[erlang-questions] Is it possible to add an filter property for the erlang process?

litao cheng <>
Fri Jun 18 04:08:57 CEST 2010


Richard, Thanks for your entire explanation!
thank you all.

On Fri, Jun 18, 2010 at 6:38 AM, Richard O'Keefe <> wrote:

>
> On Jun 17, 2010, at 4:29 PM, litao cheng wrote:
>
>  hi, buddies.
>> my question is if it's possible to add an "filter" property for the erlang
>> process, so that process can drop the uninterested messages. then we can
>> reduce the needless memory consuming.
>>
>
> The idea is at least 12 years old, so you are definitely
> not the first person to think it might be useful.
> The presentation of "abstract patterns" pointed out that
> they are the idea candidates for filtering; it's safe to
> run them in the context of the sender, so it's possible
> for unwanted messages to (sometimes) be dropped even before
> they are copied.
>
> However, we don't actually need such a mechanism, because we
> can program it using another process.  Instead of
>
>        A ------> B
>
> we use
>
>        A ------> Filter ------> B
>
> where the identity of B is known only to the Filter process,
> so no other process can send to it.
>
> filtered_spawn(Fun, Predicate)
>  when is_function(Fun, 0), is_function(Predicate, 1) ->
>    spawn(fun () ->
>        Pid = spawn_link(Fun),
>        filtered_spawn_loop(Pid, Predicate)
>    end).
>
> filtered_spawn_loop(Pid, Predicate) ->
>    receive Message ->
>        case Predicate(Message)
>          of true -> Pid ! Message
>           ; false -> ok
>        end,
>        filtered_spawn_loop(Pid, Predicate)
>    end.
>
> If you want process B to be able to change the Predicate,
> it's not _enormously_ hard except that now B must know
> Filter as well as Filter knowing B.
>
> filtered_spawn(Fun, Predicate)
>  when is_function(Fun, 0), is_function(Predicate, 1) ->
>    spawn(fun() ->
>        Pid = spawn_link(fun () -> filtered_spawn_target(Fun) end),
>        Pid ! self(),
>        filtered_spawn_loop(Pid, Predicate)
>    end).
>
> filtered_spawn_target(Fun) ->
>    receive Filter ->
>        put(my_filter_pid, Filter),
>        Fun()
>    end.
>
> set_filter(Predicate)
>  when is_function(Predicate, 1) ->
>    get(my_filter_pid) ! {set_filter,self(),Predicate}.
>
> filtered_spawn_loop(Pid, Predicate) ->
>    receive
>        {set_filter,Pid,Predicate1} ->
>            filtered_spawn_loop(Pid, Predicate1)
>      ; Message ->
>            case Predicate(Message)
>              of true -> Pid ! Message
>               ; _ -> ok
>            end,
>            filtered_spawn_loop(Pid, Predicate)
>    end.
>
> This is of course UNTESTED CODE meant solely for the purpose of
> illustrating an idea.
>
> Concurrency ALWAYS seems to introduce nasty subtle points,
> and there's one here.  There is in fact a difference between
> filtering done inside the message sending machinery and
> filtering done using a filter process.  The filter process
> discards messages *only* when it is running; while it is not
> scheduled, any number of messages could build up in its mailbox.
> With filtering done inside the machinery, the filtering cannot
> be delayed like that.
>
> I once read a science fiction story in Analog magazine
> where the plot gimmick was that a machine had been developed
> for restoring people's health by restoring the balance of
> their fields (or some such bafflegab) rather than by means of
> antibiotics &c.  There was a growing problem of "Box addicts";
> people who felt sick if they didn't take a treatment with the
> Box at least every day.  It turned out that some "extinct" disease
> (smallpox? plague?) had been released from an archaeological dig,
> and the "addicts" didn't have a psychological problem, they were
> genuinely sick with a fatal disease that the Box could help them
> survive with but not actually cure.
>
> The point here is that a filtering process or filtering machinery
> is just like the Box.  It helps a seriously sick system keep
> running despite a bug that would otherwise have taken it down.
> I can't help thinking that simply dropping bad messages and
> stumbling on will *prevent* the necessary detection and repair of
> the problem for longer than anyone would want.
>
> The Erlang philosophy is not "keep going at all costs, including
> sanity" but "let it crash".  We want bad messages to *crash*
> something so that we find out.  (This is one of the things UBF
> is for.)
>
> Messages that are a *legal* part of the protocol but currently have
> no effect really ought to be handled by explicit code that accepts
> them and does nothing with them.  It can be hard enough for a
> maintenance programmer to find out what a protocol is without moving
> part of it elsewhere and hiding it.
>
>
>
>> like this:
>> % spawn a process with filter : {foo, _}
>> PidA = spawn(?MODULE, do_something, [], [{filter, {foo, _}}]),
>>
>> % the message {foo, hello} will be saved to PidA message queue, because it
>> match the filter
>> PidA ! {foo, hello},
>>
>> % this message will be dropped
>> PidA ! {bar, world},
>>
>> % reset the filter
>> process_flag(filter, [_|_]),
>>
>> % this message will be stored in message queue
>> PidA ! "hello world",
>> ok.
>>
>> by this feature,  I can write the code like this ( in my logging library,
>> which is similar with python logging module):
>> {ok, _} = logging:start_logger("logname"),
>> Logger = logging:get_logger("logname"),
>> Logger:set_level(?CRITICAL),
>> Logger:debug("log"),
>> Logger:warning("log"),
>> Logger:critical("log"),
>> ok
>>
>> logger is an parameterized module, logging is an gen_server process, in
>> logging module:
>> handle_call({set_level, Level}, _From, State) ->
>>   process_flag(filter, #log_record{level = MsgLevel} when MsgLevel >
>> Level),
>>   {reply, ok, State};
>>
>> in my logging library,  I have two methods to resolve this problem:
>> * dynamic compile the logger module, in the debug, info, warning, error
>> function check if the log allowed
>> * in logging gen_server process handl_xxx function, test if the log record
>> is allowed
>> all two solutions have flew.
>>
>> I want to known if this process filter feature is valuable?
>> thanks
>>
>
>


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