Update policy overview

When you trigger an update policy with a command that adds data to a source table, data also appends to a target table. The target table can have a different schema, retention policy, and other policies from the source table. For example, a high-rate trace source table can contain data formatted as a free-text column. The target table can include specific trace lines, with a well-structured schema generated from a transformation of the source table's free-text data using the parse operator.

An update policy is subject to the same restrictions and best practices as regular ingestion. The policy scales-out according to the cluster size, and is more efficient when handling bulk ingestion.

Note

The source and target table must be in the same database. The update policy function schema and the target table schema must match in their column names, types, and order.

Ingesting formatted data improves performance, and CSV is preferred because of it's a well-defined format. Sometimes, however, you have no control over the format of the data, or you may want to enrich ingested data, for example by joining records with a static dimension table in your database.

Update policy query

If the update policy is defined on the target table, multiple queries can run on data ingested into a source table. If there are multiple update policies, the order of execution is not necessarily known.

Query limitations

• The policy-related query can invoke stored functions, but can't access data external to the database; specifically:
  • It can't perform cross-database queries.
  • It can't perform cross-cluster queries.
  • It can't access external data or external tables.
  • It can't make callouts (by using a plugin).
• The query doesn't have read access to tables that have the RestrictedViewAccess policy enabled or with a Row Level Security policy enabled.
• For update policy limitations in streaming ingestion, see streaming ingestion limitations.

Warning

An incorrect query might prevent data ingestion into the source table.

The limitations above, and the "compatibility" between the query results and the schema of the source and destination tables, might cause an incorrect query to prevent data ingestion into the source table.

Those limitations are validated when the policy is created and executed, but not when arbitrary stored functions that the query might reference are updated. Therefore it's important to make such changes with an eye towards keeping the update policy intact.

When referencing the Source table in the Query part of the policy, or in functions referenced by the Query part:

• Don't use the qualified name of the table. Instead, use TableName.
• Don't use database("DatabaseName").TableName or cluster("ClusterName").database("DatabaseName").TableName.

The update policy object

A table may have zero or more update policy objects associated with it. Each such object is represented as a JSON property bag, with the following properties defined.

Property	Type	Description
IsEnabled	bool	States if update policy is true - enabled, or false - disabled
Source	string	Name of the table that triggers invocation of the update policy
Query	string	A query used to produce data for the update
IsTransactional	bool	States if the update policy is transactional or not, default is false). If transactional and the update policy fails, the source table is not updated.
PropagateIngestionProperties	bool	States if properties specified during ingestion to the source table, such as extent tags and creation time, apply to the target table.

Note

In production systems, set IsTransactional:true to ensure that the target table doesn't lose data in transient failures.

Note

Cascading updates are allowed, for example from table A, to table B, to table C. However, if update policies are defined in a circular manner, this is detected at runtime, and the chain of updates is cut. Data is ingested only once to each table in the chain.

Control commands

Update policy control commands include:

• .show table *TableName* policy update shows the current update policy of a table.
• .alter table *TableName* policy update defines the current update policy of a table.
• .alter-merge table *TableName* policy update appends definitions to the current update policy of a table.
• .delete table *TableName* policy update deletes the current update policy of a table.

Update policy is initiated following ingestion

Update policies take effect when data is ingested or moved to a source table, or extents are created in a source table, using any of the following commands:

• .ingest (pull)
• .ingest (inline)
• .set | .append | .set-or-append | .set-or-replace
• .move extents
• .replace extents
  • The PropagateIngestionProperties command only takes effect in ingestion operations. When the update policy is triggered as part of a .move extents or .replace extents command, this option has no effect.

Warning

When the update policy is invoked as part of a .set-or-replace command, by default data in derived tables is replaced in the same way as in the source table. Data may be lost in all tables with an update policy relationship if the replace command is invoked. Consider using .set-or-append instead.

Remove data from source table

After ingesting data to the target table, you may want to remove it from the source table. Set a soft-delete period of 0sec (or 00:00:00) in the source table's retention policy, and the update policy as transactional. The following conditions apply:

• The source data isn't queryable from the source table
• The source data doesn't persist in durable storage as part of the ingestion operation
• Operational performance improves. Post-ingestion resources are reduced for background grooming operations on extents in the source table.

Note

When the source table has a soft delete period of 0sec (or 00:00:00), any update policy referencing this table must be transactional.

Performance impact

Update policies can affect cluster performance, and ingestion for data extents is multiplied by the number of target tables. It's important to optimize the policy-related query. You can test an update policy's performance impact by invoking the policy on already-existing extents, before creating or altering the policy, or on the function used with the query.

Evaluate resource usage

Use .show queries, to evaluate resource usage (CPU, memory, and so on) with the following parameters:

• Set the Source property, the source table name, as MySourceTable
• Set the Query property to call a function named MyFunction()

// '_extentId' is the ID of a recently created extent, that likely hasn't been merged yet.
let _extentId = toscalar(
    MySourceTable
    | project ExtentId = extent_id(), IngestionTime = ingestion_time()
    | where IngestionTime > ago(10m)
    | top 1 by IngestionTime desc
    | project ExtentId
);
// This scopes the source table to the single recent extent.
let MySourceTable = 
    MySourceTable
    | where ingestion_time() > ago(10m) and extent_id() == _extentId;
// This invokes the function in the update policy (that internally references `MySourceTable`).
MyFunction

Failures

With the default setting of IsTransactional:false, data can still be ingested to the source table even if the policy doesn't run.

Setting IsTransactional:true guarantees consistency between data in the source and target table. However, if the policy conditions fail, data isn't ingested to the source table. Alternatively, depending on conditions, sometimes data is ingested to the source table, but not to the target table. However, if your policy is defined incorrectly, or there is a schema mismatch, data will not be ingested to the source or target table. For example, a mismatch between the query output schema and the target table could be caused by dropping a column from the target table.

You can view failures using the .show ingestion failures command.

.show ingestion failures 
| where FailedOn > ago(1hr) and OriginatesFromUpdatePolicy == true

Treatment of failures

Non-transactional policy

When set to IsTransactional:false, any failure to run the policy is ignored. Ingestion is not automatically retried. You can manually retry ingestion.

Transactional policy

When set to IsTransactional:true, if the ingestion method is pull, the Data Management service is involved, and ingestion is automatically retried according to the following conditions:

• Retries are performed until one of following configurable limit settings is met: DataImporterMaximumRetryPeriod or DataImporterMaximumRetryAttempts
• By default the DataImporterMaximumRetryPeriod setting is 2 days, and DataImporterMaximumRetryAttemptsis 10
• The backoff period starts at 2 minutes, and doubles. So the wait starts with 2 min, then increases to 4 min, to 8 min, to 16 min and so on.

In any other case, you can manually retry ingestion.

Example of extract, transform, load

You can use update policy settings to perform extract, transform, load (ETL).

In this example, use an update policy in conjunction with a simple function to perform ETL. First, we create two tables:

• The source table - Contains a single string-typed column into which data is ingested.
• The target table - Contains the desired schema. The update policy is defined on this table.

1. Let's create the source table:

   .create table MySourceTable (OriginalRecord:string) 
   

2. Next, create the target table:

   .create table MyTargetTable (Timestamp:datetime, ThreadId:int, ProcessId:int, TimeSinceStartup:timespan, Message:string)
   

3. Then create a function to extract data:

   .create function
    with (docstring = 'Parses raw records into strongly-typed columns', folder = 'UpdatePolicyFunctions')
        ExtractMyLogs()  
       {
       MySourceTable
       | parse OriginalRecord with "[" Timestamp:datetime "] [ThreadId:" ThreadId:int "] [ProcessId:" ProcessId:int "] TimeSinceStartup: " TimeSinceStartup:timespan " Message: " Message:string
       | project-away OriginalRecord
   }
   

4. Now, set the update policy to invoke the function that we created:

   .alter table MyTargetTable policy update 
   @'[{ "IsEnabled": true, "Source": "MySourceTable", "Query": "ExtractMyLogs()", "IsTransactional": true, "PropagateIngestionProperties": false}]'
   

5. To empty the source table after data is ingested into the target table, define the retention policy on the source table to have 0s as its SoftDeletePeriod.

    .alter-merge table MySourceTable policy retention softdelete = 0s