一. 我會接觸Erlang的緣由
1.RFID Middleware
2.jabber (xml::stream http://zh.wikipedia.org/wiki/Jabber)
3.ejabber (http://www.process-one.net/en/ )
二. 現在的商業環境(web server)所面臨的問題
1.連線的數量不斷的攀升
2.連線的時間很長
傳統上httpd 使用Prefork的方式來解決,短時間時密集連線的問題,在現在的環境愈到了嚴重的挑戰,比如: HTTP_Streaming、Server Push、COMET 這些需要長時間連線的架構,使得httpd 能夠服務的連線變少了,而fork process 最大的問題是,他所需要佔用記憶體的空間過於龐大,於是其他的伺服器架構崛起(lighthttpd ghttpd …)
The C10K problem( http://www.kegel.com/c10k.html )
It's time for web servers to handle ten thousand clients simultaneously, don't you think? After all, the web is a big place now.
And computers are big, too. You can buy a 1000MHz machine with 2 gigabytes of RAM and an 1000Mbit/sec Ethernet card for $1200 or so. Let's see - at 20000 clients, that's 50KHz, 100Kbytes, and 50Kbits/sec per client. It shouldn't take any more horsepower than that to take four kilobytes from the disk and send them to the network once a second for each of twenty thousand clients. (That works out to $0.08 per client, by the way. Those $100/client licensing fees some operating systems charge are starting to look a little heavy!) So hardware is no longer the bottleneck???
三. Concurrent Programming
1. fork
原始的程式
(程式+資料) --fork(複製一份)(程式+資料)
當程式fork 後,child 繼承原來的資料,此後彼此不相關,如果要傳遞資訊,需要使用pipe sharememory 或是 unix socket 來做資料交換
2. thread
事實上在Linux 系統下，執行緒只是一個light weight process：Linux 核心是以fork() system call 來產生一個新的行程（process），而執行緒是以clone() system call 產生的。fork()和clone()的差別只是在clone()可以指定和父行程共用的資源有哪些，當所有資源都和父行程共用時就相當於一個執行緒了。因為Thread 的使用會讓子父行程共用資源,因此非常容易引發dead lock / race condition …這類的問題
3. lightweight Threads ( http://www.defmacro.org/ramblings/concurrency.html)
Erlang process 是一個輕量級的Thread,因此他可以非常輕易的去開啟或是結束且快速在彼此做切換,因為掀開他的底層,他只是一個簡單的function罷了,process節省了大量的context switching浪費僅在一些function上做切換的動作(Erlang 的Thread 是 vm level thread)
這份文件簡單的提到了Erlang的概觀
http://mirror.linux.org.au/pub/linux.conf.au/2007/video/talks
/252.pdf
四. Erlang ( http://www.erlang.org/ )
1.以下是 about Erlang 對他自己的簡述
Erlang is a programming language which has many features more commonly associated with an operating system than with a programming language: concurrent processes, scheduling, memory management, distribution, networking, etc.
The initial open-source Erlang release contains the implementation of Erlang, as well as a large part of Ericsson's middleware for building distributed high-availability systems.
Erlang is characterized by the following features:
Concurrency - Erlang has extremely lightweight processes whose memory requirements can vary dynamically. Processes have no shared memory and communicate by asynchronous message passing. Erlang supports applications with very large numbers of concurrent processes. No requirements for concurrency are placed on the host operating system.
Distribution - Erlang is designed to be run in a distributed environment. An Erlang virtual machine is called an Erlang node. A distributed Erlang system is a network of Erlang nodes (typically one per processor). An Erlang node can create parallel processes running on other nodes, which perhaps use other operating systems. Processes residing on different nodes communicate in exactly the same was as processes residing on the same node.
Soft real-time - Erlang supports programming "soft" real-time systems, which require response times in the order of milliseconds. Long garbage collection delays in such systems are unacceptable, so Erlang uses incremental garbage collection techniques.
Hot code upgrade - Many systems cannot be stopped for software maintenance. Erlang allows program code to be changed in a running system. Old code can be phased out and replaced by new code. During the transition, both old code and new code can coexist. It is thus possible to install bug fixes and upgrades in a running system without disturbing its operation.
Incremental code loading - Users can control in detail how code is loaded. In embedded systems, all code is usually loaded at boot time. In development systems, code is loaded when it is needed, even when the system is running. If testing uncovers bugs, only the buggy code need be replaced.
External interfaces - Erlang processes communicate with the outside world using the same message passing mechanism as used between Erlang processes. This mechanism is used for communication with the host operating system and for interaction with programs written in other languages. If required for reasons of efficiency, a special version of this concept allows e.g. C programs to be directly linked into the Erlang runtime system.
2.Erlang 語言上的概觀
書籍: ( http://pragmaticprogrammer.com/titles/jaerlang/index.html )
[ Sequential Erlang ]
Exam1:
Consider the factorial function N! defined by:
N!=N*(N-1) when N>0
N!=1 when N=0
-module(math1).
-export([fac/1]).
fac(N) when N > 0 -> N * fac(N-1);
fac(0)-> 1.
Exam2:
-module(math2).
-export([sum1/1, sum2/1]).
sum1([H T]) -> H + sum1(T);
sum1([]) -> 0.
sum2(L) -> sum2(L, 0).
sum2([], N) -> N;
sum2([H T], N) -> sum2(T, H+N).
[ Concurrency Programming ]
Exam3:
-module(concurrency).
-export([start/0, say /2]).
say (What, 0) ->
done;
say (What, Times) ->
io:format("~p~n", [What]),
say_something(What, Times - 1).
start() ->
spawn(tut14, say, [hello, 3]),
spawn(tut14, say, [goodbye, 3]).
Exam4:
-module(area_server).
-export([loop/0]).
loop() ->
receive
{rectangle, Width, Ht} ->
io:format("Area of rectangle is ~p~n",[Width * Ht]),
loop();
{circle, R} ->
io:format("Area of circle is ~p~n", [3.14159 * R * R]),
loop();
Other ->
io:format("I don't know what the area of a ~p is ~n",[Other]),
loop()
end.
We can create a process which evaluates loop/0 in the shell:
Pid = spawn(area_server,loop,[]).
Pid ! {rectangle, 6, 10}.
Pid ! {circle, 23}.
Pid ! {triangle,2,4,5}.
4. Erlang –style process or event-based model for actors ( http://lambda-the-ultimate.org/node/1615 )
( http://lamp.epfl.ch/~phaller/doc/haller07coord.pdf )
Message passing
Each process has its own input queue for messages it receives. New messages received are put at the end of the queue. When a process executes a receive, the first message in the queue is matched against the first pattern in the receive, if this matches, the message is removed from the queue and the actions corresponding to the the pattern are executed.
However, if the first pattern does not match, the second pattern is tested, if this matches the message is removed from the queue and the actions corresponding to the second pattern are executed. If the second pattern does not match the third is tried and so on until there are no more pattern to test. If there are no more patterns to test, the first message is kept in the queue and we try the second message instead. If this matches any pattern, the appropriate actions are executed and the second message is removed from the queue (keeping the first message and any other messages in the queue). If the second message does not match we try the third message and so on until we reach the end of the queue. If we reach the end of the queue, the process blocks (stops execution) and waits until a new message is received and this procedure is repeated.
Of course the Erlang implementation is "clever" and minimizes the number of times each message is tested against the patterns in each receive.
五. Erlang相關資源
Website:
Open Source Erlang
http://www.erlang.org
http://www.process-one.net/en/projects/
Mail List:
Erlang-questions -- Erlang/OTP discussions
http://www.erlang.org/mailman/listinfo/erlang-questions
BOOK:
Concurrent programming in Erlang
http://www.erlang.org/download/erlang-book-part1.pdf
Programming Erlang Software for a Concurrent World
http://pragmaticprogrammer.com/titles/jaerlang/index.html
