Enum IsolationLevel

src/sql/src/session/vars/value.rs:866–915

/// List of valid isolation levels.
#[derive(Clone, Copy, Debug, Eq, PartialEq, Serialize, Deserialize)]
pub enum IsolationLevel {
    ReadUncommitted,
    ReadCommitted,
    RepeatableRead,
    Serializable,
    /* TODO(jkosh44) Move this comment to user facing docs when this isolation level becomes available to users.
     * The Strong Session Serializable isolation level combines the Serializable isolation level
     * (https://jepsen.io/consistency/models/serializable) with the Sequential consistency model
     * (https://jepsen.io/consistency/models/sequential). See
     * http://dbmsmusings.blogspot.com/2019/06/correctness-anomalies-under.html and
     * https://cs.uwaterloo.ca/~kmsalem/pubs/DaudjeeICDE04.pdf. Operations within a single session
     * are linearizable, but operations across sessions are not linearizable.
     *
     * Operations in sessions that use Strong Session Serializable are not linearizable with
     * operations in sessions that use Strict Serializable. For example, consider the following
     * sequence of events in order:
     *
     *   1. Session s0 executes read at timestamp t0 under Strong Session Serializable.
     *   2. Session s1 executes read at timestamp t1 under Strict Serializable.
     *
     * If t0 > t1, then this is not considered a consistency violation. This matches with the
     * semantics of Serializable, which can execute queries arbitrarily in the future without
     * violating the consistency of Strict Serializable queries.
     *
     * All operations within a session that use Strong Session Serializable are only
     * linearizable within operations within the same session that also use Strong Session
     * Serializable. For example, consider the following sequence of events in order:
     *
     *   1. Session s0 executes read at timestamp t0 under Strong Session Serializable.
     *   2. Session s0 executes read at timestamp t1 under I.
     *
     * If I is Strong Session Serializable then t0 > t1 is guaranteed. If I is any other isolation
     * level then t0 < t1 is not considered a consistency violation. This matches the semantics of
     * Serializable, which can execute queries arbitrarily in the future without violating the
     * consistency of Strict Serializable queries within the same session.
     *
     * The items left TODO before this is considered ready for prod are:
     *
     * - Add more tests.
     * - Linearize writes to system tables under this isolation (most of these are the side effect
     *   of some DDL).
     */
    StrongSessionSerializable,
    StrictSerializable,
    /// Bounded staleness — pick `T` such that `T >= now_ms - D` and the
    /// query does not wait on input frontiers. Errors if no such `T` exists
    /// in the no-wait window. See
    /// `doc/developer/design/20260429_bounded_staleness_isolation.md`.
    BoundedStaleness(std::time::Duration),
}

impl IsolationLevel {
    const READ_UNCOMMITTED: &'static str = "read uncommitted";