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A database-enforced constraint can stop two concurrent booking attempts from claiming the same unique booking key, even if both requests passed an earlier availability check. PostgreSQL integrates the uniqueness check with insertion of the new index entry, which is what makes the constraint—not the application-side check—the authority. “Without locks” means you do not need to rely on an application lock for this invariant; it does not mean PostgreSQL uses no internal synchronization.
Why an availability check alone can allow double-booking
Suppose two requests check whether the same booking key is available. Each can see it as absent before either request inserts it. If each application then proceeds based only on its earlier check, both may attempt to claim the key. The check and the later write are separate operations, so the check is not itself a concurrency guarantee.
PostgreSQL’s documentation explains that unique constraints are enforced using unique indexes, and that the conflict check must be integrated with inserting the new index entry to avoid race conditions. The database therefore arbitrates competing writes at the point where the invariant is enforced, rather than trusting an earlier application read. PostgreSQL 16: Index Uniqueness Checks
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A unique constraint is appropriate when the rule is that a particular key may appear only once—for example, when the booking model has a key that uniquely identifies the resource and booking slot. The exact columns depend on the actual data model; the title alone does not establish a schema or a universally correct constraint definition.
#1 Best Overall
Before implementing it, define the invariant precisely. If the rule is “only one booking for this key,” uniqueness may express it. If the rule is instead “no two bookings may overlap in time,” that is a different scheduling rule; do not assume a simple unique key captures it. Choose and verify a constraint that represents the real rule rather than copying an example designed for a different booking model.
Choose how PostgreSQL should handle a conflict
PostgreSQL’s INSERT ... ON CONFLICT lets an insert specify what to do when it conflicts with a unique or exclusion constraint. The two actions have different outcomes: DO NOTHING skips the conflicting insert, while DO UPDATE updates the conflicting row. Use the behavior that matches the application’s meaning of a conflict; an attempted duplicate booking should not silently become an update unless that is explicitly intended. PostgreSQL 19: INSERT
For ON CONFLICT DO UPDATE, PostgreSQL documents an atomic insert-or-update outcome under high concurrency, provided there is no independent error. That conditional guarantee does not eliminate the need to handle other errors or decide how the application should report the result.
What “without locks” does—and does not—mean
The database constraint lets the application avoid depending on a separate application-side lock to protect this uniqueness invariant. It does not mean that PostgreSQL performs no internal coordination: its uniqueness-check process must safely arbitrate concurrent index entries. Database-level enforcement is the point; claiming that all locking or synchronization disappears would be inaccurate.
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Serializable transactions do not remove conflict handling
Choosing Serializable isolation does not justify promising that uniqueness violations cannot happen. PostgreSQL’s concurrency documentation describes cases where overlapping Serializable transactions can still encounter a unique-constraint violation, including after a transaction checked that a key was absent. The application must be prepared to handle transaction errors according to its retry and response policy. PostgreSQL 16.11 manual: concurrency control
Quick Recap
Implementation checklist
- Write down the precise invariant: one row per booking key, or no overlapping time intervals.
- Define a database constraint that actually expresses that invariant for the application’s schema.
- Use the constraint as the authority; treat a prior availability check as advisory rather than proof that the subsequent insert will succeed.
- Select explicit conflict behavior: skip with
DO NOTHING, update withDO UPDATE, or handle the resulting constraint error in the application. - Return a clear conflict outcome to the caller when the booking could not be created; do not report success merely because the availability check passed.
- Verify SQL syntax and behavior against the PostgreSQL version deployed. The cited references span PostgreSQL 14, 16, 16.11, and 19.
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