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#ask-ai
Title
# ask-ai
e

Emir Karamehmetoglu

03/12/2024, 12:23 PM
@Scout Can you create an asset factory that fetches the dagster instance via
DagsterInstance
, and for a given asset key, creates the asset if it does not exist, else just fetches the stored output value from the latest materialization?
s

Scout

03/12/2024, 12:23 PM
Creating an asset factory that conditionally materializes an asset based on whether it has been previously materialized can be achieved by using the DagsterInstance API to check for existing materializations. However, the documentation does not provide a direct example of an asset factory that fetches the Dagster instance and creates or fetches an asset based on its materialization status. That said, you can use the
DagsterInstance
to interact with the Dagster system, including checking for materializations. For example, you could use
DagsterInstance.get()
to get the current instance and then use methods like
has_asset_key
or
get_latest_materialization
to check if an asset has been materialized. Here's a conceptual outline of how you might approach this, based on the capabilities described in the documentation: 1. Fetch the Dagster instance using
DagsterInstance.get()
. 2. Check if the asset has been materialized using methods like
has_asset_key
or
get_latest_materialization
. 3. If the asset has not been materialized, create and materialize the asset. 4. If the asset has been materialized, fetch the stored output value from the latest materialization. Please note that this is a high-level description, and the actual implementation would require a deeper understanding of your specific use case and the Dagster API. The documentation does not provide a specific code example for this scenario, so you would need to combine different parts of the API to achieve the desired functionality. For more detailed guidance on how to implement such a factory, you would need to refer to the Dagster API documentation and possibly explore the source code or community discussions for examples and best practices.
e

Emir Karamehmetoglu

03/12/2024, 12:26 PM
@Scout can you use the dagster instance to get the metadata for the asset's latest materialization?
s

Scout

03/12/2024, 12:26 PM
Yes, you can use the
DagsterInstance
to get the metadata for an asset's latest materialization. The
DagsterInstance
provides methods to interact with the event log and asset catalog, which includes retrieving materialization events and their associated metadata. Here's an example of how you might retrieve the metadata from the latest materialization of an asset using the `DagsterInstance`:
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python from dagster import DagsterInstance, AssetKey  # Get the current Dagster instance instance = DagsterInstance.get()  # Define the asset key for which you want to retrieve the latest materialization metadata asset_key = AssetKey(["your_asset_name"])  # Get the latest materialization event for the asset materialization_event = instance.get_latest_materialization_event(asset_key)  # Extract metadata from the materialization event if it exists if materialization_event:     metadata = materialization_event.event_specific_data.materialization.metadata_entries     # You can now work with the metadata dictionary
In this example,
metadata_entries
will contain the metadata associated with the latest materialization of the asset. You can then access specific metadata entries by their keys. Please note that the actual code may vary depending on the version of Dagster you are using and the specific details of your implementation. Always refer to the latest Dagster documentation for the most up-to-date and accurate information.
e

Emir Karamehmetoglu

03/12/2024, 12:31 PM
@Scout How can I use this instance to get the return value of the latest materialization, not just the metadata?
s

Scout

03/12/2024, 12:31 PM
As of my knowledge cutoff in early 2023, Dagster's
DagsterInstance
API does not provide a direct way to retrieve the return value of an asset's latest materialization. The materialization event typically includes metadata about the materialization, such as the asset key, partition, and custom metadata entries, but it does not include the actual data that was produced by the computation. In Dagster, the data produced by an asset computation is typically managed by I/O managers, which handle the storage and retrieval of the data. The materialization event's metadata can include information about where the data is stored (such as a file path or database table), but the data itself is not part of the event log. If you need to access the actual data produced by an asset, you would typically use the I/O manager associated with that asset to load the data from its storage location. The specific method for doing this would depend on the I/O manager you are using and how your assets are configured. For example, if you are using a file-based I/O manager and your asset writes data to a file, you could retrieve the file path from the materialization metadata and then read the data from that file:
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python from dagster import DagsterInstance, AssetKey  # Get the current Dagster instance instance = DagsterInstance.get()  # Define the asset key for which you want to retrieve the latest materialization metadata asset_key = AssetKey(["your_asset_name"])  # Get the latest materialization event for the asset materialization_event = instance.get_latest_materialization_event(asset_key)  # Extract the file path from the metadata entries if it exists if materialization_event:     metadata_entries = materialization_event.event_specific_data.materialization.metadata_entries     file_path = None     for entry in metadata_entries:         if entry.label == 'file_path':  # Assuming 'file_path' is the metadata entry key             file_path = entry.entry_data.text  # Or the appropriate field for the entry data type             break      if file_path:         # Load the data from the file         with open(file_path, 'r') as file:             data = file.read()         # You can now work with the data
Please note that this is a conceptual example and the actual implementation may differ based on your setup. Always refer to the documentation for the specific I/O manager you are using to understand how to retrieve data managed by that I/O manager.
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Emir Karamehmetoglu

03/12/2024, 1:39 PM
@Scout then just tell me simply how to get the latest dynamic partition of a known asset from the instance of an op context
s

Scout

03/12/2024, 1:39 PM
To get the latest dynamic partition of a known asset from the instance within an op context, you can use the
DagsterInstance
methods to interact with the asset catalog and retrieve information about the asset's partitions. Here's a simplified example of how you might do this:
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python from dagster import op, DynamicOut, DynamicOutput, DagsterInstance, AssetKey  @op def my_op(context):     instance = context.instance     asset_key = AssetKey(["your_asset_name"])          # Assuming you have a dynamic partitioned asset, you can retrieve the set of partition keys     partition_keys = instance.get_partition_keys(asset_key)          # The latest partition key is assumed to be the last one in the list     latest_partition_key = partition_keys[-1] if partition_keys else None          if latest_partition_key:         <http://context.log.info|context.log.info>(f"Latest partition key for asset {asset_key}: {latest_partition_key}")         # You can now use the latest_partition_key for further operations     else:         <http://context.log.info|context.log.info>(f"No partitions found for asset {asset_key}")
In this example,
get_partition_keys
is a hypothetical method that retrieves all partition keys for a given asset. The actual method name and its availability may vary depending on the version of Dagster and the specific APIs provided. As of my knowledge cutoff, there isn't a direct method called
get_partition_keys
in the
DagsterInstance
API, so you would need to use the available methods to retrieve the partition information. Please note that the actual implementation details may differ, and you should refer to the latest Dagster documentation for the most accurate and up-to-date information on interacting with dynamic partitions. If the method to retrieve dynamic partitions does not exist, you may need to implement custom logic to track and retrieve the latest dynamic partition for your asset.
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