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https://issues.apache.org/jira/browse/CASSANDRA-12791?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15581110#comment-15581110
 ] 

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

The patch is fine, I'm not opposed to removing {{ConstructionTime}} given that 
the meaning of {{isCrossNode}} now becomes a property of the message rather 
than the timestamp. Also, I think 3.X is reasonable for this patch because 
CASSANDRA-10580 was added in 3.2, see below.

{quote}
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.
{quote}

The confusion is due to the fact that {{ConstructionTime.isCrossNode}} was 
intended to be a property of the construction timestamp, as it indicates 
whether the timestamp originated at the sender or at the receiver, the intent 
was not to indicate that the message itself is cross node, although comparing 
the timestamps to determine this was incorrect. CASSANDRA-10580 added the 
metrics in messaging service later on, and the comments in the code indicate 
that the metrics refer to the actual messages being local vs. cross node, but 
using {{isCrossNodeTimeout}} in messaging service was not correct. 

{quote}
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.
{quote}

{{timestamp != crossNodeTimestamp}} orginates from this 
[comment|https://issues.apache.org/jira/browse/CASSANDRA-9793?focusedCommentId=14635441&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14635441]
 from CASSANDRA-9793. It is not only used for the metrics in MessagingService 
but also for distinguishing messages dropped due to a cross node timeout vs. a 
local timeout in the logs. The intent is to help operators work out if messages 
are dropped because of clock skew. So, at this line 
[here|https://github.com/pcmanus/cassandra/commit/0f3d7f6318be11b095bbe21d0c848da6409d1a93#diff-af09288f448c37a525e831ee90ea49f9R1204],
 we need to also check {{DD.hasCrossNodeTimeout()}}, a message originating 
cross node is not sufficient. 

{{isCrossNodeTimeout}} is probably a misleading name now, and should become 
simply {{isCrossNode}}.

I would like to give a heads up to [~brandon.williams] to make sure he agrees 
that it's OK, from an operator point of view, to classify a dropped message in 
the logs as dropped due to cross node timeout if:

* the message originates from a different node
* {{DD.hasCrossNodeTimeout()}} is true

given that it is extremely rare for a message to be received in the same 
millisecond and have the machines perfectly synchronized and, if the machines 
are not synchronized, then we can still have an identical timestamp due to 
chance, so the best thing we can do is look at the yaml property.

As for the rest of the patch:

* There a typo at the end of this 
[line|https://github.com/pcmanus/cassandra/commit/0f3d7f6318be11b095bbe21d0c848da6409d1a93#diff-2578da7d6bbdd276157604856543cbecR43],
 the {{:}} should be {{;}}, this is the reason for the failures in 
{{MonitoringTaskTest}} and all of the dtests.

* Another typo 
[here|https://github.com/pcmanus/cassandra/commit/0f3d7f6318be11b095bbe21d0c848da6409d1a93#diff-70a9824fd63a5f3970840e376918da3eR137]
 in the comments: {{lower}} -> {{higher}}.

> 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.



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