SQL Server Relations#

The SQL Server relation allows you to access MS SQL Server and Azure SQL databases using a JDBC driver. It uses the spark-sql-connector from Microsoft to speed up processing. The sqlserver relation will also make use of a global temporary table as an intermediate staging target and then atomically replace the contents of the target table with the contents of the temp table within a single transaction.


This relation type is provided as part of the flowman-mssql plugin, which needs to be enabled in your namespace.yml file. See namespace documentation for more information for configuring plugins.


# First specify a connection. This can be used by multiple SQL Server relations
    kind: jdbc
    url: "$frontend_db_url"
    username: "$frontend_db_username"
    password: "$frontend_db_password"

    kind: sqlserver
    # Specify the name of the connection to use
    connection: frontend
    # Specify the table
    table: "users"
      kind: avro
      file: "${project.basedir}/schema/users.avsc"
      - user_id
      - name: "users_idx0"
        columns: [user_first_name, user_last_name]

It is also possible to directly embed the connection as follows:

    kind: sqlserver
    # Specify the name of the connection to use
      kind: jdbc
      url: "$frontend_db_url"
      username: "$frontend_db_username"
      password: "$frontend_db_password"
    # Specify the table
    table: "users"
    # Specify storage format
    storageFormat: COLUMNSTORE

For most cases, it is recommended not to embed the connection, since this prevents reusing the same connection in multiple places.


  • kind (mandatory) (type: string): jdbc

  • schema (optional) (type: schema) (default: empty): Explicitly specifies the schema of the JDBC source. Alternatively Flowman will automatically try to infer the schema.

  • primaryKey (optional) (type: list) (default: empty): List of columns which form the primary key. This will be used when Flowman creates the table, and this will also be used as the fallback for merge/upsert operations, when no mergeKey and no explicit merge condition is specified.

  • mergeKey (optional) (type: list) (default: empty): List of columns which will be used as default condition for merge and upsert operations. The main difference to primaryKey is that these columns will not be used as a primary key for creating the table.

  • description (optional) (type: string) (default: empty): A description of the relation. This is purely for informational purpose.

  • connection (mandatory) (type: string): The connection field specifies the name of a JDBC Connection object which has to be defined elsewhere.

  • database (optional) (type: string) (default: empty): Defines the Hive database where the table is defined. When no database is specified, the table is accessed without any specific qualification, meaning that the default database will be used, or the one specified in the connection.

  • table (mandatory) (type: string): Specifies the name of the table in the relational database.

  • stagingTable (optional) (type: string) (default: empty): Specifies the name of the staging table to use for write operations. This table will be created and filled with data, and the final table will be populated from this table inside a transaction. If not table is specified (the default), then Flowman will use a global temporary table.

  • properties (optional) (type: map:string) (default: empty): Specifies any additional properties passed to the JDBC connection. Note that both the JDBC relation and the JDBC connection can define properties. So it is advisable to define all common properties in the connection and more table specific properties in the relation. The connection properties are applied first, then the relation properties. This means that a relation property can overwrite a connection property if it has the same name.

  • indexes (optional) (type: list:index) (default: empty): Specifies a list of database indexes to be created. Each index has the properties name, columns, unique (default=false) and clustered (default=false). Note that clustered indexes are currently only supported by MS Flowman for SQL Server and Azure SQL.

  • storageformat (optional) (type: string) (default: empty): Specifies the internal storage format, which can either be ROWSTORE or COLUMNSTORE. Internally MS SQL Server uses ROWSTORE as the default format. COLUMNSTORE will actually create a CLUSTERED COLUMNSTORE INDEX and is preferable for typical OLAP workloads.

  • migrationPolicy (optional) (string) (default: empty) Can be one of RELAXED or STRICT. If left empty, then the value of the Flowman configuration property flowman.default.relation.migrationPolicy will be used instead.

  • migrationStrategy (optional) (string) (default: empty) Can be one of ALTER, ALTER_REPLACE, REPLACE, NEVER or FAIL. If left empty, then the value of the Flowman configuration property flowman.default.relation.migrationStrategy will be used instead.

Staging Tables#

When using the sqlserver relation, Flowman will always use staging tables when writing to a SQL database. This means, Flowman will first create this special staging table (which technically is just a normal table, but without any index or primary key), and then copy the table into the real target table. Afterward, the staging table will be dropped. This approach helps to ensure consistency, since the copy process is performed within a single SQL transaction. Moreover, since no primary key or index is present in the staging table, this will also avoid locks on the database server side, which may lead to timeouts or other failures during the parallel write process that Spark uses under the hood.

You can either explicitly specify the name of the staging table via stagingTable, or Flowman will automatically use a global temporary table. In both cases, Flowman will automatically remove the staging table after the write operation has finished (either successfully or with an error).

Automatic Migrations#

Flowman supports some automatic migrations, specifically with the migration strategies ALTER, ALTER_REPLACE and REPLACE (those can be set via the property migrationStrategy or the global config variable flowman.default.relation.migrationStrategy, see configuration for more details).

The migration strategy ALTER supports the following alterations for JDBC relations:

  • Changing nullability

  • Adding new columns

  • Dropping columns

  • Changing the column type

  • Adding / dropping indexes

  • Changing the primary key

Note that although Flowman will try to apply these changes, not all SQL databases support all of these changes in all variations. Therefore, it may well be the case, that the SQL database will fail performing these changes. If the migration strategy is set to ALTER_REPLACE, then Flowman will fall back to trying to replace the whole table altogether on any non-recoverable exception during migration.

Schema Conversion#

The JDBC relation fully supports automatic schema conversion on input and output operations as described in the corresponding section of relations.

Output Modes#

The sqlserver relation supports the following output modes in a relation target:

Output Mode Supported Comments
errorIfExists yes Throw an error if the JDBC table already exists
ignoreIfExists yes Do nothing if the JDBC table already exists
overwrite yes Overwrite the whole table or the specified partitions
overwrite_dynamic no -
append yes Append new records to the existing table
update yes -

In addition, the sqlserver relation also supports complex merge operations in a merge target.


When using SQL Server tables as data sinks in a relation target, then Flowman will manage the whole lifecycle for you. This means that

  • SQL Server tables will be created and migrated during CREATE phase, but only if a schema is provided

  • SQL Server tables will be populated with records and partitions will be added during BUILD phase, but only if the relation target contains a mapping.

  • SQL Server tables will be truncated, or individual partitions will be dropped during TRUNCATE phase

  • SQL Server tables will be removed during DESTROY phase

This means that you can

  • Externally manage tables by omitting the schema. Then Flowman will not create or migrate the table for any relation target referring to this relation.

  • Only manage the tables by Flowman but not populate it with data by omitting a mapping in the relation target.

Mocking SQL server relations#

Note that Flowman will rely on schema inference in some important situations, like mocking and generally for describing the schema of a relation. This might create unwanted connections to the physical data source, particular in case of self-contained tests. To prevent Flowman from creating a connection to the physical data source, you simply need to explicitly specify a schema, which will then be used instead of the physical schema in all situations where only schema information is required.

Using staging tables#

Since version 0.23.0 Flowman will always use a global temporary table as a staging table for any write/update operation for SQL Server relations.

While this two-step approach might slow down write processes, it is often required when performing update/merge operations since these could result in database deadlocks otherwise when Spark performs these operations in parallel from multiple processes into a single database.