[erlang-questions] Erlang processes and state management design

Alexander Lamb <>
Wed Oct 24 12:00:31 CEST 2007

Hello again,

So, I am gradually understanding the design philosophy of an Erlang  
On the one hand, functionnal programming is about having no state.  
However, in the situation of a Web application, you obviously need to  
keep some state otherwise you are back to PhP style (in the sense you  
are recalculating everything at each request).

I went back to Joe's book and read again the chapter on processes.  
And, as someone suggested earlier, I realized the state of an Erlang  
program can actually be kept in a process. So when designing an  
application, you indeed model the processes, then the state of the  
processes, then the messages.

So for example, if I create a process for each user who logs in to a  
program. Upon creation of the process I could fetch all the features  
and worklists the user has, then keep them in his process until he  
logs out (or after a given long timeout).

I can do that for every task which needs to keep a state between  

Therefore, I have a question: where is it usuall or reasonnable to  
keep the state of a process?

1) in the parameters of the function we spawn
2) in the process dictionnary
3) in an ETS structure
4) in a in-memory mnesia structure

I suppose it really depends on the amount of data and how it will be  
modified (and if we want to keep persistent some session data after  
the process is killed).

Tell me if I am wrong, but the first two solutions will put data on  
the process stack and will be destroyed when the process is killed.  
The two other solutions will have to be cleaned out when the process  
is killed.

Are there some guidelines?

For a user process, I might have 30 or 40 features in a list, some  
values about his account, etc...
For a process managing (for example) browsing patients in a list, I  
will have the list of patients with the visits for the currently  
selected patient, etc...

Thanks for any hints!

Alexander Lamb
Founding Associate

4 ch. de la Tour de Champel
1206 Geneva

Tel:  022 347 77 37
Fax: 022 347 77 38


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