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From "Sylvain Lebresne (JIRA)" <>
Subject [jira] [Updated] (CASSANDRA-12791) MessageIn logic to determine if the message is cross-node is wrong
Date Fri, 14 Oct 2016 15:49:20 GMT


Sylvain Lebresne updated CASSANDRA-12791:
    Reviewer: Stefania
      Status: Patch Available  (was: Open)

I'm attaching a patch with the changes described above:
| [12791-trunk|] | [utests|]
| [dtests|] |

The patch actually does 2 (less important) things on top of the points above:
# it uses {{ApproximateTime.currentTimeMillis()}} for anything related to the message "construction
time" timestamp. This was currently a bit inconsistent in that the timestamp was set using
{{System.currentTimeMillis()}} but later compared to {{ApproximateTime.currentTimeMillis()}}.
This is not really a problem in practice but I'm a stickler for consistency so that was bugging
# the patch gets rid of the {{ConstructionTime}} class. This is arguably a bit of a personal
opinion here since the behavior of that class is still there, it's just somewhat "inlined"
in {{MessageIn}}/{{Monitorable}}. The reason is that after this patch, grouping the 2 infos
of that class (timestamp and isCrossNode boolean) together doesn't make sense to me: the {{isCrossNode}}
property is one of message/operation, not of the timestamp.

Also, the patch is currently against trunk. It's probably worth at least putting it 3.X though,
so unless object to that I'll rebase to that branch once I have test results for trunk. I'm
not sure it's worth bothering with 3.0 though: as said in the description, I consider it kind
of a bug in that not having "crossNode" metrics be incremented by default will almost surely
surprise users, but it's also fairly minor and could be called a "weird behavior".

> MessageIn logic to determine if the message is cross-node is wrong
> ------------------------------------------------------------------
>                 Key: CASSANDRA-12791
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: Sylvain Lebresne
>            Assignee: Sylvain Lebresne
>            Priority: Minor
> {{MessageIn}} has the following code to read the 'creation time' of the message on the
receiving side:
> {noformat}
> public static ConstructionTime readTimestamp(InetAddress from, DataInputPlus input, long
timestamp) throws IOException
> {
>     // make sure to readInt, even if cross_node_to is not enabled
>     int partial = input.readInt();
>     long crossNodeTimestamp = (timestamp & 0xFFFFFFFF00000000L) | (((partial &
0xFFFFFFFFL) << 2) >> 2);
>     if (timestamp > crossNodeTimestamp)
>     {
>         MessagingService.instance().metrics.addTimeTaken(from, timestamp - crossNodeTimestamp);
>     }
>     if(DatabaseDescriptor.hasCrossNodeTimeout())
>     {
>         return new ConstructionTime(crossNodeTimestamp, timestamp != crossNodeTimestamp);
>     }
>     else
>     {
>         return new ConstructionTime();
>     }
> }
> {noformat}
> where {{timestamp}} is really the local time on the receiving node when calling that
> The incorrect part, I believe, is the {{timestamp != crossNodeTimestamp}} used to set
the {{isCrossNode}} field of {{ConstructionTime}}. A first problem is that this will basically
always be {{true}}: for it to be {{false}}, we'd need the low-bytes of the timestamp taken
on the sending node to coincide exactly with the ones taken on the receiving side, which is
_very_ unlikely. It is also a relatively meaningless test: having that test be {{false}} basically
means the lack of clock sync between the 2 nodes is exactly the time the 2 calls to {{System.currentTimeMillis()}}
(on sender and receiver), which is definitively not what we care about.
> What the result of this test is used for is to determine if the message was crossNode
or local. It's used to increment different metrics (we separate metric local versus crossNode
dropped messages) in {{MessagingService}} for instance. And that's where this is kind of a
bug: not only the {{timestamp != crossNodeTimestamp}}, but if {{DatabaseDescriptor.hasCrossNodeTimeout()}},
we *always* have this {{isCrossNode}} false, which means we'll never increment the "cross-node
dropped messages" metric, which is imo unexpected.
> That is, it is true that if {{DatabaseDescriptor.hasCrossNodeTimeout() == false}}, then
we end using the receiver side timestamp to timeout messages, and so you end up only dropping
messages that timeout locally. And _in that sense_, always incrementing the "locally" dropped
messages metric is not completely illogical. But I doubt most users are aware of those pretty
specific nuance when looking at the related metrics, and I'm relatively sure users expect
a metrics named {{droppedCrossNodeTimeout}} to actually count cross-node messages by default
(keep in mind that {{DatabaseDescriptor.hasCrossNodeTimeout()}} is actually false by default).
> Anyway, to sum it up I suggest that the following change should be done:
> # the {{timestamp != crossNodeTimestamp}} test is definitively not what we want. We should
at a minimum just replace it to {{true}} as that's basically what it ends up being except
for very rare and arguably random cases.
> # given how the {{ConstructionTime.isCrossNode}} is used, I suggest that we really want
it to mean if the message has shipped cross-node, not just be a synonymous of {{DatabaseDescriptor.hasCrossNodeTimeout()}}.
It should be whether the message shipped cross-node, i.e. whether {{from == BroadcastAdress()}}
or not.

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