For example, a Hadoop cluster using Hive would describe a Hive table where the data source is effectively a path to a set of files in HDFS. The data store only manages the schema of the data and applies the schema on read. These are referred to as external tables because the data does not reside in storage managed by the data store itself, but on some external scalable storage such as Azure data lake store or Azure blob storage. In general, a schema is overlaid on the flat file data at query time and stored as a table, enabling the data to be queried like any other table in the data store. In practice, the target data store is a data warehouse using either a Hadoop cluster (using Hive or Spark) or a SQL dedicated pools on Azure Synapse Analytics. This approach skips the data copy step present in ETL, which often can be a time consuming operation for large data sets. This data store reads directly from the scalable storage, instead of loading the data into its own proprietary storage. The key point with ELT is that the data store used to perform the transformation is the same data store where the data is ultimately consumed.

Technologies, such as Spark, Hive, or Polybase, can then be used to query the source data. For example, you might start by extracting all of the source data to flat files in scalable storage, such as a Hadoop distributed file system, an Azure blob store, or Azure Data Lake gen 2 (or a combination).

Typical use cases for ELT fall within the big data realm. However, ELT only works well when the target system is powerful enough to transform the data efficiently. Another benefit to this approach is that scaling the target data store also scales the ELT pipeline performance. This simplifies the architecture by removing the transformation engine from the pipeline. Instead of using a separate transformation engine, the processing capabilities of the target data store are used to transform data. In the ELT pipeline, the transformation occurs in the target data store.

Azure Data Factory & Azure Synapse PipelinesĮxtract, load, and transform (ELT) differs from ETL solely in where the transformation takes place.

For example, while data is being extracted, a transformation process could be working on data already received and prepare it for loading, and a loading process can begin working on the prepared data, rather than waiting for the entire extraction process to complete. Often, the three ETL phases are run in parallel to save time. The data transformation that takes place usually involves various operations, such as filtering, sorting, aggregating, joining data, cleaning data, deduplicating, and validating data. The transformation work in ETL takes place in a specialized engine, and it often involves using staging tables to temporarily hold data as it is being transformed and ultimately loaded to its destination. It then transforms the data according to business rules, and it loads the data into a destination data store. Extract, transform, and load (ETL) processĮxtract, transform, and load (ETL) is a data pipeline used to collect data from various sources. The following sections highlight the common methods used to perform these tasks. No matter the process used, there's a common need to coordinate the work and apply some level of data transformation within the data pipeline. Various tools, services, and processes have been developed over the years to help address these challenges. Often the format is different, or the data needs to be shaped or cleaned before loading it into its final destination. The destination might not be the same type of data store as the source. Then you'd need to move it to one or more data stores. A common problem that organizations face is how to gather data from multiple sources, in multiple formats.

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