Death by Accidental Complexity

37 Pages·2010·1.72 MB·English

Erlang Solutions Ltd
Death by Accidental Complexity
QCon
London, March 12, 2010
Ulf Wiger, CTO Erlang Solutions Ltd.
Ulf Wiger
[email protected]
Brain-dead concurrency
(cid:1)
A.k.a. Embarrassingly Parallel
(cid:1)
E.g. Plain web server
(cid:2)
Minimal dependencies, request/reply pattern
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Mind-blowing concurrency
(cid:1)
Complex, multi-way signalling
(cid:1)
Multiple failure domains
Presence
Billing
Registry
Gateway/broker
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Natural product evolution
(cid:1)
From simple to more complex
(cid:1)
Structuring and encapsulation
should allow us to support this
Copyright 2010 – Erlang Solutions Ltd
Message ordering (1)
(cid:1)
All messages from A to B
must arrive in the same order as they were sent.
A B
a3 a2 a1
a1
a2
a3
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Message ordering (2)
(cid:1)
Oft forgotten rule:
(cid:2)
The language should not force a specific
global order upon the programmer
C
(cid:1)
Common (naïve)
iimmpplleemmeennttaattiioonn::
AA BB
a3 a2 a1
(cid:2)
Event loop dispatches in order of arrival
a1
(cid:2)
Easy to implement
c1
(cid:2)
But forces the programmer to deal with
a2
all possible orderings of events
a3
c2
c3
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Selective Message Reception
(cid:1) (cid:1)
Separate message queues One queue per process
(cid:2) (cid:2)
Ability to select msgs + ability to select msgs
out of order out of order
C
CC
A B
a3 a2 a1
A B a1
a3 a2 a1
c1
a1
a2
Ch 1 a2 c1
a3
a3 c2 Ch 2
c2
c3
c3
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Session Broker Scenario
Telecom ”Half-Call” model
One or more stateful processes
Control/Charging, ...
Interaction with multiple
uncoordinated message sources
MMeessssaaggee sseeqquueenncceess mmaayy
AA BB
(and invariably will) interleave
Resources
A = originating side
B = terminating side
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Programming Experiment
”POTS”: Plain Ordinary
Demo system used in Ericsson’s
Telephony System –
Introductory Erlang Course
Trivial schoolbook
(cid:2) assignment: write a control program example of telephony
for a POTS subscriber loop (as simple as it gets)
We will re-write the control
lloooopp uussiinngg ddiiffffeerreenntt sseemmaannttiiccss..
(cid:2)
Selective message passing
(cid:2)
Event dispatch
Note well: no error handling
(usually the most complex part)
http://github.com/uwiger/pots
Demo...
Copyright 2010 – Erlang Solutions Ltd
Erlang – two approaches
Event-based programming Selective receive FSM
loop(Module, S) -> call(Server, Request) ->
receive Server ! {call, self(), Request},
Msg -> % matches any message receive
S1 = Module:event(Msg, S), {Server, reply, Reply} ->
lloooopp((MMoodduullee,, SS11)) RReeppllyy
end. after 5000 -> % milliseconds
exit(timeout)
end.
We can use Erlang syntax to illustrate both models
(hooray!!)
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