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  1. Cassandra
  2. CASSANDRA-4861

Consider separating tracing from log4j



    • Improvement
    • Status: Resolved
    • Normal
    • Resolution: Fixed
    • 1.2.0 beta 3
    • None
    • None


      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:

      1. 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:
             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);

          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:

             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());

          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

             if (t instanceof CompactionInterruptedException)
                 logger.debug("Full interruption stack trace:", t);

          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

             logger.debug("Snapshot for " + table + " keyspace data file " + ssTable.getFilename() + " created in " + snapshotDirectory);

          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.
      2. 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.
      3. 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.


        1. 4861.txt
          44 kB
          Sylvain Lebresne
        2. 4861-v2.txt
          44 kB
          Sylvain Lebresne



            slebresne Sylvain Lebresne
            slebresne Sylvain Lebresne
            Sylvain Lebresne
            Jonathan Ellis
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