Delta Table Relations

The deltaTable relation is used for creating Delta Lake tables stored in the Hive metastore. If you want to use a Delta Lake table, but not store its meta data in Hive, then use the deltaFile relation instead.

Plugin

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

Example

relations:
  some_delta_table:
    kind: deltaTable
    database: default
    table: financial_transactions
    # Specify the physical location where the data files should be stored at. If you leave this out, the Hive
    # default location will be used
    location: /warehouse/default/financial_transactions
    # Add partition column
    partitions:
      - name: business_date
        type: string
    # Specify the default key to use in upsert operations. Normally this should match the primary key 
    # (except partition columns, which will be added implicitly)
    mergeKey:
      - id  
    # Specify a schema, which is mandatory for creating the table during CREATE phase
    schema:
      kind: inline
      fields:
        - name: id
          type: string
        - name: amount
          type: double

Fields

  • kind (mandatory) (string): deltaTable
  • schema (optional) (schema) (default: empty): Explicitly specifies the schema of the Delta Lake table. Alternatively Flowman will automatically use the schema of the Hive table, if it already exists.
  • description (optional) (string) (default: empty): A description of the relation. This is purely for informational purpose.
  • options (optional) (map:string) (default: empty): All key-value pairs specified in options are directly passed to Apache Spark for reading and/or writing to this relation. The options will not be persisted in the Hive metastore. If that is what you want, then have a closer look at properties below.
  • database (mandatory) (string): 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.
  • table (mandatory) (string): Contains the name of the Hive table.
  • location (optional) (string) (default: empty): Specifies the location of the files stored in this Hive table. This setting is only used when Flowman is used to create the Delta table within Hive table and is ignored otherwise. This corresponds to the LOCATION in a CREATE TABLE statement.
  • partitions (optional) (list:partition) (default: empty): Specifies all partition columns. This is used both for creating Hive tables, but also for writing and reading to and from them. Therefore if you are working with partitioned Hive tables you have to specify partition columns, even if Flowman is not used for creating the table. Normally the partition columns are separate from the schema, but you may also include the partition column in the schema, although this is not considered to be best practice. But it turns out to be quite useful in combination with dynamically writing to multiple partitions.
  • properties (optional) (map:string) (default: empty): Specifies additional properties of the Hive table. This setting is only used when Flowman is used to create the Hive table and is ignored otherwise. This corresponds to the TBLPROPERTIES in a CREATE TABLE statement.
  • mergeKey (optional) (list:string) (default: empty): List of column names specifying the key to identify matching records on update operations.

Automatic Migrations

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

The migration strategy ALTER supports the following alterations:

  • Changing nullability
  • Changing the comment
  • Adding new columns

Other changes (like changing the data type or dropping columns) is not supported in the ALTER strategy and will require either REPLACE or ALTER_REPLACE - but this will remove all existing data in that table!

Schema Conversion

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

Output Modes

Batch Writing

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

Output Mode Supported Comments
errorIfExists yes Throw an error if the Delta table already exists
ignoreIfExists yes Do nothing if the Delta table already exists
overwrite yes Overwrite the whole table or the specified partitions
overwrite_dynamic yes Overwrites all partitions where records are provided in the input data. Other partitions remain untouched
append yes Append new records to the existing table
update yes Updates existing records, either using mergeKey or the primary key of the specified schema

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

Note that support overwrite_dynamic is not perfect and is implemented in a two phase approach: First all partition values are calculated, then the data is inserted. This implies that the input data is scanned twice, which might be an expensive operation depending on the computational complexity and the amount of data.

Stream Writing

In addition to batch writing, the Delta table relation also supports stream writing via the stream target with the following semantics:

Output Mode Supported Comments
append yes Append new records from the streaming process once they don't change any more
update yes Append records every time they are updated
complete yes -

Remarks

Schema Inference

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.

Writing to Dynamic Partitions

Beside explicitly writing to a single Hive partition, Flowman also supports to write to multiple partitions where the records need to contain values for the partition columns. Note that currently Delta table do not support the relation output mode dynamic_overwrite with dynamic partitions, instead you can only use append and overwrite. This means, that whenever you write to a partitioned Delta table without explicitly specifying the target partition in the relation target, then the table is truncated first and all normally unchanged partitions will be lost.