19.13 Gateway for Load Balancing

This section describes a gateway, called gatewaymonitor , used for load balancing. Start with tunnelserver of Section 19.6. The gatewaymonitor program takes two ports as command-line arguments: a listening port for client requests and a listening port for server registration requests . The gatewaymonitor program acts like tunnelserver of Section 19.6 except that instead of directing all requests to a particular server, it maintains a list of servers with identical resources and can direct the request to any of those servers. The gatewaymonitor program keeps track of how many requests it has directed to each of the servers. If a connection request to a particular server fails, gatewaymonitor outputs an error message to standard error, reporting which server failed and providing usage statistics for that server. The gatewaymonitor program removes the failed server from its list and sends the request to another server. If the server list is empty, gatewaymonitor sends an HTTP error message back to the client.

A server can add itself to gatewaymonitor 's list of servers by making a connection request to the server listening port of gatewaymonitor . The server then registers itself by sending its host name and its request listening port number. The gatewaymonitor program monitors the client listening port as before but also monitors the server request listening port. (Use select here.) If a request comes in on the server listening port, gatewaymonitor accepts the connection, reads the port information from the server, adds the host and port number to the server list, and closes the connection. The server should send the port number as a string to avoid byte-ordering problems.

Write a server program called registerserver that registers a server with gatewaymonitor as described above. The registerserver takes three or four command-line arguments. The first two arguments are the host name and server registration port number of the gatewaymonitor . The third parameter is the port number that the registered server will listen on for client requests. The optional fourth command-line argument is the name of a host to register. When called with four command-line arguments, registerserver exits after registering the specified host. The four-argument version of registerserver can be used to register an existing web server. If only three command-line arguments are given, registerserver registers itself and waits for requests.

The registerserver should have a canned HTTP response (with a resource) to send in response to all requests. The host name and process ID should be embedded in the resource so that you can tell how the request to the gateway monitor was serviced. Test your program by using a browser with as many as five servers registering with the gateway. Kill various servers and make sure that gatewaymonitor responds correctly.