Camtool - monitoring realtime information

1. Camtool - monitoring realtime information • webcam images for currently observing stations • taken from remote URL, cached in Erlang DB • webcam definitions stored in MySQL database • URL, associated databasekey, refreshrate • TSYS plots • taken from logfile monitoring utility’s gnuplot frontend • also cached locally in Erlang DB • generate webpage(s) with dynamic content • serve the images from Erlang i.s.o. disk • fault tolerant: if process dies, restart it

2. Startup of the utility start() -> % gen_servers make interfacing to a % piece of code very simple! % They are trivial to create and GREAT % our mnesia database interface, a gen_server dumpstore:start(), % virtual time server, also a gen_server nowserver:start_link(), % these are supervisors webcammonitor:start_link(), tsysmonitor:start_link(), % webserver, mapping URLs to Erlang calls webtool:start( standard_path, [{port, 8000}, {bind_address, {0,0,0,0}}] ).

3. a supervisor: webcammonitor init(_) -> {one_for_one, 10, 1}, [ % This is the list of (static) ChildSpecifications % {Module, Function, [Arguments]} % the database connection first {dbconn, {mysql, start_link,[pool,”Host”, “User”, “Pass”, “DB”]}, permanent, 1000, worker, []}, % Poll(*) JSON from “.py” URL for current status % implemented as a gen_server {jsonneer, {jsonserver, start_link, []}, permanent, 1000, worker, []}, % Poll(*) the current status from the jsonserver % also implemented as a gen_server {monitorloop, {webcamloop, start_link, []}, permanent, 1000, worker, []} ] }. (*) In future version maybe publish-subscribe architecture

4. a gen_server: jsonserver % Handle a clock-tick, fetch new “current” data. handle_cast(tick, State) -> % Get the current time {{Y,M,D}, {HH,MM,SS}} = nowserver:datetime(), % Call a pythonscript over HTTP (queries SQLite DB and returns result as JSON) B = “http://beoserv-m.jivepci:8090/current_vlbi.py?datetime=%d-%d-%d %d:%d:%d”, Url = io_lib:format(B, [Y,M,D,HH,MM,SS]), % Actually GET the json JSON = case http:request(Url) of % only accept http-status “200” (succesfully retrieved url) {ok, {{_200,_}, _, Body}} -> Body; _ -> [] end, % And parse it Obj = case rfc4627:decode(JSON) of {ok, {obj, KVList}, _} -> KVList; % Parsed into a [{Key, Value}[,{Key,Value}]] {ok, List, _} -> [{stations, List}]; _ -> [{stations, []}] end, {noreply, dict:store(currentjson, Obj, State)}.

5. Dynamic supervision: add/remove childs otf % Experiment stayed the same, now add/remove watchers update(NewExp, NewExp, CurrentStations, State) -> OldSet = sets:from_list(dict:fetch(stations, State)), NewSet = sets:from_list(CurrentStations), Removed = sets:subtract(OldSet, NewSet), Added = sets:subtract(NewSet, sets:intersection(OldSet,NewSet)), S0 = unwatch_tsys(sets:to_list(Removed), State), S1 = watch_tsys(sets:to_list(Added), S0) % dynamically add a child to the supervisor watch_tsys([Atom|Rest], State) -> % call helper function which creates a “ChildSpecification” Child = tsysgetter:mk(Atom, Expt, Station, DBKey), supervisor:start_child(?TSYSMONITOR, Child) % And remove it from supervision unwatch_tsys([Atom|Rest], State) -> supervisor:terminate_child(?TSYSMONITOR, Atom), supervisor:delete_child(?TSYSMONITOR, Atom),

6. mnesia - the database It is transactional - enforced and working. get_data(Id) -> % query the datenbank for a record where the lumpdata is still alive % (i.e. its “age” is <= Now + records’ “time-to-live” (ttl) Fun = fun() -> Now = nowfunc(), qlc:eval( qlc:q( [X || X <- mnesia:table(Table), X#lump.id =:= Id, Now<X#lump.last_refresh+X#lump.ttl] ) ) end, case mnesia:transaction( Fun ) of % data in record “too old” {atomic, []} -> {reply, stale_data, Table}; % Still up-to-date {atomic, [R]} -> {reply, R#lump.data, Table}; % something went wrong {aborted, Reason} -> {reply, {error, Reason}, Table} end;

7. mnesia - the database It stores binary data, actually any data. % From the webcam-image-getter-process Url = “http://www.jb.man.ac.uk/common/upgrade15m.jpg”, case http:request(Url) of % Only accept statuscode 200 {ok, {{_,200,_}, _Headers, Jpeg}} -> dumpstore:update(DBKey, Jpeg, Now); _ -> error end, % From the tsys-image-getter-process case rpc:call('plotserver@services.jive.nl', gnuplot, make_plot, [....]) of {ok, Png} -> dumpstore:update(DBKey, Png, Now); {error, Why} -> Why; _ -> fail_but_buggered_if_we_know_why end,

8. webtool - mapping URLs to Erlang fn calls -compile(export_all). % return configdata for the “webtool” application webtool(_Args) -> {camtool, [{web_data,{"CamTool","/webcams/camtool/camtool"}}, {alias, {erl_alias, "/webcams", [camtool]}}]}. % The {alias, {erl_alias ...}} statement maps URLs of the form % “/webcams/camtool/function?arguments” % into the Erlang function call [Module:Function(Arguments)] % camtool:function(WebserverEnvironment, Arguments) % http://services.jive.nl:8000/webcams/camtool/image?id=tsys_Mc&foo=bar camtool:image(Env, Args) -> case get_input_data(Args, "id") of undefined -> [head(),”Please give an Id”,closing()]; ImgId -> case dumpstore:get_lump(list_to_atom(ImgId)) of undefined -> [head(), “Invalid ImageId given”, closing()]; R -> MimeType=R#lump.mimetype, Data=R#lump.data, "Content-Type: "++MimeType++"\r\n\r\n"++Data end end.