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https://issues.apache.org/jira/browse/CASSANDRA-4861?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13484158#comment-13484158
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Sylvain Lebresne commented on CASSANDRA-4861:
---------------------------------------------
bq. ISTM that over half of your objections are addressed by considering that
only debug messages on a request-handling thread are traced.
It's a good point, and I did forgot that it was only logged on request-handling
threads, but that doesn't really "addressed" the problem, just make my
particular examples not relevant. As a developer, I still have to worry when I
want to add a debug logging if I'm on a request-handling thread or not, which I
shouldn't have. Do I want to log that and do I want to add this to the query
tracing is two different questions and having one implying the other is
limiting. But granted, the fact that it's only request-handling thread probably
makes fixing this less urgent than I made it sound.
The other thing is that this thread local business we are forced to use
currently seems also a bit limiting/ugly. By design, a lot of what happen in
Cassandra doesn't happen on the request-handling thread, but rather on some
other stage. We might be good for an initial version, but I'd be surprised if
we don't end up wanting to trace things happening on other threads very soon.
Which we could do with the current implementation by passing the trace uuid
around and making sure every thread that need to trace things first initialize
it's thread local TraceState based on said trace uuid, but it would be much
simpler to just be able to pass the TraceState object around.
bq. we're not tied to this implementation at all
That's definitively true and I didn't pretended otherwise. What I'm wondering
however is if using a log4j appender bring any benefits? Because it seems to me
it only bring limitations. As long as we agree on that last part, I'm willing
to spend the < 2 hours (there really isn't much to change) needed to remove the
dependency of tracing to log4j before 1.2 just for the sake of not having
people wonder what that TracingAppender in log4j-server.properties is and why
it seems that the rootLogger is at debug even though they don't get DEBUG logs.
> Consider separating tracing from log4j
> --------------------------------------
>
> Key: CASSANDRA-4861
> URL: https://issues.apache.org/jira/browse/CASSANDRA-4861
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Sylvain Lebresne
> Fix For: 1.2.0 beta 2
>
>
> Currently, (as far as I understand) tracing is implemented as a log4j
> appender that "intercepts" all log messages and write them to a system table.
> I'm sorry to not have bring that up during the initial review (it's hard to
> follow every ticket) but before we release this I'd like to have a serious
> discussion on that choice because I'm not convinced (at all) that it's a good
> idea. Namely, I can see the following drawbacks:
> # the main one is that this *forces* every debug messages to be traced and
> conversely, every traced message to be logged at debug. But I strongly think
> that debug logging and query tracing are not the same thing. Don't get me
> wrong, there is clearly a large intersection between those two things (which
> is fine), but I do think that *identifying* them is a mistake. More
> concretely:
> ** Consider some of the messages we log at debug in CFS:
> {noformat}
> logger.debug("memtable is already frozen; another thread must be flushing
> it");
> logger.debug("forceFlush requested but everything is clean in {}",
> columnFamily);
> logger.debug("Checking for sstables overlapping {}", sstables);
> {noformat}
> Those messages are useful for debugging and have a place in the log at
> debug, but they are noise as far as query tracing is concerned (None have any
> concrete impact on query performance, they just describe what the code has
> done). Or take the following ones from CompactionManager:
> {noformat}
> logger.debug("Background compaction is still running for {}.{} ({}
> remaining). Skipping", new Object[] {cfs.table.name, cfs.columnFamily,
> count});
> logger.debug("Scheduling a background task check for {}.{} with {}", new
> Object[] {cfs.table.name, cfs.columnFamily,
> cfs.getCompactionStrategy().getClass().getSimpleName()});
> logger.debug("Checking {}.{}", cfs.table.name, cfs.columnFamily);
> logger.debug("Aborting compaction for dropped CF");
> logger.debug("No tasks available");
> logger.debug("Expected bloom filter size : " + expectedBloomFilterSize);
> logger.debug("Cache flushing was already in progress: skipping {}",
> writer.getCompactionInfo());
> {noformat}
> It is useful to have that in the debug log, but how is any of that useful
> to users in query tracing? (it may be useful to trace if a new compaction
> start or stop, because that does influence query performance, but those
> message do not). Also take the following message logged when a compaction is
> user
> interrupted:
> {noformat}
> if (t instanceof CompactionInterruptedException)
> {
> logger.info(t.getMessage());
> logger.debug("Full interruption stack trace:", t);
> }
> {noformat}
> I can buy that you may want the first log message in the query tracing,
> but the second one is definitively something that only make sense for debug
> logging but not for query tracing (and as a side note, the current
> implementation don't do something sensible as it traces "Full interruption
> stack trace:" but completely ignore the throwable).
> Lastly, and though that's arguably more a detail (but why would we settle
> for something good enough if we can do better) I believe that in some cases
> you want an event to be both logged at debug and traced but having different
> messages could make sense. For instance, in CFS we have
> {noformat}
> logger.debug("Snapshot for " + table + " keyspace data file " +
> ssTable.getFilename() + " created in " + snapshotDirectory);
> {noformat}
> I'm not convinced that snapshot should be part of query tracing given it
> doesn't really have an impact on queries, but even if we do trace it, we
> probably don't care about having one event for each snapshoted file (2
> events, one for the start of the snapshot, one for the end would be enough).
> As it stands, I think query tracing will have a lot of random noises,
> which will not only be annoying but I'm also sure will make users spend time
> worrying about events that have no impact whatsoever. And I've only looked at
> the debug message of 2 classes ...
> ** I also think there could be case where we would want to trace something,
> but not have it in the debug log. For instance, it makes sense in the query
> trace to know how long parsing the query took. But logging too much info per
> query like that will make the debug log unmanageable in many case. And if you
> say, let's log that at TRACE, you have to put the TracingAppender at trace
> and now you get all the junk (junk as far as query tracing is concerned) that
> trace logging have.
> # I find it rather unintuitive. Query tracing is enable per query and it
> writes its trace in a system table. How come changing some settings in the
> log4j config file can disable that feature for me? I agree it's not a big
> deal, but it does is some form of leaking an implementation detail.
> # It doesn't seem very future-proof. For instance (and that's only an
> example), I think it could make sense to later add a tracing level. I might
> want a very detailed tracing mode where I get very fine grained details like
> what sstable was hit, and how many seeks on that sstable we did and whatnot.
> But as said above, using the log4j TRACE level for that is not convenient
> because it logs lots of stuff that are completely unrelated to my query.
> And the only advantages of using log4j that I can see is that it that we
> don't have to go through all our debug statements to check which ones make
> sense to also add to query traces. But as lengthly explained above, that's
> not a real advantage as it end up generating trace that are less useful/user
> friendly as they could be.
> Now maybe there is killer advantages that I don't see, and the goal of this
> ticket is to discuss those. But if there isn't, I'm very much in favor of
> moving from log4j for this.
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