Details
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Improvement
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Status: Open
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Major
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Resolution: Unresolved
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None
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Description
There is a weakness in the P2P/DirectChannel messaging architecture, in that it never gives up on a request (in a request-response scenario). As a result a bug (software fault) anywhere from the point where the requesting thread hands off the DistributionMessage e.g. to ClusterDistributionManager.putOutgoing(DistributionMessage), to the point where that request is ultimately fulfilled on a (one) receiver, can result in a hang (of some task on the send side, which is waiting for a response).
Well it's a little worse than that because any code in the return (response) path can also cause disruption of the (response) flow, thereby leaving the requesting task hanging.
If the code in the request path (primarily in P2P messaging) and the code in the response path (P2P messaging and TBD higher-level code) were perfect this might not be a problem. But there is a fair amount of code there and we have some evidence that it is currently not perfect, nor do we expect it to become perfect and stay that way.
This is a sketch of the situation. The left-most column is the request path or the originating member. The middle column is the server-side of the request-response path. And the right-most column is the response path back on the originating member.
You can see that Geode product code, JDK code, and hardware components all lie in the end-to-end request-response messaging path.
That being the case it seems prudent to institute response timeouts so that bugs of this sort (which disrupt request-response message flow) don't result in hangs.
It's TBD if we want to go a step further and institute retries. The latter would entail introducing duplicate-suppression (conflation) in P2P messaging. We might also add exponential backoff (open-loop) or back-pressure (closed-loop) to prevent a flood of retries when the system is at or near the point of thrashing.
But even without retries, a configurable timeout might have good ROI as a first step. This would entail:
- adding a configuration parameter to specify the timeout value
- changing ReplyProcessor21 and others TBD to "give up" after the timeout has elapsed
- changing higher-level code dependent on request-reply messaging so it properly handles the situations where we might have to "give up"
This issue affects all versions of Geode.
Counterpoint
Not everybody thinks timeouts are a good idea. This section has the highlights.
Timeouts Will Result in Data-Inconsistency
If we leave most the surrounding code as-is and introduce timeouts, then we risk data inconsistency. TODO: describe in detail why data inconsistency is inherent in using timeouts.
Narrow The Vulnerability Cross-Section Without Timeouts
The proposal (above) seeks to solve the problem using end-to-end timeouts since any component in the path can, in general, have faults. An alternative approach, would be to assume that some of the components can be made "good enough" (without adding timeouts) and that those "good enough" components can protect themselves (and user applications) from faults in the remaining components.
With this approach, the Cluster Distribution Manager, and P2P / TCP Conduit / Direct Channel framework would be enhanced so that it was less susceptible to bugs in:
- the 341 Distribution Message classes
- the 68 Reply Message classes
- the 95 Reply Processor classes
The question is: what form would that enhancement take, and also, would it be sufficient to overcome faults in remaining components (JDK, and the host+network layers).
Alternatives Discussed
These alternatives have been discussed, to varying degrees.
Baseline: no timeouts; members waiting for replies do "the right thing" if recipient departs view
Give-up-after-timeout
Retry-after-timeout-and-eventually-give-up
Retry-after-forcing-receiver-out-of-view
