Moving Legacy ETL to the Cloud Without Downtime

The phased approach we use to migrate SAS, Informatica and hand-written ETL estates while reports keep running.

Legacy ETL estates are the last thing anyone wants to touch and the first thing that blocks every new initiative. Migrating them does not have to mean a risky big-bang weekend. This is the approach we have used for banking and telecom estates of a hundred or more jobs.

Inventory before you plan

Start by listing every job, table and report, who consumes each output and when it last ran. In most estates a third of the jobs are unused. Retiring them is the cheapest migration there is.

Build lineage from the code

Documentation is usually missing or wrong. Parse the code to trace every output field back to its sources. This tells you the order to migrate in, what breaks if a source changes, and which fields carry business logic that must be preserved exactly.

Choose the target per workload

Not everything needs the same treatment. Heavy transformations move to PySpark on Databricks or Snowflake. Simple SQL logic moves to dbt. Some reports are better rebuilt directly in the BI tool. Deciding per workload avoids over-engineering.

Migrate in waves, reconcile every wave

Group jobs by dependency and business area. Rebuild one wave, run old and new side by side, and compare outputs row by row until they match. Only then switch consumers. This is slower than a cut-over but it is the reason nobody notices.

Automate the comparison

Manual reconciliation does not scale past the first wave. Build a comparison harness that runs both versions and reports differences automatically. It becomes your regression test for the rest of the programme.

Retire and stop paying

The migration is not finished until the old platform is switched off and the licence cancelled. Track the savings; they are usually the strongest argument for the next phase.

Where it goes wrong

Skipping the inventory, migrating in dependency-unaware order, and declaring success before reconciliation. Each one turns a quiet programme into a crisis. Our Legacy Modernization service exists to avoid all three.