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Webinar: ETL Patterns
ETL vs ELT - these two methods are not in contradiction, rather they have different use cases.
ETL stands for Extract-Transfer-Load and is used when the transform better be performed on a fly. Indications for ELT are absence of environments, need to scale in transit, need to transform or mask in transit. The transforms are done on-a-fly, in the data flow by creating for example SSIS packages or Biztalk messages.
ELT as a pattern is often used when the real-time is not a requirement, processors and memory in transit are scarce, and there is plenty of additional hard drive. This is often a preferred migration method or data masking on the golden copy method. It is done by backing up production environment, loading into special development environment and performing the transformation on the static data base.
Sometimes mixed techniques are used when the copy is loaded, but only specific data is transformed on-a-fly into static copy from production environment.