gproc

James Churchman <>
Sat Feb 12 20:35:21 CET 2011


Firstly, gprocs great :-) should really come as part of the erlang distro!

but i have two questions :

"First" : 

-------
first/1

first(Type::type()) -> key() | '$end_of_table'


Behaves as ets:first(Tab) for a given type of registration object.

See http://www.erlang.org/doc/man/ets.html#first-1. The registry behaves as an ordered_set table.

-------

Why does it behave as an ordered set? does this have any advantages ? i see with select/123 you can pattern match but even in this case does it help much ? and does begin ordered strongly effect performance, especially as i am only ever using it as a (single) process registry? can you pick an unordered set if you don't need it?


"Second" :

I want to do a fairly common pattern of registering a process and using it, unless its already registered where i use the registered one

so for a local registration : 

gproc:reg({n, l,"ID"}).

however on a second (and therefor "failing" ) registration it gives 

exception error: bad argument: [{gproc,chk_reply,{reg,{n,l,ok2},undefined}},         {erl_eval,do_apply,5},     {shell,exprs,7},     {shell,eval_exprs,7},      {shell,eval_loop,3}]

i can obviously catch this, but it would be nicer if it did the genserver style of returning {error , already_exists , PID / Value_Stored_At_That_Key}

the alternate is to do a lookup first, but as ~98 % of the time in my case it wont be registered this seems a bit counter intuitive

also in either case, there is a small race condition in that :

1) i look up the process, its not there, but before i register it, someone else did (so it throws... ) 
2) register the process, it exists, look it up, and in that time the process has ended its usefulness and has shutdown, so when i message it, its gone

in both cases i can add extra code to register a 2nd time etc... but it seems a tad excessive

are there better ways of doing things that im missing, or is this simply the way things are in a distributed multithreaded environment :-)


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