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#ask-ai
Title
# ask-ai
j

Josh

03/13/2024, 7:08 AM
@Scout is there a way to declare assets with dependents instead of dependencies?
s

Scout

03/13/2024, 7:08 AM
In Dagster, assets are typically defined with dependencies, specifying which assets they depend on to compute their value. However, the concept of declaring assets with dependents (i.e., specifying which assets are downstream of a given asset) is not directly supported in the same way. The standard way to define dependencies is by using the
deps
parameter in the
@asset
decorator to specify upstream assets. This creates a clear lineage from upstream to downstream assets, which Dagster uses to determine execution order and to display lineage information in the UI. Here's an example from the documentation on how to define an asset with dependencies:
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python from dagster import asset  @asset def sugary_cereals() -> None:     # logic to create sugary_cereals asset     ...  @asset(deps=[sugary_cereals]) def shopping_list() -> None:     # logic to create shopping_list asset, which depends on sugary_cereals     ...
In this example,
shopping_list
is defined as depending on
sugary_cereals
. Dagster does not provide a built-in way to invert this relationship and declare
sugary_cereals
as having
shopping_list
as a dependent directly in the asset definition. However, dependencies are inherently a two-way relationship: if
shopping_list
depends on
sugary_cereals
, then
sugary_cereals
is a dependency of
shopping_list
. While Dagster's API is designed to specify dependencies from the perspective of the downstream asset, the system understands and manages these relationships bidirectionally under the hood. If you need to track dependents for a particular use case, you would typically need to do this outside of the asset definitions, perhaps by maintaining a separate mapping or using naming conventions that make the relationships clear. Dagster's asset graph and lineage tools in the UI can help visualize these relationships based on the defined dependencies.
j

Josh

03/13/2024, 7:33 AM
@Scout is there a way to define dependencies outside of the asset definition? maybe in the Definitions object?
s

Scout

03/13/2024, 7:33 AM
Yes, Dagster allows you to define dependencies outside of the asset definition using the
Definitions
object. This can be particularly useful when dealing with external assets or when you want to define dependencies separately from the asset's implementation. For external assets, which are assets not materialized by Dagster but tracked in the asset graph and asset catalog, you can use
external_assets_from_specs
to create an external assets definition from a sequence of
AssetSpec
objects. Here's an example from the documentation:
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python from dagster import AssetSpec, Definitions, external_assets_from_specs  raw_logs = AssetSpec("raw_logs") processed_logs = AssetSpec("processed_logs", deps=[raw_logs])  defs = Definitions(assets=external_assets_from_specs([raw_logs, processed_logs]))
In this example,
processed_logs
is defined as having a dependency on
raw_logs
outside of the asset definition itself. For software-defined assets, you can define dependencies using the
deps
parameter within the
@asset
decorator, as shown in the previous examples. However, if you want to define dependencies separately, you might consider using Dagster's graph-based APIs, where you can define dependencies between ops within a graph, and then create assets from that graph. This allows you to define the execution logic and dependencies separately from the asset definitions. Here's an example of how you might define dependencies within a graph and then create assets from that graph:
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python from dagster import op, graph, GraphOut, AssetsDefinition  @op def make_asset1():     ...  @op def make_asset2(asset1):     ...  @graph(out={"asset1": GraphOut(), "asset2": GraphOut()}) def make_assets():     asset1 = make_asset1()     return asset1, make_asset2(asset1)  assets = AssetsDefinition.from_graph(make_assets)
In this example,
make_assets
is a graph that defines the dependencies between
make_asset1
and
make_asset2
. The
AssetsDefinition.from_graph
method is then used to create assets from this graph, with the dependencies defined within the graph being preserved in the resulting assets. Please note that the specific APIs and methods available may evolve over time, so it's always a good idea to check the latest Dagster documentation for the most up-to-date information on defining dependencies and working with assets.
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