Retention policy

Article
10/08/2024

Applies to: ✅ Azure Data Explorer

The retention policy controls the mechanism that automatically removes data from tables or materialized views. It's useful to remove data that continuously flows into a table, and whose relevance is age-based. For example, the policy can be used for a table that holds diagnostics events that may become uninteresting after two weeks.

The retention policy can be configured for a specific table or materialized view, or for an entire database. The policy then applies to all tables in the database that don't override it. When the policy is configured both at the database and table level, the retention policy in the table takes precedence over the database policy.

Setting up a retention policy is important when continuously ingesting data, which will limit costs.

Data that is "outside" the retention policy is eligible for removal. There's no specific guarantee when removal occurs. Data may "linger" even if the retention policy is triggered.

The retention policy is most commonly set to limit the age of the data since ingestion. For more information, see SoftDeletePeriod.

Note

The deletion time is imprecise. The system guarantees that data won't be deleted before the limit is exceeded, but deletion isn't immediate following that point.
A soft-delete period of 0 can be set as part of a table-level retention policy, but not as part of a database-level retention policy.
When this is done, the ingested data won't be committed to the source table, avoiding the need to persist the data. As a result, Recoverability can only be set to Disabled.
Such a configuration is useful mainly when the data gets ingested into a table. A transactional update policy is used to transform it and redirect the output into another table.
The retention policy on a materialized view affects only the view, not the source table. The source data remains unaffected.

The policy object

A retention policy includes the following properties:

SoftDeletePeriod:
- Time span for which it's guaranteed that the data is kept available to query. The period is measured starting from the time the data was ingested.
- Defaults to 1,000 years.
- When altering the soft-delete period of a table or database, the new value applies to both existing and new data.
Recoverability:
- Data recoverability (Enabled/Disabled) after the data was deleted.
- Defaults to Enabled.
- If set to Enabled, the data will be recoverable for 14 days after it's been soft-deleted.
- It is not possible to configure the recoverability period.

Management commands

Use .show policy retention to show the current retention policy for a database, table, or materialized view.
Use .alter policy retention to change current retention policy of a database, table, or materialized view.

Defaults

By default, when a database or a table is created, it doesn't have a retention policy defined. Normally, the database is created and then immediately has its retention policy set by its creator according to known requirements. When you run a .show command for the retention policy of a database or table that hasn't had its policy set, Policy appears as null.

The default retention policy, with the default values mentioned above, can be applied using the following command.

.alter database DatabaseName policy retention "{}"
.alter table TableName policy retention "{}"
.alter materialized-view ViewName policy retention "{}"

The command results in the following policy object applied to the database or table.

{
  "SoftDeletePeriod": "365000.00:00:00", "Recoverability":"Enabled"
}

Clearing the retention policy of a database or table can be done using the following command.

.delete database DatabaseName policy retention
.delete table TableName policy retention

Examples

For an environment that has a database named MyDatabase, with tables MyTable1, MyTable2, and MySpecialTable.

Soft-delete period of seven days and recoverability disabled

Set all tables in the database to have a soft-delete period of seven days and disabled recoverability.

Option 1 (Recommended): Set a database-level retention policy, and verify there are no table-level policies set.

.delete table MyTable1 policy retention        // optional, only if the table previously had its policy set
.delete table MyTable2 policy retention        // optional, only if the table previously had its policy set
.delete table MySpecialTable policy retention  // optional, only if the table previously had its policy set
.alter-merge database MyDatabase policy retention softdelete = 7d recoverability = disabled
.alter-merge materialized-view ViewName policy retention softdelete = 7d

Option 2: For each table, set a table-level retention policy, with a soft-delete period of seven days and recoverability disabled.

.alter-merge table MyTable1 policy retention softdelete = 7d recoverability = disabled
.alter-merge table MyTable2 policy retention softdelete = 7d recoverability = disabled
.alter-merge table MySpecialTable policy retention softdelete = 7d recoverability = disabled

Soft-delete period of seven days and recoverability enabled

Set tables MyTable1 and MyTable2 to have a soft-delete period of seven days and recoverability disabled.
Set MySpecialTable to have a soft-delete period of 14 days and recoverability enabled.

Option 1 (Recommended): Set a database-level retention policy, and set a table-level retention policy.

.delete table MyTable1 policy retention   // optional, only if the table previously had its policy set
.delete table MyTable2 policy retention   // optional, only if the table previously had its policy set
.alter-merge database MyDatabase policy retention softdelete = 7d recoverability = disabled
.alter-merge table MySpecialTable policy retention softdelete = 14d recoverability = enabled

Option 2: For each table, set a table-level retention policy, with the relevant soft-delete period and recoverability.

.alter-merge table MyTable1 policy retention softdelete = 7d recoverability = disabled
.alter-merge table MyTable2 policy retention softdelete = 7d recoverability = disabled
.alter-merge table MySpecialTable policy retention softdelete = 14d recoverability = enabled

Soft-delete period of seven days, and `MySpecialTable` keeps its data indefinitely

Set tables MyTable1 and MyTable2 to have a soft-delete period of seven days, and have MySpecialTable keep its data indefinitely.

Option 1: Set a database-level retention policy, and set a table-level retention policy, with a soft-delete period of 1,000 years, the default retention policy, for MySpecialTable.

.delete table MyTable1 policy retention   // optional, only if the table previously had its policy set
.delete table MyTable2 policy retention   // optional, only if the table previously had its policy set
.alter-merge database MyDatabase policy retention softdelete = 7d
.alter table MySpecialTable policy retention "{}" // this sets the default retention policy

Option 2: For tables MyTable1 and MyTable2, set a table-level retention policy, and verify that the database-level and table-level policy for MySpecialTable aren't set.

.delete database MyDatabase policy retention   // optional, only if the database previously had its policy set
.delete table MySpecialTable policy retention   // optional, only if the table previously had its policy set
.alter-merge table MyTable1 policy retention softdelete = 7d
.alter-merge table MyTable2 policy retention softdelete = 7d

Option 3: For tables MyTable1 and MyTable2, set a table-level retention policy. For table MySpecialTable, set a table-level retention policy with a soft-delete period of 1,000 years, the default retention policy.
```
.alter-merge table MyTable1 policy retention softdelete = 7d
.alter-merge table MyTable2 policy retention softdelete = 7d
.alter table MySpecialTable policy retention "{}"
```