More Logs - Structured Logging and Exception Handling
=====================================================

This library provides two main features

-  **Structured logging** - output is all JSON (with options to
   serialize to text for humans)
-  **Exception handling weaved in** - Good logs must represent what
   happened, and that can not be done if the logging library is not
   intimately familiar with the (exceptional) code paths taken.

Motivation
----------

Exception handling and logging are undeniably linked. There are many
instances where exceptions are raised and must be logged, and others
where the subsuming system can fully handle the exception, and no log
should be emitted. Exception handling semantics are great because they
decouple the cause from the solution, but this can be at odds with clean
logging - which couples raising and catching to make appropriate
decisions about what to emit to the log.

This logging module is additionally responsible for raising exceptions,
collecting the trace and context, and then deducing if it must be
logged, or if it can be ignored because something can handle it.

More Reading
~~~~~~~~~~~~

-  **Structured Logging is Good** -
   https://sites.google.com/site/steveyegge2/the-emacs-problem

Basic Usage
-----------

Use ``Log.note()`` for all logging
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. code:: python

        Log.note("Hello, World!")

There is no need to create logger objects. The ``Log`` module will keep
track of what, where and who of every call.

Use named parameters
~~~~~~~~~~~~~~~~~~~~

Do not use Python's formatting operator "``%``\ " nor it's ``format()``
function. Using them will create a string at call time, which is a
parsing nightmare for log analysis tools.

.. code:: python

        Log.note("Hello, {{name}}!", name="World!")

All logs are structured logs; the parameters will be included,
unchanged, in the log structure. This library also expects all parameter
values to be JSON- serializable so they can be stored/processed by
downstream JSON tools.

.. code:: javascript

        {//EXAMPLE STRUCTURED LOG
            "template": "Hello, {{name}}!",
            "param": {"name": "World!"},
            "timestamp": 1429721745,
            "thread": {
                "name": "Main thread"
            },
            "location": {
                "line": 3,
                "file": "hello.py",
                "method": "hello"
            },
            "machine": {
                "python": "CPython",
                "os": "Windows10",
                "name": "ekyle-win"
            }
        }

Instead of ``raise`` use ``Log.error()``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. code:: python

        Log.error("This will throw an error")

The actual call will always raise an exception, and it manipulates the
stack trace to ensure the caller is appropriately blamed. Feel free to
use the ``raise`` keyword (as in ``raise Log.error("")``), if that looks
nicer to you.

Always chain your exceptions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``cause`` parameter accepts an ``Exception``, or a list of
exceptions. Chaining is generally good practice that helps you find the
root cause of a failure.

.. code:: python

        try:
            # Do something that might raise exception
        except Exception, e:
            Log.error("Describe what you were trying to do", cause=e)

Always catch all ``Exceptions``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Catching all exceptions is preferred over the
*only-catch-what-you-can-handle* strategy. First, exceptions are not
lost because we are chaining. Second, we catch unexpected ``Exceptions``
early and we annotate them with a description of what the local code was
intending to do. This annotation effectively groups the possible errors
(known, or not) into a class, which can be used by callers to decide on
appropriate mitigation.

To repeat: When using dependency injection, callers can not reasonably
be expected to know about the types of failures that can happen deep
down the call chain. This makes it vitally important that methods
summarize all exceptions, both known and unknown, so their callers have
the information to make better decisions on appropriate action.

For example: An abstract document container, implemented on top of a SQL
database, should not emit SQLExceptions of any kind: A caller that uses
a document container should not need to know how to handle SQLExceptions
(or any other implementation-specific exceptions). Rather, in this
example, the caller should be told it "can not add a document", or "can
not remove a document". This allows the caller to make reasonable
decisions when they do occur. The original cause (the SQLException) is
in the causal chain.

Another example, involves *nested exceptions*: If you catch a particular
type of exception, you may not expect to catch the that same type of
exception from deeper in the call chain. Narrow exception handling is an
illusion. Broad exception handling will force you to consider a variety
of failures early; what it means when a block of code fails, no matter
the reason.

Use named parameters in your error descriptions too
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Error logging accepts keyword parameters just like ``Log.note()`` does

.. code:: python

        def worker(value):
            try:
                Log.note("Start working with {{key1}}", key1=value1)
                # Do something that might raise exception
            except Exception, e:
                Log.error("Failure to work with {{key2}}", key2=value2, cause=e)

No need to formally type your exceptions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

An exception can be uniquely identified by the first-parameter string
template it is given; exceptions raised with the same template are the
same type. You should have no need to create new exception sub-types.

Testing for exception "types"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This library advocates chaining exceptions early and often, and this
hides important exception types in a long causal chain. MoLogs allows
you to easily test if a type (or string, or template) can be found in
the causal chain by using the ``in`` keyword:

.. code:: python

        def worker(value):
            try:
                # Do something that might raise exception
            except Exception, e:
                if "Failure to work with {{key2}}" in e:
                    # Deal with exception thrown in above code, no matter
                    # how many other exception handlers where in the chain

If you can deal with an exception, then it will never be logged
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When a caller catches an exception from a callee, it is the caller's
responsibility to handle that exception, or re-raise it. There are many
situations a caller can be expected to handle exceptions; and in those
cases logging an error would be deceptive.

.. code:: python

        def worker(value):
            try:
                Log.error("Failure to work with {{key3}}", key3=value3)
            except Exception, e:
                # Try something else

Use ``Log.warning()`` if your code can deal with an exception, but you still
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

want to log it as an issue

.. code:: python

        def worker(value):
            try:
                Log.note("Start working with {{key4}}", key4=value4)
                # Do something that might raise exception
            except Exception, e:
                Log.warning("Failure to work with {{key4}}", key4=value4, cause=e)

Don't loose your stack trace!
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Be aware your ``except`` clause can also throw exceptions: In the event
you catch a vanilla Python Exception, you run the risk of loosing its
stack trace. To prevent this, wrap your exception in an ``Except``
object, which will capture your trace for later use. Exceptions thrown
from this ``Log`` library need not be wrapped because they already
captured their trace. If you wrap an ``Except`` object, you simply get
back the object you passed.

``python     try:         # DO SOME WORK             except Exception, e:         e = Except.wrap(e)         # DO SOME FANCY ERROR RECOVERY``

Other forms
-----------

All the ``Log`` functions accept a ``default_params`` as a second
parameter, like so:

.. code:: python

        Log.note("Hello, {{name}}!", {"name": "World!"})

this is meant for the situation your code already has a bundled
structure you wish to use as a source of parameters. If keyword
parameters are used, they will override the default values. Be careful
when sending whole data structures, they will be logged!

Please, never use locals()
~~~~~~~~~~~~~~~~~~~~~~~~~~

.. code:: python

        def worker(value):
            name = "tout le monde!"
            password = "123"
            Log.note("Hello, {{name}}", locals())      # DO NOT DO THIS!

Despite the fact using ``locals()`` is a wonderful shortcut for logging
it is dangerous because it also picks up sensitive local variables. Even
if ``{{name}}`` is the only value in the template, the whole
``locals()`` dict will be sent to the structured loggers for recording.

Log 'Levels'
------------

The ``logs`` module has no concept of logging levels it is expected that
debug variables (variables prefixed with ``DEBUG_`` are used to control
the logging output.

.. code:: python

        # simple.py
        DEBUG_SHOW_DETAIL = True

        def worker():
            if DEBUG_SHOW_DETAIL:
                Log.note("Starting")

            # DO WORK HERE

            if DEBUG_SHOW_DETAIL:
                Log.note("Done")

        def main():
            try:
                settings = startup.read_settings()
                Log.start(settings.debug)

                # DO WORK HERE

            except Exception, e:
                Log.error("Complain, or not", e)
            finally:
                Log.stop()

These debug variables can be set by configuration file:

.. code:: javascript

        // settings.json
        {
            "debug":{
                "constants":{"simple.DEBUG_SHOW_DETAILS":false}
            }
        }

Configuration
-------------

The ``logs`` module will log to the console by default.
``Log.start(settings)`` will redirect the logging to other streams, as
defined by the settings:

-  **log** - List of all log-streams and their parameters
-  **trace** - Show more details in every log line (default False)
-  **cprofile** - Used to enable the builtin python c-profiler (default
   False)
-  **profile** - Used to enable pyLibrary's simple profiling (default
   False) (eg with Profiler("some description"):)
-  **constants** - Map absolute path of module constants to the values
   that will be assigned. Used mostly to set debugging constants in
   modules.

Of course, logging should be the first thing to be setup (aside from
digesting settings of course). For this reason, applications should have
the following structure:

.. code:: python

        def main():
            try:
                settings = startup.read_settings()
                Log.start(settings.debug)

                # DO WORK HERE

            except Exception, e:
                Log.error("Complain, or not", e)
            finally:
                Log.stop()

::

        "log": [
            {
                "class": "logging.handlers.RotatingFileHandler",
                "filename": "examples/logs/examples_etl.log",
                "maxBytes": 10000000,
                "backupCount": 100,
                "encoding": "utf8"
            },
            {
                "log_type": "email",
                "from_address": "klahnakoski@mozilla.com",
                "to_address": "klahnakoski@mozilla.com",
                "subject": "[ALERT][DEV] Problem in ETL Spot",
                "$ref": "file://~/private.json#email"
            },
            {
                "log_type": "console"
            }
        ]

Problems with Python Logging
----------------------------

| `Python's default ``logging``
module <https://docs.python.org/2/library/logging.html#logging.debug>`__
comes close to doing the right thing, but fails:
|  \* It has keyword parameters, but they are expanded at call time so
the values are lost in a string.
|  \* It has ``extra`` parameters, but they are lost if not used by the
matching ``Formatter``.
|  \* It even has stack trace with ``exc_info`` parameter, but only if
an exception is being handled. \* Python 2.x has no builtin exception
chaining, but `Python 3
does <https://www.python.org/dev/peps/pep-3134/>`__
