to proplist. Which way should be used in a web
> framework?

I would strongly advise against using the process dictionary to pass
data between part of an erlang web framework.[1]

You say that "normally one HTTP request is processed by one process" -
by using the process dictionary you require that this is the case for
all code that uses your framework. By making this decision, you are
tying the hands of the users of your framework. They can no longer
choose the process model that suits their problem and must use a one
process per request design. In erlang it's quite common to hand requests
off to other processes, possibly on other nodes, for execution to
balance load, to move computation closer to needed resources, to turn
synchronous tasks into asynchronous ones, to alter process memory
profile, to isolate failures and so on. The use
of the process dictionary precludes all these approaches.

You also say that using the process dictionary "makes application code
which uses the framework appear to be clean". From the point of view of
the maintenance programmer, nothing could be further from the
truth.

Good erlang code does not use the process dictionary. Erlang programmers
usually only have to think about the function body and arguments to work
out what its going to do. Sprinkling 'get' and 'put' through the code
means that an erlang programmer trying to understand your code now has
to read all the code to figure out why something is happening. The order
in which functions are called becomes important. The behaviour of
functions in other modules becomes important because now there's a
back-channel to propagate bugs, er, state between parts of the code.


As a (curmudgeonly) future web framework user, I would almost certainly
not choose a framework based on the use of non-erlangy features[2] such
as the process dictionary firstly because the code would be more difficult to
understand when someone would need to maintain it and secondly because
the process dictionary would prevent me from using a different process
model if I needed to.

Using a proplist or 'dict' or some opaque datastructure and an API
module is the natural, erlangy way to solve your problem.

Good luck with your framework,
-- 
Geoff Cant

[1] More generally, I would strongly advise against using the process
    dictionary.
[2] Ditto for parameterized modules and hierarchal module names[3]
[3] I'm already guilty of this, but promise not to do it again.



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