Managing Space: Purge Query Results¶

Alation Cloud Service Applies to Alation Cloud Service instances of Alation

Customer Managed Applies to customer-managed instances of Alation

Applies from version 2021.4

Alation server admins can use the alation_action purge_execution_results action to purge query execution results and free some space.

Actions described in this section require access to the Alation server.

Note

Alation Cloud Service customers can request server configuration changes through Alation Support.

Understanding Query Execution Results Storage¶

Compose query execution results are stored in the data_storage_blobaccesspostgres table of the internal Alation database. The storage rules are based on two alation_conf parameters:

alation.query_exec.results_auto_persist_bytes

alation.query_exec.results_expiration_time

alation.query_exec.results_auto_persist_bytes¶

This parameter configures storing query results that are smaller than the parameter value perpetually.

From version 2021.4, the default value is 0 MB. This means that no queries will be stored in Alation forever unless they are explicitly saved by users on the Results tab in Compose or on the Execution Results page in the catalog. Query results that are not saved by users are only stored for a specific time period that is set by the second parameter, alation.query_exec.results_expiration_time. No small execution results will accumulate over time potentially leading to storage space issues.

Note

The default value in version 2021.3 and earlier versions was 1 MB. By default, results smaller than 1 MB were stored permanently.

alation.query_exec.results_expiration_time¶

This parameter configures storing query results that are larger than alation.query_exec.results_auto_persist_bytes in Alation for the duration of the time period specified as the parameter value. The default value is seven days, set in minutes (10080 minutes). This means that query results that are not explicitly saved by users are stored in Alation for seven days and then purged automatically.

Determining Current Execution Result Storage Size¶

You can retrieve the current size of the query execution result data from the internal Alation database table data_storage_blobaccesspostgres:

SSH to the Alation server.
Enter the Alation shell.
sudo /etc/init.d/alation shell
Use the alation_psql command to access the internal Alation database.
alation_psql
Run the following query to determine the total size of the data_storage_blobaccesspostgres table. The query will return the table size in MB.
SELECT pg_size_pretty( pg_total_relation_size('data_storage_blobaccesspostgres') );
Exit the Postgres shell.
\q
Exit the Alation shell.
exit

Purging Execution Results¶

You can use the alation_action purge_execution_results action to purge the query execution results storage and free some space. The script clears query execution results data from the data_storage_blobaccesspostgres table in the internal Alation database for a time range specified by the user.

The alation_action purge_execution_results action works in 2 modes: 'Scan' or 'Purge'. Begin with scanning the volume of results set data. Then you can purge the results data if space is an issue.

Mode 1: Scanning for Records to Purge¶

The 'Scan' mode is used to find how much space can potentially be reclaimed. Use the steps below to scan for query execution result data that can be purged from data_storage_blobaccesspostgres:

SSH to the Alation server.
Enter the Alation shell.
sudo /etc/init.d/alation shell
Change user to alation.
sudo su alation
Use the alation_action purge_execution_results command.
alation_action purge_execution_results
Respond to all prompts. Pressing Enter skips to the next prompt and applies the default value. The following prompts will be displayed:
- Enter the start date from which data is to be removed: Specify the start date of the time range that you want to scan. The input to this prompt is required and cannot be skipped. Use the format yyyy-mm-dd to specify the date.
- Enter the end date till which data is to be removed: Specify the end date of the time range that you want to scan. The input to this prompt is required and cannot be skipped. Use the format yyyy-mm-dd to specify the date.
- Enter the batch size: For the 'Scan' mode, leave the default value by pressing Enter.
- Do you want to Scan or Purge? Type 'Scan' to use this script for scanning.
- Log file path: specify the path to the output file or press Enter to accept the default value. By default, the output file will be written to /opt/alation/site/logs/ inside the Alation shell, but you can specify a different location. The name of the output file will be similar to purge_execution_results_2021-11-12T11:29:55.607535.txt
Example:
(env) PROD [alation@ip-10-13-49-1 /]$ alation_action purge_execution_results Enter the start date from which data is to be removed (format : yyyy-mm-dd) : 2021-01-01 Enter the end date till which data is to be removed(format : yyyy-mm-dd) : 2021-10-31 Enter the batch size (in days, default is 10): Do you want to Scan or Purge? (default is 'Scan'): 'Scan' Log file path (Default is: /opt/alation/site/logs/): Scan revealed 900 records that can be removed and recover a total of 20.0MB Scan complete, set to `Purge` mode to proceed with the purge
View the scan output to determine if the data should be purged. Then, if needed, proceed with purging the data. If not, exit the Alation shell.
exit

Mode 2: Purging Records¶

Use the 'Purge' action with caution. It purges all query results for a specified time period, including the results saved in the catalog by users. Some results saved in the catalog may still be in use and referenced in articles or other objects. After purging, a warning will be displayed to users in both Compose and the catalog to inform them that a result set they are trying to view is no longer available.

Warning

On version 2021.4, the purge action requires a downtime of the Alation server. Plan this activity during a period when a downtime is possible and warn your users that Alation will be temporarily unavailable. The length of the downtime depends on the amount of results data that you will be purging; however, the purging action affects one internal table and should not take a long time.

In version 2022.1 and later versions, the downtime is not required.

Use the steps below to purge query execution result data from the data_storage_blobaccesspostgres table:

SSH to the Alation server.
Enter the Alation shell.
sudo /etc/init.d/alation shell
Change user to alation:
sudo su alation
Run the alation_action purge_execution_results action.
alation_action purge_execution_results
Respond to all prompts. Pressing Enter skips to the next prompt and applies the default value. The following prompts will be displayed:
- Enter the start date from which data is to be removed: Specify the start date of the time range that you want to scan. The input to this prompt is required and cannot be skipped. Use the format yyyy-mm-dd to specify the date.
- Enter the end date till which data is to be removed: Specify the end date of the time range that you want to scan. The input to this prompt is required and cannot be skipped. Use the format yyyy-mm-dd to specify the date.
- Enter the batch size: Result set data volume can be very large. Purging all data at once would be taxing on the system. Alation will purge the data in batches specified in this prompt.
- Do you want to Scan or Purge? Type 'Purge' to use this script for purging.
- Log file path: specify the path to the output file or press Enter to accept the default value. By default, the output file will appear in /opt/alation/site/logs/ inside the Alation shell, but you can specify a different location.
Example:
(env) PROD [alation@ip-10-13-49-1 /]$ alation_action purge_execution_results Enter the start date from which data is to be removed (format : yyyy-mm-dd) : 2021-01-01 Enter the end date till which data is to be removed(format : yyyy-mm-dd) : 2021-10-31 Enter the batch size (in days, default is 10): Do you want to Scan or Purge? (default is 'Scan'): 'Purge' Log file path (Default is: /opt/alation/site/logs/): . . . Batch processing completed. Overall time taken : 0:00:00 - Overall space saved : 20.0MB Purge complete. Disk space will not be released by the operating system until a `VACUUM FULL` has been run in your Postgres database.
Depending on your Alation version, perform additional steps to return disk space to the operating system.
Still as user alation in the Altion shell, run the following command:

repack_table public.data_storage_blobaccesspostgres

Progress will be indicated by the following message:

INFO: repacking table "public.data_storage_blobaccesspostgres"

After repacking the table, you can check its size by running the query given below. Enter the Postgres shell.

alation_psql

Run the query.

SELECT pg_size_pretty( pg_total_relation_size('data_storage_blobaccesspostgres') );

Exit the Postgres shell.

\q

Exit the Alation shell:

exit
Warning

The steps below will cause Alation to be temporarily unavailable.

Exit from the user alation. You will proceed as user alationadmin.

exit

Stop Alation.

alation_action stop_alation

Start Postgres.

alation_action start_postgres

Enter the Postgres shell.

alation_psql

Run VACUUM FULL on the data_storage_blobaccesspostgres table.

VACUUM FULL data_storage_blobaccesspostgres;

When the VACUUM action is completed, you can check on the table size by running the query given below:

SELECT pg_size_pretty( pg_total_relation_size('data_storage_blobaccesspostgres') );

Exit the Postgres shell.

\q

Restart Alation.

alation_action restart_alation

Exit the Alation shell:

exit