1. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Erlang Erricsson and Ellemtel Computer Science Laboratories http://www.erlang.org Heather Conboy and Shangzhu Wang University of Massachusetts Amherst

2. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 2 What is Erlang for ? For programming concurrent, real-time, and distributed fault-tolerant systems

3. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 3 Erlang Highlights Declarative High abstraction level Dynamic data typing Single assignment variables Pattern matching (symbolic) Real time garbage collection Concurrency Robustness Distribution

4. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 4 A Sequential Erlang Program -module(tut1). -export([fac/1, mult/2, area/1], list_len/1, new_person/2). -record(person, {name, age}). fac(1) -> 1; fac(N) -> N * fac(N - 1). mult(X, Y) -> X * Y. area({square, Side}) -> Side * Side; area({circle, Radius}) -> 3 * Radius * Radius; area(Other) -> {invalid_object, Other}. list_length([]) -> 0; list_length([First | Rest]) -> 1 + list_length(Rest). new_person(Name, Age) -> #person{name=Name, age=Age}. Module Function Variable Atom Tuple List Record > c(tut1). > tut1:area({square,3}). > 9 Pattern matching

5. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 5 Erlang Highlights Declarative Concurrency Simple abstraction Massive concurrency and lightweight processes Efficient message passing (asynchronous) Processes share nothing Robustness Distribution

6. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 6 Creating a New Process Pid2 = spawn(Mod, Fun, Args). %Pid2 is only known to Pid1 register(myprocessname, Pid2). unregister(myprocessname). whereis(myprocessname). registered().

7. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 7 Creating a New Process (an example) -module(tut2). -export([start/0, say_something/2]). say_something(What, 0) -> done; say_something(What, Times) -> io:format ("~p~n", [What]), say_something(What, Times - 1). start() -> spawn(tut2, say_something, [hello, 3]), spawn(tut2, say_something, [goodbye, 3]).

8. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 8 Message Passing From and Msg become bound when the message is received (From = A, Msg = foo) Selective message passingWildcard message passing

9. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 9 Message Passing – an echo process -module(echo). -export([go/0, loop/0]). go() -> Pid2 = spawn(echo, loop, []), Pid2 ! {self(), hello}, receive {Pid2, Msg} -> io:format("P1 ~w~n",[Msg]) end, Pid2 ! stop. loop() -> receive {From, Msg} -> From ! {self(), Msg}, loop(); stop -> true end. AB AB {A, Msg} Msg

10. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 10 Message Passing – Guards and Timeouts receive -> msg1 when BoolExp -> … msg2 -> … after 5000 -> … end. Receive operations can have boolean guards In fact, functions, too. Timeout after infinity after 0

11. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 11 Processes - Micellanies Process scheduling process_flag(priority, Pri) Pri = normal/low Process groups All Erlang processes are arranged in a tree group_leader(), group_leader(Leader, Pid) Process dictionary Local to each process

12. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 12 -module(bank_server). -export([start/0, server/1]). start() -> register(bank_server, spawn(bank_server, server, [[]])). server(Data) -> receive {From, {deposit, Who, Amount}} -> From ! {bank_server, ok}, server(deposit(Who, Amount, Data)); {From, {ask, Who}} -> From ! {bank_server, lookup(Who, Data)}, server(Data); {From, {withdraw, Who, Amount}} -> case lookup(Who, Data) of undefined -> From ! {bank_server, no}, server(Data); Balance when Balance > Amount -> From ! {bank_server, ok}, server(deposit(Who, -Amount, Data)); _ -> From ! {bank_server, no}, server(Data) end end. A Banking Example A Concurrent Banking Example bank_server

13. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 13 -module(bank_client). -export([ask/1, deposit/2, withdraw/2]). ask(Who) -> call_bank({ask, Who}). deposit(Who, Amount) -> call_bank({deposit, Who, Amount}). withdraw(Who, Amount) -> call_bank({withdraw, Who, Amount}). call_bank(Msg) -> bank_server ! {self(), Msg}, receive {bank_server, Reply} -> Reply end. A Banking Example (cont’d) A Concurrent Banking Example bank_client

14. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 14 -module(bank_client). -export([ask/1, deposit/2, withdraw/2]). ask(Who) -> call_bank({ask, Who}). deposit(Who, Amount) -> call_bank({deposit, Who, Amount}). withdraw(Who, Amount) -> call_bank({withdraw, Who, Amount}). call_bank(Msg) -> bank_server ! {self(), Msg}, receive {bank_server, Reply} -> Reply end. A Banking Example (cont’d) What if the client or server crashes? A Concurrent Banking Example bank_client

15. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 15 Erlang Highlights Declarative Concurrency Robustness Explicit error handling mechanisms Allow errors to be caught, propagated, trapped Distribution

16. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 16 Types of Errors A match operation may fail A BIF may be evaluated with incorrect arguments Arithmetic expression errors A process may crash …

17. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 17 Error Handling Mechanisms Monitoring the evaluation of an expression catch, throw Monitoring the behavior of other processes link/1, spawn_link/3 process_flag(trap_exit, true) Trapping evaluation of undefined functions error_handler:undefined_call(Module, Func, Args)

18. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 18 Catch and Throw Protecting sequential code from errors (catch Expression) If there is an evaluation error, returns {‘Exit’, Reason} Otherwise, returns the value of Expression Exit directly from a function with an error (catch combined with throw) foo(1) -> hello; foo(2) -> throw({myerror, abc}); foo(3) -> tuple_to_list(a); foo(4) -> exit({myExit, 222}). demo(X) -> case catch foo(X) of {myerror, Args} -> throw({user_error, Args}); {'EXIT', What} -> throw({caught_error, What}); Other -> Other end.

19. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 19 Termination of processes Termination of Erlang processes Normal exit exit(normal) runs out of things to do Abnormal exit When encounters a runtime error exit(Reason) Links (bidirectional) link(Other_pid), spawn_link When a process terminates it sends a signal (pid and reason) to all the processes it’s linked with A normal exit signal gets ignored (by default) An abnormal exit signal gets propagated, processes linked are killed, messages are dropped Exit signals can be trapped by a receiving process process_flag(trap_exit, true)

20. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 20 Error Propagation and Trapping Error propagation Error trapping receive {'EXIT', P1, Why} ->... end

21. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 21 Error Handling in Layers Error management in layers

22. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 22 - module(bank_server). -export([start/0, start_server/1]). start() -> register(bank_server, spawn(bank_server, start_server, [[]])). start_server(Data) -> process_flag(trap_exit, true), server(Data). server(Data) -> receive {From, {deposit, Who, Amount}} -> link(From), From ! {bank_server, ok}, server(deposit(Who, Amount, Data)); {From, {ask, Who}} -> link(From), From ! {bank_server, lookup(Who, Data)}, server(Data); {From, {withdraw, Who, Amount}} -> link(From), case lookup(Who, Data) of … end {‘EXIT’, From, _} -> unlink(From), io:format(“Bank client ~p is down.~n”, [From]), server(Data) end. The Banking Example with Error Handling A Concurrent Banking Example with error handling the server

23. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 23 Trapping Evaluation of Undefined Functions The error_handler module error_handler:undefined_call(Module, Func, Args) is called when an undefined function is called Specialized error handler module can be defined Set process_flag(error_handler, MyErrorHandlerModule) Then, MyErrorHandlerModule:underfined_call will be called instead

24. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 24 Erlang Highlights Declarative Concurrency Robustness Distribution Transparent distribution Run on a network of Erlang nodes

25. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 25 Distributed Programming System runs on a network of Erlang nodes nodes: an executing Erlang system which can take part in distributed transactions Create processes on a specific node

26. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 26 Distributed Programming - BIFs spawn(Node, Mod, Func, Args) spawn_link(Node, Mod, Func, Args) monitor_node(Node, Flag) be notified with a {nodedown, Node} message if Node fails or if the network connection to Node fails. node() Returns our own node name. nodes() Returns a list of the other known node names. node(Item) Returns the node name of the origin of Item where Item can be a Pid, reference or a port. disconnect_node(Nodename) Disconnects us from the node Nodename.

27. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 27 Distributing the Banking Example A Distributed Banking Example The server - module(bank_server). -export([start/0, server/1]). start() -> register(bank_server, spawn(bank_server, start_server, [[]])). start_server(Data) -> process_flag(trap_exit, true), server(Data). server(Data) -> receive {From, {deposit, Who, Amount}} -> link(From), From ! {bank_server, ok}, server(deposit(Who, Amount, Data)); {From, {ask, Who}} -> link(From), From ! {bank_server, lookup(Who, Data)}, server(Data); {From, {withdraw, Who, Amount}} -> link(From), case lookup(Who, Data) of … end {‘EXIT’, From, _} -> unlink(From), io:format(“Bank client ~p is down.~n”, [From]), server(Data) end.

28. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 28 -module(bank_client). -export([ask/1, deposit/2, withdraw/2]). head_office() -> 'bank_server@super.eua.ericsson.se'. ask(Who) -> call_bank({ask, Who}). deposit(Who, Amount) -> call_bank({deposit, Who, Amount}). withdraw(Who, Amount) -> call_bank({withdraw, Who, Amount}). call_bank(Msg) -> Headoffice = head_office(), {bank_server, Headoffice} ! {self(), Msg}, receive {bank_server, Reply} -> Reply end. A Distributed Banking Example the client Distributing the Banking Example

29. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 29 -module(bank_client). -export([ask/1, deposit/2, withdraw/2]). head_office() -> 'bank@super.eua.ericsson.se'. ask(Who) -> call_bank({ask, Who}). deposit(Who, Amount) -> call_bank({deposit, Who, Amount}). withdraw(Who, Amount) -> call_bank({withdraw, Who, Amount}). call_bank(Msg) -> Headoffice = head_office(), monitor_node(Headoffice, true), {bank_server, Headoffice} ! {self(), Msg}, receive {bank_server, Reply} -> monitor_node(Headoffice, false), Reply; {nodedown, Headoffice} -> no end. Distributing the Banking Example A Distributed Banking Example with error handling the client

30. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 30 Other Erlang Features Runs on virtual machines Can be compiled natively Interpreter Incremental real-time garbage collector Support soft real-time programming with responsive time in milliseconds Hot code loading Primitives to allow code to be replaced in a running system Allow old and new versions of code to execute at the same time

31. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 31 Other Erlang Features External interfaces (ports) Provide byte stream interfaces to external UNIX processes (binary) Ports act as normal Erlang processes open_port(Portname, Portsettings) Erlang Application Outside world port

32. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 32 Other Erlang Features (cont’d) Last call optimization Allows functions to be evaluated in constant space A tail recursive function do not accumulate any pending operations before a recursive call length([_ | T]) -> 1 + length(T); length([]) -> 0. length1(L) -> length1(L, 0). length1([_|T], N) -> length1(T, 1 + N); length1([], N) -> N.

33. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 33 Other Erlang Features (cont’d) References R = make_ref(). Guaranteed globally unique objects e.g., can be used as end_to_end confirmation in a client-server model Binaries Used to store an area of untyped memory Can be converted to and from other data types

34. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 34 Erlang Domains Originally targeted at telecom applications Massively multi-player online games Banking system Robust, scalable web services Highly multi-threaded user interfaces

35. U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 35 References http://www.erlang.org