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Re: Question about message passing paradigm

by Edwin Fine :: Rate this Message:

I'm a bit of an Erlang newbie so the gurus may come up with something much better than this, but I can think of these possible approaches:

Create a gen_fsm that controls all the collections. The collections could be ETS tables or gen_servers wrapping ETS tables. Under normal use, messages are sent to the fsm to update the collections individually. When the time comes to require consistency across the collections, send a message to the fsm to get the collective state data. The fsm goes into a different state while it gathers the data. This state would reject requests to update the collections (or wait until the state changes), although reads would still be allowed. On getting the result, the state changes back to allow updates again.
Create a memory-only Mnesia table for each collection, and use Mnesia transactions to get the multiple values atomically.
Change the architecture of the current lock-oriented program to make better use of Erlang's features.

Hope this helps.

2008/6/29 Mike T <talmage.news@...>:

Hello,

I was not sure where best to post this and hope someone is able to
help with this question.

After being bitten by the pitfalls of lock-oriented multi threading I
am interested in switching to message passing oriented concurrency.

I have read that erlang has a per-process ordering guarantee (that is
if A sends messages 1 and 2 to B 1 will arrive before 2 at B. However,
there is no guarantee that messages from C and D will not be placed in
between 1 and 2.

So my question is this:
In my current lock oriented program design I have threads dedicated to
managing different collections and actions on those collections
(hashmaps), lets call them A, B, C, and D. At several points in the
program one thread needs to get data from the other collections and
make a decision based on the collective state of the values it gathers
form those other collections.

An example situation would be A needing data from B, C, and D. In that
case A would attempt to lock A, B, C, and D then gather the items it
needs and release the lock.

How would I accomplish this same task in a message passing manner?

I had though of doing: A sends a message to B, C, and D asking for the
data. However, B, C, and D may have each changed independently of each
other by the time they receive the request for data and/or by the time
they are able to send the message to A. So, how do you deal with
situations like this in a message passing paradigm?
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