服务间单向通信的选项

Question

I'm searching for different options for implementing communication between a service and other services/applications.

What I would like to do :

I have a service that is constantly running, polling a device connected to a serial port. At certain points, this service should send a message to interested clients containing data retrieved from the device. Data is uncomplicated, most likely just a single string.

Ideally, the clients would not have to subscribe to receive these messages, which leads me to some sort of event 'broadcast' setup (similar to Windows events). The message sending process should not block, and does not need a response from any clients (or that there even are any clients for that matter).

I've been reading about IPC (COM in particular) and windows events, but am yet to come across something that really fits with what I want to do.

So is this possible? If so, what technologies should I be using? If not, what are some viable communication alternatives?

Here's the particulars of the setup:

• Windows 2000/XP environments
• 'Server' service is a windows service, using VC++2005
• Clients would vary, but always be in the windows environment (usual clients would be VC++6 windows services, VB6 applications)

Any help would be appreciated!

Answer 1

Windows supports broadcasting messages, check here . You can SendMessage to HWND_BROADCAST from the service, and receive it in each client.

Answer 2

There are a number of ways to do a broadcast system, but you'll have to either give up reliability (ie, some messages must be lost) or use a proper subscription system.

If you're willing to give up reliability, you can create a shared memory segment and named manual-reset event object. When a new message arrives, write it to the shared memory segment, signal the event object, then close the event object and create a new one with a different name (the name should be in the shmem segment somewhere). Clients open the shmem segment, find the current event object, wait for it to be signaled, then read off the message and new event segment.

In this option, you must be careful to deal with the case of a client reading at the same time as the shmem segment is updated properly. One way to do this is to have two sequence number fields in the shmem segment - one is updated before the new message is written, one after. Clients read the second sequence number prior to reading the message, then re-read both sequence numbers after, and check that they are all equal (and discard the message and retry after a delay if they are not). Be sure to place memory barriers around accesses to these sequence numbers to ensure the compiler does not reorder them!

Of course, this is all a bit hairy. Named pipes are a lot simpler, but a subscription (of a sort) is required. The server calls CreateNamedPipe , then accepts connections with ConnectNamedPipe . Clients use CreateFile to connect to the server's pipe. The server then just loops to send data (using WriteFile ) to all of its clients. Note that you will need to create addititonal instance of the pipe using CreateNamedPipe each time you accept a connection. An example of a named pipe server can be found here: http://msdn.microsoft.com/en-us/library/aa365588(v=vs.85).aspx