How would you make Spring Boot startup safe during a rolling deployment?

Coordinates startup work, readiness, schema compatibility, warmup, and mixed-version traffic during a rolling release.

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TL;DR

I keep the instance unready until required startup work is complete and the application can serve requests. Database migrations are backward compatible with the old version because both versions run during a rolling deployment. Heavy one-time migrations or data backfills are separate controlled jobs, not repeated independently by every application replica.

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Application startup, readiness transition, migrations, warmup, backward compatibility, probes, and deployment evidence.

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Core answer
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A new process being alive is not proof that it can safely serve traffic alongside the old version.
I keep the instance unready until required startup work is complete and the application can serve requests. Database migrations are backward compatible with the old version because both versions run during a rolling deployment. Heavy one-time migrations or data backfills are separate controlled jobs, not repeated independently by every application replica.
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Stronger answer
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Startup validates configuration and critical dependencies, initializes only required state, and has bounded time. Optional caches can warm after readiness or use a small controlled warmup so deployment does not create a dependency spike. The readiness probe changes only when the service is genuinely safe to route, and shutdown removes the old instance before its process exits. I test cold starts with production-like data and dependency latency.

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Running a destructive migration on application startup.
Marking readiness true before required initialization.
Warming every cache from every new replica at once.

How would you make Spring Boot startup safe during a rolling deployment?

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