Enable ordered responses for ADS delta watches #752
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This is only an initial review of the PR. I need to spend more time on the impact of this new shared channel.
My initial analysis is that there is a potential deadlock case as a watch can have queued a response prior to being cancelled. In some edge cases when the client has sent a lot of requests in a short sequence, I believe this channel could end up overflowing the muxed one and then itself. The previous model guarantees that the muxed channel being filled does not block other processes (as they each dequeue within their own goroutine), but here it is no longer the case
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pkg/server/delta/v3/server.go
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| watch.responses = make(chan cache.DeltaResponse, 1) | ||
| if ordered { | ||
| // Use the shared channel to keep the order of responses. | ||
| watch.UseSharedResponseChan(watches.deltaMuxedResponses) |
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I globally like the state of this PR, but I think that there is an issue with how the channel is buffered.
This channel will take potentially multiple responses for each type (e.g. if a cache update triggers while a new request is retrieved and tries to enqueue in the same goroutine). As the channel is only buffered for the nb of types this will deadlock
The issue is that theoretically there could be more than 2 responses per type, as the request/response channels are in a select (and therefore are not ordered). In this case even buffering for two times would not guarantee to not deadlock
I think this can be solved if it is guaranteed that enqueued responses would be processed prior to new requests, but I'm not sure that'd be simple
Another possibility is to, like for the sotw, purge the response queue prior to processing a request. In this case I believe buffering with 2x would be enough to guarantee there is no deadlock. I'm honestly not a huge fan as I think it makes the state machine more complex, but it might be okay
@alecholmez do you think this would cover this issue?
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@valerian-roche If we handle the responses and requests in two different go routines, will that solve the deadlock problem?
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@valerian-roche I tried to follow your suggestion to guarantee that the responses ar processed prior to new requests. Could you please take another look?
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Some tests failed because of this new change. If this approach is acceptable, I'll go on and fix them. |
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Signed-off-by: huabing zhao <zhaohuabing@gmail.com>
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LGTM, thanks a lot for your patience on this!
Were you able to test this on a running environments? I might be able to test it on some of our test systems next week if you cannot easily run it
Can you expand in the description describing a bit the implementation and the reasoning behind if people need to take a look later on?
@valerian-roche Thanks. I can test it since I have already encountered this issue in my local development environment. But it may take a few days since I'll be attending KubeCon CN next week and I'm preparing on my talk. Description updated with reasons and hows. |
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@valerian-roche Tested in my local development environment. |
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thanks @zhaohuabing for fixing this, can we get this merged ? Envoy Gateway (based on delta xDS) needs this |
This import brings in envoyproxy/go-control-plane#752 which should ensure resources pushed via delta ADS follow a specific order to ensure the traffic chain doesnt transiently break Signed-off-by: Arko Dasgupta <arko@tetrate.io>
* Bring in go-control-plane fixes This import brings in envoyproxy/go-control-plane#752 which should ensure resources pushed via delta ADS follow a specific order to ensure the traffic chain doesnt transiently break Signed-off-by: Arko Dasgupta <arko@tetrate.io> * rerun go generate Signed-off-by: Arko Dasgupta <arko@tetrate.io> --------- Signed-off-by: Arko Dasgupta <arko@tetrate.io>
Fix #705
Why we need this?
go-control-plane doesn't guarantee the orders of responses for delta ADS. If there are two requests, the response to the second request may be sent before the first request. This causes problems for us since our application generates xDS resources for clusters and listeners in sequence. Sometimes, the the listener resources are sent to Envoy before the cluster resources. As a result, Envoy complains that it can't find referenced clusters in the Listener configuration and fail to apply the configuration.
How this PR fix this issue?
This PR follow an approach similar to #544 to enable ordered responses for ADS delta watches.
deltaMuxedResponseschannel to guarantee that the orders won't be changeddeltaMuxedResponseschannel is created with a buffer size of 2x the number of types.