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Export data to cloud storage

  • Artikkeli

Applies to: ✅ Microsoft Fabric ✅ Azure Data Explorer

Executes a query and writes the first result set to an external cloud storage, specified by a storage connection string.

Permissions

You must have at least Database Viewer permissions to run this command.

Syntax

.export [async] [compressed] to OutputDataFormat ( StorageConnectionString [, ...] ) [with ( PropertyName = PropertyValue [, ...] )] <| Query

Learn more about syntax conventions.

Parameters

Name Type Required Description
async string If specified, the command runs in asynchronous mode. See asynchronous mode.
compressed string If specified, the output storage artifacts are compressed as .gz files. See the compressionType supported property for compressing Parquet files as snappy.
OutputDataFormat string ✔️ Indicates the data format of the storage artifacts written by the command. Supported values are: csv, tsv, json, and parquet.
StorageConnectionString string One or more storage connection strings that indicate which storage to write the data to. More than one storage connection string might be specified for scalable writes. Each such connection string must indicate the credentials to use when writing to storage. For example, when writing to Azure Blob Storage, the credentials can be the storage account key, or a shared access key (SAS) with the permissions to read, write, and list blobs.
PropertyName, PropertyValue string A comma-separated list of key-value property pairs. See supported properties.

Note

We highly recommended exporting data to storage that is co-located in the same region as the database itself. This includes data that is exported so it can be transferred to another cloud service in other regions. Writes should be done locally, while reads can happen remotely.

Supported properties

Property Type Description
includeHeaders string For csv/tsv output, controls the generation of column headers. Can be one of none (default; no header lines emitted), all (emit a header line into every storage artifact), or firstFile (emit a header line into the first storage artifact only).
fileExtension string Indicates the "extension" part of the storage artifact (for example, .csv or .tsv). If compression is used, .gz is appended as well.
namePrefix string Indicates a prefix to add to each generated storage artifact name. A random prefix is used if left unspecified.
encoding string Indicates how to encode the text: UTF8NoBOM (default) or UTF8BOM.
compressionType string Indicates the type of compression to use. Possible values are gzip or snappy. Default is gzip. snappy can (optionally) be used for parquet format.
distribution string Distribution hint (single, per_node, per_shard). If value equals single, a single thread writes to storage. Otherwise, export writes from all nodes executing the query in parallel. See evaluate plugin operator. Defaults to per_shard.
persistDetails bool Indicates that the command should persist its results (see async flag). Defaults to true in async runs, but can be turned off if the caller doesn't require the results). Defaults to false in synchronous executions, but can be turned on in those as well.
sizeLimit long The size limit in bytes of a single storage artifact written before compression. Valid range: 100 MB (default) to 4 GB.
parquetRowGroupSize int Relevant only when data format is Parquet. Controls the row group size in the exported files. Default row group size is 100,000 records.
distributed bool Disable or enable distributed export. Setting to false is equivalent to single distribution hint. Default is true.
parquetDatetimePrecision string Specifies the precision to use when exporting datetime values to Parquet. Possible values are millisecond and microsecond. Default is millisecond.

Authentication and authorization

The authentication method is based on the connection string provided, and the permissions required vary depending on the authentication method.

The following table lists the supported authentication methods and the permissions needed for exporting data to external storage by storage type.

Authentication method Azure Blob Storage / Data Lake Storage Gen2 Data Lake Storage Gen1
Impersonation Storage Blob Data Contributor Contributor
Shared Access (SAS) token Write Write
Microsoft Entra access token No extra permissions required No extra permissions required
Storage account access key No extra permissions required No extra permissions required

Returns

The commands return a table that describes the generated storage artifacts. Each record describes a single artifact and includes the storage path to the artifact and how many records it holds.

Path NumRecords
http://storage1.blob.core.windows.net/containerName/export_1_d08afcae2f044c1092b279412dcb571b.csv 10
http://storage1.blob.core.windows.net/containerName/export_2_454c0f1359e24795b6529da8a0101330.csv 15

Asynchronous mode

If the async flag is specified, the command executes in asynchronous mode. In this mode, the command returns immediately with an operation ID, and data export continues in the background until completion. The operation ID returned by the command can be used to track its progress and ultimately its results via the following commands:

For example, after a successful completion, you can retrieve the results using:

.show operation f008dc1e-2710-47d8-8d34-0d562f5f8615 details

Examples

In this example, Kusto runs the query and then exports the first recordset produced by the query to one or more compressed CSV blobs, up to 1 GB before compression. Column name labels are added as the first row for each blob.

.export
  async compressed
  to csv (
    h@"https://storage1.blob.core.windows.net/containerName;secretKey",
    h@"https://storage1.blob.core.windows.net/containerName2;secretKey"
  ) with (
    sizeLimit=1000000000,
    namePrefix="export",
    includeHeaders="all",
    encoding="UTF8NoBOM"
  )
  <| 
  Logs | where id == "1234"

Failures during export commands

Export commands can transiently fail during execution. Continuous export automatically retries the command. Regular export commands (export to storage, export to external table) don't perform any retries.

  • When the export command fails, artifacts that were already written to storage aren't deleted. These artifacts remain in storage. If the command fails, assume the export is incomplete, even if some artifacts were written.
  • The best way to track both completion of the command and the artifacts exported upon successful completion is by using the .show operations and .show operation details commands.

Storage failures

By default, export commands are distributed such that there might be many concurrent writes to storage. The level of distribution depends on the type of export command:

  • The default distribution for regular .export command is per_shard, which means all extents that contain data to export write to storage concurrently.

  • The default distribution for export to external table commands is per_node, which means the concurrency is the number of nodes.

When the number of extents/nodes is large, this might lead to high load on storage that results in storage throttling, or transient storage errors. The following suggestions might overcome these errors (by order of priority):

  • Increase the number of storage accounts provided to the export command or to the external table definition. The load is evenly distributed between the accounts.

  • Reduce the concurrency by setting the distribution hint to per_node (see command properties).

  • Reduce concurrency of number of nodes exporting by setting the client request property query_fanout_nodes_percent to the desired concurrency (percent of nodes). The property can be set as part of the export query. For example, the following command limits the number of nodes writing to storage concurrently to 50% of the nodes:

    .export async  to csv
    ( h@"https://storage1.blob.core.windows.net/containerName;secretKey" ) 
    with
    (
        distribution="per_node"
    ) 
    <| 
    set query_fanout_nodes_percent = 50;
    ExportQuery

  • Reduce concurrency of number of threads exporting in each node when using per shard export, by setting the client request property query_fanout_threads_percent to the desired concurrency (percent of threads). The property can be set as part of the export query. For example, the following command limits the number of threads writing to storage concurrently to 50% on each of the nodes:

    .export async  to csv
    ( h@"https://storage1.blob.core.windows.net/containerName;secretKey" ) 
    with
    (
        distribution="per_shard"
    ) 
    <| 
    set query_fanout_threads_percent = 50;
    ExportQuery

  • If exporting to a partitioned external table, setting the spread/concurrency properties can reduce concurrency (see details in the command properties.

  • If neither of the previous recommendations work, you can completely disable distribution by setting the distributed property to false. However, we don't recommend doing so, as it might significantly affect the command performance.

Authorization failures

Authentication or authorization failures during export commands can occur when the credentials provided in the storage connection string aren't permitted to write to storage. If you're using impersonate or a user-delegated SAS token for the export command, the Storage Blob Data Contributor role is required to write to the storage account. For more information, see Storage connection strings.

Data types mapping

Parquet data types mapping

On export, Kusto data types are mapped to Parquet data types using the following rules:

Kusto Data Type Parquet Data Type Parquet Annotation Comments
bool BOOLEAN
datetime INT64 TIMESTAMP_MICROS
dynamic BYTE_ARRAY UTF-8 Serialized as JSON string
guid BYTE_ARRAY UTF-8
int INT32
long INT64
real DOUBLE
string BYTE_ARRAY UTF-8
timespan INT64 Stored as ticks (100-nanosecond units) count
decimal FIXED_LENGTH_BYTE_ARRAY DECIMAL

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