# ask-ai

Alan Bohannon

03/01/2024, 6:28 PM
Can you describe what the use of a backfill is and how it works?


03/01/2024, 6:28 PM
Certainly! A backfill is a process where you take a data asset that's normally updated incrementally and update historical parts of it. For example, if you have a table that you add records to each day corresponding to events that happened during that day, backfilling the table means filling in or overwriting data for days in the past. Backfills are common in scenarios such as: 1. Setting up a pipeline for the first time with historical data that needs to be materialized to bring the assets up to date. 2. When you’ve changed the logic for an asset and need to update historical data with the new logic. The process of backfilling can involve several steps: - Step 0: Manage your data in a way that makes it easy to backfill. This often means using partitions to organize your data and track its changes. It also involves tracking what code and upstream data you've used to generate your data so you know what parts are out-of-date. Lastly, it means tracking the dependencies between your data assets, so you can determine what's impacted by a change. - Step 1: Plan your backfill. Determine what data assets (i.e., tables and files) you need to backfill and what partitions of those data assets the backfill needs to include. The better a job you've done with Step 0, the easier this step will be. - Step 2: Execute the backfill. This involves running the necessary processes to update the historical data. This could be done as a single big query or as separate tasks per partition, depending on the strategy you choose. Backfilling becomes dramatically easier when you think of your data in terms of partitions. Partitioning is an approach to incremental data management that views each data asset as a collection of partitions. For example, a table of events might correspond to a set of hourly partitions, each of which contains the events that occurred within a single hour. When you run a backfill, you overwrite partitions for hours in the past. In Dagster, you can launch backfills for a partitioned asset using the Materialize button on either the Asset details or the Global asset lineage page. The backfill modal will display, allowing you to select the partitions you want to backfill. Dagster orchestrators are helpful in managing these kinds of backfills, as they can track dependencies and kick off work to backfill downstream assets after the upstream asset's backfill is complete. It's important to note that backfills can be complex and potentially costly operations, so they should be planned and executed carefully to avoid issues such as targeting the wrong subset of data, resource overload, cost overruns, or getting lost in the middle of a backfill.