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Delta Lake on Databricks

Delta Lake is an open-source storage layer that brings reliability to data lakes. It was initially developed by Databricks in 2016 and open-sourced to the Linux Foundation in 2019.  Delta Lake provides ACID transactions, scalable metadata handling, and unifies streaming and batch data processing. It is a storage layer on top of cloud object stores (Amazon S3, Azure Data Lake Storage, Google Cloud Storage and etc) or distributed filesystems like HDFS, which describes and defines the formats of data object and transaction logs, and a set of access protocols enabling DBMS-like features.  It is fully compatible with Apache Spark APIs.  Its schema enforcement automatically handles schema variations to prevent insertion of bad records during ingestion.  The Databricks Lakehouse platform provides data engineering, SQL analytics, data science and Machine Learning on Azure Databricks, Databricks on AWS and google Cloud.

 

Apache Spark SQL in Databricks is designed to be compatible with  Apache Hive, including metastore connectivity, SerDes, and UDFs.  So every Databricks deployment has a central Hive metastore accessible by all clusters to persist table metadata, and Hackolade has been supporting Hive for several years.  

 

To perform data modeling for Delta Lake on Databricks with Hackolade, you must first download the DeltaLake plugin.  

 

Hackolade was specially adapted to support the data modeling of Delta Lake, including the Databricks storage structure of clusters, databases, tables, views, and indexes.  It leverages Hive primitive and complex data types, plus user-defined types.  And combines it all with the usual capabilities of forward-engineering of HiveQL scripts, reverse-engineering, documentation generation, model comparison, command-line interface integration with CI/CD pipelines, etc...The application closely follows the Delta Lake terminology.

 

The data model in the picture below results from the modeling of an application described in this tutorial.

 

Delta Lake on Databricks workspace

 

 

Clusters

A Databricks cluster is a set of computation resources and configurations on which you run data engineering, data science, and data analytics workloads, such as production ETL pipelines, streaming analytics, ad-hoc analytics, and machine learning.  

 

Databases

A Databricks database is a collection of tables.

 

Tables

A Databricks table is a collection of structured data. Tables be can queried with Spark APIs and Spark SQL.  The table is logically made up of the data being stored in cloud object stores (Amazon S3, Azure Data Lake Storage, Google Cloud Storage and etc) or distributed filesystems like HDFS.  The table metadata describes the layout of the data in the table and defines the formats of data object and transaction logs, and a set of access protocols enabling DBMS-like features.  It is fully compatible with Apache Spark APIs.  Its schema enforcement automatically handles schema variations to prevent insertion of bad records during ingestion.

 

Every Databricks deployment has a central Hive metastore accessible by all clusters to persist table metadata. Instead of using the Databricks Hive metastore, users have the option to use an existing external Hive metastore instance or the AWS Glue Catalog.

 

Delta Lake does not support multi-table transactions, primary or foreign keys. The only constraints supported by Delta Lake are NOT NULL and CHECK.

 

Databricks table properties

 

 

Data types

Delta Lake supports different data types to be used in table columns. The Hive data types supported by Delta Lake can be broadly classified in Primitive and Complex data types.

 

Hackolade was specially adapted to support the data types and attributes behavior of Delta Lake, including arrays, maps and structs.

 

Databricks data types

 

Views

A Databricks view is a searchable object in a database, which can be defined by a query.  Data cannot be stored in a view, as it is a sort of virtual table.  By using joins, it is possible to combine data from one or more tables. It may also hold a subset of information.  More info can be found here.

 

Hackolade supports Delta Lake views, via a SELECT of columns of the underlying base tables.

 

Forward-Engineering

Hackolade dynamically generates the HiveQL script to create databases, tables, columns and their data types, as well as views for the structure created with the application.

 

Databricks forward-engineering

 

By pressing the button "Apply to instance" the system will automatically creates the database, tables, views, columns, including constraints and indexes.

 

Note: As foreign keys and primary keys are not supported in Delta Lake, they can be used in Hackolade for informational purposes, but are not generated in the HQL script.

 

As many people store JSON within text columns, Hackolade allows for the schema design of those documents.  That JSON structure is not forward-engineered, but is useful for developers, analysts and designers.  It is often integrated into a CI/CD pipeline, using the Command-Line Interface.

Reverse-Engineering

The connection is established using a connection string including the Cloud provider and a personal access token.

 

The Hackolade process for reverse-engineering of Delta Lake databases includes the execution of HQL SHOW statements to discover databases, tables, columns and their data types.  If JSON is detected in text columns, Hackolade performs statistical sampling of records followed by probabilistic inference of the JSON document schema.

 

For more information on Delta Lake in general, please consult the website.  Here is the links to the Databricks website.