NAME
    Log::Defer - Deferred logs and timers

SYNOPSIS
        use Log::Defer;
        use JSON::XS; ## or whatever

        sub my_logger_function {
          my $msg = shift;
          print JSON::XS->new->pretty(1)->encode($msg);
        }

        my $logger = Log::Defer->new({
                                       cb => \&my_logger_function,
                                       verbosity => 30,
                                     });

        $logger->info("hello world");

        my $timer = $logger->timer('some timer');
        undef $timer; ## stops timer

        undef $logger; ## write out log message

    Prints:

        {
           "start" : 1340421702.16684,
           "end" : 0.000249,
           "logs" : [
              [
                 0.000147,
                 30,
                 "hello world"
              ]
           ]
        }

DESCRIPTION
    I believe a lot of log processing is done too early. This module helps
    you to defer log processing in two ways:

    Defer recording of log messages until a "transaction" has completed
        Typically this transaction is something like an HTTP request or a
        cron job. Generally log messages are easier to read if they are
        recorded atomically and are not intermingled with log messages
        created by other transactions.

    Defer rendering of log messages
        Sometimes you don't know how logs should be rendered until long
        after the message has been written. If you aren't sure what
        information you'll want to display, or you expect to display logs in
        multiple different formats, it makes sense to store your logs in a
        highly structured format so they can be processed as late as
        possible.

    This module doesn't actually write out logs! To use this module for
    normal logging purposes you also need a logging library (some of them
    are mentioned in "SEE ALSO").

USAGE
    The simplest use case is outlined in the SYNOPSIS. You create a new
    Log::Defer object and pass in a code ref callback. This callbac will be
    called once the Log::Defer object is destroyed or once all references to
    the object go out of scope.

    With Log::Defer, if a transaction has several possible code paths it can
    take, there is no need to manually ensure that every possible path ends
    up calling your logging routine at the end. The log writing will be
    deferred until the logger object is destroyed or goes out of scope.

    In an asynchronous application where multiple asynchronous tasks are
    kicked off concurrently, each task can keep a reference to the logger
    object and the log writing will be deferred until all tasks are
    finished.

    Log::Defer makes it easy to gather timing information about the various
    stages of your request. This is explained further below.

STRUCTURED LOGS
    Free-form line-based log protocols are probably the most common log
    formats by far. The formats are usually just happenstance -- whatever
    happened to be convenient for the programmer.

    Unfortunately, doing analysis on ad-hoc unstructured multi-line formats
    requires a lot of time-consuming parsing work. As well as being a perl
    module, Log::Defer is also a specification for a structured logging
    format.

    Although this module doesn't impose any external encoding for log
    messages on you, some tools like the visualisation tool only support
    JSON at this time.

    FIXME: QQQ Normally all unnecessary white-space would be removed and it
    would be stored on a single line so that ad-hoc command-line "grep"ing
    still works.

LOG MESSAGES
    Log::Defer objects provide a very basic "log level" system. In order of
    increasing verbosity, here are the normal logging methods and their
    numeric log level:

        $logger->error("...");  # 10
        $logger->warn("...");   # 20
        $logger->info("...");   # 30
        $logger->debug("...");  # 40

    You can also use custom log levels:

        $logger->add_log(25, "...");

    If you pass in a "verbosity" argument to the Log::Defer constructor,
    messages with a higher log level will not be included in the final log
    message. Otherwise, all log messages are included.

    Even if you include noisy debug logs you can filter them out with the
    visualisation tool at display time. The "verbosity" argument is only
    useful for reducing the size of log messages or eliminating unnecessary
    processing overhead (see the no-overhead debug logs section below).

    Note that you can pass in multiple items to a log message and they don't
    even need to be strings:

        $logger->warn("something weird happened: $@", { username => $username });

    In the deferred logging callback, the log messages are recorded in the
    "logs" element of the $msg hash. It is an array ref and here would be
    the element pushed onto "logs" by the "warn" method call above:

        [ 0.201223, 20, "something weird happened: peer timeout", { username => "jimmy" } ]

    The first element is a timestamp of how long the "warn" method was
    called after the "start" in seconds (see TIMERS below). The second
    element is the verbosity level of this message.

NO-OVERHEAD DEBUG LOGS
    If you would like to compute complex messages in debug mode but don't
    want to burden your production systems with this overhead, you can use
    delayed message generation:

        $logger->debug(sub { "Connection: " . dump_connection_info($conn) });

    The sub won't be invoked unless the logger object is instantiated with
    "verbosity" of 40 or higher (or you omit "verbosity" altogether).

DATA
    Instead of log messages, you can directly add items to a "data" hash
    reference with the "data" method:

        $log->data->{ip} = $ENV{REMOTE_ADDR};

    This is a useful place to recording info that needs to be extracted
    since you don't need to crawl through log message entries. Anything you
    put in the "data" hash reference will be passed along untouched to your
    defered callback.

TIMERS
    When the logger object is first created, the current time is recorded as
    a Time::HiRes absolute timestamp and is stored in the "start" element of
    the log hash. All other times are relative offsets from "start".

    When the logger object is destroyed, the time elapsed since "start" is
    stored in "end".

    In addition to start and duration of the entire transaction, you can
    also record timing data of sub-portions of your transaction by using
    timer objects.

    Timer objects are created by calling the "timer" method on the logger
    object. This method should be passed a description of what you are
    timing.

    The timer starts as soon as the timer object is created and stops once
    the last reference to the timer is destroyed or goes out of scope. If
    the logger object itself is destroyed or goes out of scope then all
    outstanding timers are terminated at that point.

    "start" is a Time::HiRes absolute timestamp. All other times are
    relative offsets from this "start" time. Everything is in seconds.

    Here is a fairly complicated example showing how to use concurrent
    timers:

        sub handle_request {
          my $request = shift;
          my $logger = Log::Defer->new(\&my_logging_function);

          my $headers = do {
            my $parse_timer = $logger->timer('parsing request');
            parse_request($request);
          };

          my $fetch_timer = $logger->timer('fetching results');
          async_fetch_results($headers, sub {

            ## stop first timer by undefing ref, then start new timer
            undef $fetch_timer; $fetch_timer = $logger->timer('fetching stage 2');

            async_fetch_stage_2($headers, sub {

              $logger; ## keep reference alive
              undef $fetch_timer;
              send_response();

            });

            my $update_cache_timer = $logger->timer('update cache');

            async_update_cach(sub {

              $logger; ## keep reference alive
              undef $update_cache_timer;

            });

          });
        }

EXAMPLE LOG MESSAGE
    Each structured log message will be passed into the callback provided to
    "new" as a perl hash reference that contains various other perl
    data-structures. What you do at this point is up to you.

    Here is a prettified example of a JSON-encoded message:

        {
           "start" : 1340353046.93565,
           "end" : 0.202386,
           "logs" : [
              [
                 0.000158,
                 30,
                 "This is an info message (log level=30)"
              ],
              [
                 0.201223,
                 20,
                 "Warning! \n\n Here is some more data:",
                 {
                     "whatever" : 987
                 }
              ]
           ],
           "data" : {
              "junkdata" : "some data"
           },
           "timers" : {
              "junktimer" : [
                 0.000224,
                 0.100655
              ],
              "junktimer2" : [
                 0.000281,
                 0.202386
              ]
           }
        }

Visualisation
    See the Log::Defer::Viz module for a command line utility that renders
    Log::Defer logs. Timers are shown something like this:

         download file |===============================================|
          cache lookup |==============|
          update cache                |=========================================|
             DB lookup                |======================|
            sent reply                                                 X
        ________________________________________________________________________________
        times in ms    0.2            32.4                             100.7
                                                             80.7              119.2

SEE ALSO
    As mentioned above, this module doesn't actually log messages to
    disk/syslog/anything so you still must use some other module to record
    your log messages. There are many libraries on CPAN that can do this and
    there should be at least one that fits your requirements. Some examples
    are: Sys::Syslog, Log::Dispatch, Log::Handler, Log::Log4perl, Log::Fast,
    AnyEvent::Log.

    Additionally, this module doesn't provide any official serialization
    format. There are many choices for this, including JSON::XS, Sereal,
    Storable, and Data::MessagePack.

    Currently the timestamp generation system is hard-coded to
    "Time::HiRes::time". You should be aware of some caveats related to
    non-monotonous clocks that are discussed in Time::HiRes.

AUTHOR
    Doug Hoyte, "<doug@hcsw.org>"

COPYRIGHT & LICENSE
    Copyright 2012-2013 Doug Hoyte.

    This module is licensed under the same terms as perl itself.

