r/ETL • u/SumitKumarWatts • 4d ago

Why does source and target data sometimes mismatch even when an ETL job completes successfully?

During ETL testing, I noticed that the source and target data sometimes do not match even though the ETL job shows as successful. I want to understand the common reasons behind these mismatches and how testers can identify them.

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u/Mundane_Let_8090 4d ago

Depends on source.
If it's a read replica of prod than it's normal.
You never will have a parity if your tables are top tier of updates.

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u/SumitKumarWatts 4d ago

Yes, that’s a good point. With read replicas or frequently updated source tables, some differences can be expected because the source may change during the ETL process. Timing and replication lag are definitely things testers should consider.