[erlang-questions] Message protocol vs. Function call API

Joe Armstrong erlang@REDACTED
Thu Oct 1 09:39:27 CEST 2009

Think of it as a protocol stack.

Users Programs
------------------ API ---------------       (A)
------------ Messaging interface -----  (B)

The users program call functions in a module. These are defined in an API (A).
The library which implements the API works together with a messaging interface

The top-level interface (A) is defined in terms of typed function calls.
The messaging interface (B) is defined in terms of message flows.

Now (A) is usually well defined and should be stable over many years.
(B) might change with
time. This has the advantage that we can change B as we learn more
while not damaging
applications that use A.

There are some problems with this: Usually the interface (B) is not
well specified and
has to be inferred by reading the code. Also for efficiency reasons a
user program might like
to bypass (A) and talk to the (B) interface directly, the latter will
cause problems if the (B)
protocol is changed.

This is exactly how the erlang system is structured. A user program doing io
will call io:read(Pid, X)   (ie a function according to the (A) interface) the
implementation of io:read will send a
{get_until,unicode,X,erl_scan,tokens,[1]} message
across the (B) interface, and so on ...

The (A) interface is well defined and stable. The (B) interface is not
well documented (read the code Luke).

The is not too bad. Usually only one person has to understand the (B)
interface (ie the person who implements the library) - whereas large
numbers of people have to understand the (A) interface.

Taking a problem and partitioning it into (A) and (B) interfaces is
not easy. You can start with (A) and "invent" the necessary (B)
interface (this is top-down design). or you can start with
B and invent A (bottom-up design).

In the case where B is given we'd do bottom up design (for example B
*is* TCP/IP or a given hardware interface like an SSD). For user
convenience we'd like to start with A and derive B.

Sometimes we design A with a set of specific set of properties and B
is given - so here
we just have to write some glue in the middle (what I call a middle
man) - an example of
this might be AMQP (the Advanced Message Queue Protocol). The designer of
AMQP things "what would be a nice API to write messaging applications
with?" - answer
"reliable messaging with, security and notifications" - so (A) is
given. (B) might be TCP/IP
or UDP or whatever ...

For some projects I start by defining (A) then invent (B), or use a
given (B). others B is
given and I figure out an (A) that seems appropriate.

"It depends ..." :-)


On Thu, Oct 1, 2009 at 4:52 AM, Mihai Balea <mihai@REDACTED> wrote:
> This came up on a different thread, but I believe it is an interesting
> subject on its own.
> On Sep 30, 2009, at 9:36 PM, Robert Virding wrote:
>> P.S. One long, never-resolved discussion Joe and I had for many years was
>> whether you should have an API like is done in gen_XXX or to explicitly
>> specify the message protocol. Having the message protocol would allow you to
>> increase concurrency.
> I've been mulling this question myself lately. Besides increased
> concurrency, a message protocol would also allow for other nifty things,
> like versioning. Function calls make synchronous APIs easier. So I was
> wondering what folks on this list think... besides the obvious "it depends
> on the situation".
> Mihai
> ________________________________________________________________
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