Isolation levels and concurrency

Derby provides
four transaction isolation levels. Setting the transaction isolation level
for a connection allows a user to specify how severely the user’s transaction
should be isolated from other transactions.

For example, it allows you to specify whether transaction A is allowed
to make changes to data that have been viewed by transaction B before transaction
B has committed.

A connection determines its own isolation level, so JDBC provides an application
with a way to specify a level of transaction isolation. It specifies four
levels of transaction isolation. The higher the transaction isolation, the
more care is taken to avoid conflicts; avoiding conflicts sometimes means
locking out transactions. Lower isolation levels thus allow greater concurrency.

Inserts, updates, and deletes always behave the same no matter what the
isolation level is. Only the behavior of select statements varies.

To set isolation levels you can use the JDBC Connection.setIsolationLevel method
or the SQL SET ISOLATION statement. The names of the isolation levels are
different, depending on whether you use a JDBC method or SQL statement. Mapping of JDBC transaction isolation levels to Derby isolation levels shows the equivalent
names for isolation levels whether they are set through the JDBC method or
an SQL statement.


Table 1. Mapping of JDBC transaction
isolation levels to Derby isolation
levels
Isolation levels for JDBC Isolation levels for SQL
Connection.TRANSACTION_READ_ UNCOMMITTED (ANSI level
0)		 UR, DIRTY READ, READ UNCOMMITTED
Connection.TRANSACTION_READ_COMMITTED (ANSI level
1) 		CS, CURSOR STABILITY, READ COMMITTED
Connection.TRANSACTION_REPEATABLE_READ (ANSI level
2) 		RS
Connection.TRANSACTION_SERIALIZABLE (ANSI level
3) 		RR, REPEATABLE READ, SERIALIZABLE

These levels allow you to avoid particular kinds of transaction anomalies,
which are described in Transaction Anomalies.


Table 2. Transaction Anomalies
Anomaly Example
Dirty Reads

A dirty read happens when a transaction
reads data that is being modified by another transaction that has not yet
committed.

 Transaction A begins. 
UPDATE employee SET salary = 31650WHERE empno = '000090'

Transaction B begins. 

SELECT * FROM employee

(Transaction
B sees data updated by transaction A. Those updates have not yet been committed.)
Non-Repeatable Reads

Non-repeatable reads happen when
a query returns data that would be different if the query were repeated within
the same transaction. Non-repeatable reads can occur when other transactions
are modifying data that a transaction is reading.

 Transaction A begins. 
SELECT * FROM employeeWHERE empno = '000090'

Transaction B begins. 

UPDATE employee SET salary = 30100WHERE empno = '000090'

(Transaction B updates rows viewed by transaction
A before transaction A commits.) If Transaction A issues the same SELECT statement,
the results will be different.
Phantom Reads

Records that appear in a set being read
by another transaction. Phantom reads can occur when other transactions insert
rows that would satisfy the WHERE clause of another transaction’s statement.

 Transaction A begins. 
SELECT * FROM employee WHERE salary > 30000

Transaction B begins. 

INSERT INTO employee(empno, firstnme, midinit,lastname, job,salary) VALUES ('000350', 'NICK','A','GREEN','LEGAL COUNSEL',35000)

Transaction B inserts a row
that would satisfy the query in Transaction A if it were issued again.

The transaction isolation level is a way of specifying whether these transaction
anomalies are allowed. The transaction isolation level thus affects the quantity
of data locked by a particular transaction. In addition, a DBMS’s locking
schema might also affect whether these anomalies are allowed. A DBMS can lock
either the entire table or only specific rows in order to prevent transaction
anomalies.

When Transaction Anomalies Are Possible shows
which anomalies are possible under the various locking schemas and isolation
levels.


Table 3. When Transaction Anomalies
Are Possible
Isolation Level Table-Level Locking Row-Level Locking
TRANSACTION_READ_UNCOMMITTED Dirty reads, nonrepeatable reads, and phantom reads possible Dirty reads, nonrepeatable reads, and phantom reads possible
TRANSACTION_READ_COMMITTED Nonrepeatable reads and phantom reads possible Nonrepeatable reads and phantom reads possible
TRANSACTION_REPEATABLE_READ Phantom reads not possible because entire table is locked Phantom reads possible
TRANSACTION_SERIALIZABLE None None

The following java.sql.Connection isolation levels are supported:

  • TRANSACTION_SERIALIZABLE

    RR, SERIALIZABLE,
    or REPEATABLE READ from SQL.

    TRANSACTION_SERIALIZABLE means
    that Derby treats the transactions
    as if they occurred serially (one after the other) instead of concurrently. Derby issues locks to prevent
    all the transaction anomalies listed in Transaction Anomalies from
    occurring. The type of lock it issues is sometimes called a range lock.

  • TRANSACTION_REPEATABLE_READ

    RS from
    SQL.

    TRANSACTION_REPEATABLE_READ means that Derby issues
    locks to prevent only dirty reads and non-repeatable reads, but not phantoms.
    It does not issue range locks for selects.

  • TRANSACTION_READ_COMMITTED

    CS or CURSOR
    STABILITY     from SQL.

    TRANSACTION_READ_COMMITTED means
    that Derby issues locks
    to prevent only dirty reads, not all the transaction anomalies listed in Transaction Anomalies.

    TRANSACTION_READ_COMMITTED is
    the default isolation level for transactions.

  • TRANSACTION_READ_UNCOMMITTED

    UR, DIRTY
    READ    , or READ UNCOMMITTED from SQL.

    For a
    SELECT INTO, FETCH with a read-only cursor, full select used in an INSERT,
    full select/subquery in an UPDATE/DELETE, or scalar full select (wherever
    used), READ UNCOMMITTED allows:

    • Any row that is read during the unit of work to be changed by other application
      processes.
    • Any row that was changed by another application process to be read even
      if the change has not been committed by the application process.

    For other operations, the rules that apply to READ COMMITTED also
    apply to READ UNCOMMITTED.