Click here to close now.

Welcome!

Java Authors: Kyle Samani, Elizabeth White, Kathy Thomas, Carmen Gonzalez, Asad Ali

Related Topics: Java

Java: Article

Clustered Timers

For Robust Scalable Systems

Often, when someone asks how we are going to scale the Web application we're about to develop, we look at them, smile, and say, "Not a problem - we'll just cluster the application servers." Clustering our application across multiple servers provides us with the ability to handle large volumes of traffic and to scale systems by adding additional servers to the cluster.

In addition to providing scalability, application clusters make the system more robust by allowing for automatic system failover when a server fails. This way when one server goes down the application continues to run, albeit with slightly decreased performance. While it is true that the current generation of application servers makes it relatively pain-free to create a cluster, there are still several significant, if often overlooked, design issues that must be taken into account now that the system is clustered.

When we run our Web application in a cluster, we have the exact same software running on each machine in the cluster. While this eliminates a host of configuration management difficulties, it does create other problems. While we don't have to write different code for every possible machine in the cluster, there are times when this simplicity actually makes things more complex; the running of scheduled tasks is typically one of these areas. Scheduled tasks are used to execute procedures that need to run at certain fixed times or at fixed intervals. Typical examples of scheduled tasks within a Web application are report-generation tasks and tasks that send data to external systems that are only available within a certain time frame.

To understand why clustering affects how we design our application to handle scheduled tasks, let's consider a generic e-commerce Web application. To allow management to analyze sales trends, profits, inventory, etc., the system has been set up to periodically compile a set of reports and e-mail them to management. Clearly, management doesn't want to receive multiple e-mails containing the same reports, yet this is what we will get if we simply write a scheduled task and then cluster our system. When the appointed time to run the report comes up, all machines in the cluster will generate the same report and send it to management. This can be seen visually in Figure 1.

Perhaps the most straightforward way to solve this problem is to package the code that runs the scheduled tasks into a separate JAR file within the EAR file that contains the WAR file for the Web application. This EAR file is deployed to all the servers in the cluster; however, the JAR containing the scheduled tasks is configured to run on only one of the servers. This solves the problem by preventing the scheduled tasks from ever running on multiple machines. However, there is a significant downside to these solutions. First, you have now created additional configuration management problems. You need to carefully track which servers are set up to run the scheduled tasks and the exact deployment procedures that were used so that when additional servers are added to the cluster, the application is properly deployed on those servers.

The second problem is that you have effectively taken the scheduled tasks out of the cluster. Now, if the machine that is set up with the scheduled tasks fails, or its connection to the network fails, there is no backup or failover system. The tasks won't run. The remainder of this article investigates solutions to this problem that allow the scheduled tasks to remain part of the cluster and don't involve additional configuration management.

To stop every system in the cluster from performing the same scheduled task, report generation in this case, we have to utilize something outside of the application server cluster to track the state of our scheduled task. A perfect candidate for maintaining the state of our scheduled tasks is a shared database, and since nearly all applications already have access to a shared database, this is the resource we will use to solve this problem in our example (see Figure 2). It's worth mentioning that while a shared database is an ideal resource for solving this problem, it's not the only option. The solution presented here could be adapted to use flat files or some other shared resource external to the cluster.

Our external resource, the database in this case, will act as a mediator between competing machines in the cluster. We will create a table in the database that tracks scheduled tasks and their status. When a machine in the cluster wants to run one of the scheduled tasks, it first checks the status of that task in the database to see if some other machine is already running that task. If no other machine is running the task, the status of the task will be updated and that machine will run the task.

Another way of thinking about this solution is to think in terms of a concurrent method running on a single machine. If we see the scheduled task in these terms, it becomes clear that the best way to keep multiple threads from running the task at the same time is to use some sort of semaphore. Again, if this was a single method on one machine, we could easily do this by creating a synchronized block around the code that we wanted to protect. When a thread first attempts to enter the synchronized block, it has to attempt to get the lock. If it fails to get the lock, it can't run. In our distributed system, we are using the database as the lock.

We will call our database table "Tasks" and it will have three columns. The first column will be the name of the task, the second the status of the task, and the third the date and time that the task last changed status. The generic SQL script to produce this table is shown below.

CREATE TABLE 'Tasks' (
'TaskName' varchar(50) NOT NULL,
'Status' varchar(25) NOT NULL,
'StatusTime' datetime,
PRIMARY KEY ('TaskName')
) ;

Now that we have created our database table to serve as our mediator, we can create the class that accesses this table in order to determine if a particular instance of a Task can execute. We'll call this class TaskMonitor. (The source code for this article can be downloaded from www.sys-con.com/java/sourcec.cfm.) The class exposes two methods to the public, public static boolean acquireLock(String taskName) and public static void releaseLock(String taskName). Before a Task runs, it will need to call the acquireLock method of the TaskMonitor. If this method returns True, it's safe for the Task to run. If it returns False, then it's not safe for the Task to run as some other instance of this Task in the cluster is already executing. The key to understanding the TaskMonitor class is to understand the ACQUIRE_LOCK SQL query on lines 5-7.

What needs to be done is to determine if the Task in question, as identified by the field TaskName, is currently Idle, and if so, change its Status to Active. The crucial aspect of this is that it needs to happen atomically, that is, it must all happen as one single step. That's why we use a single update statement instead of writing both a select statement to see if the Task is currently Idle and an update to change its Status. In the case where we use the select statement first, it would be possible for the same select statement to be run by the other machines in the cluster before the update is executed. This would result in multiple Tasks running since they would all see the Idle state. By performing the entire process in an update statement, we take advantage of the automatic exclusive row locking that takes place in the database whenever an update statement is executed.

Now that we understand how the ACQUIRE_LOCK query works, the rest of the acquireLock method of the TaskMonitor is easy to follow. On line 23 the query is executed and the results are examined. The executeUpdate method returns the number of rows that were affected by the query. When the ACQUIRE_LOCK query successfully changes the Task from Idle to Active (as will be the case when this particular query is the first one in the cluster to run), one row will have been affected and the lockAcquired flag will be set to true. Otherwise, no rows will be affected and the lockAcquired flag will remain false.

The releaseLock method of TaskMonitor is meant to be called when a Task has finished executing. This method simply changes the status of the Task back to Idle. Both the releaseLock and the acquireLock methods also update the StatusTime field with the current date and time for record-keeping purposes.

One final note on the TaskMonitor class: the getConnection method shown in lines 75-85 should be upgraded before placing this class into production. As written, the method creates a connection to an instance of a MySQL database. A better practice in production would be to retrieve a connection from an existing connection pool.

Together the Tasks database table and the TaskMonitor class provide a framework for ensuring that only one instance of a given Task is running at a particular time, no matter how many instances of the application are running within the clustered system. At this point we're ready to create our report generating Task.

Because we're concerned with managing Tasks in a clustered environment, and not with creating reports or using the javax.mail APIs, we'll create a simple Task, called ReportTask, to illustrate the concept. Because we want this Task to execute automatically on a schedule, we need to extend java.util.TimerTask. TimerTask is an abstract class that has one method that we have to implement, public void run(). This is the method where all the Task's work is done. For our simple example, ReportTask, we'll output some text to show that the Task is running. The code for this class is shown below.

1) import java.util.TimerTask;
2) public class ReportTask extends TimerTask {
3) public void run() {
4) if(TaskMonitor.acquireLock("ReportTask") == false)
5) return;
6) System.out.println("Creating report to be emailed...");
7) TaskMonitor.releaseLock("ReportTask");
8) }
9) }

The key thing to note here is that before the ReportTask actually performs its work, printing some text in this case, it first attempts to acquire the lock for this Task by making the call to acquireLock on line 4. If it fails to acquire the lock, it simply returns without performing its work. However, if it does successfully acquire the lock, then it's free to perform its work and it goes ahead and prints out its message on line 6. Once the Task is complete, it's vital that the lock be released. This is accomplished by calling releaseLock on line 7. If the lock is never released, this Task will never run again on any machine in the cluster. Ensuring that the lock is properly released is clearly not an issue with this simple example; however, in more complex tasks it can be tricky. Consider a Task where several different error conditions could cause the Task to terminate before running to completion. There are now potentially several places where the lock will have to be released.

At this point, you've probably noticed a serious problem with our Task. We never populated the Tasks table with any tasks. As things stand, our ReportTask will never be able to acquire a lock and will never run, and this step needs to take place for every Task that's going to be managed in this way. To rectify this situation we need to insert the ReportTask information into the Tasks table using the following SQL script:

insert into Tasks values (‘ReportTask', ‘Idle', null);

We've nearly finished setting up our system for managing clustered tasks. So far we've created an external resource and a TimerTask called ReportTask that will run in our cluster. All that remains to be done is to create a Timer for running our ReportTask. Because we want to start the Timer for our task as soon as the application starts, we'll create a servlet called StartupServlet that does the work of creating our Timer. We will ensure that StartupServlet is loaded immediately by adding the following lines to web.xml:

<servlet>
<servlet-name>StartupServlet</servlet-name>
<display-name>StartupServlet</display-name>
<description>Used to create the Timers</description>
<servlet-class>StartupServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>

As our simple StartupServlet is not designed to handle requests, it doesn't need to override any method other than init(). When we create the Timer for running our ReportTask, it's important that we use one of the overloaded constructors to create the Timer as a daemon thread. If we don't specify that the Timer should be a daemon thread and use the default no argument constructor, the Timer will not be a daemon thread. By making it a daemon thread, we ensure that the Timer will continue to run for as long as our Web application runs and that it will terminate when the application terminates. We don't want to try to generate reports if the application has been stopped for some reason.

After calculating how many milliseconds are in a day (we want our ReportTask to run once a day), we schedule the ReportTask to run daily, starting now. On line 12 we place the Timer that we created in the ServletContext. While this is not strictly necessary to keep the ReportTask running, by keeping a reference to the Timer available we are able to check easily on the status of the ReportTimer or cancel it entirely should the need arise.

With the StartupServlet in place, we now have a very basic but workable system for running scheduled Tasks in a clustered environment, without having to worry about the same task running on all of the machines in the cluster simultaneously. It's important to note that if this scheme is used as presented and the tasks being executed complete in a very short period of time, you could still see duplicate executions of the same task if the clocks on all of the machines are not in synch with each other. While it is possible to extend this approach to address this problem, it's outside the scope of this article. With a little bit of effort, this system can also be extended to allow for such things as programmatic modification of the running tasks, robust error handling, and recovery of frozen tasks.

More Stories By Clark D. Richey Jr.

Clark is a principal consultant with the RABA Technologies RiSC group for advanced research and development. In his spare time, he teaches the Java platform to students at Loyola College, where as an associate professor, he shares his experiences with much enthusiasm. Clark is the founder of both JUGaccino, a Maryland-based JUG, and the StopLight and PermissionSniffer open source projects. He is also involved in implementing highly scalable, highly secure, service-oriented architectures using Jini.

Comments (2) View Comments

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.


Most Recent Comments
cbellonch 05/12/05 11:04:17 AM EDT

Hi,
Thanks for the article, it would be useful for our project. We've tried to download the code in:
· www.sys-con.com/java/sourcec.cfm
· http://www.sys-con.com/java/archives3/0903/Richey0903.zip

without success, are the links correct?

tbb 03/10/04 08:44:10 AM EST

I believe a class that implements ServletContextListener would be a better way to solve this problem than a servlet that loads on startup. (If your servlet container implements the servlet 2.3+ spec).

@ThingsExpo Stories
The world's leading Cloud event, Cloud Expo has launched Microservices Journal on the SYS-CON.com portal, featuring over 19,000 original articles, news stories, features, and blog entries. DevOps Journal is focused on this critical enterprise IT topic in the world of cloud computing. Microservices Journal offers top articles, news stories, and blog posts from the world's well-known experts and guarantees better exposure for its authors than any other publication. Follow new article posts on Twitter at @MicroservicesE
SYS-CON Events announced today that Litmus Automation 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. Litmus Automation’s vision is to provide a solution for companies that are in a rush to embrace the disruptive Internet of Things technology and leverage it for real business challenges. Litmus Automation simplifies the complexity of connected devices applications with Loop, a secure and scalable cloud platform.
Internet of Things (IoT) will be a hybrid ecosystem of diverse devices and sensors collaborating with operational and enterprise systems to create the next big application. In their session at @ThingsExpo, Bramh Gupta, founder and CEO of robomq.io, and Fred Yatzeck, principal architect leading product development at robomq.io, will discuss how choosing the right middleware and integration strategy from the get-go will enable IoT solution developers to adapt and grow with the industry, while at the same time reduce Time to Market (TTM) by using plug and play capabilities offered by a robust I...
SYS-CON Events announced today the IoT Bootcamp – Jumpstart Your IoT Strategy, being held June 9–10, 2015, in conjunction with 16th Cloud Expo and Internet of @ThingsExpo at the Javits Center in New York City. This is your chance to jumpstart your IoT strategy. Combined with real-world scenarios and use cases, the IoT Bootcamp is not just based on presentations but includes hands-on demos and walkthroughs. We will introduce you to a variety of Do-It-Yourself IoT platforms including Arduino, Raspberry Pi, BeagleBone, Spark and Intel Edison. You will also get an overview of cloud technologies s...
Containers and microservices have become topics of intense interest throughout the cloud developer and enterprise IT communities. Accordingly, attendees at the upcoming 16th Cloud Expo at the Javits Center in New York June 9-11 will find fresh new content in a new track called PaaS | Containers & Microservices Containers are not being considered for the first time by the cloud community, but a current era of re-consideration has pushed them to the top of the cloud agenda. With the launch of Docker's initial release in March of 2013, interest was revved up several notches. Then late last...
The WebRTC Summit 2014 New York, to be held June 9-11, 2015, at the Javits Center in New York, NY, announces that its Call for Papers is open. Topics include all aspects of improving IT delivery by eliminating waste through automated business models leveraging cloud technologies. WebRTC Summit is co-located with 16th International Cloud Expo, @ThingsExpo, Big Data Expo, and DevOps Summit.
SOA Software has changed its name to Akana. With roots in Web Services and SOA Governance, Akana has established itself as a leader in API Management and is expanding into cloud integration as an alternative to the traditional heavyweight enterprise service bus (ESB). The company recently announced that it achieved more than 90% year-over-year growth. As Akana, the company now addresses the evolution and diversification of SOA, unifying security, management, and DevOps across SOA, APIs, microservices, and more.
Wearable technology was dominant at this year’s International Consumer Electronics Show (CES) , and MWC was no exception to this trend. New versions of favorites, such as the Samsung Gear (three new products were released: the Gear 2, the Gear 2 Neo and the Gear Fit), shared the limelight with new wearables like Pebble Time Steel (the new premium version of the company’s previously released smartwatch) and the LG Watch Urbane. The most dramatic difference at MWC was an emphasis on presenting wearables as fashion accessories and moving away from the original clunky technology associated with t...
SYS-CON Events announced today that robomq.io 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. robomq.io is an interoperable and composable platform that connects any device to any application. It helps systems integrators and the solution providers build new and innovative products and service for industries requiring monitoring or intelligence from devices and sensors.
The list of ‘new paradigm’ technologies that now surrounds us appears to be at an all time high. From cloud computing and Big Data analytics to Bring Your Own Device (BYOD) and the Internet of Things (IoT), today we have to deal with what the industry likes to call ‘paradigm shifts’ at every level of IT. This is disruption; of course, we understand that – change is almost always disruptive.
SYS-CON Events announced today that Solgenia 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, and the 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. Solgenia is the global market leader in Cloud Collaboration and Cloud Infrastructure software solutions. Designed to “Bridge the Gap” between Personal and Professional Social, Mobile and Cloud user experiences, our solutions help large and medium-sized organizations dr...
SYS-CON Events announced today that SafeLogic has been named “Bag Sponsor” of SYS-CON's 16th International Cloud Expo® New York, which will take place June 9-11, 2015, at the Javits Center in New York City, NY. SafeLogic provides security products for applications in mobile and server/appliance environments. SafeLogic’s flagship product CryptoComply is a FIPS 140-2 validated cryptographic engine designed to secure data on servers, workstations, appliances, mobile devices, and in the Cloud.
After making a doctor’s appointment via your mobile device, you receive a calendar invite. The day of your appointment, you get a reminder with the doctor’s location and contact information. As you enter the doctor’s exam room, the medical team is equipped with the latest tablet containing your medical history – he or she makes real time updates to your medical file. At the end of your visit, you receive an electronic prescription to your preferred pharmacy and can schedule your next appointment.
GENBAND has announced that SageNet is leveraging the Nuvia platform to deliver Unified Communications as a Service (UCaaS) to its large base of retail and enterprise customers. Nuvia’s cloud-based solution provides SageNet’s customers with a full suite of business communications and collaboration tools. Two large national SageNet retail customers have recently signed up to deploy the Nuvia platform and the company will continue to sell the service to new and existing customers. Nuvia’s capabilities include HD voice, video, multimedia messaging, mobility, conferencing, Web collaboration, deskt...
SYS-CON Media announced today that @WebRTCSummit Blog, the largest WebRTC resource in the world, has been launched. @WebRTCSummit Blog offers top articles, news stories, and blog posts from the world's well-known experts and guarantees better exposure for its authors than any other publication. @WebRTCSummit Blog can be bookmarked ▸ Here @WebRTCSummit conference site can be bookmarked ▸ Here
SYS-CON Events announced today that Cisco, the worldwide leader in IT that transforms how people connect, communicate and collaborate, has been named “Gold 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 City, NY. Cisco makes amazing things happen by connecting the unconnected. Cisco has shaped the future of the Internet by becoming the worldwide leader in transforming how people connect, communicate and collaborate. Cisco and our partners are building the platform for the Internet of Everything by connecting the...
Temasys has announced senior management additions to its team. Joining are David Holloway as Vice President of Commercial and Nadine Yap as Vice President of Product. Over the past 12 months Temasys has doubled in size as it adds new customers and expands the development of its Skylink platform. Skylink leads the charge to move WebRTC, traditionally seen as a desktop, browser based technology, to become a ubiquitous web communications technology on web and mobile, as well as Internet of Things compatible devices.
Docker is an excellent platform for organizations interested in running microservices. It offers portability and consistency between development and production environments, quick provisioning times, and a simple way to isolate services. In his session at DevOps Summit at 16th Cloud Expo, Shannon Williams, co-founder of Rancher Labs, will walk through these and other benefits of using Docker to run microservices, and provide an overview of RancherOS, a minimalist distribution of Linux designed expressly to run Docker. He will also discuss Rancher, an orchestration and service discovery platf...
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.
SYS-CON Events announced today that Liaison Technologies, a leading provider of data management and integration cloud services and solutions, has been named "Silver 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. Liaison Technologies is a recognized market leader in providing cloud-enabled data integration and data management solutions to break down complex information barriers, enabling enterprises to make smarter decisions, faster.