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Event-driven command execution |
The example classes discussed in this section appear in the subdirectorypolling of the example directory |
In the preceding examples each participant in a communication had to get ready to send or receive at specific stages of its life. Although this did not preclude asynchronous communication, it is sometimes desirable to make the scheme even more asynchronous, and control more decentralized, by letting each system simply specify certain communication events that it wants to monitor, and certain commands to be executed on occurrence of the specified events.
The commands are objects, instances of a general-purpose class COMMAND or its proper descendants. Class COMMAND has, among its features, a procedureexecute which executes the current command; some commands are undoable and have anundo procedure.
In EiffelNet the possible events associated with a socket will be of three kind: a read event; a write event; or a special event (out of bounds operation). The command classes will be descendants of POLL_COMMAND, an heir of COMMAND.
The example uses four command classes: CLIENT_DATAGRAM_READER, CLIENT_DATAGRAM_WRITER and their counterpart for servers, representing operations that must be triggered in the case of a read event and a write event.
Here is the reader command for clients:
class CLIENT_DATAGRAM_READER inherit POLL_COMMAND redefine active_medium end creation make feature active_medium: NETWORK_DATAGRAM_SOCKET execute is -- Obtain a packet of ten characters and print them. local rec_pack: DATAGRAM_PACKET i: INTEGER do rec_pack := active_medium.received (10, 0) io.putint (rec_pack.packet_number) io.new_line from i := 0 until i > 9 loop io.putchar (rec_pack.element (i)); i := i + 1 end io.new_line end end
Theexecute procedure reads a packet of ten characters and prints these characters. Its counterpart in the writing command will produce these ten packets:
class CLIENT_DATAGRAM_WRITER inherit POLL_COMMAND redefine active_medium end BASIC_ROUTINES creation make feature active_medium: NETWORK_DATAGRAM_SOCKET execute is -- Make a packet with characters 'a' to 'k' in successive positions. local sen_pack: DATAGRAM_PACKET ccode: INTEGER do create sen_pack.make (10) from ccode := charcode ('a') until ccode > charcode ('k') loop sen_pack.put_element (charconv (ccode), ccode -- charcode ('a')) ccode := ccode + 1 end sen_pack.set_packet_number (1) active_medium.send (sen_pack, Void, 0) end end
Once the commands have been defined, it suffices for the server and the client to associate instances of these commands with the appropriate.
The abstraction needed for this purpose is provided by class MEDIUM_POLLER. An instance of this class knows about a number of commands, each associated with a certain socket in read, write or special event mode. By applying procedureexecute to such a medium poller, you direct it to monitor these sockets for the corresponding events and to execute the command associated with each event that will be received. Procedureexecute takes two integer arguments: the maximum number of sockets to monitor, and the timeout in milliseconds.
Here is the server built with this mechanism:
class POLLING_SERVER creation make feature make is -- Create read and write commands, attach them to a poller, -- set up the poller for execution. local soc: NETWORK_DATAGRAM_SOCKET poller: MEDIUM_POLLER readcomm: SERVER_DATAGRAM_READER writecomm: SERVER_DATAGRAM_WRITER do create soc.make_server_by_port (6530) create poller.make create readcomm.make (soc); poller.put_read_command (readcomm) create writecomm.make (soc); poller.put_write_command (writecomm) poller.make_read_only; poller.execute (15, 20000) poller.make_write_only; poller.execute (15, 20000) soc.close end end
Proceduremake creates three objects: a socket, which it associates with a specific port; a poller; and a read command (an instance of SERVER_DATAGRAM_READER), which it attaches to the socket. It then enters the read command into the poller, and does the same thing with a write command. It sets up the poller to accept read commands only and then executes the poller; this will enable the server to get the read event triggered by the client's write command (as it appears below in the text of class POLLING_CLIENT). Then the server reverses the poller's set-up to write-only, and callsexecute again.
The proceduresmake_read_only andmake_write_only are creation procedures, so that it is possible in a single instruction to create a poller and set it up for read-only or write-only, as increatepollerl.make_read_only. For clarity, however, the above class and the next separate calls to these procedures from the creation of the poller, which usesmake as creation procedure.
The client follows the same scheme, reversing the order of read and write operations:
class POLLING_CLIENT creation make feature make is -- Create read and write commands, attach them to a poller, -- set up the poller for execution. local soc: NETWORK_DATAGRAM_SOCKET poller: MEDIUM_POLLER readcomm: DATAGRAM_READER writecomm: DATAGRAM_WRITER do create soc.make_client_by_port (6530, "serverhost") create poller.make create readcomm.make (soc) poller.put_read_command (readcomm) create writecomm.make (soc) poller.put_write_command (writecomm) poller.make_write_only poller.execute (15, 20000) poller.make_read_only poller.execute (15, 20000) soc.close rescue if soc /= Void and then not soc.is_closed then soc.close end end end
Although the example uses the event-driven mechanisms of EiffelNet, it is still relatively deterministic in that it follows a precise protocol defined by a strict sequence of read and write operations on both sides. This is why every call toexecute is preceded by a call to eithermake_read_only ormake_write_only to set up the poller in the appropriate mode.
A less deterministic scheme may often be desirable, where you simply enter a number of commands (read, write, out of bounds processing) into a poller and then wait for arbitrary events to occur and trigger commands. There is no need with this scheme to know in advance the order in which events may occur: a read event will trigger the command entered into the poller throughput_read_command; a write event will trigger the command entered throughput_write_command.
To achieve this behavior, simply create the poller usingmake as creation procedure. This will set up the poller so as to accept all socket events, and enter into event-driven command execution by callingexecute on the poller.
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