Isolation Level

Isolation is one of the ACID (Atomicity, Consistency, Isolation, Durability) properties. It determines how transaction integrity is visible to other users and systems. For example, when a user is creating a Purchase Order and has created the header, but not the Purchase Order lines, is the header available for other systems/users (carrying out concurrent operations, such as a report on Purchase Orders) to see?

A lower isolation level increases the ability of many users to access the same data at the same time, but increases the number of concurrency effects (such as dirty reads or lost updates) users might encounter. Conversely, a higher isolation level reduces the types of concurrency effects that users may encounter, but requires more system resources and increases the chances that one transaction will block another.

Most DBMSs offer a number of transaction isolation levels, which control the degree of locking that occurs when selecting data. For many database applications, the majority of database transactions can be constructed to avoid requiring high isolation levels (e.g. SERIALIZABLE level), thus reducing the locking overhead for the system. The programmer must carefully analyze database access code to ensure that any relaxation of isolation does not cause software bugs that are difficult to find. Conversely, if higher isolation levels are used, the possibility of deadlock is increased, which also requires careful analysis and programming techniques to avoid.

Since each isolation level is stronger than those below, in that no higher isolation level allows an action forbidden by a lower one, the standard permits a DBMS to run a transaction at an isolation level stronger than that requested (e.g., a “Read committed” transaction may actually be performed at a “Repeatable read” isolation level).

The isolation levels defined by the ANSI/ISO SQL standard are listed as follows.

Serializable

This is the highest isolation level.

With a lock-based concurrency control DBMS implementation, serializability requires read and write locks (acquired on selected data) to be released at the end of the transaction. Also range-locks must be acquired when a SELECT query uses a ranged WHERE clause, especially to avoid the phantom reads phenomenon.

When using non-lock based concurrency control, no locks are acquired; however, if the system detects a write collision among several concurrent transactions, only one of them is allowed to commit. See snapshot isolation for more details on this topic.

Per the SQL-92 standard:

The execution of concurrent SQL-transactions at isolation level SERIALIZABLE is guaranteed to be serializable. A serializable execution is defined to be an execution of the operations of concurrently executing SQL-transactions that produces the same effect as some serial execution of those same SQL-transactions. A serial execution is one in which each SQL-transaction executes to completion before the next SQL-transaction begins.

Repeatable Read

In this isolation level, a lock-based concurrency control DBMS implementation keeps read and write locks (acquired on selected data) until the end of the transaction. However, range-locks are not managed, so phantom reads can occur.

Write skew is possible at this isolation level, a phenomenon where two writes are allowed to the same column(s) in a table by two different writers (who have previously read the columns they are updating), resulting in the column having data that is a mix of the two transactions.

Repeatable reads is the default isolation level for MySQL’s InnoDB engine.

Read Committed

In this isolation level, a lock-based concurrency control DBMS implementation keeps write locks (acquired on selected data) until the end of the transaction, but read locks are released as soon as the SELECT operation is performed (so the non-repeatable reads phenomenon can occur in this isolation level). As in the previous level, range-locks are not managed.

Putting it in simpler words, read committed is an isolation level that guarantees that any data read is committed at the moment it is read. It simply restricts the reader from seeing any intermediate, uncommitted, ‘dirty’ read. It makes no promise whatsoever that if the transaction re-issues the read, it will find the same data; data is free to change after it is read.

Read Uncommitted

This is the lowest isolation level. In this level, dirty reads are allowed, so one transaction may see not-yet-committed changes made by other transactions.

Snapshot Isolation

We mentioned 4 different isolation levels above, but TiDB doesn’t adopt any of them. Instead, TiDB uses snapshot isolation as its default ioslation level. The main reason for it is that it allows better serializability, yet still avoids most of the concurrency anomalies that serializability avoids (but not always all).

TiDB is not alone: snapshot isolation also has been adopted by major database management systems such as InterBase, Firebird, Oracle, MySQL, PostgreSQL, SQL Anywhere, MongoDB and Microsoft SQL Server (2005 and later).

Snapshot isolation is a guarantee that all reads made in a transaction will see a consistent snapshot of the database, and the transaction itself will successfully commit only if no updates it has made conflict with any concurrent updates made since that snapshot.

In practice snapshot isolation is implemented within multiversion concurrency control (MVCC), where generational values of each data item (versions) are maintained: MVCC is a common way to increase concurrency and performance by generating a new version of a database object each time the object is written, and allowing transactions’ read operations of several last relevant versions (of each object). The prevalence of snapshot isolation has been seen as a refutation of the ANSI SQL-92 standard’s definition of isolation levels, as it exhibits none of the “anomalies” that the SQL standard prohibited, yet is not serializable (the anomaly-free isolation level defined by ANSI).

A transaction executing under snapshot isolation appears to operate on a personal snapshot of the database, taken at the start of the transaction. When the transaction concludes, it will successfully commit only if the values updated by the transaction have not been changed externally since the snapshot was taken. Such a write-write conflict will cause the transaction to abort.

In a write skew anomaly, two transactions (T1 and T2) concurrently read an overlapping data set (e.g. values V1 and V2), concurrently make disjoint updates (e.g. T1 updates V1, T2 updates V2), and finally concurrently commit, neither having seen the update performed by the other. Were the system serializable, such an anomaly would be impossible, as either T1 or T2 would have to occur “first”, and be visible to the other. In contrast, snapshot isolation permits write skew anomalies.

As a concrete example, imagine V1 and V2 are two balances held by a single person, James. The bank will allow either V1 or V2 to run a deficit, provided the total held in both is never negative (i.e. V1 + V2 ≥ 0). Both balances are currently $100. James initiates two transactions concurrently, T1 withdrawing $200 from V1, and T2 withdrawing $200 from V2.

If the database guaranteed serializable transactions, the simplest way of coding T1 is to deduct $200 from V1, and then verify that V1 + V2 ≥ 0 still holds, aborting if not. T2 similarly deducts $200 from V2 and then verifies V1 + V2 ≥ 0. Since the transactions must serialize, either T1 happens first, leaving V1 = -$100, V2 = $100, and preventing T2 from succeeding (since V1 + (V2 - $200) would be -$200), or T2 happens first and similarly prevents T1 from committing.

If the database is under snapshot isolation (MVCC), however, T1 and T2 operate on private snapshots of the database: each deducts $200 from an account, and then verifies that the new total is zero, using the other account value that held when the snapshot was taken. Since neither update conflicts, both commit successfully, leaving V1 = V2 = -$100, and V1 + V2 = -$200.

In TiDB, you can use SELECT … FOR UPDATE statement to avoid write skew anomaly. In this case, TiDB will use locks to serialize writes together with MVCC to gain some of the performance gains and still support the stronger “serializability” level of isolation.