Home > Articles > Programming

This chapter is from the book

This chapter is from the book

Connection Pooling

Opening a connection to a database is generally much more expensive than executing an SQL statement. A connection pool is used to minimize the number of connections opened between application and database. It serves as a librarian, checking out connections to application code as needed. Much like a library, your application code needs to be strict about returning connections to the pool when complete, for if it does not do so, your application will run out of available connections.

Starving a Pool

When using connection pooling, it is important to remember that a chunk of bad code that neglects to return connections can starve the rest of the application, causing it to eventually run out of connections and hang (potentially failing nowhere near the actual problem). To test for this, set the maximum connections in your pool to a small number (as low as 1), and use tools like p6spy and IronTrack SQL (described above) to look for statements that fail to close.

This problem can be avoided by always using a finally block to close your connection, as shown throughout this book.

Hibernate supports a variety of connection pooling mechanisms. If you are using an application server, you may wish to use the built-in pool (typically a connection is obtaining using JNDI). If you can't or don't wish to use your application server's built-in connection pool, Hibernate supports several other connection pools, as shown in Table 10.1.

Table 10.1. Hibernate-Supported Connection Pools

c3p0

http://sourceforge.net/projects/c3p0

Distributed with Hibernate

Apache DBCP

http://jakarta.apache.org/commons/dbcp/

Apache Pool

Proxool

http://proxool.sourceforge.net/

JDBC Pooling Wrapper


Statement Cache

Certain connection pools, drivers, databases, and other portions of the system may provide an additional cache system, known as a statement cache. This cache stores a partially compiled version of a statement in order to increase performance. By reusing the parsed or precompiled statement, the application is able to trade an increase in memory usage for a boost in performance.

You should consider using a statement cache if one is available, but keep in mind that a statement cache is not the same thing as the other forms of caching described later in this chapter.

The choice of a connection pool is up to you, but be sure to remember that a connection pool is necessary for every production use.

If you wish to use c3p0, the version distributed with Hibernate 2.1.2 (0.8.3) is out of date (and GPL is a problem if you wish to distribute a non-GPL application). If you wish to distribute an application that makes use of c3p0, make sure to download the latest (LGPL) release, c3p0-0.8.4-test1 or later.

Because Hibernate ships with c3p0, configuration is a simple matter of adding a few Hibernate configuration properties to your hibernate.properties (or hibernate.cfg.xml) file. Listing 10.2 shows an example of the configuration of c3p0.

Listing 10.2 Sample Hibernate c3p0 Configuration

hibernate.connection.driver_class=com.mysql.jdbc.Driver
hibernate.connection.url=jdbc:mysql://localhost/hibernate
hibernate.connection.username=root
hibernate.connection.password= 
hibernate.dialect=net.sf.hibernate.dialect.MySQLDialect
hibernate.show_sql=false

hibernate.c3p0.max_size=1
hibernate.c3p0.min_size=0
hibernate.c3p0.timeout=5000
hibernate.c3p0.max_statements=100
hibernate.c3p0.idle_test_period=300
hibernate.c3p0.acquire_increment=2

The properties shown in Listing 10.2 are as described in Table 10.2.

Table 10.2. c3p0 Configuration Options

Property Meaning

Property

Example

Maximum number of database connections to open

hibernate.c3p0.max_size

15

Initial number of database connections

hibernate.c3p0.min_size

3

Maximum idle time for a connection (in seconds)

hibernate.c3p0.timeout

5000

Maximum size of c3p0 statement cache (0 to turn off)

hibernate.c3p0.max_ statements

0

Number of connections in a clump acquired when pool is exhausted

hibernate.c3p0.acquire_ increment

3

Idle time before a c3p0 pooled connection is validated (in seconds)

hibernate.c3p0.idle_test period _

300

Validate the connection on checkout. Recommend setting the hibernate.c3p0.idle_test_period property instead. Defaults to false

hibernate.c3p0.validate

true |false


If you prefer to use Apache DBCP, make sure that the Apache DBCP library is on your class path, and add the properties to your hibernate.properties file, as shown in Table 10.3.

Table 10.3. Apache DBCP Configuration Options

Property Meaning

Property

Example

Maximum number of checked-out database connections

hibernate.dbcp.maxActive

8

Maximum number of idle database connections for connection pool

hibernate.dbcp.maxIdle

8

Maximum idle time for connections in connection pool (expressed in ms).

hibernate.dbcp.max Wait

-1

Set to -1 to turn off

Action to take in case of an exhausted DBCP connection pool. Set to 0 to fail, 1 to block until a connection is made available, or 2 to grow)

hibernate.dbcp.whenExhaustedAction

1

Validate connection when borrowing connection from pool (defaults to true)

hibernate.dbcp.test OnBorrow

true |false

Validate connection when returning connection to pool (optional, true, or false)

hibernate.dbcp.test OnReturn

true |false

Query to execute for connection validation (optional, requires either hibernate.dbcp.testOn Borrow or hibernate.dbcp.testOnReturn)

hibernate.dbcp.validationQuery

Valid SQL SELECT statement (e.g., SELECT 1+1)

Maximum number of checked-out statements

hibernate.dbcp.ps.maxActive

8

Maximum number of idle statements

hibernate.dbcp.ps.maxIdle

8

Maximum idle time for statements (in ms)

hibernate.dbcp.ps.maxWait

1000 * 60 * 30

Action to take in case of an exhausted statement pool. Set to 0 to fail, 1 to block until a statement is made available, or 2 to grow)

hibernate.dbcp.ps.whenExhaustedAction

1


Finally, if you wish to use Proxool as your connection pool provider, you will need to specify hibernate.properties values as shown in Table 10.4. Unlike c3p0 and DBCP, you will need to include additional configuration options as described at http://proxool.sourceforge.net/configure.html.

Table 10.4. Proxool Configuration Options

Property Meaning

Property

Example

Configure Proxool provider using an XML file

hibernate.proxool.xml

/path/to/file.xml

Configure the Proxool provider using a properties file.properties

hibernate.proxool.properties

/path/to/proxool

Configure the Proxool provider from an existing pool

hibernate.proxool.existing_pool

true | false

Proxool pool alias to use (required for hibernate.proxool.existing_pool, hibernate.proxool .properties, hibernate .proxool.xml)

hibernate.proxool.pool_alias

As set by Proxool configuration


InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020