Continuous data export overview

Applies to: ✅ Microsoft Fabric ✅ Azure Data Explorer

This article describes continuous export of data from Kusto to an external table with a periodically run query. The results are stored in the external table, which defines the destination, such as Azure Blob Storage, and the schema of the exported data. This process guarantees that all records are exported "exactly once", with some exceptions.

By default, continuous export runs in a distributed mode, where all nodes export concurrently, so the number of artifacts depends on the number of nodes. Continuous export isn't designed for low-latency streaming data.

To enable continuous data export, create an external table and then create a continuous export definition pointing to the external table.

Permissions

All continuous export commands require at least Database Admin permissions.

Continuous export guidelines

• Output schema:

  • The output schema of the export query must match the schema of the external table to which you export.

• Frequency:

  • Continuous export runs according to the time period configured for it in the intervalBetweenRuns property. The recommended value for this interval is at least several minutes, depending on the latencies you're willing to accept. The time interval can be as low as one minute, if the ingestion rate is high.

    Note

    The intervalBetweenRuns serves as a recommendation only, and isn't guaranteed to be precise. Continuous export isn't suitable for exporting periodic aggregations. For example, a configuration of intervalBetweenRuns=1h with an hourly aggregation (T | summarize by bin(Timestamp, 1h)) won't work as expected, since the continuous export won't run exactly on-the-hour. Therefore, each hourly bin will receive multiple entries in the exported data.

• Number of files:

  • The number of files exported in each continuous export iteration depends on how the external table is partitioned. For more information, see export to external table command. Each continuous export iteration always writes to new files, and never appends to existing ones. As a result, the number of exported files also depends on the frequency in which the continuous export runs. The frequency parameter is intervalBetweenRuns.

• External table storage accounts:

  • For best performance, the database and the storage accounts should be colocated in the same Azure region.
  • Continuous export works in a distributed manner, such that all nodes are exporting concurrently. On large databases, and if the exported data volume is large, this might lead to storage throttling. The recommendation is to configure multiple storage accounts for the external table. For more information, see storage failures during export commands.

Exactly once export

To guarantee "exactly once" export, continuous export uses database cursors. The continuous export query shouldn't include a timestamp filter - the database cursors mechanism ensures that records aren't processed more than once. Adding a timestamp filter in the query can lead to missing data in exported data.

IngestionTime policy must be enabled on all tables referenced in the query that should be processed "exactly once" in the export. The policy is enabled by default on all newly created tables.

The guarantee for "exactly once" export is only for files reported in the show exported artifacts command. Continuous export doesn't guarantee that each record is written only once to the external table. If a failure occurs after export begins and some of the artifacts were already written to the external table, the external table might contain duplicates. If a write operation was aborted before completion, the external table might contain corrupted files. In such cases, artifacts aren't deleted from the external table, but they aren't reported in the show exported artifacts command. Consuming the exported files using the show exported artifacts command guarantees no duplications and no corruptions.

Export from fact and dimension tables

By default, all tables referenced in the export query are assumed to be fact tables. As such, they're scoped to the database cursor. The syntax explicitly declares which tables are scoped (fact) and which aren't scoped (dimension). See the over parameter in the create command for details.

The export query includes only the records that joined since the previous export execution. The export query might contain dimension tables in which all records of the dimension table are included in all export queries. When using joins between fact and dimension tables in continuous-export, keep in mind that records in the fact table are only processed once. If the export runs while records in the dimension tables are missing for some keys, records for the respective keys are either missed or include null values for the dimension columns in the exported files. Returning missed or null records depends on whether the query uses inner or outer join. The forcedLatency property in the continuous-export definition can be useful in such cases, where the fact and dimensions tables are ingested during the same time for matching records.

Note

Continuous export of only dimension tables isn't supported. The export query must include at least a single fact table.

Monitor continuous export

Monitor the health of your continuous export jobs using the following export metrics:

• Continuous export max lateness - Max lateness (in minutes) of continuous exports in the database. This is the time between now and the min ExportedTo time of all continuous export jobs in database. For more information, see .show continuous export command.
• Continuous export result - Success/failure result of each continuous export execution. This metric can be split by the continuous export name.

Use the .show continuous export failures command to see the specific failures of a continuous export job.

Warning

If a continuous export fails for over 7 days due to a permanent failure, the export will be automatically disabled by the system. Permanent errors include: external table not found, mismatch between schema of continuous export query and external table schema, storage account is not accessible. After the error has been fixed, you can re-enable the continuous export using the .enable continuous export command.

Resource consumption

• The impact of the continuous export on the database depends on the query the continuous export is running. Most resources, such as CPU and memory, are consumed by the query execution.
• The number of export operations that can run concurrently is limited by the database's data export capacity. For more information, see Management commands throttling. If the database doesn't have sufficient capacity to handle all continuous exports, some start lagging behind.
• The show commands-and-queries command can be used to estimate the resources consumption.
  • Filter on | where ClientActivityId startswith "RunContinuousExports" to view the commands and queries associated with continuous export.

Export historical data

Continuous export starts exporting data only from the point of its creation. Records ingested before that time should be exported separately using the non-continuous export command. Historical data might be too large to be exported in a single export command. If needed, partition the query into several smaller batches.

To avoid duplicates with data exported by continuous export, use StartCursor returned by the show continuous export command and export only records where cursor_before_or_at the cursor value. For example:

.show continuous-export MyExport | project StartCursor

StartCursor
636751928823156645

Followed by:

.export async to table ExternalBlob
<| T | where cursor_before_or_at("636751928823156645")

Continuous export from a table with Row Level Security

To create a continuous export job with a query that references a table with Row Level Security policy, you must:

• Provide a managed identity as part of the continuous export configuration. For more information, see Use a managed identity to run a continuous export job.
• Use impersonation authentication for the external table to which the data is exported.

Continuous export to delta table - Preview

Continuous export to a delta table is currently in preview.

Important

Delta table partitioning isn't supported in continuous data export.

Kusto won't write to existing delta tables if the delta protocol writer version is higher than 1.

To define continuous export to a delta table, do the following steps:

1. Create an external delta table, as described in Create and alter delta external tables on Azure Storage.

   Note

   If the schema isn't provided, Kusto will try infer it automatically if there is already a delta table defined in the target storage container.
   Delta table partitioning isn't supported.

2. Define continuous export to this table using the commands described in Create or alter continuous export.

   Important

   The schema of the delta table must be in sync with the continuous export query. If the underlying delta table changes, the export might start failing with unexpected behavior.

Limitations

General:

• The following formats are allowed on target tables: CSV, TSV, JSON, and Parquet.
• Continuous export isn't designed to work over materialized views, since a materialized view might be updated, while data exported to storage is always appended and never updated.
• Continuous export can't be created on follower databases since follower databases are read-only and continuous export requires write operations.
• Records in source table must be ingested to the table directly, using an update policy, or ingest from query commands. If records are moved into the table using .move extents or using .rename table, continuous export might not process these records. See the limitations described in the Database Cursors page.
• If the artifacts used by continuous export are intended to trigger Event Grid notifications, see the known issues section in the Event Grid documentation.

Cross-database and cross-Eventhouse:

• Continuous export doesn't support cross-Eventhouse calls.
• Continuous export supports cross-database calls only for dimension tables. All fact tables must reside in the local database. See more details in Export from fact and dimension tables.

Policies:

• Continuous export can't be enabled on a table with Row Level Security policy unless specific conditions are met. For more information, see Continuous export from a table with Row Level Security.
• Continuous export can't be configured on a table with restricted view access policy.

Related content

• .create or alter continuous-export
• External tables