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 Python, REST, R API, and Java API 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 for the run, if it was run from an MLflow Project.
Start & End Time
Start and end time of the run
Name of the file to launch the run, or the project name and entry point for the run if run 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 records and lets 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), and 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 Tracking UI or the MLflow API.

Where Runs Are Recorded

MLflow runs can be recorded to local files, to a SQLAlchemy compatible database, 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.
  • Database encoded as <dialect>+<driver>://<username>:<password>@<host>:<port>/<database>. Mlflow supports the dialects mysql, mssql, sqlite, and postgresql. For more details, see SQLAlchemy database uri.
  • 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.

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.

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, a database connection string, 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 automatically starts 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 single key-value param in the currently active run. The key and value are both strings. Use mlflow.log_params() to log multiple params at once.

mlflow.log_metric() logs a single key-value metric. The value must always be a number. MLflow remembers the history of values for each metric. Use mlflow.log_metrics() to log multiple metrics at once.

mlflow.set_tag() sets a single key-value tag in the currently active run. The key and value are both strings. Use mlflow.set_tags() to set multiple tags at once.

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 launch 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.

Performance Tracking with Metrics

You log MLflow metrics with log methods in the Tracking API. The log methods support two alternative methods for distinguishing metric values on the x-axis: timestamp and step.

timestamp is an optional long value that represents the time that the metric was logged. timestamp defaults to the current time. step is an optional integer that represents any measurement of training progress (number of training iterations, number of epochs, and so on). step defaults to 0 and has the following requirements and properties:

  • Must be a valid 64-bit integer value.
  • Can be negative.
  • Can be out of order in successive write calls. For example, (1, 3, 2) is a valid sequence.
  • Can have “gaps” in the sequence of values specified in successive write calls. For example, (1, 5, 75, -20) is a valid sequence.

If you specify both a timestamp and a step, metrics are recorded against both axes independently.


with mlflow.start_run():
    for epoch in range(0, 3):
        mlflow.log_metric(key="quality", value=2*epoch, step=epoch)
Java and Scala
MlflowClient client = new MlflowClient();
RunInfo run = client.createRun();
for (int epoch = 0; epoch < 3; epoch ++) {
    client.logMetric(run.getRunId(), "quality", 2 * epoch, System.currentTimeMillis(), epoch);

Visualizing Metrics

Here is an example plot of the quick start tutorial with the step x-axis and two timestamp axes:


X-axis step


X-axis wall time - graphs the absolute time each metric was logged


X-axis relative time - graphs the time relative to the first metric logged, for each run

Automatic Logging from TensorFlow and Keras (experimental)

Call mlflow.tensorflow.autolog() or mlflow.keras.autolog() before your training code to enable automatic logging of metrics and parameters without the need for explicit log statements. See example usages with Keras and TensorFlow.

Autologging captures the following information:

Framework Metrics Parameters Tags Artifacts
Keras Training loss; validation loss; user-specified metrics Number of layers; optimizer name; learning rate; epsilon Model summary MLflow Model (Keras model), TensorBoard logs; on training end
tf.keras Training loss; validation loss; user-specified metrics Number of layers; optimizer name; learning rate; epsilon Model summary MLflow Model (Keras model), TensorBoard logs; on training end
tf.estimator TensorBoard metrics MLflow Model (TF saved model); on call to tf.estimator.export_saved_model
TensorFlow Core All tf.summary.scalar calls

Note that autologging for tf.keras is handled by mlflow.tensorflow.autolog(), not mlflow.keras.autolog().

Note: this feature is experimental - the API and format of the logged data are subject to change.

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 name for a individual run using the CLI (for example, mlflow run ... --experiment-name [name]) or the MLFLOW_EXPERIMENT_NAME environment variable. Alternatively, you can use the experiment ID instead, via the --experiment-id CLI flag or the MLFLOW_EXPERIMENT_ID environment variable.

# Set the experiment via environment variables
export MLFLOW_EXPERIMENT_NAME=fraud-detection

mlflow experiments create --experiment-name fraud-detection
# Launch a run. The experiment is inferred from the MLFLOW_EXPERIMENT_NAME environment
# variable, or from the --experiment-name 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. A tag can only have a single unique value mapped to it at a time. 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 log runs to a local mlruns directory, run mlflow ui in the directory above it, and it loads 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

You can access all of the functions in the Tracking UI 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.

Referencing Artifacts

When you specify the location of an artifact in MLflow APIs, the syntax depends on whether you are invoking the Tracking, Models, or Projects API. For the Tracking API, you specify the artifact location using a (run ID, relative path) tuple. For the Models and Projects APIs, you specify the artifact location in the follow ways:

  • /Users/me/path/to/local/model
  • relative/path/to/local/model
  • <scheme>/<scheme-dependent-path>. For example:
    • s3://my_bucket/path/to/model
    • hdfs://<host>:<port>/<path>
    • runs:/<mlflow_run_id>/run-relative/path/to/model

For example:

Tracking API

mlflow.log_artifacts("<mlflow_run_id>", "/path/to/artifact")

Models API


MLflow Tracking Servers

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

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


An MLflow tracking server has two components for storage: a backend store and an artifact store.

The backend store is where MLflow Tracking Server stores experiment and run metadata as well as params, metrics, and tags for runs. MLflow supports two types of backend stores: file store and database-backed store.

Use --backend-store-uri to configure the type of backend store. You specify a file store backend as ./path_to_store or file:/path_to_store and a database-backed store as SQLAlchemy database URI. The database URI typically takes the format <dialect>+<driver>://<username>:<password>@<host>:<port>/<database>. MLflow supports the database dialects mysql, mssql, sqlite, and postgresql. Drivers are optional. If you do not specify a driver, SQLAlchemy uses a dialect’s default driver. For example, --backend-store-uri sqlite:///mlflow.db would use a local SQLite database.


mlflow server will fail against a database-backed store with an out-of-date database schema. To prevent this, upgrade your database schema to the latest supported version using mlflow db upgrade [db_uri]. Schema migrations can result in database downtime, may take longer on larger databases, and are not guaranteed to be transactional. You should always take a backup of your database prior to running mlflow db upgrade - consult your database’s documentation for instructions on taking a backup.

By default --backend-store-uri is set 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). artifact_location is a property recorded on mlflow.entities.Experiment for default location to store artifacts for all runs in this experiment. Additional, artifact_uri is a property on mlflow.entities.RunInfo to indicate location where all artifacts for this run are stored.

Use --default-artifact-root (defaults to local ./mlruns directory) to configure default location to server’s artifact store. This will be used as artifact location for newly-created experiments that do not specify one. Once you create an experiment, --default-artifact-root is no longer relevant to that experiment.

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-location 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 probably refer to different physical locations (that is, the same path on different disks).

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 must set one of these variables on both your client application and your MLflow tracking server. Finally, you must run 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 must run 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

To store artifacts in a FTP server, specify a URI of the form ftp://user@host/path/to/directory . 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 be 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.


To store artifacts in HDFS, specify a hdfs: URI. It can contain host and port: hdfs://<host>:<port>/<path> or just the path: hdfs://<path>.

There are also two ways to authenticate to HDFS:

  • Use current UNIX account authorization
  • Kerberos credentials using following environment variables:
export MLFLOW_KERBEROS_TICKET_CACHE=/tmp/krb5cc_22222222
export MLFLOW_KERBEROS_USER=user_name_to_use

Most of the cluster contest settings are read from hdfs-site.xml accessed by the HDFS native driver using the CLASSPATH environment variable.

Optionally you can select a different version of the HDFS driver library using:

export MLFLOW_HDFS_DRIVER=libhdfs3

The default driver is libhdfs.


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. You can then pass authentication headers to MLflow using these environment variables.

Additionally, you should ensure that the --backend-store-uri (which defaults to the ./mlruns directory) points to a persistent (non-ephemeral) disk or database connection.

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)

In addition to the MLFLOW_TRACKING_URI environment variable, the following environment variables allow passing HTTP authentication to the tracking server:

  • MLFLOW_TRACKING_USERNAME and MLFLOW_TRACKING_PASSWORD - username and password to use with HTTP Basic authentication. To use Basic authentication, you must set both environment variables .
  • MLFLOW_TRACKING_TOKEN - token to use with HTTP Bearer authentication. Basic authentication takes precedence if set.
  • MLFLOW_TRACKING_INSECURE_TLS - if set to the literal true, MLflow does not verify the TLS connection, meaning it does not validate certificates or hostnames for https:// tracking URIs. This flag is not recommended for production environments.

System Tags

You can annotate runs with arbitrary tags. Tag keys that start with mlflow. are reserved for internal use. The following tags are set automatically by MLflow, when appropriate:

Key Description
mlflow.runName Human readable name that identifies this run.
mlflow.parentRunId The ID of the parent run, if this is a nested run.
mlflow.user Identifier of the user who created the run.
mlflow.source.type Source type. Possible values: "NOTEBOOK", "JOB", "PROJECT", "LOCAL", and "UNKNOWN" Source identifier (e.g., GitHub URL, local Python filename, name of notebook)
mlflow.source.git.commit Commit hash of the executed code, if in a git repository.
mlflow.source.git.branch Name of the branch of the executed code, if in a git repository.
mlflow.source.git.repoURL URL that the executed code was cloned from.
mlflow.project.env The runtime context used by the MLflow project. Possible values: "docker" and "conda".
mlflow.project.entryPoint Name of the project entry point associated with the current run, if any. Name of the Docker image used to execute this run. ID of the Docker image used to execute this run.