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From "James Clampffer (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-9118) Add logging system for libdhfs++
Date Wed, 09 Mar 2016 20:01:40 GMT

    [ https://issues.apache.org/jira/browse/HDFS-9118?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15187815#comment-15187815
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James Clampffer commented on HDFS-9118:
---------------------------------------

Thanks for the feedback [~bobthansen], I hadn't thought about some of these things.

{quote}
Add a TRACE level for finer than Debug. My general mapping is

ERROR: Something went wrong that is catastrophic (invalid internal states, etc.)
WARN: Something went wrong that you'll always want to know about, but doesn't halt operations
(recoverable errors, messages out of order, etc.)
INFO: Here's a high-level list of what's going on with libhdfs++ (opening and closing files,
retrying operations, etc.)
DEBUG: Here's a low-level stream of libhfds++ operations (operation by operation)
TRACE: Here's a packet-by-packet stream of libhdfs++ operations (function entry/exit, each
packet sent, etc.)
{quote}

I'll do this, right now there aren't a whole lot of messages that I'd consider trace level,
but I'll go through and push anything down that looks like it's more detail than needed for
typical debugging.  Either way I'll add the appropriate stuff to handle a trace level. 

{quote}
Add a LogEnabled(level, component)-->bool function that can be checked before constructing
the LogMessage. This allows client code to circumvent expensive logging operations. Make it
pluggable so that we can hook it into the current Google logging settings, etc.
{quote}
Yea it looks like I can have the macro instantiate a dummy object if logging isn't enabled.
 I was thinking of putting this off until I had a chance to do some decent profiling, but
it doesn't look hard to extend the macros.  This still ends up evaluating any function calls
in the log, but at least gets rid of the stringstream allocations and LogManager::Write (which
grabs a lock) calls.

I'll look into having the LogManager delegate to a plugin.  Depending on how large the patch
starts getting I might save that for later.

{quote}
Logging should be disabled by default. This is a low-level library; it shouldn't spew if not
asked to
{quote}
Good point.  It was getting annoying to look through all the logs for the tests I'm writing.

{quote}
Can we make the LogMessage a real ostream? That way consumers could do LogMessage(...) <<
hex << my_pointer rather than LogHelpers::PointerToHex(...). It's a bit more idiomatic,
but I don't want to create a lot more work.
{quote}
It looks implementing a new ostream correctly involves a fair amount of work (according to
stackoverflow).  For now I think I can just overload operator<<(void*) to deal with
hex, and put in a operator<<(const char *) to specialize for c style strings.  Does
that sound like a decent solution?

{quote}
Rather than require the consumers to call hdfsFreeLogData, always have the library free the
object. If the consumer wants to retain the data, they can copy the structure. We could provide
an hdfsCopyLogData method if we wanted to be very nice.
{quote}
This sounds like a good improvement, I assume most people just want to get the info to stick
into their own log systems on the C side.  This will also help cut back on heap allocations
with short lives.









> Add logging system for libdhfs++
> --------------------------------
>
>                 Key: HDFS-9118
>                 URL: https://issues.apache.org/jira/browse/HDFS-9118
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: hdfs-client
>    Affects Versions: HDFS-8707
>            Reporter: Bob Hansen
>            Assignee: James Clampffer
>         Attachments: HDFS-9118.HDFS-8707.000.patch, HDFS-9118.HDFS-8707.001.patch
>
>
> With HDFS-9505, we've starting logging data from libhdfs++.  Consumers of the library
are going to have their own logging infrastructure that we're going to want to provide data
to.  
> libhdfs++ should have a logging library that:
> * Is overridable and can provide sufficient information to work well with common C++
logging frameworks
> * Has a rational default implementation 
> * Is performant



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