The listener determines whether a database service and its service handlers are available through service registration. During registration, the Listener Registration (LREG) process provides the listener with information about the following:

• Names of the database services provided by the database

• Name of the database instance associated with the services and its current and maximum load

• Service handlers (dispatchers and dedicated servers) available for the instance, including their type, protocol addresses, and current and maximum load

The preceding information enables the listener to direct a client request appropriately.

Figure 5-2 shows two database instances registering information with two listeners. The figure does not represent all the information that can be registered. For example, listening endpoints, such as the port numbers, can be dynamically registered with the listener.

If the listener is not running when an instance starts, then the LREG process cannot register the service information. LREG attempts to connect to the listener periodically, but it may take up to 60 seconds before LREG registers with the listener after it has been started. To initiate service registration immediately after the listener is started, use the SQL statement ALTER SYSTEM REGISTER. This statement is especially useful in high availability configurations.

Each listener is configured with one or more protocol addressesthat specify its listening endpoints. The protocol address defines the protocol the listener listens on and any other protocol-specific information. For example, the listener could be configured to listen at the following protocol address:

(DESCRIPTION=
   (ADDRESS=(PROTOCOL=tcp)(HOST=sales-server)(PORT=1521)))

The preceding example shows a TCP/IP address that specifies the host of the listener (sales-server) and a port number (1521).

Clients configured with a protocol address can send connection requests to the listener. When a client request reaches the listener, it selects an appropriate service handler to service the request and forwards the request to the handler. A service handler is a dispatcher or a dedicated server process that acts as a connection point to a database.

Figure 5-3 illustrates the role of the listener during the establishment of a connection. The figure shows a browser making an HTTP connection and a client making a database connection.

1. The browser or client send a connection request to the listener.

2. The listener parses the request and forwards it to the service handler for the database service requested.

3. The browser or client connect to the database.

Oracle Restart enhances the availability of Oracle databases in a single-instance environment. Using the Server Control (SRVCTL) utility, you can add components such as the listener to an Oracle Restart configuration. The configuration enables the listener to start automatically when the listener fails or is not running.

When using Oracle Restart, note the following:

• Use the SRVCTL utility to start and stop the listener. Do not use the listener control utility LSNRCTL.

• Each listener must have a unique name.

Blocked connection requests can occur when an incoming request occurs before the respective instance has been registered, or when a database is in restricted mode, such as when a shutdown of the database is in progress. If a database instance is in restricted mode, then LREG instructs the listener to block all connections to the instance. Clients attempting to connect receive one of the following errors:

• ORA-12526: TNS:listener: all appropriate instances are in restricted mode

• ORA-12527: TNS:listener: all appropriate instances are in restricted mode or blocking new connections

• ORA-12528: TNS:listener: all appropriate instances are blocking new connections

The ORA-12528 error occurs when a database instance is not yet registered with the listener.

Based on the service handler type registered with the listener, the listener forwards requests to either a shared server or dedicated server process. The shared server architecture enables a database server to allow many client processes to share server processes. In a dedicated server configuration, the listener starts a separate dedicated server process for each incoming client connection request dedicated to servicing the client.

• About Shared Server Processes
  
• About Dedicated Server Processes
  

Oracle Connection Manager is a gateway through which client connection requests are sent either to the next hop or directly to the database server. Clients who relay connection requests through an Oracle Connection Manager can take advantage of the session multiplexing and access control features configured on Oracle Connection Manager. It carries no service information until a LREG process registers its services.

Oracle Connection Manager consists of three components:

• listener

• CMGW (Oracle Connection Manager Gateway)

• CMADMIN (Oracle Connection Manager Administration)

The listener receives client connections and evaluates against a set of rules whether to deny or allow access. If it allows access, then the listener forwards a request to a gateway process, selecting the one with the fewest connections. The CMGW process, in turn, forwards the request to another Oracle Connection Manager or directly to the database server, relaying data until the connection terminates. If a connection to the server already exists, then the gateway multiplexes, or funnels, its connections through the existing connection. CMADMIN monitors the state of the gateway processes and the listener, shutting down or starting up processes as needed. In addition, it registers the location and load of the gateway processes with the listener, and it answers requests from the Oracle Connection Manager Control utility.

In Figure 5-6, the listener screens connection requests. A gateway process registers with the CMADMIN process., and the CMADMIN process registers with the listener. Finally, the listener forwards the connection requests to the gateway process. After receiving the three valid client connections, the gateway process multiplexes them through a single network protocol connection to the database. The fourth connection is denied when it is evaluated against the set of rules.

Oracle Net provides an architectural solution that allows for greater scalability in Internet and intranet environments.

Figure 5-7 shows how multiple connections to an Oracle database server are made more scalable with Oracle Connection Manager and a shared server architecture. Oracle Connection Manager is used to offload some of the network I/O of the application web servers, and a shared server is used to serve more concurrent users.