Management of message credits should be on a per subscription basis (not on a
per session basis)
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Key: QPID-3613
URL: https://issues.apache.org/jira/browse/QPID-3613
Project: Qpid
Issue Type: Bug
Components: Java Client
Affects Versions: 0.14
Reporter: Rajith Attapattu
Assignee: Rajith Attapattu
Fix For: 0.15
We currently calculate credit replenishment at the session level completely
ignoring the capacity of each individual subscription. Further more as per
QPID-3612 the completions are tied to message-acks further complicating the
issue.
The general formula used for calculating the replenishment rate is as follows.
{code}
if (unackedCount >= prefetch/2 || maxAckDelay <= 0 || _acknowledgeMode ==
javax.jms.Session.AUTO_ACKNOWLEDGE)
{
flushAcknowledgments();
}
{code}
There are several issues with this.
1. If the application doesn't ack (in CLIENT_ACK) or commit a transaction
before it exhaust the credits, the consumer will starve. See QPID-3612. A
recent change done in rev 1195213 fixes the TX case.
2. Ineffective flow control as the calculation happens globally at the session
level instead at the subscription level. Due to this, slow consumers can affect
fast consumers.
Ex . Assume there are two fast consumers and a slow consumer each with an
individual capacity of 20 and the max_prefetch for the connection to be 100.
The threshold for sending acks (and therefore completions) is 50. If the fast
consumers have already done with their 20 messages, they will have to wait for
the slow consumer to do atleast 10 messages before the threshold is hit.
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