The table below shows where JavaGroups fits in. In unicast communication,
where one sender sends a message to one receiver, there is UDP and TCP.
UDP is unreliable, packets may get lost, duplicated, may arrive out of
order, and there is a maximum packet size restriction. TCP is also unicast,
but takes care of message retransmission for missing messages, weeds out
duplicates, fragments packets that are too big and present messages to
the application in the order in which they were sent. In the multicast
case, where one sender sends a message to many receivers, IP Multicast
extends UDP: a sender sends messages to a multicast address and the receivers
have to join that multicast address to receive them. Like in UDP, message
transmission is still unreliable, and there is no notion of membership
(who has currently joined the multicast address).
Unreliable | Reliable | |
Unicast | UDP | TCP |
Multicast | IP Multicast | JavaGroups |
JavaGroups extends reliable unicast message transmission (like in TCP) to multicast settings. As described above it provides reliability and group membership on top of IP Multicast. Since every application has different reliability needs, JavaGroups provides a flexible protocol stack architecture that allows users to put together custom-tailored stacks, ranging from unreliable but fast to highly reliable but slower stacks. As an example, a user might start with a stack only containing IP Multicast. To add loss-less transmission, he might add the NAKACK protocol (which also weeds out duplicates). Now messages sent from a sender are always received by the recipients, but the order in which they will be received is undefined. Therefore, the user might choose to add the FIFO layer to impose per/sender ordering. If ordering should be imposed over all the senders, then the TOTAL protocol may be added. The Group Membership Service (GMS) and FLUSH protocols provide for group membership: they allow the user to register a callback function that will be invoked whenever the membership changes, e.g. a member joins, leaves or crashes. In the latter case, a failure detector (FD) protocol is needed by the GMS to announce crashed members. If new members want to obtain the current state of existing members, then the STATE_TRANSFER protocol has to be present in this custom-made stack. Finally, the user may want to encrypt messages, so he adds the CRYPT protocol (which encrypts/decrypts messages and takes care of re-keying using a key distribution toolkit).
Using composition of protocols (each taking care of a different aspect) to build reliable multicast communication has the benefit that
String props="UDP:PING:FD:STABLE:NAKACK:UNICAST:" +
"FRAG:FLUSH:GMS:VIEW_ENFORCER:STATE_TRANSFER:QUEUE";
Message send_msg, recv_msg;
Channel channel=new JChannel(props);channel.Connect("MyGroup");
send_msg=new Message(null, null, "Hello world");
channel.Send(send_msg);
recv_msg=(Message)channel.Receive(0);
System.out.println("Received " + recv_msg);channel.Disconnect();
channel.Close();
The channel's properties are specified when creating it. They include
IP Multicast (UDP), failure detection (FD), distributed message garbage
collection (STABLE), multicast (NAKACK) and unicast (UNICAST) loss-less
and FIFO delivery, fragmentation (FRAG), group membership (GMS, FLUSH,
VIEW_ENFORCER, QUEUE) and state transfer (STATE_TRANSFER). Each protocol
"X" results in the creation of an instance of a Java class X. All protocol
instances are kept in a linked list (the protocol stack), which messages
traverse up/down.
To join a group, Connect() is called. It returns when the member has successfully joined the group named "MyGroup", or when it has created a new group (if it is the first member).
Then a message is sent using the Send() method. A message contains the receiver's address ('null' = all group members), the sender's address ('null', will be filled in by the protocol stack before sending the message) and a byte buffer. In the example, the String object "Hello world" is set to be the message's contents. It is serialized into the message's byte buffer.
Since the message is sent to all members, the sender will also receive it (unless the socket option 'receive own multicasts' is set to 'true'). This is done using the Receive() method.
Finally, the member disconnects from the channel and then closes it.
This results in a notification being sent to all members who are registered
for membership change notifications.
public class RpcDispatcherTest {
Channel
channel;
RpcDispatcher disp;
RspList
rsp_list;
String
props="UDP:PING:FD:STABLE:NAKACK:UNICAST:FLUSH:GMS:VIEW_ENFORCER:QUEUE";
public int Print(int number) {
System.out.println("Print("
+ number + ")");
return number * 2;
}
public void Start() throws Exception {
channel=new JChannel(props);
disp=new RpcDispatcher(channel,
null, null, this);
channel.Connect("RpcDispatcherTestGroup");
for(int i=0; i < 100;
i++) {
Util.Sleep(1000);
rsp_list=disp.CallRemoteMethods(null, "Print", new Integer(i),
GroupRequest.GET_ALL, 0);
System.out.println("Responses: " + rsp_list);
}
channel.Close();
}
}
As before, the example creates a channel specifying the properties.
It also defines a method Print() which will be called by the RpcDispatcher
later on. Then an instance of RpcDispatcher is created on top of the channel
and the channel connected (this joins the new member to the group). Now,
messages can be sent and received. But instead of sending/receiving messages
using the channel, the application invokes remote method call using RpcDispatcher's
CallRemoteMethods().
The first argument 'null' means send to all group members, "Print" is the
name of the method to be invoked, 'new Integer(i)' is the argument to the
Print() method, GET_ALL means wait until the responses from all
group members have been received and '0' specifies the timeout (in this
case, it means wait forever). RpcDispatcher sends a multicast message (containing
the method call) to all members (e.g. 4 members, including itself) and
waits for 4 replies. If one or more of the members crash in the meantime,
the call nevertheless returns and has those replies marked as 'suspected'
in the response list. The response list contains an entry for each expected
reply, which has the address of the replier, the value (if any, in our
case it is an integer), and a flag (received, not received (in case of
timeouts) or suspected). If this member is the only group member, then
the method call will call its own Print() method.
The current set of protocols shipped with JavaGroups provide Virtual
Synchrony properties: messages are sent/received in views. Each
member has a view which is a set of members that constitute the current
membership. When the membership changes, a new view will be installed by
all members. Views are installed in the same order at all members. The
set of messages between 2 consecutive views will be the same at all receivers.
Messages sent in view V1 will be received by all non-faulty members in
view V1.
Through its flexible protocol stack architecture, JavaGroups can be adapted to any environment. This can be done by replacing, removing or modifying existing protocols, or by adding new protocols. JavaGroups is an ideal testbed for development and experimentation of new reliable multicast protocols written in Java. As JavaGroups is an OpenSource project, new protocols are always welcome and will be integrated.