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    <p>I think the first thing you're going to have to consider is: after your first server has crashed and your client has successfully reconnected to the backup server, how would your client ever know that that the first server has come back on line?</p> <p>I can think of two possibilities: one might be that the backup server might notify the client about the re-appearance of the primary server (e.g. by sending some sort of PRIMARY_SERVER_ONLINE message over the TCP connection, or perhaps simply by closing the TCP connection, with the expectation that that would cause the client to try to connect to the primary server again). </p> <p>The other approach would be to make your client smart enough that it can periodically (e.g. once per minute) try to reconnect to the primary server even while it is using the TCP connection to the backup server. That is doable, but not with a single thread and blocking I/O like your posted code has... (because if your program is blocked in a recv() call, there is no way for it to do anything else like try to connect a TCP connection). You'd need to either use non-blocking I/O and select() (or similar), or asynchronous I/O, or multiple threads, in order to do it properly.</p>
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    1. USJeremy Friesner
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    1. COFrom a bandwidth perspective the first option is almost certainly better. That way only the backup server(s) are polling the primary server. Will your primary server be out of sync once it comes back up? If so it may need to talk to the backup before it restores service anyway. If not then are your servers basically stateless? If so why do you have a primary and a backup...they are just peers. In that case you should be trying to keep clients distributed between them for load reasons and to ease the strain when one fails.
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    2. CO@JeremyFriesner I will try second approach, what I think is that I will try to connect to the main server before I send message to the back up server. Also can you elaborate more on blocking I/O that I have, I'm not really familiar with that. Thanks
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    3. CO@swiftk sockets are created in blocking mode by default, which means that if they can't send/recv data at the moment send()/recv() is called, then the send()/recv() call will not return until there is some data sent/received (i.e. possibly not for a long time). If you'd rather have those calls never block (i.e. so your thread can do something else while you are waiting for the socket to become ready to send/recv), then you can set the socket to non-blocking mode after you create it, via fcntl(fd, F_SETFL, O_NONBLOCK); or under Windows it's unsigned long m=1; ioctlsocket(fd, FIONBIO, &m);
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