Each virtual server provides connections between the server and clients through one or more HTTP listeners. Each HTTP listener is a listen socket that has an IP address, a port number, a server name, and a default virtual server.
HTTP listeners must have a unique combination of port number and IP address. For example, an HTTP listener can listen on all configured IP addresses on a given port for a machine by specifying the IP address 0.0.0.0. Alternatively, the HTTP listener can specify a unique IP address for each listener, but use the same port.
Since an HTTP listener is a combination of IP address and port number, you can have multiple HTTP listeners with the same IP address and different port numbers (for example, 18.104.22.168:8081 and 22.214.171.124:8082), or with different IP addresses and the same port number (for example, 126.96.36.199:8081 and 188.8.131.52:8081, if your machine was configured to respond to both these addresses).
However, if an HTTP listener uses the 0.0.0.0 IP address, which listens on all IP addresses on a port, you cannot create HTTP listeners for additional IP addresses that listen on the same port for a specific IP address. For example, if an HTTP listener uses 0.0.0.0:8080 (all IP addresses on port 8080), another HTTP listener cannot use 184.108.40.206:8080.
Because the system running the Application Server typically has access to only one IP address, HTTP listeners typically use the 0.0.0.0 IP address and different port numbers, with each port number serving a different purpose. If the system does have access to more than one IP address, each address can serve a different purpose.
By default, when the Application Server starts, it has the following HTTP listeners:
Two HTTP listeners named http-listener-1 and http-listener-2, associated with the virtual server named server. The listener named http-listener-1 does not have security enabled; http-listener-2 has security enabled.
An HTTP listener named admin-listener, associated with the virtual server named __asadmin. This listener has security enabled.
All these listeners use the IP address 0.0.0.0 and the port numbers specified as the HTTP server port numbers during installation of the Application Server. If the Application Server uses the default port number values, http-listener-1 uses port 8080, http-listener-2 uses port 8181, and admin-listener uses port 4849.
Each HTTP listener has a default virtual server. The default virtual server is the server to which the HTTP listener routes all request URLs whose host component does not match any of the virtual servers that are associated with the HTTP listener (a virtual server is associated with an HTTP listener by listing the HTTP listener in its http-listeners attribute).
In addition, specify the number of acceptor threads in the HTTP listener. Acceptor threads are threads that wait for connections. The threads accept connections and put them in a queue, called the connection queue, where they are then picked up by worker threads. Configure enough acceptor threads so that there is always one available when a new request comes in, but few enough so that they do not provide too much of a burden on the system.The connection queue includes both the new connections just accepted by acceptor threads and persistent connections managed by the Keep-Alive connection management subsystem.
A set of request processing threads retrieves incoming HTTP requests from the connection queue and processes the requests. These threads parse the HTTP headers, select the appropriate virtual server, and run through the request processing engine to service the request. When there are no more requests to process, but the connection can be kept persistent (either by using HTTP/1.1 or sending a Connection: keep-alive header), the request processing thread assumes the connection to be idle and passes the connection to the Keep-Alive connection management subsystem.
The Keep-Alive subsystem periodically polls such idle connections and queues those connections with activity into the connection queue for future processing. From there, a request processing thread again retrieves the connection and processes its request. The Keep-Alive subsystem is multi-threaded, as it manages potentially tens of thousands of connections. Efficient polling techniques are used, by dividing the number of connections into smaller subsets, to determine which connections are ready with requests and which of those connections have idled for sufficient time to deem them closed (beyond a maximum permissible Keep-Alive timeout).
The HTTP listener’s server name is the host name that appears in the URLs the server sends to the client as part of a redirect. This attribute affects URLs the server automatically generates; it does not affect the URLs for directories and files stored in the server. This name is normally the alias name if the server uses an alias. If a client sends a Host: header, that host name supersedes the HTTP listener’s server name value in redirects.
Specify a redirect port to use a different port number from that specified in the original request. A redirect occurs in one of these situations:
If a client tries to access a resource that no longer exists at the specified URL (that is, the resource has moved to another location), the server redirects the client to the new location (instead of returning a 404), by returning a designated response code and including the new location in the response’s Location header.
If a client tries to access a resource that is protected (for example, SSL) on the regular HTTP port, the server redirects the request to the SSL-enabled port. In this case, the server returns a new URL in the Location response header, in which the original insecure port has been replaced with the SSL-enabled port. The client then connects to this new URL.
Specify also whether security is enabled for an HTTP listener and what kind of security is used (for example, which SSL protocol and which ciphers).
To access a web application deployed on the Application Server, use the URL http://localhost:8080/ (or https://localhost:8181/ if it is a secure application), along with the context root specified for the web application. To access the Administration Console, use the URL https://localhost:4849/ or https://localhost:4849/asadmin/ (its default context root).
Because a virtual server must specify an existing HTTP listener, and because it cannot specify an HTTP listener that is already being used by another virtual server, create at least one HTTP listener before creating a new virtual server.