Logging
=======

To add your own lines to the node's output you should use the ``CCF_APP_*`` macros defined in ``ccf/ds/logger.h``:

.. code-block:: cpp

    #include "ccf/ds/logger.h"

    int x = 5;
    CCF_APP_INFO("x is currently {}", x);

Applications written in JavaScript and TypeScript can produce similar log lines using standard functions ``console.log``, ``console.info``, ``console.warn``, and ``console.error``:

.. code-block:: js

    x = 5
    console.info(`x is ${x}`)

Either approach will produce a line in the node's stdout like::

    2022-07-12T12:34:56.626262Z        0   [info ][app] ../src/my_app/my_app.cpp:42    | x is 5

These logging functions do several things:

- Variable substitution. See `libfmt <https://fmt.dev/latest/>`_ for more details of the formatting syntax used in C++
- Declare the severity of the entry. CCF defines 5 levels (``trace``, ``debug``, ``info``, ``fail``, and ``fatal``), and production nodes will generally ignore entries below ``info``
- Prefix formatted metadata. The produced log line will include a timestamp, the name and line number where the line was produced, and an ``[app]`` tag
- Queue writes to a ringbuffer for the host to process, so diagnostic logging should not cause significant performance drops

.. note:: The app's logging entries will be interleaved (line-by-line) with the framework's logging messages. Filter for entries containing ``[app]`` to extract only application log lines.

.. note:: Since these logs are produced during execution, they will generally only appear on a single node and not every replica. They may also log information about uncommitted or re-executed transactions, as they are emitted independently of transaction commit.