CPL 2016, week 9 Erlang fault tolerance and distributed programming Oleg Batrashev Institute of Computer Science, Tartu, Estonia April 4, 2016
Overview Previous week ◮ Erlang: functional core and agents Today ◮ Erlang fault tolerance and distributed programming Next weeks ◮ Clojure language and asynchronous Javascript
Fault tolerance 27/50 - General ideas ◮ system process – handles exit signals from linked processes ◮ traps exit messages {’EXIT’,Pid,Why} A
Fault tolerance 27/50 - General ideas ◮ system process – handles exit signals from linked processes ◮ traps exit messages {’EXIT’,Pid,Why} ◮ default: usual process dies if Why � = normal A B
Fault tolerance 27/50 - General ideas ◮ system process – handles exit signals from linked processes ◮ traps exit messages {’EXIT’,Pid,Why} ◮ default: usual process dies if Why � = normal ◮ link – connection between 2 processes ◮ symmetric , must be set explicitly A B
Fault tolerance 27/50 - General ideas ◮ system process – handles exit signals from linked processes ◮ traps exit messages {’EXIT’,Pid,Why} ◮ default: usual process dies if Why � = normal ◮ link – connection between 2 processes ◮ symmetric , must be set explicitly ◮ exit signal – sent to the set of linked processes ◮ when process dies {’EXIT’,B,Why} ◮ when process finishes {’EXIT’,B,normal} A B {’EXIT’,B,Why}
Fault tolerance 28/50 - Links ◮ defines error propagation path between 2 processes ◮ if one dies then another gets exit signal ◮ links are established with ◮ link(B) or ◮ spawn_link(Fun)
Fault tolerance 29/50 - Signals Exit signal ◮ generated when process dies or finishes ◮ {’EXIT’,Pid,Why} ◮ Why=normal if a process just finishes (i.e. recursion ends) ◮ Why=<exception desc> if there was a problem ◮ exit(Why) may be called to stop itself ◮ sent to all linked processes Faking death: exit(Pid2, Why) ◮ sends {’EXIT’,Pid,Why} to process Pid2 ◮ continues exection
Fault tolerance 30/50 - System processes ◮ usual process ◮ dies if receives exit signal from any linked process where Why � = normal ◮ system process ◮ set with process_flag(trap_exit,true) ◮ traps exit signals from linked processes ◮ messages {’EXIT’,Pid,Why} are added to its mailbox ◮ exit signals with Why=kill are not caught at all! ◮ process is killed, even system process ◮ {’EXIT’,Pid,killed} broadcasted to all linked processes (notice that kill is propagated as killed )
Fault tolerance 31/50 - Example (1) on_exit(Pid , Fun) -> spawn(fun () -> process_flag (trap_exit , true), link(Pid), receive {'EXIT ',Pid ,Why} -> Fun(Why) end end ). ◮ creates a process that “monitors” the process with given Pid ◮ upon exit calls the given function Fun
Fault tolerance 32/50 - Example (2) F = fun () -> receive X -> list_to_atom (X) end end. Pid = spawn(F). on_exit(Pid , fun(Why) -> io:format("~p died with ~p~n",[Pid ,Why]) end ). ◮ create a process that transforms lists to atoms ◮ add error handler with on_exit ◮ Now sending Pid ! hello. ◮ results in <0.61.0> died with:{badarg,[{ · · ·
Fault tolerance 33/50 - Summary of exit signals What happens if ◮ a process with given trap_exit (i.e. system or not) ◮ receives the given Exit signal trap_exit Exit signal Action true kill Die: broadcast the exit signal killed to the link set true X Add {’EXIT’,Pid,X} to the mailbox false normal Continue: Do nothing, signal vanishes false kill Die: broadcast the exit signal killed to the link set false X Die: broadcast the exit signal X to the link set
Fault tolerance 34/50 - Idioms for trapping exits 1. Don’t care about new process Pid=spawn(fun () -> ... end) 2. Want to die if new process dies Pid= spawn_link (fun () -> ... end) 3. Want to handle errors if new process dies ... process_flag (trap_exit , true), Pid= spawn_link (fun () -> ... end), loop (...). loop(State) -> receive {'EXIT ', SomePid , Reason} -> %% do something with the error loop(State1) ... end.
Distributed programming 35/50 - Overview ◮ In trusted environment – allow to run any code remotely ◮ Distributed Erlang – all message passing and error handling work automatically ◮ In non-trusted environment – restrict what can be run ◮ lib_chan library – RPC like Erlang specific, automates serialization of Erlang objects; ◮ socket based programming – low-level, but can be used with other languages ◮ Binary data manipulation in Erlang – allows to encode/decode messages for socket based programming
Distributed programming 36/50 Erlang specific - Outline Fault tolerance Distributed programming Erlang specific Distributed Erlang Chan library Binary data manupulation Socket programming Erlang support libraries
Distributed programming 37/50 Erlang specific - Distributed Erlang Erlang node ◮ Erlang node is a separate Erlang VM ◮ may be run on the same or different host ◮ if it fails/exits then other VMs are not directly affected ◮ all communication between two or more nodes is transparent ◮ programmer does not see a difference ◮ except when creating processes or making explicit remote calls ◮ values (numeric, tuples, etc) are copied ◮ agents are provided with proxies to communicate
Distributed programming 38/50 Erlang specific - Distributed Erlang Running nodes ◮ run erl with the argument -sname <nodename> ◮ may run any code from other local node ◮ run erl with the argument -name <nodename@host> ◮ may run on local or remote network ◮ use the same version of the code ◮ nodes must have the same cookie -setcookie <abc> ◮ make sure Erlang Port Mapper Daemon port is available (4369) ◮ choose a range of ports to be used ( -kernel inet_dist_listen_min <port>)
Distributed programming 39/50 Erlang specific - Distributed Erlang Example ◮ rmt.erl -module(rmt ). -export([runme /0]). runme () -> io:format("Running on ~p~n", [node ()]). ◮ Run node a erl -sname a 1> c(rmt ). 2> rmt:runme (). ◮ Run node b erl -sname b 1> rpc:call(a@mycomputer ,rmt ,runme ,[]). Running on a@mycomputer ◮ replace mycomputer with your host name
Distributed programming 40/50 Erlang specific - Distributed Erlang Distribution primitives ◮ built in rpc module rpc:call(Node ,Mod ,Function ,Args) -> Result|{badrpc ,Reason} ◮ built in global module ◮ extended spawn functions, etc spawn(Node ,Fun) -> Pid spawn(Node ,Mod ,Func ,ArgList) -> Pid spawn_link (Node ,Fun) -> Pid spawn_link (Node ,Mod ,Func ,ArgList) -> Pid disconnect_node (Node) -> bool ()| ignored node () -> Node node(Arg) -> Node nodes () -> [Node] Pid ! Msg {RegName ,Node} ! Msg
Distributed programming 41/50 Binary data manupulation - Outline Fault tolerance Distributed programming Erlang specific Distributed Erlang Chan library Binary data manupulation Socket programming Erlang support libraries
Distributed programming 42/50 Binary data manupulation - Binary data ◮ used with data from external programs/sources ◮ double bracket syntax to define binaries (arrays) 2> R=31,G=0,B=0. 3> Color = <<R:5, G:6, B:5>>. <<248,0>> ◮ Integer:NumOfBits ◮ tricky with endianess (big or little) ◮ shorthand for strings 4> <<64,65,66>>. <<"@AB">> ◮ IoList is a list of integers (0..255), binaries, or IoLists 5> Lst = [ <<1,2,3>>,4,[ <<5,6>>,7] ,8]. ◮ convenient, if we want to defer combining into the final binary 6> list_to_binary (Lst ). <<1,2,3,4,5,6,7,8>>
Distributed programming 43/50 Binary data manupulation - BIFs for binaries BIF – built-in function in Erlang ◮ list_to_binary(IoList) ->binary() ◮ many IO functions accept IoLists, so no need to transform ◮ split_binary(Bin,Pos) ->{Bin1,Bin2} ◮ pattern matching for binaries may be more convenient 7> <<Z:3/ binary , Rest/binary >> = <<1,2,3,4,5,6,7,8>>. <<1,2,3,4,5,6,7,8>> 8> Z. <<1,2,3>> 9> Rest. <<4,5,6,7,8>> ◮ term_to_binary(Term) ->Bin ◮ binary_to_term(Bin) ->Term ◮ size(Bin) ->Int
Distributed programming 44/50 Binary data manupulation - The bit syntax Full syntax for binaries ◮ <<E1,E2,...,En>> where Ei are the elements ◮ each element is one of the forms Ei = Value | Value:Size | Value/ TypeSpecifierList | Value:Size/ TypeSpecifierList ◮ where TypeSpecifierList is hypnen separated list of ◮ endianess: big | little | native ◮ sign: signed | unsigned ◮ type: integer | float | binary ◮ Size is in bits for integer/float and in bytes for binaries 13> <<3:16/big -unsigned -integer , 6,7>>. <<0,3,6,7>> 14> <<3:16/ integer , 6,7, 2.1415:64/ float >>. < <0 ,3,6,7 ,64 ,1,33,202,192 ,131 ,18 ,111 >> 15> <<A:2/ binary ,B:16/ little -integer >> = <<1,2,3,0>>.
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