High availability network components
The foundation for a highly available environment is dependent on the network that is created for this environment. The WAS high availability manager, by default, uses a channel framework network protocol model to establish network connections. This model enables network ports to be shared among all of the channels within a transport chain. A unicast protocol, which supports communication between a specific receiver and a specific sender, or a multicast protocol, which supports open communication to all receivers, can also be used.
The following components are important parts of a high availability network:
- Distributed Consistency Services (DCS).
This component implements JVM level group services. It is responsible for detecting when JVMs in a core group start and stop. It also tracks the set of servers that have joined the core group. The set of servers currently in the core group is a called the view. A JVM typically joins the view in 20-30 seconds after it initializes. This delay allows multiple JVMs to start together and to join the view together rather as a sequence of new views, each with a single additional member. High availability resources can only be activated or deactivated in a JVM after the JVM has joined the view.
The high availability issues message HMGR218I to the system out logs whenever a view changes. This message occurs simultaneously across all current view members, and indicates the number of current view members. A view changes whenever a JVM stops or fails, and whenever a server fails or leaves the view. A view change enables the high availability manager to quickly react to failures and recover any services running on the failed server.
- Reliable Multicast Messaging (RMM).
This component is a very high-speed publish and subscribe message transport that DCS and the high availability manager use for JVM to JVM communication. It can operate using any of the following low-level transports:
- Reliable Multicast. This is true multicast.
- TCP unicast. This emulates multicast over unicasts.
- Channel Framework. This emulates multicast over channel framework point to point streams.
A high availability environment ties all of the network components together. It provides the basis for providing peer-to-peer failover. The moment a component loses its network connection to the rest of the infrastructure, the high availability manager assumes a failure has occurred and assumes the work assigned to that component.
Thousands of high availability groups can exist within a core group. The policies that are associated with a high availability group control how and when members of that group are activated. A group services mechanism is used to communicate high availability group membership information between members of a core group. This information includes the group services that are available and the lightweight component groups that are part of a high availability group.
The group services mechanism also supports the following types of messaging between core group members.
- High speed First In First Out (FIFO) messaging between members of the lightweight component groups.
- High speed view synchronous messaging.
- Java Virtual Machine (JVM) heartbeats or pulses, which are used to indicate that the associated application server is still healthy.
Transport protocol for a high availability manager
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