Op Events and Exceptions#

Within the body of an op, it is possible to communicate with the Dagster framework either by yielding an event, logging an event, or raising an exception. This page describes these different possibilities and the scenarios in which you might use them.

Relevant APIs#

Name	Description
Output	Dagster event used to yield an output from an op
AssetMaterialization	Dagster event indicating that an op has materialized an asset
AssetObservation	Dagster event indicating that an op has observed an asset
ExpectationResult	Dagster event representing the result of a data quality check
Failure	Dagster exception indicating that a failure has occurred
RetryRequested	Dagster exception requesting the step to be retried

Overview#

Within the body of an op, a stream of structured events can be yielded or logged. These events will be processed by Dagster and recorded in the event log, along with some additional context about the op that emitted it.

It is also possible to raise Dagster-specific exceptions, which will indicate to the framework to halt the op execution entirely and perform some action.

Event Metadata#

Often, it may be useful to attach some arbitrary information to an event or exception that is not captured by its basic parameters. Through the MetadataEntry and MetadataValue classes, we provide a consistent interface for specifying this metadata as a set of "entries". Individual entries are instances of the MetadataEntry class, which is a simple container for a string label and a typed value. The available value types are accessible through a static API defined on MetadataValue. These include simple datatypes (MetadataValue.float, MetadataValue.int, MetadataValue.text), as well as more complex information such as markdown and json (MetadataValue.md, MetadataValue.json). Depending on the type of its value, a MetadataEntry will be rendered in Dagit in a more useful format than a simple unstructured string.

Metadata is attached to events at construction time. There are currently two available metadata specification keyword arguments on all event constructors: (1) metadata takes a dictionary mapping string labels to MetadataValue objects. Under the hood, this is resolved to a list of MetadataEntry objects. (2) metadata_entries takes a list of MetadataEntry objects directly. The metadata_entries argument is deprecated-- new code should use the metadata dictionary form.

Events#

Yielding events from within the body of an op is a useful way of communicating with the Dagster framework. The most critical event to the functionality of Dagster is the Output event, which allows output data to be passed on from one op to the next. However, we also provide interfaces to inform Dagster about external assets and data quality checks during the run of an op.

Outputs#

Because returning a value from an op is such a fundamental part of creating a data pipeline, we have a few different interfaces for this functionality, to help ease transition into writing Dagster-specific code.

For ops with a single output, you can simply return a value directly from the compute_fn. Internally, this will be converted to a Dagster Output event with the default output name result:

from dagster import op


@op
def my_simple_return_op(context):
    return 1

While this is perhaps the most intuitive way to return a value from a function, once you have multiple outputs defined on your op, or want to yield additional, non-output information from the body of your op, explicitly returning a value is no longer an option. In these cases, you'll want to explicitly yield Output events. With that in mind, the above example can be converted to the equivalent yield pattern like so:

from dagster import Output, op


@op
def my_simple_yield_op(context):
    yield Output(1)

or, if you have a specific output name other than the default result:

from dagster import Output, op


@op(out={"my_output": Out(int)})
def my_named_yield_op(context):
    yield Output(1, output_name="my_output")

Check out the docs on Op Outputs to learn more.

Attaching Metadata to Outputs

Experimental

#

If there is information specific to an Output that you would like to log, you may optionally represent that by passing in a metadata parameter containing a mapping of string labels to metadata values.

The EventMetadata class contains a set of static wrappers to customize the display of certain types of structured metadata.

The following example demonstrates how you might use this functionality:

from dagster import MetadataValue, Output, op


@op
def my_metadata_output(context):
    df = get_some_data()
    yield Output(
        df,
        metadata={
            "text_metadata": "Text-based metadata for this event",
            "dashboard_url": MetadataValue.url("http://mycoolsite.com/url_for_my_data"),
            "raw_count": len(df),
            "size (bytes)": calculate_bytes(df),
        },
    )

Asset Materializations#

AssetMaterialization events tell Dagster that you have written some data asset to an external system. The classic example would be writing to a table in a database, but really any sort of persisted object that you would want to keep track of can be considered an asset.

Generally, you'd want to send this event directly after you persist the asset to your external system. All AssetMaterialization events must define an asset_key, which is a unique identifier to describe the asset you are persisting.

from dagster import AssetMaterialization, op


@op
def my_asset_op(context):
    df = get_some_data()
    store_to_s3(df)
    context.log_event(
        AssetMaterialization(
            asset_key="s3.my_asset",
            description="A df I stored in s3",
        )
    )

    result = do_some_transform(df)
    return result

Asset materializations can also be yielded:

from dagster import AssetMaterialization, Output, op


@op
def my_asset_op_yields():
    df = get_some_data()
    store_to_s3(df)
    yield AssetMaterialization(
        asset_key="s3.my_asset",
        description="A df I stored in s3",
    )

    result = do_some_transform(df)
    yield Output(result)

When yielding asset materializations, outputs must also be yielded via an Output.

To learn more about assets and how they are surfaced once you send this event, check out the Asset Catalog documentation.

Attaching Metadata to Asset Materializations#

Attaching metadata to Asset Materializations is an important way of tracking aspects of a given asset over time. This functions essentially identically to other events which accept a metadata parameter, allowing you to attach a set of structured labels and values to display.

from dagster import op, AssetMaterialization, MetadataValue


@op
def my_metadata_materialization_op(context):
    df = read_df()
    remote_storage_path = persist_to_storage(df)
    context.log_event(
        AssetMaterialization(
            asset_key="my_dataset",
            description="Persisted result to storage",
            metadata={
                "text_metadata": "Text-based metadata for this event",
                "path": MetadataValue.path(remote_storage_path),
                "dashboard_url": MetadataValue.url(
                    "http://mycoolsite.com/url_for_my_data"
                ),
                "size (bytes)": calculate_bytes(df),
            },
        )
    )
    return remote_storage_path

Asset Observations#

AssetObservation events record metadata about assets. Unlike asset materializations, asset observations do not signify that an asset has been mutated.

Within ops and assets, you can log or yield AssetObservation events at runtime. Similar to attaching metadata to asset materializations, asset observations accept a metadata parameter, allowing you to track specific properties of an asset over time.

from dagster import AssetObservation, op


@op
def observation_op(context):
    df = read_df()
    context.log_event(
        AssetObservation(asset_key="observation_asset", metadata={"num_rows": len(df)})
    )
    return 5

In the example above, an observation tracks the number of rows in an asset persisted to storage. This information can then be viewed on the Asset Details page.

To learn more about asset observations, check out the Asset Observation documentation.

Expectation Results#

Ops can emit structured events to represent the results of a data quality test. The data quality event class is the ExpectationResult. To generate an expectation result, we can log or yield an ExpectationResult event in our op.

from dagster import ExpectationResult, op


@op
def my_expectation_op(context, df):
    do_some_transform(df)
    context.log_event(
        ExpectationResult(success=len(df) > 0, description="ensure dataframe has rows")
    )
    return df

Attaching Metadata to Expectation Results#

Like many other event types in Dagster, there are a variety of types of metadata that can be associated with an expectation result event, all through the MetadataEntry class. Each expectation event optionally takes a list of metadata entries that are then displayed in the event log.

This example shows metadata entries of different types attached to the same expectation result:

from dagster import ExpectationResult, MetadataValue, op


@op
def my_metadata_expectation_op(context, df):
    df = do_some_transform(df)
    context.log_event(
        ExpectationResult(
            success=len(df) > 0,
            description="ensure dataframe has rows",
            metadata={
                "text_metadata": "Text-based metadata for this event",
                "dashboard_url": MetadataValue.url(
                    "http://mycoolsite.com/url_for_my_data"
                ),
                "raw_count": len(df),
                "size (bytes)": calculate_bytes(df),
            },
        )
    )
    return df

Exceptions#

Dagster also provides some op-specific exception classes, which can be raised to halt the execution of a op. The behavior after an exception is raised depends on the exception that you use. The exceptions are documented below.

Failures#

Failure exceptions may be raised to indicate that an op has failed in an unrecoverable way, and that execution should stop. This may be useful if, in the body of your op, you detect an issue that would not raise an error on its own, but would still cause problems if the execution were to continue.

from dagster import Failure, op


@op
def my_failure_op():
    path = "/path/to/files"
    my_files = get_files(path)
    if len(my_files) == 0:
        raise Failure(
            description="No files to process",
            metadata={
                "filepath": MetadataValue.path(path),
                "dashboard_url": MetadataValue.url("http://mycoolsite.com/failures"),
            },
        )
    return some_calculation(my_files)

Attaching Metadata to Failures#

As is the case with many Dagster-provided classes, you can attach a list MetadataEntry denoting an arbitrary set of metadata relevant to the failure.

from dagster import Failure, op


@op
def my_failure_metadata_op():
    path = "/path/to/files"
    my_files = get_files(path)
    if len(my_files) == 0:
        raise Failure(
            description="No files to process",
            metadata={
                "filepath": MetadataValue.path(path),
                "dashboard_url": MetadataValue.url("http://mycoolsite.com/failures"),
            },
        )
    return some_calculation(my_files)

Retry Requests#

RetryRequested exceptions are useful when you experience failures that are possible to recover from. For example, if you have a flaky operation that you expect to throw an exception once in a while, you can catch the exception and throw a RetryRequested to make Dagster halt and re-start op execution.

You can configure the number of retries to be attempted with the max_retries parameter.

from dagster import RetryRequested, op


@op
def my_retry_op():
    try:
        result = flaky_operation()
    except Exception as e:
        raise RetryRequested(max_retries=3) from e
    return result