[QPID-2341] Annotate replicated broker classes with assertions.

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.6
• Fix Version/s: 0.8
• Component/s: C++ Broker
• Assignee: Alan Conway
• Reporter: Alan Conway
• Created: 15/Jan/10 21:43
• Updated: 20/Aug/13 20:49
• Resolved: 20/Jan/10 17:09

Description

A clustered broker maintains consistency of replicated objects by only modifying them in a "replication safe" thread context: while receiving an update or dispatching cluster events.

A repeated source of cluster bugs is broker code that unwittingly modifies replicated objects in an unsafe context such as a timer thread. These bugs are intermittent race conditions that are hard to track down.

Proposal: annotate broker code with assertions to identify code that modifies replicated state and log/abort if such code is called in an unsafe context:

// New class:
namespace broker {
class Replicated {
protected:
void assertReplicationSafe();
}

// Existing classes
class Queue : public Replicated { // Mark Queue as state that may be replicated.
void someQueueModifier()

{ assertReplicationSafe(); // This function should only be called in replication-safe context. }

The assertion is cheap: just testing a thread-local boolean value. In a non-clustered broker it does nothing.

This technique has already proven valuable in debugging a recent bug, putting the assertions permanently in the code should speed debugging of future bugs.

This would be the beginning of a formal contract between the broker code and the cluster that should make things more maintainable in the long run.