Did
you know that Db2 provides a method to change the way a program or SQL
statement acquires locks? That way is known as the isolation level and it can
be set to specify the locking behavior for a transaction or statement. Standard
SQL defines four isolation levels that can be set using the SET TRANSACTION
ISOLATION LEVEL statement:
· Serializable
· Repeatable read
· Read committed
· Read uncommitted
The
isolation level determines the mode of page or row locking implemented by the
program as it runs.
Db2
supports a variation of the standard isolation levels. Db2 implements page and
row locking at the program execution level, which means that all page or row
locks are acquired as needed during the program run. Page and row locks are
released as the program run depending on the isolation level.
In
Db2 you can specify the following four isolation levels: cursor stability (CS),
repeatable read (RR), read stability (RS), and uncommitted read (UR).
Using
the ISOLATION parameter of the BIND command you can set the isolation level of
a package or plan. You also can use the WITH parameter on a SELECT statement to
set the isolation level of a single SQL statement.
Cursor stability is the Db2 implementation of the SQL
standard read committed isolation level. CS is perhaps the
most common DB2 isolation level in use in production applications because it
offers a good tradeoff between data integrity and concurrency. When CS is
specified the transaction will never read data that is not yet committed; only
committed data can be read. Cursor stability is the current default isolation
level if none is specified at BIND time.
A
higher level of integrity is provided with repeatable
read. Under an RR isolation level all page locks are held until they
are released by a COMMIT (or ROLLBACK), whereas with CS read-only page locks
are released as soon as another page is accessed.
An
RR page locking strategy is useful when an application program requires
consistency in rows that may be accessed twice in one execution of the program,
or when an application program requires data integrity that cannot be achieved
with CS.
For
example, of a good reason to use RR page locking, consider a reporting program
that scans a table to produce a detail report, and then scans it again to
produce a summarized managerial report. If the program is bound using CS, the
results of the first report might not match the results of the second.
Suppose
that you are reporting the estimated completion dates for project activities.
The first report lists every project and the estimated completion date. The
second, managerial report lists only the projects with a completion date
greater than one year.
The
first report indicates that two activities are scheduled for more than one
year. After the first report but before the second, however, an update occurs.
A manager realizes that she underestimated the resources required for a
project. She invokes a transaction to change the estimated completion date of
one of her project's activities from 8 months to 14 months. The second report
is produced by the same program, but reports 3 activities.
If
the program used an RR isolation level rather than CS, an UPDATE that occurs
after the production of the first report but before the second would not have
been allowed. The program would have maintained the locks it held from the
generation of the first report and the updater would be locked out until the
locks were released.
How
about another example? Consider a program that is looking for pertinent
information about employees in the information center and software support
departments who make more than $30,000 in base salary. In the DB2 sample tables
department 'C01' is the information center and department 'E21' is software
support.
The program opens a cursor based on the following SELECT statement:
SELECT EMPNO, FIRSTNME, LASTNAME,
WORKDEPT, SALARY
FROM DSN8B10.EMP
WHERE WORKDEPT IN ('C01', 'E21')
AND SALARY > 30000;
The
program then begins to FETCH employee rows. Assume further, as would probably
be the case, that the statement uses the XEMP2 index on the WORKDEPT column. An
update program that implements employee modifications is running concurrently.
The program handles transfers by moving employees from one department to
another, and implements raises by increasing the salary.
Assume
that Sally Kwan, one of your employees, has just been transferred from the
information center to software support. Assume further that another information
center employee, Heather Nicholls, received a 10 percent raise. The update
program running concurrently with the report program implements both of these
modifications.
If
the report program were bound with an isolation level of CS, the second program
could move Sally from 'C01' to 'E21' after she was reported to be in department
'C01' but before the entire report was finished. Thus, she could be reported
twice: once as an information center employee and again as a software support
employee. Although this circumstance is rare, it can happen with programs that
use cursor stability. If the program were bound instead with RR, this problem
could not happen. The update program probably would not be allowed to run
concurrently with a reporting program, however, because it would experience too
many locking problems.
Now
consider Heather's dilemma. The raise increases her salary 10 percent, from
$28,420 to $31,262. Her salary now fits the parameters specified in the WHERE
condition of the SQL statement. Will she be reported? It depends on whether the
update occurs before or after the row has been retrieved by the index scan,
which is clearly a tenuous situation. Once again, RR avoids this problem.
You
might be wondering, "If CS has the potential to cause so many problems,
why is it used so ubiquitously? Why not trade the performance and concurrency
gain of CS for the integrity of RR?"
The
answer is simple: the types of problems outlined are rare. The expense of using
RR, however, can be substantial in terms of concurrency. So the tradeoff
between the concurrency expense of RR and the efficiency of CS usually is not a
sound one.
The
third isolation level provided by DB2 is read
stability (RS). Read
stability is similar in functionality to the RR isolation level, but a little
less. A retrieved row or page is locked until the end of the unit of work; no
other program can modify the data until the unit of work is complete, but other
processes can insert values that might be read by your application if it
accesses the row a second time.
Consider
using read stability over repeatable read only when your program can handle
retrieving a different set of rows each time a cursor or singleton SELECT is
issued. If using read stability, be sure your application is not dependent on
having the same number of rows returned each time.
Finally,
we come to the last, and most maligned isolation level, uncommitted read (UR). The UR isolation level
provides read-through locks, also know as dirty
read or read uncommitted. Using UR can
help to overcome concurrency problems. When you're using an uncommitted read,
an application program can read data that has been changed but is not yet
committed.
UR
can be a performance booster, too, because application programs bound using the
UR isolation level will
read data without taking locks. This way, the application program can read data
contained in the table as it is being manipulated. Consider the following
sequence of events:
1. To change a specific value, at 9:00
a.m. a transaction containing the
following SQL is executed:
UPDATE DSN8B10.EMP
SET FIRSTNME =
ˈMICHELLEˈ
WHERE EMPNO = 10020;
The
transaction is a long-running one and continues to execute without issuing a
COMMIT.
2. At 9:01 a.m., a second transaction
attempts to SELECT the data that was
changed, but not committed.
If
the UR isolation level were used for the second transaction, it would read the
changed data even though it had yet to be committed. Obviously, if the program
doesn't need to wait to take a lock and merely reads the data in whatever state
it happens to be at that moment, the program will execute faster than if it had
to wait for locks to be taken and resources to be freed before processing.
The
implications of reading uncommitted data, however, must be carefully examined
before being implemented. Several types of problems can occur. Using the
previous example, if the long-running transaction rolled back the UPDATE to
EMPNO 10020, the program using dirty reads may have picked up the wrong name
("MICHELLE") because it was never committed to the database.
Inaccurate
data values are not the only problems that can be caused by using UR. A dirty
read can cause duplicate rows to be returned where none exist. Alternatively, a
dirty read can cause no rows to be returned when one (or more) actually exists.
Additionally, an ORDER BY clause does not guarantee that rows will be returned
in order if the UR isolation level is used. Obviously, these problems must be
taken into consideration before using the UR isolation level.
Keep
in mind, too, that the UR isolation level applies to read-only operations:
SELECT, SELECT INTO, and FETCH from a read-only result table. Any application
plan or package bound with an isolation level of UR will use uncommitted read
functionality for any read-only SQL. Operations contained in the same plan or
package and are not read-only will use an isolation level of CS.
When
is it appropriate to use UR isolation? The general rule of thumb is to avoid UR
whenever the results must be 100 percent accurate. Following are examples of
when this would be true:
- Calculations that must balance are being performed on the selected data
- Data is being retrieved from one source to insert to or update another
- Production, mission-critical work is being performed that cannot contain or cause data integrity problems
In
general, most production Db2 applications are not serious candidates for dirty reads. In a
few specific situations, however, the dirty read capability will be of major
benefit. Consider the following cases in which the UR isolation level could prove to be
useful:
- Access is required to a reference, code, or look-up table that basically is static in nature. Due to the non-volatile nature of the data, a dirty read would be no different than a normal read the majority of the time. In those cases when the code data is being modified, any application reading the data would incur minimum, if any, problems.
- Statistical processing must be performed on a large amount of data. Your company, for example, might want to determine the average age of female employees within a certain pay range. The impact of an uncommitted read on an average of multiple rows will be minimal because a single value changed will not greatly impact the result.
- Dirty reads can prove invaluable in a data warehousing environment that uses DB2 as the DBMS. A data warehouse is a time-sensitive, subject-oriented, store of business data that is used for online analytical processing. Other than periodic data propagation and/or replication, access to the data warehouse is read-only. Because the data is generally not changing, an uncommitted read is perfect in a read-only environment due to the fact that it can cause little damage. More data warehouse projects are being implemented in corporations worldwide and DB2 with dirty read capability is a very wise choice for data warehouse implementation.
- In those rare cases when a table, or set of tables, is used by a single user only, UR can make a lot of sense. If only one individual can be modifying the data, the application programs can be coded such that all (or most) reads are done using UR isolation level, and the data will still be accurate.
- Finally, if the data being accessed already is inconsistent, little harm can be done using a dirty read to access the information.
Although
the dirty read capability can provide relief to concurrency problems and
deliver faster performance in specific situations, it also can cause data
integrity problems and inaccurate results. Be sure to understand the
implications of the UR isolation level and the problems it can cause before
diving headlong into implementing it in your production applications.
Summary
It
is important for Db2 DBAs and application programmers to know the four
isolation levels and their impact on SQL. Using the isolation levels is an
effective way to control concurrency and locking for your Db2 applications.
