Time to Live (TTL)

This section describes how the TTL mechanism works and what its limits are. It also gives examples of commands and code snippets that can be used to enable, configure, and disable TTL.

How it works

YDB lets you specify a table column (TTL column), whose values set the lifetime of items. TTL automatically deletes the item from your table once the specified number of seconds passes after the time set in the TTL column.

Warning

An item with the NULL value in the TTL column is never deleted.

The timestamp for deleting a table item is determined by the formula:

expiration_time = valueof(ttl_column) + expire_after_seconds

Note

TTL doesn't guarantee that the item will be deleted exactly at expiration_time, it might happen later. If it's important to exclude logically obsolete but not yet physically deleted items from the selection, use query-level filtering.

Data is deleted by the Background Removal Operation (BRO), consisting of two stages:

  1. Checking the values in the TTL column.
  2. Deleting expired data.

The BRO has the following properties:

Guarantees

  • At every point in time, a BRO is run in no more than 1 instance per table.
  • BROs are run no more than once per hour for the same shard.
  • Data consistency is guaranteed. The TTL column value is re-checked during the delete stage. This means that if the TTL column value is updated between stages 1 and 2 (for example, with UPDATE) and ceases to meet the delete criteria, the row will not be deleted.

Limitations

  • The TTL column must be of one of the following types:
    • Date.
    • Datetime.
    • Timestamp.
    • Uint32.
    • Uint64.
    • DyNumber.
  • The value in the TTL column with a numeric type (Uint32, Uint64, or DyNumber) is interpreted as a Unix time value. The following units are supported (set in the TTL settings):
    • Seconds.
    • Milliseconds.
    • Microseconds.
    • Nanoseconds.
  • TTL setup using YQL is only possible for the Date, Datetime, and Timestamp columns.
  • You can't specify multiple TTL columns.
  • You can't delete the TTL column. However, if this is required, you should first disable TTL for the table.

Setup

Currently, you can manage TTL settings using:

Enabling TTL for an existing table

In the example below, the items of the mytable table will be deleted an hour after the time set in the created_at column:

ALTER TABLE `mytable` SET (TTL = Interval("PT1H") ON created_at);
$ ydb -e <endpoint> -d <database> table ttl set --column created_at --expire-after 3600 mytable
session.AlterTable(
    "mytable",
    TAlterTableSettings()
        .BeginAlterTtlSettings()
            .Set("created_at", TDuration::Hours(1))
        .EndAlterTtlSettings()
);
session.alter_table('mytable', set_ttl_settings=ydb.TtlSettings().with_date_type_column('created_at', 3600))

Tip

When setting up TTL using YQL, an Interval is created from a string literal in ISO 8601 format.

The example below shows how to use the modified_at column with a numeric type (Uint32) as a TTL column. The column value is interpreted as the number of seconds since the Unix epoch:

$ ydb -e <endpoint> -d <database> table ttl set --column modified_at --expire-after 3600 --unit seconds mytable
session.AlterTable(
    "mytable",
    TAlterTableSettings()
        .BeginAlterTtlSettings()
            .Set("modified_at", TTtlSettings::EUnit::Seconds, TDuration::Hours(1))
        .EndAlterTtlSettings()
);
session.alter_table('mytable', set_ttl_settings=ydb.TtlSettings().with_value_since_unix_epoch('modified_at', UNIT_SECONDS, 3600))

Enabling TTL for a newly created table

For a newly created table, you can pass TTL settings along with the table description:

CREATE TABLE `mytable` (
    id Uint64,
    expire_at Timestamp,
    PRIMARY KEY (id)
) WITH (
    TTL = Interval("PT0S") ON expire_at
);
session.CreateTable(
    "mytable",
    TTableBuilder()
        .AddNullableColumn("id", EPrimitiveType::Uint64)
        .AddNullableColumn("expire_at", EPrimitiveType::Timestamp)
        .SetPrimaryKeyColumn("id")
        .SetTtlSettings("expire_at")
        .Build()
);
session.create_table(
    'mytable',
    ydb.TableDescription()
    .with_column(ydb.Column('id', ydb.OptionalType(ydb.DataType.Uint64)))
    .with_column(ydb.Column('expire_at', ydb.OptionalType(ydb.DataType.Timestamp)))
    .with_primary_key('id')
    .with_ttl(ydb.TtlSettings().with_date_type_column('expire_at'))
)

Disabling TTL

ALTER TABLE `mytable` RESET (TTL);
$ ydb -e <endpoint> -d <database> table ttl drop mytable
session.AlterTable(
    "mytable",
    TAlterTableSettings()
        .BeginAlterTtlSettings()
            .Drop()
        .EndAlterTtlSettings()
);
session.alter_table('mytable', drop_ttl_settings=True)

Getting TTL settings

The current TTL settings can be obtained from the table description:

$ ydb -e <endpoint> -d <database> scheme describe mytable
auto desc = session.DescribeTable("mytable").GetValueSync().GetTableDescription();
auto ttl = desc.GetTtlSettings();
desc = session.describe_table('mytable')
ttl = desc.ttl_settings