JBoss EAP 6 provides
two operating modes for JBoss EAP 6 instances: standalone server or
managed domain.
The two modes differ
in how servers are managed, not in their capacity to service end-user requests.
It is important to note that the high-availability (HA) cluster functionality
is available via either operating mode. A group of standalone servers can be
configured to form an HA cluster.
1.About Standalone Servers
Standalone server mode
is an independent process and is analogous to the only running mode available
in previous JBoss EAP versions.
A JBoss EAP 6 instance
running as a standalone server is a single instance only but can optionally run
in a clustered configuration.
2. About Managed
Domains
The managed domain
operating mode allows for management of multiple JBoss EAP 6 instances from a
single control point. Centrally managed JBoss EAP 6 server collections are
known as members of a domain. All JBoss EAP 6 instances in a domain share a
common management policy.
A domain consists of one domain controller, one or more host
controller(s), and zero or more server groups per host.
A domain controller is the central point from which the domain is
controlled. It ensures that each server is configured according to the
management policy of the domain. The domain controller is also a host
controller. One host controller instance is configured to act as a domain
controller.
By default, the
central management policy is stored in the domain/configuration/domain.
xml file. This is the main configuration file for a managed domain. Only
the domain master reads this file. On other domain members, it can be removed.
The host controller interacts with the
domain controller to help manage the communication between the servers and the
domain controller and control the lifecycle of the application server instances
running on its host and to assist the domain controller to manage them.
Multiple host
controllers of a domain can interact with only a single domain controller.
Hence, all the host controllers and server instances running on a single domain
mode have a single domain controller and must belong to the same domain.
By default each host controller reads
its configuration from the domain/configuration/host.xml
file located in the unzipped JBoss EAP 6 installation file on its host's
filesystem. The host.xml file contains the following configuration information
that is specific to the particular host:
- How the host controller contacts the domain controller to register itself and access the domain configuration.
- How to find and contact a remote domain controller.
- That the host controller is to act as the domain controller
A server group is a set of server instances which have JBoss EAP 6
installed on them and are managed and configured as one. The domain controller
manages the configuration of and applications deployed onto server groups.
Consequently, each server in a server group shares the same configuration and
deployments.
Each server group
needs a unique name and a reference to one profile of the domain configuration
The server instances are independent from
the domain controller and started by Process Controller. These are application
server instance.
Every server instance(Application servers) of a domain belongs to a virtual
server group. The idea is, that all servers of the same server group, perform
the same tasks. When you deploy an application you will not deploy it to a
server, you will deploy it to a whole server group. It is also possible to mange
different server groups in one domain, e.g. production, staging or a test sever
group.
The controller and the
server instances of each host are separate JVM processes, which will be
monitored by a process controller.
The process controller is also a separate JVM process that is responsible for
spawning the other processes and monitoring their lifecycle. If, the host
controller process crashed, the process controller will start up the host
controller (If you kill host controller, process control will start it again.) and
each server that is configured with the auto-start parameter. However, if the
server process of a server instance is terminated unexpectedly, the process
controller will not restart the server instance automatically.
If the domain
controller crashes, the host controller and server instances will be still
available. Only the management and configuration of the server is not possible
at this time. The host controller tried in a certain interval a re-connect.
After the domain controller is available, the host controller establishes a new
connection.
What happens when a JBoss server starts in domain mode?
JBoss goes through a
few steps to load the right configuration. There are three components at work:
1. process controller,
2. host controller
3. domain
controller.
The process controller
is responsible for starting other processes and monitoring them. The host
controller is responsible for managing configuration on the physical host and
making sure the correct server instances are started by the process controller
and the domain controller is responsible for holding the domain configuration.
When JBoss is started
in domain mode, by calling either domain.bat or domain.sh, the following steps
are performed:
1. The Process
Controller (PC) is started in its own JVM(separate process)
2. The PC starts the
Host Controller (HC) in a separate JVM
3. The HC parses the
host.xml to see if this host is the Domain Controller (DC)
If this host is the DC:
The local domain.xml and local
host.xml settings are combined to make a single configuration for the machine.
If this host is not the DC:
The HC tries to connect to the DC
and combines the remote domain.xml with the local host.xml to make a single
configuration for the machine.
4.Each server instance
defined in the host.xml (if any) is started in its own JVM.
