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Implementing a data retention policy in CrateDB using Apache Airflow

Welcome to the second article of our series on leveraging the full potential of Apache Airflow with CrateDB, by Niklas Schmidtmer and Marija Selakovic from CrateDB's Customer Engineering team.

What is a Data Retention Policy?

A data retention policy describes the practice of storing and managing data for a designated period of time. Once a data set completes its retention period, it should be deleted or archived, depending on requirements. Implementing data retention policies in the right way ensures compliance with existing guidelines and regulations, such as data privacy law, optimizes storage space by discarding outdated data and reduces storage costs.

Specification of a Data Retention Policy in CrateDB

In the previous tutorial, we illustrated how to use CrateDB and Apache Airflow to automate periodic data export to a remote filesystem with the infrastructure provided by Astronomer. In this tutorial, we focus on a more complex use case: the implementation of an effective retention policy for time-series data. To define retention policies we create a new table in CrateDB with the following schema:

CREATE TABLE IF NOT EXISTS "doc"."retention_policies" (
   "table_schema" TEXT,
   "table_name" TEXT,
   "partition_column" TEXT NOT NULL,
   "retention_period" INTEGER NOT NULL,
   "strategy" TEXT NOT NULL,
   PRIMARY KEY ("table_schema", "table_name")
);

The retention policy requires the use of a partitioned table, as in CrateDB, data can be deleted in an efficient way by dropping partitions. Therefore, for each retention policy, we store table schema, table name, the partition column, and the retention period defining how many days data should be retained.

The strategy column is reserved for future implementations of additional data retention policies. For now, we will always set it to the value delete.

Next, define the table for storing demo data:

You may also use a different table. The important part is that data should be partitioned: in our case, we partition the table on the ts_day column. Finally, we store the retention policy of 1 day for demo data in the retention_policies table:

Implementation in Apache Airflow

To automate the process of deleting expired data we use Apache Airflow. Our workflow implementation does the following: once a day, fetch policies from the database, and delete all data for which the retention period expired.

Retrieving Retention Policies

The first step consists of a task that queries partitions affected by retention policies. We do this by joining retention_policies and information_schema.table_partitions tables and selecting values with expired retention periods. In CrateDB, information_schema.table_partitions [documentation] contains information about all partitioned tables including the name of the table, schema, partition column, and the values of the partition.

The resulting query is constructed as:

SELECT QUOTE_IDENT(p.table_schema) || '.' || QUOTE_IDENT(p.table_name),
       QUOTE_IDENT(r.partition_column),
       TRY_CAST(p.values[r.partition_column] AS BIGINT)
FROM information_schema.table_partitions p
JOIN doc.retention_policies r ON p.table_schema = r.table_schema
  AND p.table_name = r.table_name
  AND p.values[r.partition_column] < %(day)s::TIMESTAMP - (r.retention_period || ' days')::INTERVAL
WHERE r.strategy = 'delete';

To separate SQL logic from orchestration logic, we save the query as a file to include/data_retention_retrieve_delete_policies.sql.

In the query, we use the %(day)s placeholder which will be substituted with the logical execution date. This is especially useful in case of failing workflow: the next time Airflow will pick up the date on which the job failed. This makes job runs consistent.
To implement the query above we use a regular Python method, annotated with @taskto make it executable by Airflow. The most important reason behind choosing this type of operator is the need to pass the query result to the next operator. In our case that would be the list of affected partitions. However, it would be natural to expect that we want to execute a query on CrateDB as an SQLExecuteQueryOperator, but since this operator always returns None as a result, we would not be able to access the query result outside the operator.

The implementation of the corresponding tasks looks as follows:

The first step is to create the function get_policies that takes as a parameter the logical date. The SQL statement gets loaded from a file. The PostgresHook establishes the connection with CrateDB. A PostgresHook takes the information from the postgres_conn_id and hooks us up with the CrateDB service. Then, the function executes the query and returns the result.

Cross-Communication Between Tasks

Before we continue into the implementation of the next task in Apache Airflow, we would like to give a brief overview of how the data is communicated between different tasks in a DAG. For this purpose, Airflow introduces the XCom system. Simply speaking XCom can be seen as a small object with storage that allows tasks to push data into that storage that can be later used by a different task in the DAG.

The key thing here is that it allows the exchange of a small amount of data between tasks. From Airflow 2.0, the return value of a Python method used as a task will be automatically stored in XCom. For our example, this means that the get_policies return value is available from the next task after the get_policies operator executes. To access the data from another task, a reference to the previous task can be passed to the next task when defining dependencies between tasks.

Applying Retention Policies

Now that we retrieved the policies and Airflow automatically saved them via XCom, we need to create another task that will go through each element in the list and delete expired data.

The get_policies task returns tuples with a positional index. As this makes further processing not very readable, we map tuples to a list with named indexes:

In the DAG’s main method, we can now make use of Airflows’ dynamic task mapping which allows executing the same task several times, with different parameters:

SQLExecuteQueryOperator.partial(
    task_id="delete_partition",
    conn_id="cratedb_connection",
    sql="DELETE FROM  WHERE  = ;",
).expand(params=get_policies().map(map_policy))

get_policies returns a set of policies. On each policy, the map_policy is applied. The return value of map_policy is finally passed as params to the SQLExecuteQueryOperator.

This leads us already to the final version of the DAG:

On the SQLExecuteQueryOperator, a certain set of attributes are passed via partial instead of expand. These are static values that are the same for each DELETE statement, like the connection and task ID.

The full DAG implementation of the data retention policy can be found in our GitHub repository. To run the workflow, we rely on Astronomer infrastructure with the same setup as shown in the first part of the CrateDB and Apache Airflow tutorial.


Summary

This tutorial gives a guide on how to delete data with expired retention policies. The first part shows how to design policies in CrateDB and then, how to use Apache Airflow to automate data deletion. The DAG implementation is fairly simple: the first task performs the extraction of relevant policies, while the second task makes sure that affected partitions are deleted. In the following tutorial, we will focus on another real-world example that can be automated with Apache Airflow and CrateDB.