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Software Architecture Guide

Last Responsible Moment

Explanation

Section titled “Explanation”

A team makes many choices while building software. Some are costly to undo. The Last Responsible Moment (LRM) is a timing rule for such choices. It names the point where failing to decide eliminates an important alternative. The advice is to delay commitment until that point.

Most Responsible Moment. A related idea is the Most Responsible Moment (MRM). It was proposed as a reaction to the lean principle “Decide as late as possible”. The MRM asks for decisions to be taken when the time is right. They should be neither forced nor delayed. LRM thinking can invite late decisions, which is the risk the MRM addresses.

Too early and too late. Deciding too early harms flexibility. Deciding too late is also bad. Postponing a decision beyond its MRM can signal uncertainty or even fear.

Readiness check. A decision readiness check helps with timing. It asks three things:

  • Is the problem important and urgent?
  • Has the MRM of the decision come?
  • Is the outcome useful and needed now?

The Architectural Decision Definition of Ready also stresses hitting the MRM. It links this to the lean idea of deciding late.

Big decisions. Some architectural decisions have an early MRM, so they cannot wait. Rules of thumb exist for such “big” decisions to guide their timing.

Example

Section titled “Example”

A team must pick a database for a new product. Nothing built in the next few weeks depends on the choice, so the team keeps working with a simple storage layer.

Then a planned feature needs a kind of query that only some databases support. If the team does not choose now, a good option will be lost. That is the last responsible moment. The team runs the readiness check: the problem is urgent and the outcome is needed now. The team decides.

A decision that shapes the whole system may reach its right moment much earlier. Such a decision cannot wait for the feature.

Trade-offs

Section titled “Trade-offs”

Waiting keeps options open, but it has costs. Deciding too early harms flexibility. Deciding too late is also bad. Pure “decide as late as possible” thinking can invite late decisions, so the MRM asks for the right time instead. Procrastinating can also hide uncertainty or fear. Big decisions with an early MRM cannot wait at all.

Checks

Section titled “Checks”
  • Each open decision states whether its most responsible moment has come.
  • Decisions are checked for being useful and needed now before they are made.
  • Big architectural decisions with an early most responsible moment are identified and not postponed.
  • Decisions are not delayed past their most responsible moment without a stated reason.
Section titled “Related”

Sources

Section titled “Sources”