CBGT: Dead node detection autofailover

[zgramana]

As documented here, couchbaselabs/cbgt#22,

CBGT does provide a way to do this via an API. However, that would only apply for SG nodes taken down intentionally.

[tleyden]

After speaking to @steveyen, it appears there are a few options:

• Tight integration with ns_server would allow this feature to be inherited -- of course, this is probably a long way off.
• This problem has already been solved by CBFS in a simple and elegant manner that requires no inter-node communication. That part can be refactored out of CBFS into a shared library that CBGT could use.

[tleyden]

Requirements

• A timeout is specified (eg, 30s), and if the node is deemed as being down outside of the range of the timout (eg, hasn't been seen for > 30s), then any other node who notices this will remove the node from the CBGT Cfg so it's Pindexes are re-distributed. This either has to be idempotent, or nodes will need to coordinate.
• If the node comes online again (perhaps it restarted, perhaps there was a temporary network partition), it will need to be re-added back into the CBGT Cfg. Not sure if this can be done from the node itself, or another node would need to do this.
• Some protection should be added such that an SG node in a "Reboot loop" would not cause havoc to the system.

[tleyden]

@steveyen -- need some help debugging this.

I have two sync gateways running, and the CFG is currently:

https://gist.githubusercontent.com/tleyden/bffa2eae29f1dfa9ece8/raw/60c8528ad34da2c09da9d4465cf48153e0344967/gistfile1.txt

Immediately after killing one of the Sync Gateways, the CFG is:

https://gist.github.com/tleyden/c5e1c452fa3e95a6900c

and in the remaining Sync Gateway, the following is shown in the logs:

https://gist.github.com/tleyden/0bc928851cf96343f536

and the final CFG is:

https://gist.github.com/tleyden/e9b481d63f3fb4144b81

the full logs for SG1 are here:

https://gist.github.com/tleyden/f956a682dfc7f659c8da

[steveyen]

Hi Traun, Looks like the invocation of cbgt.CfgRemoveNodeDef() with NODE_DEFS_KNOWN worked, but the invocation of cbgt.CfgRemoveNodeDef() with NODE_DEFS_WANTED didn't work.

Are the calls to cbgt.CfgRemoveNodeDef() returning any errors?

Or, if not already, added some error logging for more diagnosis?

More info on the two calls with NODE_DEFS_KNOWN and NODE_DEFS_WANTED are here...

https://github.com/couchbaselabs/cbgt/issues/26

[tleyden]

Here's the code:

https://github.com/couchbase/sync_gateway/blob/feature/distributed_index_autofailover/src/github.com/couchbase/sync_gateway/base/sgw_pindex.go#L306-L326

it has error checking, and I don't see the error being emitted.

invocation of cbgt.CfgRemoveNodeDef() with NODE_DEFS_WANTED didn't work.

btw, how did you determine that? Let me know if you have any tips on how to debug aside from checking the errors.

[tleyden]

WHOOPS, I see a bug.

Should be

for kind := range kinds {
        log.Printf("call cbgt.CfgRemoveNodeDef with nodeuuid: %v cfg: %+v", nodeUuid, h.Cfg)
        if err := cbgt.CfgRemoveNodeDef(
            h.Cfg,
            kind,  <----!!
            nodeUuid,
            h.CbgtVersion,
        ); err != nil {
            log.Printf("Warning: attempted to remove %v (%v) from CBGT but failed: %v", nodeUuid, kind, err)
        }

    }

[steveyen]

On the debugging, from the Cfg snapshots that you had gist'ed, I saw that one node had disappeared from nodesKnown but not from the nodesWanted section of the Cfg.

[tleyden]

@steveyen - The autofailover is still not working -- here are the steps to reproduce:

• Start sg1 with clean data dir and empty couchbase buckets
• Add a user and a doc
• Start sg2
• Add a doc
• Kill sg2
• Capture CFG json
• Add more docs

Actual sg1 is only getting DCP updates for vbuckets where vbucket id >= 512, but nothing for vbucketid < 512, which are the vbuckets that sg2 was handling. Expected sg1 should be getting DCP updates for all vbuckets after sg2 was shut down

• sg1 logs
• sg2 logs

What debug info can I collect?

[steveyen]

First suspicion... looks like a legit cbgt bug (!), as I just saw something similar with a cbft distributed cluster.

[steveyen]

Yeah, I think there's maybe a bug in the cbgt code that recalculates the DCP streams (CalcFeedsDelta). Looking through it.

[steveyen]

Please see CBGT fix, commit a119e02 which corrects the CalcFeedsDelta for the scenario of a shrinking cluster.

[tleyden]

Cool! Thanks, will try it and let you know.

[tleyden]

It works now!

• Bring sg1 online
• Add user and docs
• Bring sg2 online
• Add docs
• Take sg2 offline
• Verify that sg1 now handles DCP messages for all vbuckets

Here are the Sync Gw logs:

• sg1
• sg2