Coexistence Patterns

Running old and new together for years without corrupting either.

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Questions

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A migration will take eighteen months, during which old and new systems both serve production. How should the coexistence period be designed?

2 min answer
advanced

During a multi-year migration, old and new systems must coexist. Which coexistence patterns are sustainable and which accumulate debt?

2 min answer
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Eighteen months into a coexistence period, customer records are authoritative in the new system and order records are still authoritative in the old one. A refund credits the customer and cancels the order line. What happens the first time the credit commits and the cancellation is rejected?

3 min answer
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Old and new stores both hold 42 million customer records during a coexistence period. The nightly full-row comparison now takes 6 hours 40 minutes against a 4-hour window, and the legacy side serves the export at about 1800 rows per second. Roughly what does a hash-bucket comparison cost instead, and which assumption decides whether it fits?

3 min answer
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Your cutover plan says switch reads, then switch writes, then decommission. A reviewer asks what happens at 02:00 if the new store turns out to be wrong after writes have moved. What mechanism makes the write switch reversible, and what is the honest unavailability window?

3 min answer

Terminology

4 terms in this topic

concept

Coexistence Period

The interval during which old and new systems both operate and must be kept consistent, which is longer, more expensive and riskier than most plans assume.

pattern

Per-Entity Migration State

A field on each business entity naming the system currently authoritative for it, which makes the unit of migration one row rather than one capabilit…

pattern

Reverse Replication Cutover

Keeping a replication stream flowing from the new store back to the old one after writes have moved, so that rolling back stays a routing change inst…

pattern

Write-Ownership Boundary

The line drawn during coexistence that names exactly one system as the writer for each piece of state, placed along business transaction boundaries s…

Legacy Modernization