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19.7.2. Flow Control

The flow control service tries to adapt the sending data rate and the receiving data among nodes. If a sender node is too fast, it might overwhelm the receiver node and result in dropped packets that have to be retransmitted. In JGroups, the flow control is implemented via a credit-based system. The sender and receiver nodes have the same number of credits (bytes) to start with. The sender subtracts credits by the number of bytes in messages it sends. The receiver accumulates credits for the bytes in the messages it receives. When the sender's credit drops to a threshold, the receivers sends some credit to the sender. If the sender's credit is used up, the sender blocks until it receives credits from the receiver. The flow control service is configured in the FC sub-element under the JGroups Config element. Here is an example configuration.

<FC max_credits="1000000"
down_thread="false" up_thread="false" 

The configurable attributes in the FC element are as follows.

  • max_credits specifies the maximum number of credits (in bytes). This value should be smaller than the JVM heap size.

  • min_credits specifies the threshold credit on the sender, below which the receiver should send in more credits.

  • min_threshold specifies percentage value of the threshold. It overrides the min_credits attribute.


Applications that use synchronous group RPC calls primarily do not require FC protocol in their JGroups protocol stack because synchronous communication, where the hread that makes the call blocks waiting for responses from all the members of the group, already slows overall rate of calls. Even though TCP provides flow control by itself, FC is still required in TCP based JGroups stacks because of group communication, where we essentially have to send group messages at the highest speed the slowest receiver can keep up with. TCP flow control only takes into account individual node communications and has not a notion of who's the slowest in the group, which is why FC is required.