Isolation is an element of ACID transactions that determines how concurrency is controlled, and ultimately guarantees consistency. CockroachDB offers two transaction isolation levels: [SERIALIZABLE]({% link {{ page.version.version }}/demo-serializable.md %}) and [READ COMMITTED]({% link {{ page.version.version }}/read-committed.md %}).
By default, CockroachDB executes all transactions at the strongest ANSI transaction isolation level: SERIALIZABLE, which permits no concurrency anomalies. To place all transactions in a serializable ordering, SERIALIZABLE isolation may require [transaction restarts]({% link {{ page.version.version }}/transaction-retry-error-reference.md %}) and [client-side retry handling]({% link {{ page.version.version }}/transaction-retry-error-reference.md %}#client-side-retry-handling). For a demonstration of how SERIALIZABLE prevents anomalies such as write skew, refer to [Serializable Transactions]({% link {{ page.version.version }}/demo-serializable.md %}).
{% include_cached new-in.html version="v23.2" %} CockroachDB can be configured to execute transactions at [READ COMMITTED]({% link {{ page.version.version }}/read-committed.md %}) instead of SERIALIZABLE isolation. If [enabled]({% link {{ page.version.version }}/read-committed.md %}#enable-read-committed-isolation), READ COMMITTED and READ UNCOMMITTED transactions no longer [upgrade to SERIALIZABLE]({% link {{ page.version.version }}/transactions.md %}#isolation-level-upgrades). READ COMMITTED permits some concurrency anomalies in exchange for minimizing transaction aborts and [retries]({% link {{ page.version.version }}/developer-basics.md %}#transaction-retries). Depending on your workload requirements, this may be desirable. For more information, refer to [Read Committed Transactions]({% link {{ page.version.version }}/read-committed.md %}).
{% include {{ page.version.version }}/sql/mixed-isolation-levels.md %}