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TTL

TTL (Time To Live) is a mechanism in NebulaGraph that defines the lifespan of data. Once the data reaches its predefined lifespan, it is automatically deleted from the database. This feature is particularly suitable for data that only needs temporary storage, such as temporary sessions or cached data.

OpenCypher Compatibility

This topic applies to native nGQL only.

Precautions

  • You CANNOT modify a property schema with TTL options on it.
  • TTL options and indexes have coexistence issues.

    • TTL options and indexes CANNOT coexist on a tag or an edge type. If there is an index on a property, you cannot set TTL options on other properties.
    • If there are TTL options on a tag, an edge type, or a property, you can still add an index on them.

TTL options

The native nGQL TTL feature has the following options.

Option Description
ttl_col Specifies an existing property to set a lifespan on. The data type of the property must be int or timestamp.
ttl_duration Specifies the timeout adds-on value in seconds. The value must be a non-negative int64 number. A property expires if the sum of its value and the ttl_duration value is smaller than the current timestamp. If the ttl_duration value is 0, the property never expires.
You can set ttl_use_ms to true in the configuration file nebula-storaged.conf (default path: /usr/local/nightly/etc/) to set the default unit to milliseconds.

Warning

  • Before setting ttl_use_ms to true, make sure that no TTL has been set for any property, as shortening the expiration time may cause data to be erroneously deleted.
  • After setting ttl_use_ms to true, which sets the default TTL unit to milliseconds, the data type of the property specified by ttl_col must be int, and the property value needs to be manually converted to milliseconds. For example, when setting ttl_col to a, you need to convert the value of a to milliseconds, such as when the value of a is now(), you need to set the value of a to now() * 1000.

Use TTL options

You must use the TTL options together to set a lifespan on a property.

Before using the TTL feature, you must first create a timestamp or integer property and specify it in the TTL options. NebulaGraph will not automatically create or manage this timestamp property for you.

When inserting the value of the timestamp or integer property, it is recommended to use the now() function or the current timestamp to represent the present time.

Set a timeout if a tag or an edge type exists

If a tag or an edge type is already created, to set a timeout on a property bound to the tag or edge type, use ALTER to update the tag or edge type.

# Create a tag.
nebula> CREATE TAG IF NOT EXISTS t1 (a timestamp);

# Use ALTER to update the tag and set the TTL options.
nebula> ALTER TAG t1 TTL_COL = "a", TTL_DURATION = 5;

# Insert a vertex with tag t1. The vertex expires 5 seconds after the insertion.
nebula> INSERT VERTEX t1(a) VALUES "101":(now());

Set a timeout when creating a tag or an edge type

Use TTL options in the CREATE statement to set a timeout when creating a tag or an edge type. For more information, see CREATE TAG and CREATE EDGE.

# Create a tag and set the TTL options.
nebula> CREATE TAG IF NOT EXISTS t2(a int, b int, c string) TTL_DURATION= 100, TTL_COL = "a";

# Insert a vertex with tag t2. The timeout timestamp is 1648197238 (1648197138 + 100).
nebula> INSERT VERTEX t2(a, b, c) VALUES "102":(1648197138, 30, "Hello");

Data expiration and deletion

Caution

  • When the TTL options are set for a property of a tag or an edge type and the property's value is NULL, the property never expires.
  • If a property with a default value of now() is added to a tag or an edge type and the TTL options are set for the property, the history data related to the tag or the edge type will never expire because the value of that property for the history data is the current timestamp.

Vertex property expiration

Vertex property expiration has the following impact.

  • If a vertex has only one tag, once a property of the vertex expires, the vertex expires.
  • If a vertex has multiple tags, once a property of the vertex expires, properties bound to the same tag with the expired property also expire, but the vertex does not expire and other properties of it remain untouched.

Edge property expiration

Since an edge can have only one edge type, once an edge property expires, the edge expires.

Data deletion

The expired data are still stored on the disk, but queries will filter them out.

NebulaGraph automatically deletes the expired data and reclaims the disk space during the next compaction.

Note

If TTL is disabled, the corresponding data deleted after the last compaction can be queried again.

Remove a timeout

To disable TTL and remove the timeout on a property, you can use the following approaches.

  • Drop the property with the timeout.
    nebula> ALTER TAG t1 DROP (a);
    
  • Set ttl_col to an empty string.
    nebula> ALTER TAG t1 TTL_COL = "";
    
  • Set ttl_duration to 0. This operation keeps the TTL options and prevents the property from expiring and the property schema from being modified.
    nebula> ALTER TAG t1 TTL_DURATION = 0;
    

Last update: November 3, 2023