JGroups Protocol FD ALL
External
- User Manual FD_ALL http://www.jgroups.org/manual/html/protlist.html#FD_ALL
Internal
Overview
FD_ALL implements failure detection based on simple heartbeat protocol. Every member periodically multicasts a heartbeat using the underlying transport, not a parallel mechanism. Every member also maintains a table of all members (minus itself). When data or a heartbeat from a pair are received, the member resets the timestamp for that pair to the current time. Periodically, the member checks for expired members, and suspects those.
Once a member has been suspected, a SUSPECT_MESSAGE is sent up the stack.
The protocol has two asynchronous processing threads: one that periodically sends multicast heartbeats and one that periodically analyzes the suspect table and raises SUSPECT events up the stack, when the suspect table is not empty.
!!!Configuration Sample
{{{
<FD_ALL interval="1000" timeout="3000"/>
}}}
!!!Configuration
!!interval
The periodicity (in ms) of the HEARTBEAT message is sent by a member to the cluster __AND__ with which the response timestamps are checked. In the example above, each member sends a heartbeat and check the response timestamps for previous heartbeats every second. If at any moment the difference between a specific HEARTBEAT event timestap and the response timestamp from a member is larger than 'timeout', that member is suspected.
So, for the values defined above, if heartbeat H1 doesn't receive a response, the remote member will be suspected after 3 * interval + interval (~4,000 ms). If the timeout is set to 2,500 ms, the member will be suspected after 3,000 ms.
!!timeout
Timeout after which a node is suspected by the current node if neither a heartbeat nor data were received from. Also see the 'interval' definition above.
!!ergonomics
Enables ergonomics: dynamically find the best values for properties at runtime
!!level
Sets the logger level (see javadocs)
!!msg_counts_as_heartbeat
Treat messages received from members as heartbeats. Note that this means we're updating a value in a hashmap every time a message is passing up the stack through FD_ALL, which is costly. Default is false.
!!stats
Determines whether to collect statistics (and expose them via JMX). Default is true
Recommendations
Resulted from personal experimentation
Both [FD_SOCK] and [FD] failure detection protocols, which rely on directly pinging a neighbor, have proved unreliable under certain platform-specific circumstances. If you plan to use those, test response times with your specific JVM and platform. They may work very well on certain platforms and fail on others. I would use FD_ALL as a central failure detection protocol, its coverage seems to be most generic.
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