MLflow Tracking

The MLflow Tracking component is an API and UI for logging parameters, code versions, metrics, and output files when running your machine learning code and for later visualizing the results. MLflow Tracking lets you log and query experiments using both Python and REST APIs.


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 hash used to execute the run, if it was executed from an MLflow Project.
Start & End Time
Start and end time of the run
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 MLflow Project.
Key-value input parameters of your choice. Both keys and values are strings.
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.
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 file) as artifacts.

You can record runs using MLflow Python, R, Java, and REST APIs from anywhere you run your code. 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 MLflow Project, MLflow remembers the project URI and source version.

You can optionally organize runs into experiments, which group together runs for a specific task. You can create an experiment using the mlflow experiments CLI, with mlflow.create_experiment(), or using the corresponding REST parameters. The MLflow API and UI let you create and search for experiments.

Once your runs have been recorded, you can query them using the Adding Tags to Runs or the MLflow API.

Where Runs Are Recorded

MLflow runs are recorded either locally in files or remotely to a tracking server. By default, the MLflow Python API logs runs locally to files in an mlruns directory wherever you ran your program. You can then run mlflow ui to see the logged runs.

To log runs remotely, set the MLFLOW_TRACKING_URI environment variable to a tracking server’s URI or call mlflow.set_tracking_uri().

There are different kinds of remote tracking URIs:

  • Local file path (specified as file:/my/local/dir), where data is just directly stored locally.
  • HTTP server (specified as https://my-server:5000), which is a server hosting an MLFlow tracking server.
  • Databricks workspace (specified as databricks or as databricks://<profileName>, a Databricks CLI profile. This works only in workspaces for which the Databricks MLflow Tracking Server is enabled; contact Databricks if interested.

Logging Data to Runs

You can log data to runs using the MLflow Python, R, Java, or REST API. This section shows the Python API.

Basic Logging Functions

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.

mlflow.tracking.get_tracking_uri() returns the current tracking URI.

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.

mlflow.set_experiment() sets an experiment as active. If the experiment does not exist, creates a new experiment. If you do not specify an experiment in mlflow.start_run(), new runs are launched under this experiment.

mlflow.start_run() returns the currently active run (if one exists), or starts a new run and returns a mlflow.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.

mlflow.end_run() ends the currently active run, if any, taking an optional run status.

mlflow.active_run() returns a mlflow.entities.Run object corresponding to the currently active run, if any.

mlflow.log_param() logs a key-value parameter in the currently active run. The keys and values are both strings.

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.

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.

mlflow.log_artifacts() logs all the files in a given directory as artifacts, again taking an optional artifact_path.

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 mlflow.start_run() is a Python context manager. You can “scope” each run to just one block of code as follows:

with mlflow.start_run():
    mlflow.log_param("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 allows you to group runs under experiments, which can be useful for comparing runs intended to tackle a particular task. You can create experiments using the Command-Line Interface (mlflow experiments) or the mlflow.create_experiment() Python API. You can pass the experiment ID for a individual run using the CLI (for example, mlflow run ... --experiment-id [ID]) or the MLFLOW_EXPERIMENT_ID environment variable.

# Prints "created an experiment with ID <id>
mlflow experiments create fraud-detection
# Set the ID via environment variables
# Launch a run. The experiment ID is inferred from the MLFLOW_EXPERIMENT_ID environment
# variable, or from the --experiment-id parameter passed to the MLflow CLI (the latter
# taking precedence)
with mlflow.start_run():
    mlflow.log_param("a", 1)
    mlflow.log_metric("b", 2)

Managing Experiments and Runs with the Tracking Service API

MLflow provides a more detailed Tracking Service API for managing experiments and runs directly, which is available through client SDK in the mlflow.tracking module. This makes it possible to query data about past runs, log additional information about them, create experiments, add tags to a run, and more.


from  mlflow.tracking import MlflowClient
client = MlflowClient()
experiments = client.list_experiments() # returns a list of mlflow.entities.Experiment
run = client.create_run(experiments[0].experiment_id) # returns mlflow.entities.Run
client.log_param(, "hello", "world")

Adding Tags to Runs

The mlflow.tracking.MlflowClient.set_tag() function lets you add custom tags to runs. For example:

client.set_tag(, "tag_key", "tag_value")


Do not use the prefix mlflow for a tag. This prefix is reserved for use by MLflow.

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, run mlflow ui in the directory above it, and it will load the corresponding runs. Alternatively, the MLflow 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

Querying Runs Programmatically

All of the functions in the Tracking UI can be accessed programmatically. 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. For an example of querying runs and constructing a multistep workflow, see the MLflow Multistep Workflow Example project.
  • Load artifacts from past runs as MLflow Models. For an example of training, exporting, and loading a model, and predicting using the model, see the MLFlow TensorFlow example.
  • Run automated parameter search algorithms, where you query the metrics from various runs to submit new ones. For an example of running automated parameter search algorithms, see the MLflow Hyperparameter Tuning Example project.

MLflow Tracking Servers

You run an MLflow tracking server using mlflow server. An example configuration for a server is:

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


An MLflow tracking server has two properties related to how data is stored: file store and artifact store.

The file store (exposed as --file-store) is where the server stores run and experiment metadata. It defaults to the local ./mlruns directory (the same as when running mlflow run locally), but when running a server, make sure that this points to a persistent (that is, non-ephemeral) file system location.

The artifact store is a location suitable for large data (such as an S3 bucket or shared NFS file system) and is where clients log their artifact output (for example, models). The artifact store is a property of an experiment, but the --default-artifact-root flag sets the artifact root URI for newly-created experiments that do not specify one. Once you create an experiment, --default-artifact-root is no longer relevant to it.

To allow the server and clients to access the artifact location, you should configure your cloud provider credentials as normal. For example, for S3, you can set the AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY environment variables, use an IAM role, or configure a default profile in ~/.aws/credentials. See Set up AWS Credentials and Region for Development for more info.


If you do not specify a --default-artifact-root or an artifact URI when creating the experiment (for example, mlflow experiments create --artifact-root s3://<my-bucket>), the artifact root is a path inside the file store. Typically this is not an appropriate location, as the client and server will probably be referring to different physical locations (that is, the same path on different disks).

Supported Artifact Stores

In addition to local file paths, MLflow supports the following storage systems as artifact stores: Amazon S3, Azure Blob Storage, Google Cloud Storage, SFTP server, and NFS.

Amazon S3

To store artifacts in S3, specify a URI of the form s3://<bucket>/<path>. MLflow obtains credentials to access S3 from your machine’s IAM role, a profile in ~/.aws/credentials, or the environment variables AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY depending on which of these are available. For more information on how to set credentials, see Set up AWS Credentials and Region for Development.

To store artifacts in a custom endpoint, set the MLFLOW_S3_ENDPOINT_URL to your endpoint’s URL. For example, if you have a Minio server at on port 9000:

Azure Blob Storage

To store artifacts in Azure Blob Storage, specify a URI of the form wasbs://<container>@<storage-account><path>. MLflow expects Azure Storage access credentials in the AZURE_STORAGE_CONNECTION_STRING or AZURE_STORAGE_ACCESS_KEY environment variables (preferring a connection string if one is set), so you will need to set one of these variables on both your client application and your MLflow tracking server. Finally, you will need to pip install azure-storage separately (on both your client and the server) to access Azure Blob Storage; MLflow does not declare a dependency on this package by default.

Google Cloud Storage

To store artifacts in Google Cloud Storage, specify a URI of the form gs://<bucket>/<path>. You should configure credentials for accessing the GCS container on the client and server as described in the GCS documentation. Finally, you will need to pip install google-cloud-storage (on both your client and the server) to access Google Cloud Storage; MLflow does not declare a dependency on this package by default.

FTP server ~~~ Specify a URI of the form ftp://user@host/path/to/directory to store artifacts in a FTP server. The URI may optionally include a password for logging into the server, e.g. ftp://user:pass@host/path/to/directory

SFTP Server

To store artifacts in an SFTP server, specify a URI of the form sftp://user@host/path/to/directory. You should configure the client to be able to log in to the SFTP server without a password over SSH (e.g. public key, identity file in ssh_config, etc.).

The format sftp://user:pass@host/ is supported for logging in. However, for safety reasons this is not recommended.

When using this store, pysftp must be installed on both the server and the client. Run pip install pysftp to install the required package.


To store artifacts in an NFS mount, specify a URI as a normal file system path, e.g., /mnt/nfs. This path must the same on both the server and the client – you may need to use symlinks or remount the client in order to enforce this property.


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 unencrypted), and instead putting it behind a reverse proxy like NGINX or Apache httpd, 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.

Logging to a Tracking Server

To log to a tracking server, set the MLFLOW_TRACKING_URI environment variable to the server’s URI, along with its scheme and port (for example, or call mlflow.set_tracking_uri().

The mlflow.start_run(), mlflow.log_param(), and mlflow.log_metric() calls then make API requests to your remote tracking server.

import mlflow
with mlflow.start_run():
    mlflow.log_param("a", 1)
    mlflow.log_metric("b", 2)