Use a migration tool’s durable history to make repeated deployment commands apply only pending, versioned changes. Separately, design any script that may execute again—such as a repeatable migration or a retry after partial failure—to tolerate the database’s actual state. Keep released versioned migrations immutable, and inspect both the database and migration history before retrying a failed change.
What does “safe to rerun” mean?
It can mean two different things, and they need different safeguards:
- Rerunning the migration command: the runner checks its history and skips versioned migrations already recorded as successful, applying only pending work.
- Rerunning a migration script: the script itself executes again. This may be expected for a repeatable migration or may happen during recovery from a partial failure. Its operations must be valid for the database’s current state.
For the question “How do you add a migration script that should only run once?”, use a uniquely versioned migration managed by a runner. For definitions intended to be reapplied, use a repeatable migration and make its operations safe to execute again.
Choose versioned or repeatable migrations
| Approach | Use it for | Rerun behavior and safety |
|---|---|---|
| Versioned migration | Schema changes and one-off data corrections | Flyway applies versioned migrations in order once, recording successful work in its schema history table. The table also records checksums and success status, so the runner can identify pending work and detect edits. |
| Repeatable migration | Definitions such as views and procedures that need updating when their contents change | It may be applied again when its checksum changes, so the script must be safe to apply multiple times. Flyway’s documentation states: “It is your responsibility to ensure the same repeatable migration can be applied multiple times.” |
These behaviors are documented in Flyway’s migrations guide. Check your installed tool version and configuration, since runner behavior can vary by tool and setup.
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Design one-time migrations so history stays trustworthy
Give each change a unique version
Put work that should happen once in a uniquely versioned migration and let the runner track it. Avoid manually marking a migration complete unless you have verified the database state and understand how that bookkeeping change will affect later deployments.
Keep released migrations immutable
A checksum can reveal that a versioned migration differs from the file previously applied; it is not permission to rewrite the past. Once a migration has been used in an environment, treat it as immutable. Make a new versioned migration to correct the schema or data instead of changing the old file and leaving environments with different histories.
Make the change focused and inspectable
Keep each migration focused, with clear preconditions and expected postconditions. For repeatable definitions, database-supported replacement syntax such as CREATE OR REPLACE may suit the object and engine. For data changes, choose conditions, uniqueness constraints, or an upsert pattern that matches the intended result and the database’s semantics.
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An IF NOT EXISTS guard is not proof that the database is correct: it may hide an object that exists with the wrong definition. Verify the resulting schema or data. If it has drifted, create a versioned correction rather than relying on a guard to make the migration harmless.
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Account for transactions and partial failure
A failed migration does not guarantee that nothing changed. Rollback can restore atomicity only when the migration tool, database, and statements involved support the necessary transactional behavior. Some DDL cannot run transactionally, and some databases implicitly commit around DDL.
Flyway ordinarily wraps a migration in a transaction where supported. Its documentation explains that a failed non-transactional migration can leave partial effects requiring manual cleanup and a repair of the schema-history entry. Liquibase also warns that a multi-statement changeset with runInTransaction=false can leave its changelog state invalid if it fails midway. See the Flyway project migration guidance and Liquibase’s runInTransaction reference (last updated January 21, 2026).
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Prepare non-transactional work
When an operation cannot run in a transaction, isolate that step if the tool and database allow it. Before deploying, document how to inspect whether it partially completed and how to clean up or reconcile the state. Test the procedure rather than assuming a failed command left the database untouched.
Handle PostgreSQL concurrent index creation carefully
In PostgreSQL, CREATE INDEX CONCURRENTLY has transaction and locking requirements that can affect migration execution. Flyway’s PostgreSQL reference describes a potential issue with its default transactional lock and documents an alternative session-level lock setting. Verify the current Flyway version, PostgreSQL version, and deployment configuration before using that setting: Flyway’s PostgreSQL database reference.
Recover from a failed migration by inspecting state first
- Stop automatic retries. Do not assume the failed attempt had no effect.
- Inspect the database. Check the affected schema and data for completed, partial, or unexpected changes.
- Inspect migration history. Compare the runner’s recorded status with the actual database state.
- Clean up or reconcile partial effects. Decide what must be removed or completed before another attempt can succeed.
- Repair bookkeeping only when it matches reality. Use the migration tool’s documented repair mechanism after the database state has been verified; do not use it to conceal unresolved changes.
- Retry only when the script is safe for the resulting state. If the change needs correction, create a suitable new versioned migration rather than altering a released migration.
Do not treat an undo migration as a universal recovery mechanism. A multi-statement change may fail after some statements succeed, leaving a state an undo for the entire migration cannot reliably reverse. Flyway’s guidance on rolling out updates discusses failure handling, compatibility, and recovery.
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Prevent concurrent migration runs and unsafe rollouts
Run one migration process per database change window, or rely on the migration tool’s supported locking. Flyway describes a database-level lock on its schema history table that allows only one concurrent migration invocation to proceed. This serializes migration runners; it does not eliminate every concurrency concern inside application transactions.
For example, PostgreSQL documents that under repeatable read, a transaction’s snapshot may predate a lock acquired after an earlier query. When application code uses explicit locks to protect consistency, acquire them in an order that accounts for this behavior. See PostgreSQL’s application-level consistency checks guidance.
During a staged deployment, keep the old and new application versions compatible with the database while the rollout is in progress. Prefer backward-compatible schema changes where possible, and test backup restoration rather than assuming an undo script can recover every failure. Flyway covers these rollout practices in its update rollout guidance.
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Test the clean path, retry path, and deployment races
Exercise the migration against the actual database engine and the migration-tool configuration intended for deployment. A useful test set includes:
- A fresh database that needs all migrations.
- A database already at the target version, where the command should not repeat successful versioned work.
- A controlled failure after an early statement, followed by inspection of the database and history.
- A retry after that failure, using the documented cleanup or recovery procedure.
- Two deployment processes attempting to migrate concurrently.
- Application behavior during a staged rollout with both old and new application versions.
- Restoration from backup, so recovery does not depend solely on undo migrations.
Transaction boundaries, statement support, runner locking, and database-specific DDL behavior determine what these tests need to verify; they are not interchangeable across engines.
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