When we gradually replace a legacy system, we have plenty of cases
where the legacy system and its replacement need to interact. Since these
legacy systems are often difficult, and costly, to change, we need a
mechanism that can integrate elements of the replacement while minimizing
the impact to the legacy system. Ian Cartwright, Rob Horn, and
James Lewis explain how we can use Event Interception on
state-changing events, allowing us to forward them to the
replacement.
Source: martinfowler