.. _tracking:

MLflow Tracking
===============

The MLflow Tracking component lets you log and query experiments using either REST or Python.

Concepts
--------

MLflow Tracking is organized around the concept of *runs*, which are executions of some piece of
data science code. Each run records the following information:

Code Version
    Git commit used to execute the run, if it was executed from an :ref:`MLflow Project <projects>`.

Start & End Time
    Start and end time of the run

Source
    Name of the file executed to launch the run, or the project name and entry point for the run
    if the run was executed from an :ref:`MLflow Project <projects>`.

Parameters
    Key-value input parameters of your choice. Both keys and values are strings.

Metrics
    Key-value metrics where the value is numeric. Each metric can be updated throughout the
    course of the run (for example, to track how your model's loss function is converging), and
    MLflow will record and let you visualize the metric's full history.

Artifacts
    Output files in any format. For example, you can record images (for example, PNGs), models
    (for example, a pickled scikit-learn model) or even data files (for example, a
    `Parquet <https://parquet.apache.org/>`_ file) as artifacts.

Runs can be recorded from anywhere you run your code through MLflow's Python and REST APIs: for
example, you can record them in a standalone program, on a remote cloud machine, or in an
interactive notebook. If you record runs in an :ref:`MLflow Project <projects>`, however, MLflow
remembers the project URI and source version.

Finally, runs can optionally be organized into *experiments*, which group together runs for a
specific task. You can create an experiment via the ``mlflow experiments`` CLI, with
:py:func:`mlflow.create_experiment`, or via the corresponding REST parameters. The MLflow UI and
API let you create and search for experiments.

Once your runs have been recorded, you can query them using the :ref:`tracking_ui` or the MLflow
API.

Where Runs Get Recorded
-----------------------

MLflow runs can be recorded either locally in files or remotely to a tracking server.
By default, the MLflow Python API logs runs to files in an ``mlruns`` directory wherever you
ran your program. You can then run ``mlflow ui`` to see the logged runs. Set the
``MLFLOW_TRACKING_URI`` environment variable to a server's URI or call
:py:func:`mlflow.set_tracking_uri` to log runs remotely.

You can also :ref:`run your own tracking server <tracking_server>` to record runs.

Logging Data to Runs
--------------------

You can log data to runs using either the MLflow REST API or the Python API. This section 
shows the Python API, but there are corresponding REST APIs as well.

Basic Logging Functions
^^^^^^^^^^^^^^^^^^^^^^^

:py:func:`mlflow.set_tracking_uri` connects to a tracking URI. You can also set the
`MLFLOW_TRACKING_URI` environment variable to have MLflow find a URI from there. In both cases,
the URI can either be a HTTP/HTTPS URI for a remote server, or a local path to log data to a
directory. The URI defaults to ``mlruns``.

:py:func:`mlflow.get_tracking_uri` returns the current tracking URI.

:py:func:`mlflow.create_experiment` creates a new experiment and returns its ID. Runs can be
launched under the experiment by passing the experiment ID to `mlflow.start_run`

:py:func:`mlflow.start_run` returns the currently active run (if one exists), or starts a new run
and returns a :py:class:`mlflow.tracking.ActiveRun` object usable as a context manager for the
current run. You do not need to call `start_run` explicitly: calling one of the logging functions
with no active run will automatically start a new one.

:py:func:`mlflow.end_run` ends the currently active run, if any, taking an optional run status.

:py:func:`mlflow.active_run` returns a :py:class:`mlflow.tracking.Run` object corresponding to the
currently active run, if any.

:py:func:`mlflow.log_param` logs a key-value parameter in the currently active run. The keys and
values are both strings.

:py:func:`mlflow.log_metric` logs a key-value metric. The value must always be a number. MLflow will
remember the history of values for each metric.

:py:func:`mlflow.log_artifact` logs a local file as an artifact, optionally taking an
``artifact_path`` to place it in within the run's artifact URI. Run artifacts can be organized into
directories, so you can place the artifact in a directory this way.

:py:func:`mlflow.log_artifacts` logs all the files in a given directory as artifacts, again taking
an optional ``artifact_path``.

:py:func:`mlflow.get_artifact_uri` returns the URI that artifacts from the current run should be
logged to.


Launching Multiple Runs in One Program
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Sometimes you want to execute multiple MLflow runs in the same program: for example, maybe you are
performing a hyperparameter search locally or your experiments are just very fast to run. This is
easy to do because the ActiveRun object returned by :py:func:`mlflow.start_run` is a Python
`context manager <https://docs.python.org/2.5/whatsnew/pep-343.html>`_. You can "scope" each run to
just one block of code as follows:

.. code:: python

   with mlflow.start_run():
       mlflow.log_parameter("x", 1)
       mlflow.log_metric("y", 2)
       ...

The run remains open throughout the ``with`` statement, and is automatically closed when the
statement exits, even if it exits due to an exception.

Organizing Runs in Experiments
------------------------------

MLflow allow you to group runs under experiments, which can be useful for comparing runs intended
to tackle a particular task. You can create experiments via the CLI (``mlflow experiments``) or via
the :py:func:`create_experiment` Python API. You can pass the experiment ID for a individual run
via the CLI (for example, ``mlflow run ... --experiment-id [ID]``) or via the ``MLFLOW_EXPERIMENT_ID``
environment variable.

.. code:: shell

    # Prints "created an experiment with ID <id>
    mlflow experiments create fraud-detection
    # Set the ID via environment variables
    export MLFLOW_EXPERIMENT_ID=<id>

.. code:: python

    # Launch a run. The experiment ID is inferred from the MLFLOW_EXPERIMENT_ID environment
    # variable, or from the --experiment-id parameter passed to the Databricks CLI (the latter
    # taking precedence)
    with mlflow.start_run():
        mlflow.log_parameter("a", 1)
        mlflow.log_metric("b", 2)


.. _tracking_ui:

Tracking UI
-----------

The Tracking UI lets you visualize, search and compare runs, as well as download run artifacts or
metadata for analysis in other tools. If you have been logging runs to a local ``mlruns`` directory,
simply run ``mlflow ui`` in the directory above it, and it will load the corresponding runs.
Alternatively, the :ref:`MLflow Server <tracking_server>` serves the same UI, and enables remote storage of run artifacts.

The UI contains the following key features:

* Experiment-based run listing and comparison
* Searching for runs by parameter or metric value
* Visualizing run metrics
* Downloading run results

.. _tracking_query_api:

Querying Runs Programmatically
------------------------------

All of the functions in the Tracking UI can be accessed programmatically through the
:py:mod:`mlflow.tracking` module and the REST API. This makes it easy to do several
common tasks:

* Query and compare runs using any data analysis tool of your choice, for example, **pandas**.
* Determine the artifact URI for a run to feed some of its artifacts into a new run when executing
  a workflow.
* Load artifacts from past runs as :ref:`models`.
* Run automated parameter search algorithms, where you query the metrics from various runs to
  submit new ones.

.. _tracking_server:

Running a Tracking Server
-------------------------

The MLflow tracking server launched via ``mlflow server`` also hosts REST APIs for tracking runs,
writing data to the local filesystem. You can specify a tracking server URI
via the ``MLFLOW_TRACKING_URI`` environment variable and MLflow's tracking APIs will automatically
communicate with the tracking server at that URI to create/get run information, log metrics, etc.

An example configuration for a server is as follows:

.. code:: shell

    mlflow server \
        --file-store /mnt/persistent-disk \
        --artifact-root s3://my-mlflow-bucket/ \
        --host 0.0.0.0

Storage
^^^^^^^
There are two properties related to how data is stored:

* ``--file-store`` is where the server will store run and experiment information. This should
  be a persistent (non-ephemeral) disk.
* ``--artifact-root`` causes clients to log their artifact output (e.g., models) to this
  location which is suitable for large data (such as an S3 bucket or shared NFS file system). If
  you do not provide this option, then clients will write artifacts to *their* local directories,
  which the server probably can't serve.

Note that for the clients and server to access the artifact bucket, you should configure your Cloud
Provider credentials as normal. For example, S3 can be accessed by setting the ``AWS_ACCESS_KEY_ID``
and ``AWS_SECRET_ACCESS_KEY`` environment variables, by using an IAM role, or by configuring a default
profile in `~/.aws/credentials`. See the `AWS docs <https://docs.aws.amazon.com/sdk-for-java/latest/developer-guide/setup-credentials.html>`_ for more info.

Networking
^^^^^^^^^^
The ``--host`` option exposes the service on all interfaces. If running a server in production, we
would recommend not exposing the built-in server broadly (as it is unauthenticated and unecrypted),
and instead putting it behind a reverse proxy like nginx or apache, or connecting over VPN.
Additionally, you should ensure that the ``--file-store`` (which defaults to the ``./mlruns`` directory)
points to a persistent (non-ephemeral) disk.

Connecting to a Remote Server
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Once you have a server running, simply set ``MLFLOW_TRACKING_URI`` to the server's URI, along
with its scheme and port (e.g., ``http://10.0.0.1:5000``). Then you can use mlflow as normal:

.. code:: python

    import mlflow
    with mlflow.start_run():
        mlflow.log_metric("a", 1)

The ``mlflow.start_run`` and ``mlflow.log_metric`` calls make API requests to your remote
tracking server.
