JGroups Protocol PING

External

• JGroups Manual http://www.jgroups.org/manual/html/protlist.html#PING
• JGroups Wiki http://community.jboss.org/wiki/JGroupsPING

Internal

• JGroups

TODO

• Next time I dive into this, come up with a full explanation for 'num_initial_members' below.

Overview

PING performs the initial (dirty) discovery of members, by multicasting PING request to an IP multicast address. The PING request is a Message with a PingHeader.GET_MBRS_REQ header. Each member replies with a (C, A) packet where C = coordinator address and A its own address. After a set number of milliseconds ('timeout') or replies ('num_initial_members'), the joiner determines the coordinator from responses, and sends a JOIN to it. The JOIN request is handled by the GMS layer. If nobody responds, the joiner assumes it's the first member of a group.

Configuration

  <PING timeout="2000" num_initial_members="3"/>

Difference between PING and MPING

MPING, unlike PING, uses its own IP multicast socket to send discovery requests. PING simply sends a discover event down the stack to be sent by the underlying transport protocol.

Periodic GET_MBRS_REQ Requests

The super-type Discovery protocol is in charge with sending GET_MBRS_REQ (get members) requests upon receiving a FIND_INITIAL_MBRS event. FIND_INITIAL_MBRS events are sent either by the [GMS] protocol, when initializing, or periodically by the FindSubgroupsTask of the [MERGE2] protocol.

The discovery requests are generated by the PingSenderTask, in the form of OOB messages with PingHeader.GET_MBRS_REQ headers. PingSenderTask sends these messages in a delayed loop. The delay between requests is Template:Timeout/Template:Num ping requests where Template:Timeout and Template:Num ping requests are configuration parameters described below. Normally that should be about 1.5 sec (1,500 ms).

The loop is interrupted either after a 'timeout' milliseconds (which means that all 'num_ping_requests' discovery requests have been sent), or after the first valid response - which allows determining who the coordinator is - is received. Further clarifications needed as to how 'num_initial_members' works.

A new PingSenderTask - and the loop - will be started again when a new GET_MBRS_REQ message needs to be sent.

!!Stack Boot Time Factor

The whole stack is waiting while the first discovery loop is running, setting the 'timeout' to a smaller value can reduce the stack boot time.

!!!Configuration

!timeout

The timeout to wait for the initial members. Default is 3000 ms.

!num_initial_members

!num_ping_requests

Number of discovery requests to be sent distributed over Template:Timeout. Default is 2.

!Filtering Out

If you want to filter these messages out, the header id is __6__.

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