A parallel run keeps a SaaS product and its replacement available during a defined transition period. It creates time to observe the new system under real operating conditions before the source is retired. The method can reduce uncertainty, but it also creates temporary duplication and requires explicit rules for data entry, comparison, and issue handling.
What runs in parallel
Parallel operation does not always mean that every user performs every task twice. The scope may cover selected workflows, user groups, locations, integrations, reports, or data sets. The plan defines which system is authoritative for each activity and what evidence the second system must produce.
| Parallel model | How work is handled | Main control |
|---|---|---|
| Duplicate processing | The same input is processed in both systems | Compare outputs and record differences |
| Source-led shadowing | Users work in the source while the replacement receives copied data | Prevent destination results from triggering live actions |
| Destination-led transition | Users work in the replacement while the source remains available | Keep source data current enough for fallback |
| Staged groups | A bounded group moves while others remain on SaaS | Control data and workflow boundaries between groups |
The chosen model must account for integrations. A notification, payment, stock update, or customer message must not happen twice merely because the same case exists in both systems.
What a parallel run can verify
The strongest evidence comes from differences that are expected to match. Teams compare calculations, statuses, reports, downstream messages, access decisions, and the completion of end-to-end tasks. Data validation supplies the checks for records and relationships, while operational scenarios show whether people can complete their work.
A useful parallel-run plan specifies:
- Scope: which users, workflows, records, and interfaces participate.
- Authority: where new records and changes are entered for each part of the process.
- Synchronization: how updates reach the other system and how conflicts are prevented.
- Comparison: which outputs must match and how accepted differences are documented.
- Support: who investigates discrepancies and communicates changed procedures.
- Exit criteria: what evidence permits full cutover or activates fallback.
Costs and risks of dual operation
Running two systems creates its own failure modes. Users may update the wrong record, integrations may fire twice, access can diverge, and reports can mix data from different authorities. Temporary licences, additional support, synchronization, and duplicate work also contribute to switching cost.
The main risks and controls are:
| Risk | Control |
|---|---|
| Conflicting changes | One authoritative write path per data domain |
| Duplicate external action | Disable or isolate selected destination integrations |
| User confusion | Clear task ownership, labels, and support guidance |
| Unexplained output difference | Logged comparison with an assigned reviewer |
| Endless overlap | Defined duration and evidence-based exit decision |
Parallel running is therefore not automatically safer than a direct change. It is useful when the old and new systems can coexist without corrupting data or duplicating business actions, and when the comparison produces evidence that could not be obtained through rehearsal alone.
Ending parallel operation
The cutover plan defines how the overlap ends. Before the source becomes read-only or is closed, accepted differences are documented, blocking issues are resolved, destination integrations are enabled, and any final synchronization is validated.
A parallel run also changes fallback planning. If the destination has accepted new transactions, returning to the SaaS product may require reverse synchronization or manual reconciliation. The rollback plan must describe that data path before live operation begins.
A parallel run deliberately creates a temporary version of several sources of truth. Its boundaries, authority rules, and end date keep the temporary arrangement from becoming the next permanent workaround.
