Lock-Free Reservations: High-Concurrency DML Without Conflicts

One of Oracle 23ai’s most interesting concurrency features is Lock-Free Reservations. It solves a classic high-concurrency problem: multiple sessions competing to update the same row — like decrementing inventory counts in a flash sale.

The classic problem:

-- Session 1 and Session 2 both try to decrement inventory
UPDATE products SET stock_qty = stock_qty - 1 WHERE product_id = 42;

With thousands of concurrent orders, Sessions 2 through N all block waiting for Session 1’s lock. This creates a hot row contention bottleneck.

Lock-Free Reservation solution:

-- Define the column as a reservation target
ALTER TABLE products MODIFY (
    stock_qty NUMBER RESERVABLE CONSTRAINT stock_non_negative CHECK (stock_qty >= 0)
);

Once a column is declared RESERVABLE, Oracle handles concurrent updates to it using a reservation mechanism rather than row-level locking. Each session “reserves” a portion of the value without blocking others.

How it works under the hood:

Oracle maintains a pending reservation table alongside the main table. Each concurrent update is stored as a reservation. The actual column value is the base value plus the sum of all pending reservations. When a session commits, its reservation is rolled into the base value.

The net effect:

• Multiple sessions can concrement stock_qty simultaneously without blocking each other
• The CHECK constraint (stock_qty >= 0) is still enforced — the sum of all reservations cannot drive the value below zero
• ACID guarantees are maintained

Ideal use cases: Inventory management, ticket booking, seat reservations, resource quotas — any scenario where many sessions update the same numeric column concurrently.

This is a sophisticated concurrency improvement that eliminates a well-known Oracle scalability bottleneck without requiring application-level workarounds.