How a Legacy Lease Mechanism Solved Cache Consistency During System Migration

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A developer migrating a legacy system to a modern service using the Strangler Fig pattern encountered a cache consistency issue where the legacy service continued reading stale in-memory data after the new service updated the database. Standard solutions like distributed locks, cache invalidation, or new infrastructure were ruled out due to constraints that prohibited modifying legacy or main services and avoided adding new dependencies. The team discovered that the legacy service already had a built-in lease mechanism, which they repurposed to prevent it from operating on outdated state during migrated endpoint requests. This approach allowed the new service to coordinate with the legacy system without altering its core behavior or introducing additional infrastructure. The solution also accounted for failure scenarios, providing a controlled way to handle edge cases during the incremental migration.

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