Welcome!

IBM Cloud Authors: Liz McMillan, Elizabeth White, Roger Strukhoff, Pat Romanski, Sematext Blog

Related Topics: IBM Cloud

IBM Cloud: Article

Managing JDBC Connections

Managing JDBC Connections

If application servers like WebSphere are the heart of e-business, then the database is surely the lifeblood. Whether your firm is managing thousands of airline flights a day or simply exposing a library's card catalog to the Web, you must deal with database connectivity.

Java developers have a simple way to manage database connectivity at their disposal: JDBC. It provides a convenient interface to access a variety of vendors' databases via JDBC drivers. Simply plug in a new driver and your application can talk to DB2, Oracle, Sybase, Informix, and others.

The JDBC version 2.0 extensions introduced a feature called connection pooling, providing better performance and scalability. IBM implemented a JDBC 2.0-compliant connection pool in WebSphere 3.5, and continues to enhance it in version 4.0.

In addition to connection pooling, the WebSphere connection manager offers many additional qualities to enhance an application's performance and scalability for the savvy Web developer. This article will highlight those features and show how they can be used effectively for building better Web applications. In addition, this article will describe many best practices and potential pitfalls for developing with WebSphere's connection manager.

While the following information will provide an improvement to applications, note that this is meant to aid in application design, not replace it. No amount of "tweaking" can make up for a poor design, but a well-designed application with poor implementation of database connectivity logic can be improved. This article will address the latter case.

Background
The WebSphere connection manager has many facets. At the core, however, it's simply a connection pool. Object pools are nothing new.

The basic reason for a pool is that creation of certain objects is expensive in performance terms (time, memory allocation, etc.). Rather than creating these objects over and over again, it's more efficient to create a pool of already initialized objects that can be allocated to application components, and then returned to the pool when no longer needed. Thus, the hit to performance is only incurred once, when the object is actually created.

Anyone doing performance-tuning of JDBC applications knows that creating database connections is an expensive operation. Connection pooling is a natural fit for Web applications that utilize servlets or EJBs that require many simultaneous threads to connect to databases. Figure 1 provides a more visual example. In this diagram, three application components (a servlet, JSP, and session bean) have each received a Connection object from the pool (whose current size is six).

In addition to pooling connections, the WebSphere connection manager monitors the health of a database connection. If a connection has been allocated from the pool but hasn't been used in a specified amount of time (orphan timeout), the connection manager will mark the connection as "stale" and will return it to the pool (preempt the connection).

In the event of a connectivity failure (database error, network connection dropped, etc.), the connection manager will throw a StaleConnectionException, a subclass of SQLExcep-tion, on the next JDBC operation. This allows application developers to handle failure conditions more flexibly, such as allowing the application to retry the database operation. Further, the connection manager removes all Connection objects from the pool.

WebSphere's connection manager supports a variety of databases, including DB2, Oracle, Sybase, Microsoft SQL Server, and Informix (only available in WebSphere 4.0). SQL Server and Oracle (for two-phase commit operations) require the use of the Merant SequeLink 5.1 JDBC Driver, which ships with WebSphere Application Server, Advanced Edition.

Configuration of the connection manager is simple and painless (see Figures 2 and 3). Each data source represents a connection pool to a specified database, and is linked to a single JDBC driver.

Accessing a data source programmatically requires a JNDI lookup such as:

 

Context ctx = new InitialContext();
DataSource ds = (DataSource)
ctx.lookup("jdbc/
MyDataSource");

The string, "jdbc/MyDataSource", represents the JNDI name of the datasource. In version 4.0, you may use a resource reference such as "java:comp/env/jdbc/MyDataSource", as per the J2EE specification. The DataSource object contains the methods getConnection() and getConnection(String username, String password) to allocate a Connection object from the pool.

Simply close the Connection object to return it to the pool. For example:

 

Connection con = ds.getConnection(
username, password);
...
con.close();

Now that you understand the basics of the WebSphere connection manager, let's take a look at some of the best practices.

Keep the Connection Life Span Minimized
Ideally, an application should never hold on to a Connection object beyond a single method. Nor should a single thread acquire more than one Connection. The longer one part of an application holds onto a Connection, the longer other parts cannot use that same Connection.
An application designed to acquire more than one Connection per thread and hold them for very long periods of time could easily come to a screeching halt when multiple threads collectively request more Connection objects than the maximum pool size.

The temptation here is to simply increase the size of the pool, but that will impact performance. Each Connection, whether it's in use or not, can cost over two megabytes of JVM memory. Furthermore, a higher pool size means creating more Connection objects which, as previously mentioned, is expensive. Efficient usage of fewer Connection objects is a far better design.

Additionally, when an orphaned Connection times out, the connection manager will reclaim the Connection for the pool, throwing a StaleConnectionException in the offending client on the next JDBC operation. While this is costly for the single thread, it does at least allow the Connection object to be freed. However, inside the bounds of a transaction (e.g., inside an EJB or when using UserTransaction objects), a Connection will never be orphaned. Thus, long-running transactions could keep Connection objects out of commission for a long time, and could lead to deadlock-like conditions.

While keeping connectivity life span minimized, a developer should be careful not to compromise functionality. There are times when holding a ResultSet open for an extended operation is necessary to preserve transaction isolation (row locks). Closing a ResultSet (or its creating Connection or Statement object) too early might result in your application making decisions that assumed the state of the data in the database was X when another thread has modified it to be Y.

From an application development standpoint, the best alternative is to use fewer connections in your pool, but to use them wisely. Once a method has finished its database utilization, it should free the Connection object by calling its close() method.

Cache the Datasource, Not the Connection
To take full advantage of the connection pool and make limited calls to expensive operations, an application developer should cache a datasource object when database connectivity may be utilized often. Caching the Connection has the negative effects described above and defeats the purpose of pooling.

Also, be aware that declaring Connection objects as static class variables can have unexpected and undesirable effects. For example:

 

public class MyJDBCClass {
static Connection con;
...
}

When a Connection is declared static, it's shared by all instances of the declaring class. Therefore, it's possible that two threads might attempt to utilize the same Connection object simultaneously. The results could include unexpected table locking, incorrect ResultSets, and serious data integrity problems.

Also, keep in mind that all global variables of a servlet are static by default (even if you don't declare them static). If you decide to cache a Connection, make sure not to declare it static.

Caching the datasource means creating a global instance variable in one of your classes. That class will maintain the reference to the datasource object throughout its life. When a member method needs a Connection, it creates one by calling the datasource's getConnection() method.

The advantage of caching the datasource is that you prevent multiple JNDI lookups, another expensive operation. WebSphere provides additional caching in this area, but caching it locally is advantageous.

Make Sure to Clean Up!
The WebSphere connection manager does a good job of caring for your connections, but nothing makes up for careless coding. Make sure to clean up the open database resource objects properly.

The rule of thumb when cleaning up JDBC objects is to close them in the reverse order they were created. As an example, most database read operations encompass a Connection, which creates or prepares a Statement (or PreparedStatement), which produces a ResultSet. In this case, close the ResultSet first, the Statement second, the Connection last. The order of the cleanup is important to prevent database-specific annoyances, like running out of cursors.

The best way to ensure proper cleanup is to place all cleanup logic in a finally clause of a try-catch-finally block. The code in a finally clause is guaranteed to run in all cases short of a JVM crash, so a Connection.close() in a finally clause is sure to return the Connection to the pool. Example:

 

try {
con = ds.getConnection();
ps = con.prepareStatement
("select * from mytable where mycolumn = ?");
ps.setString(1, someString);
rs = ps.executeQuery();
// do something with the ResultSet
} catch (SQLException ex) {
// handle the SQLException
} finally {
try { rs.close(); } catch(SQLException ex) {rs = null;}
try { ps.close(); } catch (SQLException ex) {ps = null;}
try { con.close(); } catch (SQLException ex) {con = null;}
}

One alternative to closing Connection objects in finally clauses is to close them in a finalize() method of an object. This approach is not recommended, as the finalize() method is only called prior to garbage collection, an asynchronous operation. Also, the only reason to postpone closing a Connection in this manner would imply that you're caching it in the object, another poor practice. Cleaning up JDBC objects correctly can prevent many unnecessary headaches.

When Good Connections Go Bad
Occasionally database connectivity breaks unexpectedly. Network cables get pulled, electricity goes out, or sometimes a database server just gets pegged. Although nothing can be done until the database is brought back online, these sorts of outages don't have to permanently bring down your application's functionality.

The WebSphere connection manager provides a convenient manner of handling temporary connectivity outages. Proper handling of the StaleConnectionException allows developers to create more fault-tolerant applications.

The StaleConnectionException is an exception that extends SQLException. This is necessary in order to follow the JDBC specification (most JDBC methods may only throw a SQLException). This is important to note because if an application is to catch a StaleConnectionException, it must be caught before catching the SQLException. The actual package name for the StaleConnectionException in version 3.5 is com.ibm.ejs.cm.portability; in 4.0 it's com.ibm.websphere.ce.cm (the 3.5 package name will work in 4.0 but is deprecated).

Just for clarification, when a Connection is marked "stale," that means that on its next operation it will throw a StaleConnectionException.

A Connection can "go bad" in any number of situations:

1.   The application attempts to acquire a Connection during a database failure or when the database has not yet been started.
2.   All of the Connection objects in the pool have gone bad due to a database failure. In this case, any new calls to getConnection() will return a stale Connection object, and all Connection objects in use would throw this exception when the application attempts to use them.
3.   The application using the Connection has previously called its close() method, but later tries to use the connection again.
4.   The application acquired a Connection but didn't use it within the orphan timeout period. In this case, the connection manager orphans (preempts) the Connection and the next time the application attempts to use it (or a JDBC resource that it created; e.g. Statement, ResultSet, etc.) a StaleConnectionException will be thrown.

Handling a StaleConnection-Exception is entirely up to the application designer. For more fault tolerance, one recommended approach is to catch the StaleConnection-Exception and retry the database operation up to a predefined number of tries. Here's an example:

 

boolean retry = false;
do {
try {
retry = false;
con = ds.getConnection();
stmt = con.createStatement();
rowsUpdated = stmt.execute("update myTable set myColumn =
5000");
} catch(com.ibm.ejs.cm.portability.StaleConnectionException sce) {
if (retryAttempts++ < MAX_RETRIES)retry = true;
} catch(SQLException sqle) {
/* handle a more severe failure and exit loop */
} finally {/*Close stmt and con */cleanupJDBCResources();
}
} while (retry);

Normal execution of this code will only execute the logic in the loop once; only in error conditions will the thread retry the operation. The connection manager could throw a StaleConnectionException on any of the operations on lines four through six. In this event, the thread will check to see if it's reached the maximum number of retries (set prior to execution), and if so retry the entire operation, starting by obtaining a new Connection object.

Handling the StaleConnectionException in this manner will allow your Web application to recover from a number of temporary database outages that would normally return an error page to your Internet customer.

Wrapping Up
The practices outlined here are recommended as best practices, and in general are good advice for building WebSphere applications. Nevertheless, none of these tips will replace a poor design; they are meant to complement a well-designed application. They're also meant to provide insight into how the connection manager functions. This information should help in future development projects as well as in maintaining your current applications.

Managing JDBC connections can be tricky, but with a good application design and by following these recommendations, you should be well on your way to delivering high-performing and scalable enterprise solutions.

Resources
Cuomo, G., A Methodology for Production Performance Tuning
Lauzon, S., and Schommer, P., Handling Database Failures
Erickson, D., Connection Manager Whitepaper
WebSphere 3.5 Handbook Redbook

These articles and whitepapers can be found online at:
www.ibm.com/software/webservers/appserv/library.html
www7b.boulder.ibm.com/wsdd and www.redbooks.ibm.com

More Stories By J. Andrew McCright

Andy McCright is a software engineer for IBM software group. His>responsibilities include development of the WebSphere Application Server>and customer engagement. Recently, he has been researching web services and how they will play into the next generation of application servers.

Comments (0)

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


@ThingsExpo Stories
A strange thing is happening along the way to the Internet of Things, namely far too many devices to work with and manage. It has become clear that we'll need much higher efficiency user experiences that can allow us to more easily and scalably work with the thousands of devices that will soon be in each of our lives. Enter the conversational interface revolution, combining bots we can literally talk with, gesture to, and even direct with our thoughts, with embedded artificial intelligence, wh...
Adobe is changing the world though digital experiences. Adobe helps customers develop and deliver high-impact experiences that differentiate brands, build loyalty, and drive revenue across every screen, including smartphones, computers, tablets and TVs. Adobe content solutions are used daily by millions of companies worldwide-from publishers and broadcasters, to enterprises, marketing agencies and household-name brands. Building on its established design leadership, Adobe enables customers not o...
Why do your mobile transformations need to happen today? Mobile is the strategy that enterprise transformation centers on to drive customer engagement. In his general session at @ThingsExpo, Roger Woods, Director, Mobile Product & Strategy – Adobe Marketing Cloud, covered key IoT and mobile trends that are forcing mobile transformation, key components of a solid mobile strategy and explored how brands are effectively driving mobile change throughout the enterprise.
In this strange new world where more and more power is drawn from business technology, companies are effectively straddling two paths on the road to innovation and transformation into digital enterprises. The first path is the heritage trail – with “legacy” technology forming the background. Here, extant technologies are transformed by core IT teams to provide more API-driven approaches. Legacy systems can restrict companies that are transitioning into digital enterprises. To truly become a lea...
What are the new priorities for the connected business? First: businesses need to think differently about the types of connections they will need to make – these span well beyond the traditional app to app into more modern forms of integration including SaaS integrations, mobile integrations, APIs, device integration and Big Data integration. It’s important these are unified together vs. doing them all piecemeal. Second, these types of connections need to be simple to design, adapt and configure...
24Notion is full-service global creative digital marketing, technology and lifestyle agency that combines strategic ideas with customized tactical execution. With a broad understand of the art of traditional marketing, new media, communications and social influence, 24Notion uniquely understands how to connect your brand strategy with the right consumer. 24Notion ranked #12 on Corporate Social Responsibility - Book of List.
Just over a week ago I received a long and loud sustained applause for a presentation I delivered at this year’s Cloud Expo in Santa Clara. I was extremely pleased with the turnout and had some very good conversations with many of the attendees. Over the next few days I had many more meaningful conversations and was not only happy with the results but also learned a few new things. Here is everything I learned in those three days distilled into three short points.
What happens when the different parts of a vehicle become smarter than the vehicle itself? As we move toward the era of smart everything, hundreds of entities in a vehicle that communicate with each other, the vehicle and external systems create a need for identity orchestration so that all entities work as a conglomerate. Much like an orchestra without a conductor, without the ability to secure, control, and connect the link between a vehicle’s head unit, devices, and systems and to manage the ...
The Jevons Paradox suggests that when technological advances increase efficiency of a resource, it results in an overall increase in consumption. Writing on the increased use of coal as a result of technological improvements, 19th-century economist William Stanley Jevons found that these improvements led to the development of new ways to utilize coal. In his session at 19th Cloud Expo, Mark Thiele, Chief Strategy Officer for Apcera, will compare the Jevons Paradox to modern-day enterprise IT, e...
Major trends and emerging technologies – from virtual reality and IoT, to Big Data and algorithms – are helping organizations innovate in the digital era. However, to create real business value, IT must think beyond the ‘what’ of digital transformation to the ‘how’ to harness emerging trends, innovation and disruption. Architecture is the key that underpins and ties all these efforts together. In the digital age, it’s important to invest in architecture, extend the enterprise footprint to the cl...
SYS-CON Events announced today that Commvault, a global leader in enterprise data protection and information management, has been named “Bronze Sponsor” of SYS-CON's 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. Commvault is a leading provider of data protection and information management solutions, helping companies worldwide activate their data to drive more value and business insight and to transform moder...
SYS-CON Events has announced today that Roger Strukhoff has been named conference chair of Cloud Expo and @ThingsExpo 2016 Silicon Valley. The 19th Cloud Expo and 6th @ThingsExpo will take place on November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. "The Internet of Things brings trillions of dollars of opportunity to developers and enterprise IT, no matter how you measure it," stated Roger Strukhoff. "More importantly, it leverages the power of devices and the Interne...
What does it look like when you have access to cloud infrastructure and platform under the same roof? Let’s talk about the different layers of Technology as a Service: who cares, what runs where, and how does it all fit together. In his session at 18th Cloud Expo, Phil Jackson, Lead Technology Evangelist at SoftLayer, an IBM company, spoke about the picture being painted by IBM Cloud and how the tools being crafted can help fill the gaps in your IT infrastructure.
Digital innovation is the next big wave of business transformation based on digital technologies of which IoT and Big Data are key components, For example: Business boundary innovation is a challenge to excavate third-party business value using IoT and BigData, like Nest Business structure innovation may propose re-building business structure from scratch, as Uber does in the taxicab industry The social model innovation is also a big challenge to the new social architecture with the design fr...
Data is an unusual currency; it is not restricted by the same transactional limitations as money or people. In fact, the more that you leverage your data across multiple business use cases, the more valuable it becomes to the organization. And the same can be said about the organization’s analytics. In his session at 19th Cloud Expo, Bill Schmarzo, CTO for the Big Data Practice at EMC, will introduce a methodology for capturing, enriching and sharing data (and analytics) across the organizati...
DevOps at Cloud Expo, taking place Nov 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with 19th Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world. The widespread success of cloud computing is driving the DevOps revolution in enterprise IT. Now as never before, development teams must communicate and collaborate in a dynamic, 24/7/365 environment. There is no time to wait for long dev...
IoT offers a value of almost $4 trillion to the manufacturing industry through platforms that can improve margins, optimize operations & drive high performance work teams. By using IoT technologies as a foundation, manufacturing customers are integrating worker safety with manufacturing systems, driving deep collaboration and utilizing analytics to exponentially increased per-unit margins. However, as Benoit Lheureux, the VP for Research at Gartner points out, “IoT project implementers often ...
Video experiences should be unique and exciting! But that doesn’t mean you need to patch all the pieces yourself. Users demand rich and engaging experiences and new ways to connect with you. But creating robust video applications at scale can be complicated, time-consuming and expensive. In his session at @ThingsExpo, Zohar Babin, Vice President of Platform, Ecosystem and Community at Kaltura, will discuss how VPaaS enables you to move fast, creating scalable video experiences that reach your ...
In his keynote at 18th Cloud Expo, Andrew Keys, Co-Founder of ConsenSys Enterprise, provided an overview of the evolution of the Internet and the Database and the future of their combination – the Blockchain. Andrew Keys is Co-Founder of ConsenSys Enterprise. He comes to ConsenSys Enterprise with capital markets, technology and entrepreneurial experience. Previously, he worked for UBS investment bank in equities analysis. Later, he was responsible for the creation and distribution of life sett...
In his session at @ThingsExpo, Kausik Sridharabalan, founder and CTO of Pulzze Systems, Inc., will focus on key challenges in building an Internet of Things solution infrastructure. He will shed light on efficient ways of defining interactions within IoT solutions, leading to cost and time reduction. He will also introduce ways to handle data and how one can develop IoT solutions that are lean, flexible and configurable, thus making IoT infrastructure agile and scalable.