Click here to close now.

Welcome!

Java Authors: Ian Khan, Harry Trott, Liz McMillan, Pat Romanski, Amit Gupta

Related Topics: Websphere

Websphere: 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
Operational Hadoop and the Lambda Architecture for Streaming Data Apache Hadoop is emerging as a distributed platform for handling large and fast incoming streams of data. Predictive maintenance, supply chain optimization, and Internet-of-Things analysis are examples where Hadoop provides the scalable storage, processing, and analytics platform to gain meaningful insights from granular data that is typically only valuable from a large-scale, aggregate view. One architecture useful for capturing and analyzing streaming data is the Lambda Architecture, representing a model of how to analyze rea...
SYS-CON Events announced today that Vitria Technology, Inc. will exhibit at SYS-CON’s @ThingsExpo, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Vitria will showcase the company’s new IoT Analytics Platform through live demonstrations at booth #330. Vitria’s IoT Analytics Platform, fully integrated and powered by an operational intelligence engine, enables customers to rapidly build and operationalize advanced analytics to deliver timely business outcomes for use cases across the industrial, enterprise, and consumer segments.
The explosion of connected devices / sensors is creating an ever-expanding set of new and valuable data. In parallel the emerging capability of Big Data technologies to store, access, analyze, and react to this data is producing changes in business models under the umbrella of the Internet of Things (IoT). In particular within the Insurance industry, IoT appears positioned to enable deep changes by altering relationships between insurers, distributors, and the insured. In his session at @ThingsExpo, Michael Sick, a Senior Manager and Big Data Architect within Ernst and Young's Financial Servi...
SYS-CON Events announced today that Open Data Centers (ODC), a carrier-neutral colocation provider, will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place June 9-11, 2015, at the Javits Center in New York City, NY. Open Data Centers is a carrier-neutral data center operator in New Jersey and New York City offering alternative connectivity options for carriers, service providers and enterprise customers.
When it comes to the Internet of Things, hooking up will get you only so far. If you want customers to commit, you need to go beyond simply connecting products. You need to use the devices themselves to transform how you engage with every customer and how you manage the entire product lifecycle. In his session at @ThingsExpo, Sean Lorenz, Technical Product Manager for Xively at LogMeIn, will show how “product relationship management” can help you leverage your connected devices and the data they generate about customer usage and product performance to deliver extremely compelling and reliabl...
SYS-CON Events announced today that CodeFutures, a leading supplier of database performance tools, has been named a “Sponsor” of SYS-CON's 16th International Cloud Expo®, which will take place on June 9–11, 2015, at the Javits Center in New York, NY. CodeFutures is an independent software vendor focused on providing tools that deliver database performance tools that increase productivity during database development and increase database performance and scalability during production.
The IoT market is projected to be $1.9 trillion tidal wave that’s bigger than the combined market for smartphones, tablets and PCs. While IoT is widely discussed, what not being talked about are the monetization opportunities that are created from ubiquitous connectivity and the ensuing avalanche of data. While we cannot foresee every service that the IoT will enable, we should future-proof operations by preparing to monetize them with extremely agile systems.
There’s Big Data, then there’s really Big Data from the Internet of Things. IoT is evolving to include many data possibilities like new types of event, log and network data. The volumes are enormous, generating tens of billions of logs per day, which raise data challenges. Early IoT deployments are relying heavily on both the cloud and managed service providers to navigate these challenges. Learn about IoT, Big Data and deployments processing massive data volumes from wearables, utilities and other machines.
The explosion of connected devices / sensors is creating an ever-expanding set of new and valuable data. In parallel the emerging capability of Big Data technologies to store, access, analyze, and react to this data is producing changes in business models under the umbrella of the Internet of Things (IoT). In particular within the Insurance industry, IoT appears positioned to enable deep changes by altering relationships between insurers, distributors, and the insured. In his session at @ThingsExpo, Michael Sick, a Senior Manager and Big Data Architect within Ernst and Young's Financial Servi...
The major cloud platforms defy a simple, side-by-side analysis. Each of the major IaaS public-cloud platforms offers their own unique strengths and functionality. Options for on-site private cloud are diverse as well, and must be designed and deployed while taking existing legacy architecture and infrastructure into account. Then the reality is that most enterprises are embarking on a hybrid cloud strategy and programs. In this Power Panel at 15th Cloud Expo (http://www.CloudComputingExpo.com), moderated by Ashar Baig, Research Director, Cloud, at Gigaom Research, Nate Gordon, Director of T...
“In the past year we've seen a lot of stabilization of WebRTC. You can now use it in production with a far greater degree of certainty. A lot of the real developments in the past year have been in things like the data channel, which will enable a whole new type of application," explained Peter Dunkley, Technical Director at Acision, in this SYS-CON.tv interview at @ThingsExpo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
SYS-CON Events announced today that Intelligent Systems Services will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Established in 1994, Intelligent Systems Services Inc. is located near Washington, DC, with representatives and partners nationwide. ISS’s well-established track record is based on the continuous pursuit of excellence in designing, implementing and supporting nationwide clients’ mission-critical systems. ISS has completed many successful projects in Healthcare, Commercial, Manufacturing, ...
PubNub on Monday has announced that it is partnering with IBM to bring its sophisticated real-time data streaming and messaging capabilities to Bluemix, IBM’s cloud development platform. “Today’s app and connected devices require an always-on connection, but building a secure, scalable solution from the ground up is time consuming, resource intensive, and error-prone,” said Todd Greene, CEO of PubNub. “PubNub enables web, mobile and IoT developers building apps on IBM Bluemix to quickly add scalable realtime functionality with minimal effort and cost.”
Sensor-enabled things are becoming more commonplace, precursors to a larger and more complex framework that most consider the ultimate promise of the IoT: things connecting, interacting, sharing, storing, and over time perhaps learning and predicting based on habits, behaviors, location, preferences, purchases and more. In his session at @ThingsExpo, Tom Wesselman, Director of Communications Ecosystem Architecture at Plantronics, will examine the still nascent IoT as it is coalescing, including what it is today, what it might ultimately be, the role of wearable tech, and technology gaps stil...
DevOps tends to focus on the relationship between Dev and Ops, putting an emphasis on the ops and application infrastructure. But that’s changing with microservices architectures. In her session at DevOps Summit, Lori MacVittie, Evangelist for F5 Networks, will focus on how microservices are changing the underlying architectures needed to scale, secure and deliver applications based on highly distributed (micro) services and why that means an expansion into “the network” for DevOps.
In the consumer IoT, everything is new, and the IT world of bits and bytes holds sway. But industrial and commercial realms encompass operational technology (OT) that has been around for 25 or 50 years. This grittier, pre-IP, more hands-on world has much to gain from Industrial IoT (IIoT) applications and principles. But adding sensors and wireless connectivity won’t work in environments that demand unwavering reliability and performance. In his session at @ThingsExpo, Ron Sege, CEO of Echelon, will discuss how as enterprise IT embraces other IoT-related technology trends, enterprises with i...
When it comes to the Internet of Things, hooking up will get you only so far. If you want customers to commit, you need to go beyond simply connecting products. You need to use the devices themselves to transform how you engage with every customer and how you manage the entire product lifecycle. In his session at @ThingsExpo, Sean Lorenz, Technical Product Manager for Xively at LogMeIn, will show how “product relationship management” can help you leverage your connected devices and the data they generate about customer usage and product performance to deliver extremely compelling and reliabl...
The Internet of Things (IoT) is causing data centers to become radically decentralized and atomized within a new paradigm known as “fog computing.” To support IoT applications, such as connected cars and smart grids, data centers' core functions will be decentralized out to the network's edges and endpoints (aka “fogs”). As this trend takes hold, Big Data analytics platforms will focus on high-volume log analysis (aka “logs”) and rely heavily on cognitive-computing algorithms (aka “cogs”) to make sense of it all.
The Internet of Everything (IoE) brings together people, process, data and things to make networked connections more relevant and valuable than ever before – transforming information into knowledge and knowledge into wisdom. IoE creates new capabilities, richer experiences, and unprecedented opportunities to improve business and government operations, decision making and mission support capabilities. In his session at @ThingsExpo, Gary Hall, Chief Technology Officer, Federal Defense at Cisco Systems, will break down the core capabilities of IoT in multiple settings and expand upon IoE for bo...
With several hundred implementations of IoT-enabled solutions in the past 12 months alone, this session will focus on experience over the art of the possible. Many can only imagine the most advanced telematics platform ever deployed, supporting millions of customers, producing tens of thousands events or GBs per trip, and hundreds of TBs per month. With the ability to support a billion sensor events per second, over 30PB of warm data for analytics, and hundreds of PBs for an data analytics archive, in his session at @ThingsExpo, Jim Kaskade, Vice President and General Manager, Big Data & Ana...