Caching policy (hot and cold cache)

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Applies to: ✅ Azure Data Explorer

To ensure fast query performance, a multi-tiered data cache system is used. Data is stored in reliable storage but parts of it are cached on processing nodes, SSD, or even in RAM for faster access.

The caching policy allows you to choose which data should be cached. You can differentiate between hot data cache and cold data cache by setting a caching policy on hot data. Hot data is kept in local SSD storage for faster query performance, while cold data is stored in reliable storage, which is cheaper but slower to access.

The cache uses 95% of the local SSD disk for hot data. If there isn't enough space, the most recent data is preferentially kept in the cache. The remaining 5% is used for data that isn't categorized as hot. This design ensures that queries loading lots of cold data won't evict hot data from the cache.

The best query performance is achieved when all ingested data is cached. However, certain data might not warrant the expense of being kept in the hot cache. For instance, infrequently accessed old log records might be considered less crucial. In such cases, teams often opt for lower querying performance over paying to keep the data warm.

Use management commands to alter the caching policy at the cluster, database, table, or materialized view level.

Tip

Your cluster is designed for ad hoc queries with intermediate result sets that fit in the cluster's total RAM. For large jobs, like map-reduce, it can be useful to store intermediate results in persistent storage. To do so, create a continuous export job. This feature enables you to do long-running batch queries using services like HDInsight or Azure Databricks.

How caching policy is applied

When data is ingested, the system keeps track of the date and time of the ingestion, and of the extent that was created. The extent's ingestion date and time value (or maximum value, if an extent was built from multiple pre-existing extents), is used to evaluate the caching policy.

Note

You can specify a value for the ingestion date and time by using the ingestion property creationTime. When doing so, make sure the Lookback property in the table's effective Extents merge policy is aligned with the values you set for creationTime.

By default, the effective policy is null, which means that all the data is considered hot. A null policy at the table level means that the policy will be inherited from the database. A non-null table-level policy overrides a database-level policy.

Scoping queries to hot cache

When running queries, you can limit the scope to only query data in hot cache.

Note

Data scoping applies only to entities that support caching policies, such as tables and materialized views. It's ignored for other entities, such as external tables and data in the row store.

There are several query possibilities:

  • Add a client request property called query_datascope to the query. Possible values: default, all, and hotcache.
  • Use a set statement in the query text: set query_datascope='...'. Possible values are the same as for the client request property.
  • Add a datascope=... text immediately after a table reference in the query body. Possible values are all and hotcache.

The default value indicates use of the default settings, which determine that the query should cover all data.

If there's a discrepancy between the different methods, then set takes precedence over the client request property. Specifying a value for a table reference takes precedence over both.

For example, in the following query, all table references use hot cache data only, except for the second reference to "T", that is scoped to all the data:

set query_datascope="hotcache";
T | union U | join (T datascope=all | where Timestamp < ago(365d)) on X

Caching policy vs retention policy

Caching policy is independent of retention policy:

  • Caching policy defines how to prioritize resources. Queries for important data are faster.
  • Retention policy defines the extent of the queryable data in a table/database (specifically, SoftDeletePeriod).

Configure this policy to achieve the optimal balance between cost and performance, based on the expected query pattern.

Example:

  • SoftDeletePeriod = 56d
  • hot cache policy = 28d

In the example, the last 28 days of data will be on the SSD and the additional 28 days of data will be stored in Azure blob storage. You can run queries on the full 56 days of data.

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