[jira] [Commented] (CASSANDRA-12791) MessageIn logic to determine if the message is cross-node is wrong

["Stefania (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-12791?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15624015#comment-15624015 ] 

Stefania commented on CASSANDRA-12791:
--------------------------------------

I was trying to preserve the behavior of CASSANDRA-9793. However, it is true that knowing if cross-node-timeout is enabled can be easily derived from yaml, and I hadn't noticed that CASSANDRA-10580 added the latency to the same log message. So I agree that it is better to have the number of dropped messages and latency match.

I've amended the log message in [this commit|https://github.com/stef1927/cassandra/commit/39168a3eb8e43815e4001521d2793d59c227f9ee].

CI still pending.

> MessageIn logic to determine if the message is cross-node is wrong
> ------------------------------------------------------------------
>
>                 Key: CASSANDRA-12791
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-12791
>             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 method.
> 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.



--
This message was sent by Atlassian JIRA
(v6.3.4#6332)