Why run a new system beside the current operation?

Run the new system beside a known fallback so the team can compare complete operating outcomes before transferring authority. Begin with explicit entry criteria, traffic allocation, stable identifiers, exception ownership, rollback, and evidence gates. Expand, restrict, or stop from those gates; do not keep both paths indefinitely.

Choose one unit of work and state where parallel operation begins and ends. The fallback should be a path the organization knows how to run and recover. It may be the current system, a manual procedure, or a narrower mode that preserves required records and decisions.

State what the new system may do in each mode. Shadow work changes no authoritative state. Supervised work may prepare an action for review. Authoritative work may complete specified actions. These labels matter only when the permitted actions and records are explicit.

Define which cases qualify, which remain excluded, and how traffic moves between paths. Allocation may start with selected case types, a controlled sample, one operating team, or a scheduled window. Preserve exclusions for cases whose data, authority, or recovery path has not been evaluated.

Give each unit a stable identifier across both paths. Record the input version, system version, decisions, actions, timestamps, exceptions, and final state. Stable identity prevents duplicate work and lets the team reconcile differences without guessing whether two records describe the same case.

A plausible answer is not an operating result. Compare whether the required record changed, the right owner received the exception, a message reached its destination, a transaction remained unique, and the item finished in a known state. Include time and review burden where they affect the operation.

Exercise dependency failure, retry, correction, rollback, and reconciliation. Name an owner for each exception class and give that person the source context needed to decide. Parallel operation earns confidence by exposing recovery behavior, not by producing two matching outputs on clean examples.

Write the triggers before the run. Evidence may permit broader case coverage, a move from shadow to supervised action, or authority over one bounded step. The same record should identify conditions that restrict scope, return work to supervision, or stop the run.

Keep the decision with an accountable operational owner. The implementation team can assemble comparisons and explain technical causes, but it should not move the acceptance standard after seeing results. Record the decision, evidence reviewed, permitted boundary, and unresolved conditions.

Before retiring the fallback, prove that operators can restrict authority, locate unfinished work, restore the fallback, and reconcile actions already taken. A paper rollback path is insufficient when the migration removes credentials, staff access, data flow, or procedural knowledge needed to use it.

Close parallel operation by choosing an authoritative path and documenting remaining supervised or excluded cases. If evidence does not support migration, narrow or stop the new path. Continuing two complete operations without a decision adds cost and increases the chance that their records diverge.

• The current operation provides a known fallback and recoverable state.
• The new path has bounded entry criteria and traffic allocation.
• Stable identifiers allow end-state comparison and reconciliation.
• A named owner can expand, restrict, stop, or roll back authority.

• Dual operation continues without an authority decision or end condition.
• The team compares model outputs instead of complete operating end states.
• The evidence gate moves after results are observed.
• The fallback is retired before recovery and reconciliation are exercised.

Set these conditions before representative work enters the new path or any authority moves from the fallback.

• The unit of work and parallel-operation boundary are explicit.

• Entry criteria, exclusions, and traffic allocation are recorded.

• Shadow, supervised, and authoritative actions are defined.

• Each unit keeps a stable identifier across comparison and retry.

• The comparison method covers final records, exceptions, review, and recovery.

• Exception classes have owners and usable decision context.

• Expand, restrict, and stop triggers are written before results are reviewed.

• A named owner makes and records each authority decision.

• Rollback, unfinished-work discovery, and reconciliation have been exercised.

• The parallel period has an end condition and a planned authoritative state.

Running beside the current operation is useful when it reveals how the new system behaves under real conditions while preserving recovery. The comparison should make authority easier to place, not create a permanent second operation.

Finish with a recorded choice to expand, restrict, stop, or migrate. Keep the evidence, accepted boundary, and recovery record available for the next change in model, provider, workflow, or authority.

Continue with the complete resource library or read our implementation approach.