The HADB management system provides built-in security and facilitates multi-platform management. As illustrated in the following figure, the HADB management architecture contains the following components:
As shown in the figure, one HADB management agent runs on every machine that runs the HADB service. Each machine typically hosts one or more HADB nodes. An HADB management domain contains many machines, similar to an Application Server domain. At least two machines are required in a domain for the database to be fault tolerant, and in genera there must be an even number of machines to form the DRU pairs. Thus, a domain contains many management agents.
As shown in the figure, a domain can contain one or more database instances. One machine can contain one or more nodes belonging to one or more database instances.
The HADB management client is a command-line utility, hadbm, for managing the HADB domain and its database instances. HADB services can run continously— even when the associated Application Server cluster is stopped—but must be shut down carefully if they are to be deleted.
You can use the asadmin command line utility to create and delete the HADB instance associated with a highly available cluster. For more information, see Chapter 9, Configuring High Availability Session Persistence and Failover, in Sun GlassFish Enterprise Server v2.1.1 High Availability Administration Guide.
The management agent is a server process (named ma) that can access resources on a host; for example, it can create devices and start database processes. The management agent coordinates and performs management client commands such as starting or stopping a database instance.
A management client connects to a management agent by specifying the address and port number of the agent. Once connected, the management client sends commands to HADB through the management agent. The agent receives requests and executes them. Thus, a management agent must be running on a host before issuing any hadbm management commands to that host. The management agent can be configured as a system service that starts up automatically.
The management agent process ensures the availability of the HADB node supervisor processes by restarting them if they fail. Thus, for deployment, you must ensure the availability of the ma process to maintain the overall availability of HADB. After restarting, the management agent recovers the domain and database configuration data from other agents in the domain.the system.
Use the host operating system (OS) to ensure the availability of the management agent. On Solaris or Linux, init.d ensures the availability of the ma process after a process failure and reboot of the operating system. On Windows, the management agent runs as a Windows service. Thus, the OS restarts the management agent if the agent fails or the OS reboots.
An HADB management domain is a set of hosts, each of which has a management agent running on the same port number. The hosts in a domain can contain one or more HADB database instances. A management domain is defined by the common port number the agents use and an identifier (called a domainkey) generated when you create or the domain or add an agent to it. The domainkey provides a unique identifier for the domain, crucial since management agents communicate using multicast. You can set up an HADB management domain to match with an Application Server domain.
Having multiple database instances in one domain can be useful in a development environment, since it enables different developer groups to use their own database instance. In some cases, it may also be useful in production environments.
All agents belonging to a domain coordinate their management operations. When you change the database configuration through an hadbm command, all agents will change the configuration accordingly. You cannot stop or restart a node unless the management agent on the node’s host is running. However, you can execute hadbm commands that read HADB state or configuration variable values even if some agents are not available.
Use the following management client commands to work with management domains:
hadbm createdomain: creates a management domain with the specified hosts.
hadbm extenddomain: adds hosts to an existing management domain
hadbm deletedomain: removes a management domain.
hadbm reducedomain: removes hosts from the management domain.
hadbm listdomain: lists all hosts defined in the management domain.
Management agents store the database configuration in a repository. The repository is highly fault-tolerant, because it is replicated over all the management agents. Keeping the configuration on the server enables you to perform management operations from any computer that has a management client installed.
A majority of the management agents in a domain must be running to perform any changes to the repository. Thus, if there are M agents in a domain, at least M/2 + 1 agents (rounded down to a whole number) must be running to make a change to the repository.
If some of the hosts in a domain are unavailable, for example due to hardware failures, and you cannot perform some management commands because you don’t have a quorum, use the hadbm disablehost command to remove the failed hosts from the domain.