[jboss-jira] [JBoss JIRA] Created: (JGRP-1154) FC: separate implementations for multicast and unicast messages

[Bela Ban (JIRA)]

FC: separate implementations for multicast and unicast messages
---------------------------------------------------------------

                 Key: JGRP-1154
                 URL: https://jira.jboss.org/jira/browse/JGRP-1154
             Project: JGroups
          Issue Type: Feature Request
            Reporter: Bela Ban
            Assignee: Bela Ban
             Fix For: 2.11


Currently, FC handles both multicast (=group) and unicast messages. This has a few disadvantages:
- Complexity
- Unicast messages are not dependent on multicast messages (and vice versa), so why should this be different for flow control ? E.g. why 
   should a multicast message block waiting for credits from X because a recent unicast message was sent to X ?
- If we have separate flow control protocols for multicasts and unicasts, we can selectively remove one or the other. This is currently
   not possible

Design (see separate documents in ./doc/design):


FC_MCAST:
-----------------
(see whether some concepts can be borrowed from SFC)

Sender:
- Every sender has a credits AtomicLong variable
- When sending a  message and the message length exceeds credits, the sender blocks (until it gets credits from all current group members)
- Else credits is decremented by the message length and the message is sent
- When credits are received from all current group members, credits is reset to the initial value and all blocked senders are unblocked
- When a new member joins, credits is also reset and all blocked senders are unblocked

Receiver:
- Maintains a hashmap of senders and credits (Map<Address,AtomicLong>)
- When a message is received from S, the credits for S are decremented by the message length
- If the remaining length is less than the threshold, new credits are sent to S and S's credits are reset to their initial value
- When a new member joins, all credits (including the newly joined member) are reset to the initial value



FC_UNICAST:
--------------------

Sender:
- A hashmap of addresses and credits is maintained: every receiver has associated credits
- When a message is sent to P, if P's credits minus the message length is less than 0, the sender blocks until it gets new credits from the receiver
- Else P's credits are decremented by the message length and the message is sent

Receiver:
-Same as for FC_MCAST

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