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79 lines
2.7 KiB
Plaintext
79 lines
2.7 KiB
Plaintext
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About the MacroExclusive application
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------------------------------------
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Steve Davies <steve@connection-telecom.com
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The MacroExclusive application was added to solve the problem of
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synchronisation between calls running at the same time.
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This is usually an issue when you have calls manipulating global
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variables or the Asterisk database, but may be useful elsewhere.
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Consider this example macro, intended to return a "next" number -
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each caller is intended to get a different number:
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[macro-next]
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exten => s,1,Set(RESULT=${COUNT})
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exten => s,n,SetGlobalVar(COUNT=$[${COUNT} + 1])
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The problem is that in a box with high activity, you can be sure
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that two calls will come along together - both will get the same
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"RESULT", or the "COUNT" value will get mangled.
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Calling this Macro via MacroExclusive will use a mutex to make sure
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that only one call executes in the Macro at a time. This ensures
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that the two lines execute as a unit.
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Note that even the s,2 line above has its own race problem. Two
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calls running that line at once will step on each other and
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the count will end up as +1 rather than +2.
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I've also been able to use MacroExclusive where I have two Macros
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that need to be mutually exclusive.
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Here's the example:
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[macro-push]
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; push value ${ARG2} onto stack ${ARG1}
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exten => s,1,Set(DB(STACK/${ARG1})=${ARG2}^${DB(STACK/${ARG1})})
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[macro-pop]
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; pop top value from stack ${ARG1}
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exten => s,1,Set(RESULT=${DB(STACK/${ARG1})})
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exten => s,n,Set(DB(STACK/${ARG1})=${CUT(RESULT,^,2)})
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exten => s,n,Set(RESULT=${CUT(RESULT,^,1)})
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All that futzing with the STACK/${ARG1} in the astdb needs protecting
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if this is to work. But neither push nor pop can run together.
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So add this "pattern":
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[macro-stack]
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exten => Macro(${ARG1},${ARG2},${ARG3})
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... and use it like so:
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exten => s,1,MacroExclusive(stack,push,MYSTACK,bananas)
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exten => s,n,MacroExclusive(stack,push,MYSTACK,apples)
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exten => s,n,MacroExclusive(stack,push,MYSTACK,guavas)
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exten => s,n,MacroExclusive(stack,push,MYSTACK,pawpaws)
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exten => s,n,MacroExclusive(stack,pop,MYSTACK) ; RESULT gets pawpaws (yum)
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exten => s,n,MacroExclusive(stack,pop,MYSTACK) ; RESULT gets guavas
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exten => s,n,MacroExclusive(stack,pop,MYSTACK) ; RESULT gets apples
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exten => s,n,MacroExclusive(stack,pop,MYSTACK) ; RESULT gets bananas
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We get to the push and pop macros "via" the stack macro. But only one call
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can execute the stack macro at a time; ergo, only one of push OR pop can
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run at a time.
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Hope people find this useful.
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Lastly, its worth pointing out that only Macros that access shared data
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will require this MacroExclusive protection. And Macro's that you call
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with macroExclusive should run quickly or you will clog up your Asterisk
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system.
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Regards,
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Steve
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