The Linux Programming Interface: a linux and unix system Programming Handbook
Using a single message queue for server and clients
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- Figure 46-2
| Using a single message queue for server and clients
Using a single message queue may be suitable when the messages exchanged between servers and clients are small. However, note the following points: z Since multiple processes may attempt to read messages at the same time, we must use the message type (mtype) field to allow each process to select only those messages intended for it. One way to accomplish this is to use the client’s process ID as the message type for messages sent from the server to the client. The client can send its process ID as part of its message(s) to the server. Fur- thermore, messages to the server must also be distinguished by a unique message type. For this purpose, we can use the number 1, which, being the process ID of the permanently running init process, can never be the process ID of a client process. (An alternative would be to use the server’s process ID as the message type; however, it is difficult for the clients to obtain this information.) This numbering scheme is shown in Figure 46-2. z Message queues have a limited capacity. This has the potential to cause a couple of problems. One of these is that multiple simultaneous clients could fill the message queue, resulting in a deadlock situation, where no new client requests can be submitted and the server is blocked from writing any responses. The other problem is that a poorly behaved or intentionally malicious client may fail to read responses from the server. This can lead to the queue becoming clogged with unread messages, preventing any communication between clients and server. (Using two queues—one for messages from clients to the server, and the other for messages from the server to the clients—would solve the first of these problems, but not the second.) Figure 46-2: Using a single message queue for client-server IPC Using one message queue per client Using one message queue per client (as well as one for the server) is preferable where large messages need to be exchanged, or where there is potential for the message queue Client sends request (mtype = 1, mtext includes client PID) 1 Server sends response (mtype = PID of client) 3 Server reads request (select msgtyp = 1) 2 Client reads response (select msgtyp = own PID) 4 Client Server System V Message Queues 955 problems listed above when using a single message queue. Note the following points regarding this approach: z Each client must create its own message queue (typically using the IPC_PRIVATE key) and inform the server of the queue’s identifier, usually by transmitting the identifier as part of the client’s message(s) to the server. z There is a system-wide limit ( MSGMNI ) on the number of message queues, and the default value for this limit is quite low on some systems. If we expect to have a large number of simultaneous clients, we may need to raise this limit. z The server should allow for the possibility that the client’s message queue no longer exists (perhaps because the client prematurely deleted it). We say more about using one message queue per client in the next section. tải về 6.93 Mb. Chia sẻ với bạn bè của bạn:
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