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Software Architecture Guide

AS-IS and TO-BE View

Explanation

Section titled “Explanation”

An AS-IS view describes the existing (legacy) system. A TO-BE view describes the target the team wants to reach. Showing both makes the planned change visible.

Two views are not always enough. Moving from one to the other can pass through intermediate configurations. These may need integrations that have no place in the final target. So the transition state can differ from both the AS-IS and the TO-BE.

Such transitional parts work like scaffolding on a building. They ease the move from the legacy system to the target. Once the target is reached, the team must remove them.

Where these views can go in an arc42 document:

  • The building block view is the static decomposition of a system into building blocks. A structural AS-IS or TO-BE view fits there.
  • Decisions can carry a status: proposed, accepted, deprecated or superseded. A proposed decision can mark something planned. A superseded one can mark something replaced. The arc42 page does not present this as an AS-IS/TO-BE technique, so treat it as a possible convention, not a rule.

Example

Section titled “Example”

A team runs an old billing system and plans to replace it. The AS-IS view shows the old system. The TO-BE view shows the new one. The team sees that during the move, a small adapter must connect old and new parts. That adapter appears in neither view. They document it as a transition state and plan to remove it once the new system runs alone.

Trade-offs

Section titled “Trade-offs”

A separate transition view costs extra effort and shows parts that will be thrown away. The benefit is that the team plans for those parts and removes them, instead of leaving them in the final system.

From the existing system to the target, with temporary transition parts in between.

Diagram source 
flowchart LR
  A[AS-IS: legacy system] --> B[Transition: temporary scaffolding]
  B --> C[TO-BE: target system]

Checks

Section titled “Checks”
  • The document states which building blocks belong to the existing system and which belong to the target.
  • Temporary transition components are identified, and a plan exists to remove them once the target is reached.
  • Planned decisions are marked as proposed or accepted, and replaced decisions as deprecated or superseded.
Section titled “Related”

Sources

Section titled “Sources”