Erlang - An Open Source Language for Robust Distributed Applications

1. Erlang - An Open Source Language for Robust Distributed Applications
   • Dr Lawrie Brown
   • Australian Defence Force Academy
2. Introduction
   • introduce Erlang
   • a functional language
   • for robust, reliable, distributed, near real-time applications
   • developed by the Ericsson Computer Science Laboratories
   • to support new generation telecommunications applications
   • released as open source in late 1998
3. Functional
   • Erlang is a functional language
   • dynamically typed, symbolic data representation
   • single assignent to variables
   • uses pattern matching for variable binding & function selection
   • applications comprise many functions with multiple clauses
   • minimal side-effects simplifies code analysis & testing

     fac(N) when N>0 -> N * fac(N-1);
     fac(0) -> 1.
     

4. Concurrent
   • focus is on soft, real-time, systems
   • have extremely cheap and efficient process spawning
   • use returned process identifier (pid) for all interactions

         Echo = spawn(echo,loop,[]), 
         Echo ! {self(),"Here is a message"}, 
         ....
         loop() ->
           receive  
             {From,Msg} -> From ! Msg, loop()
           end.
     

5. Distributed
   • remote spawn almost identical
   • given pid, all use is independent of locality
   • eases implementation of robust distributed applications

         Echo = spawn(SomeNode,echo,loop,[]), 
         ....
     

6. Language Features
   • Concurrency - extremely light-weight processes with message passing
   • Distribution - easy process creation & interaction on many nodes
   • Robustness - error and exception handlers for fault-tolerance
   • Soft real-time response - handles events in millisecs
   • Hot code upgrade - on running system
   • Incremental code loading - at boot time or as needed
   • External interfaces - to devices, files, network, other processes
7. Advantages
   • faster implementation cycle and time to market
   • more robust implementations
   • especially for distributed, near real-time applications
8. Disadvantages
   • dynamic typing and garbage collection does incur runtime overhead
   • many numerical algorithms are problematic since use mutable data
   • GUI use less efficient compared to other languages
   • can call out to external drivers or programs
   • comes down to selecting the best language for any given task
9. Open Source
   • binary distribution has been available for some time
   • released as open source in late 1998
   • under Erlang Public Licence (derivative of Mozilla licence)
   • release comprises full source for: run-time system, compilers, Mnesia DBMS, libraries and utilities
   • see http://www.erlang.org/
10. Major Library Components

    Mnesia

    fast, distributed, real-time database for Erlang

    Inets

    package of Internet tools, including full HTTP server

    Orber

    a CORBA v2.0 object request broker (ORB)

    SNMP

    extensible SNMP v1/v2 agent and MIB (ASN.1) compiler

11. A Trivial Example

        -module(bankserver).
        -export([start/1]).
        start(Sum) ->
            register(bank,self()),
            account(Sum).
        account(Sum) ->
            receive
              {Pid, Ref, Amount} ->
                NewSum = Sum+Amount,
                Pid ! {Ref,NewSum},
                account(NewSum);
              stop -> nil
            end.
    

12. A Trivial Example cont.

        % spawn a bank account process with initial Sum 1000
        Bank_pid = spawn(BankNode,bankserver,start,[1000]),
        ...
        Ref = make_ref(),
        Bank_pid ! {self(),Ref,17},
        receive
            {Ref,New_balance} ->
                ...
        end,
    

13. A Slightly More Serious Example
    • paper includes full code for minimal HTTP/0.9 server
    • minimal functionality
      • does return requested doc
      • with error handling for bad requests
    • also have a simple application monitor
      • starts the requested application
      • catches and logs errors
      • then restarts application
    • fully featured versions provided in supplied libraries
14. Erlang Applications - Mobility Server
    • intelligent call control system
    • provides personal numbers for Conson PBX mobile users
    • 486 modules, 230000 lines Erlang, 20000 lines C, 39 programmers
    • in production, on time, under budget
    • claimed as one of largest functional programs ever
15. Erlang Applications - AXD301 ATM Switch
    • new generation ATM switch
    • compact, high functionality, robust, scaleable
    • different architectures components use differet languages
      • device interfaces to switching fabric in C
      • web based management interface in Java
      • overall control and management in Erlang
      • each about 150000 lines code
    • now being marketed
16. Application Experiences
    • general comments and experiences
    • time to market
      • shorter time, better quality
    • performance
      • sufficiently good for real telecomms
      • in large systems loss of speed wrt C more than compensated for by ease of development and maintenance
    • real-time garbage collection works
17. And Where do I come In?
    • in my sabbatical in 1997 I looked at safe mobile code
    • specifically what changes would be needed in Erlang
    • interested in functional languages rather than procedural
    • changes needed are:
      • use of capabilities for forgeable pids, ports etc
      • hierarchy of sub-node to support security & usage restrictions
      • support for remote code loading in context
    • evaluated these using prototype which rewrites Erlang source
    • eventually must fold into run-time system
18. Conclusion
    • have briefly introduced Erlang
    • a very interesting, useful language
    • for building large, scaleable, robust, distributed applications
    • available as open source
    • consider it for some types of applications