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Getting Started with JXTA, Part 2
Pages: 1, 2, 3

Creating Peergroups



Any peer can create a peergroup. The shell creates a peergroup with these commands:

JXTA>mygroupadv = mkadv -g mygroup
JXTA>mkpgrp -d mygroupadv

The first of these commands creates an advertisement (mygroupadv) of a new group with a symbolic name of mygroup (advertisements are discussed in more detail later in this chapter). The mkpgrp command actually creates the group.

When you create a group in this manner, information about the group is broadcast to other peers using a discovery protocol that we discuss in the next section. In general, a peer can find a peergroup in two ways:

1. When a new group is created, an announcement of that group is broadcast, and peers who are listening will discover the new group.

2. A peer can initiate discovery to find groups. In the shell, this is accomplished with the groups -r command:

     JXTA>groups -r
     group discovery message sent

You can see which groups the shell knows about by using the groups command:

JXTA>groups
group0: mygroup
group1: anothergroup

In this last example, we know about mygroup because that's the group that we created; we know about anothergroup because another peer on the network created that group and we discovered it. Note that the NetPeerGroup does not show up in this list, even though it is also a peergroup.

Each peergroup is uniquely identified by a unique peergroup ID. The peergroup ID is guaranteed to be unique within the entire JXTA network.

Joining Groups

Once a peer has discovered or created a group, it must join the group. This entails sending an application (with any necessary credentials) to the group and awaiting a response. The membership service of the group will vote on the new application and send back either an acceptance or a rejection.

A shell joins a particular group by using the join command:

JXTA>join -d group0
Enter the identity you want to use when joining this peergroup (nobody)
1Identity: jra

After executing this command, the shell is a member of the group mygroup. A quick note about syntax here: group0 comes from the list of groups we obtained with the groups command. You can use the symbolic group name (e.g., mygroup) to join a group only if that shell created the group. To join a group that the shell discovered (such as anothergroup), however, you must use the group# that the shell has assigned. Because the shell caches information about groups, it might have created the group, exited, and restarted. In that case, you must also use the group# that the shell has assigned.

A peer can join multiple groups by executing another join command; the join command can also be used to determine which groups the peer has joined:

JXTA>join -d group1
Enter the identity you want to use when joining this peergroup (nobody)
1Identity: sdo
JXTA> join
Joined group    : mygroup
Joined group    : anothergroup    (current)

Commands that the shell executes will be associated with the current group (anothergroup); the current group can be changed with the chpgrp command:

JXTA>chpgrp mygroup

n this command, you always use the symbolic group name rather than the group# name. The current group is used to find any service that the shell needs; it is also the group printed by the whoami command.

Membership in any group created by the shell is automatic, so these examples work well when dealing with groups created by two different shells. A shell might also discover a different group, however: one that has membership criteria and requires the shell to present some special credentials. Membership in such groups requires that you know how to fill out a credential request, which is group-specific; we'll look at how this works in Chapter 7.

Canceling Group Membership

A peer can cancel its membership in a group. The shell cancels its membership in a group with the leave command:

JXTA>leave

This command cancels membership in the current group. You cannot leave the NetPeerGroup.

Pages: 1, 2, 3

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