Welcome!

Java IoT Authors: Elizabeth White, Liz McMillan, Yeshim Deniz, Pat Romanski, Yakov Fain

Related Topics: Java IoT, IBM Cloud, Weblogic

Java IoT: Article

Java Developer's Journal Feature: "Deadlocks in J2EE"

Most non-trivial applications involve high degrees of concurrency and many layers of abstraction

In the example, the updateBatchStatus method makes a "RequiresNew" EJB call to actually update the batch_status database table so the status change is immediately visible, even though the effects of the current transaction aren't yet visible. The call to executeUpdate isn't an EJB call, so it executes in the same transaction as the rest of bulkLoadData.

As described, this code will cause a deadlock even in the absence of concurrency. When bulkLoadData calls the executeUpdate method, it updates an existing database row, which involves acquiring a write-lock on that row. The nested EJB call to updateBatchStatus will execute on a separate database connection and try to execute a very similar query, but it will block because it can't acquire the necessary write-lock. From the database's perspective, this second connection will be allowed to proceed as soon as the first connection's transaction is committed or rolled back, which is why the database doesn't detect this as a deadlock. The VM, however, won't allow the call to bulkLoadData to complete before each call to updateBatchStatus completes, so we have a deadlock.

This example shows one update blocking another update, so it causes a deadlock on any database. If the initial update query was a simple select query instead, this example would only cause deadlocks on databases that use lock-based concurrency control, where one connection's read-lock can block another connection from acquiring a write-lock. In any case, a deadlock of this type is neither timing-dependent nor load-dependent, and the thread dump will show one Java thread waiting for a database response, but that thread will be associated with two active database connections. And one of those database connections will be idle, but blocking the other connection.

This scenario has many subtle variations, where more than one thread and more than two database connections may be involved. For example, the outer EJB call's database connection may have acquired a database lock that blocks another unrelated database connection from proceeding, but that unrelated database connection has acquired a lock that blocks the nested EJB call's database operation. This particular case is timing-dependent, and will show several Java threads waiting for database responses. At least one of those Java threads will be associated with two active database connections.

Cross-Resource Deadlock #3: JVM Locks Crossed with Database Locks
A third deadlock scenario occurs when mixing database locks with JVM locks. In this case, one thread holds a database lock and is trying to acquire a JVM lock (trying to enter a synchronized block), and another thread holds that JVM lock and is trying to acquire a database lock. Again, the database sees one connection blocking another, but nothing is stopping that connection from proceeding, so it won't detect a deadlock. The JVM sees one thread inside a synchronized block and another trying to enter, so even if the JVM could detect deadlocks and handle them somehow, it wouldn't detect this case.

For an example that illustrates this deadlock scenario, consider a simple (flawed) read-through cache. This cache is a HashMap backed by a database table. If a cache hit occurs, it will simply return the value from the HashMap; on a cache miss however, it will read the value from the database, add it to the HashMap, and return it as shown in Listing 2.

It's a simple read-through cache. Note that the get() method is synchronized, since we access a non-thread safe container, and we want the containsKey/put combination to be atomic in the case of a cache miss.

This cache is fairly straightforward: the contract is that if we change the data in the table backing the cache, we should call clearCache(), so the cache can avoid handing back stale data. The resulting cache misses will repopulate the cache appropriately.

Let's now consider some code that changes this data and clears the cache:

public void updateData(String key, String value) {
    executeUpdate("update cache_table set value='" + value +
       "' where key='" + key + "'");
    SimpleCache.getInstance().clearCache();
}

In a simple case, this will work with no problems. However, on a database that uses lock-based concurrency control, the query in updateData will prevent the select query in queryForValue from executing, because the update statement will have acquired a write-lock that prevents the select query from acquiring a read-lock on the same row. If the timing is just right, one thread can try to read a given value out of the cache and get a cache miss while another thread updates that value in the database. If the database executes the update statement first, it will block the select statement from continuing. However, the thread executing the select statement came through the synchronized get method, so it has acquired the lock on the SimpleCache. For the thread in updateData to return, it has to call clearCache(), but it can't acquire the lock (clearCache() is synchronized).

When dealing with an instance of this scenario, there will be one Java thread waiting for a response from the database, and one waiting to acquire a JVM lock. Each thread will be associated with a database connection, with one connection blocking the other. The fix is to avoid doing database operations while holding JVM locks: the get() method of our SimpleCache could be rewritten like this:

public Object get(String key) {
    synchronized(this) {
       if (cache.containsKey(key)) {
          return cache.get(key);
       }
    }
    Object value = queryForValue(key);
    synchronized(this) {
       cache.put(key, value);
    }
    return value;
}

Since we now know that this deadlock case can happen, we can add a check to our queryForValue method to try to avoid the deadlock condition by using Thread.holdsLock():

private Object queryForValue(String key) {
      assert(!Thread.holdsLock(this));
      return executeQuery(...);
}

While Thread.holdsLock() can be useful in this case, it only works if we know which locks we're specifically worried about. It would be useful to have a similar method that could determine whether the current thread holds any JVM locks. Then, any piece of code that made any kind of RPC call, database access, etc., could throw an exception or log a warning to indicate that these operations can be dangerous while holding a JVM lock.

Note: even though we've fixed the deadlock problem in this example, it's still flawed because the cache is cleared before updateData's transaction has been committed. If a cache miss happens after the clearCache call but before updateData's transaction is committed, the cache will load the old data because the new data isn't visible yet. The fix here is to clear the cache only after the changes have been committed. Observe that this would only happen in an MVCC database; in a lock-based database, the pending update would block the cache's read, so the cache would read the correct value after the update's transaction was committed.

Rules of Thumb
These guidelines should help you avoid these problems, or at least diagnose and fix them if they do happen.

  • Keep your transactions short and simple.
  • Understand the locking behavior of your database (and your transaction isolation level).
  • Assume any database access has a chance of failing with a database deadlock, and retry correctly.
  • Don't update any non-transactional state (in-memory state, caches, etc.) until after the transaction has completed.
  • Make sure resource pools are large enough for your peak concurrency.
  • Try not to acquire multiple resources at the same time. If you must, acquire them in the same order each time.
  • Learn how to get a full thread dump from your application server and a list of database connections from your database (including which connections are blocked by each), and how to know which Java thread each active database connection is associated with. The easiest way to understand the mapping between Java threads and database connections is to add some logging to your connection pool access code.
  • When making nested EJB calls, understand which ones will use the same database connection as the caller, and which ones will use new connections. Even if the nested call runs in the same global transaction, it may still use a different database connection, and this can cause cross-resource deadlocks.
  • Avoid making database calls and EJB calls, or doing other off-JVM operations while holding JVM locks. Use assert(!Thread.holdsLock(...)) if there are specific JVM locks you're concerned about to prevent future code changes from violating this rule unintentionally.
Conclusion
Cross-resource deadlocks in a J2EE application can be a big problem - they can cause the entire application to grind to a halt. They can also be difficult to isolate and fix, especially if developers aren't familiar with how to analyze a deadlocked environment. The scenarios we've discussed should help you understand a few common deadlocks, and give you some idea of what to look for. Even better, the rules of thumb we've outlined should give you a few conventions to follow in your code to avoid problems like this altogether.

Resources

More Stories By Michael Nonemacher

Michael Nonemacher is a lead software engineer for Lombardi Software. He has worked with Java since 1997, focusing on server-side database interaction and concurrency in Web-based enterprise applications.

Comments (3) 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
debnathm 11/18/08 11:48:20 AM EST

You mention:
"It would be nice if the EJB vendors supplied a retry-on-deadlock parameter that would do this automatically with container-managed transactions. Without this automated feature, developers end up forcing EJB calls to use bean-managed transactions just to be able to retry on deadlocks. (One of the disadvantages to making your EJB calls use bean-managed transactions is that it's not obvious how to get the same semantics as a container-managed transaction with "RequiresNew" ...)"

Is BMT strictly required to retry? Suppose all business methods pass through an initial method with no transaction attributes. This method front_bizmethod() calls the actual business method which of course
has a transaction attribute of Required/RequiresNew etc.

public void front_bizmethod()
{
int retry_count = 0;
boolean retry = false;
do
{
try
{
bizmethod();
}
catch (RetryException re)
{
retry = true;
retry_count++;
}
} while (retry && retry_count < 10);
}

/** Business method -- has transaction attribute
* Required or RequiresNew
*/
bizmethod() throws RetryException
{
// Do business calls and throw a RetryException
// if a deadlock was detected by the database
}

What I mean is that there is a way around it by
coding an extra method.

What are your thoughts on the same ?

jnorris10 07/08/06 11:10:54 PM EDT

Thanks for this great article, Mike. I wasn't able to make it to JavaOne this year so I missed your BOF at JavaOne that looks like it was talking about this stuff (BOF-0534). Are there any slides from that that I can get ahold of?

With the power and simplicity that CMT (container managed transactions) brings, is there really no simple way to automatically handle database deadlock by retrying the transaction according to some given parameters (ie: number of retries, back off time, etc)? That would really be unfortunate.

Here are a few possible solutions, but they all seem sub-optimal for various reasons: (I would like to stay within the spec (EJB3/JEE5), but I'm not adament on this).

1) JBoss has a proprietary extension for this: (org.jboss.ejb.plugins.TxRetryExceptionHandler) However, besides being outside the spec, this implementation does not allow me to easily keep track of any state regarding the retry (ie: retry count numbers, data for back off algorithms, etc.).

2) I can use the EJB3 Interceptor spec, but this is really cumbersome because it ties in *after* the CMT stuff has already been setup.

3) I'm sure I could write a jboss-aop interceptor that would do the right thing here, but again it would be outside the spec. Has anyone done this already? I certainly don't want to reinvent the wheel on this one.

4) I could switch from CMT (container managed transactions) to BMT (bean managed transactions). This would give me more control over the the transaction endpoints but then I would have to give up all the niceties that CMT give me.

I really would like a nice solution for this...

SYS-CON Australia News Desk 04/11/06 10:14:26 AM EDT

Most non-trivial applications involve high degrees of concurrency and many layers of abstraction. Concurrency is associated with resource contention and an increase in deadlock conditions. The multiple layers of abstraction make it more difficult to isolate and fix the deadlock conditions.

@ThingsExpo Stories
SYS-CON Events announced today that Super Micro Computer, Inc., a global leader in Embedded and IoT solutions, will exhibit at SYS-CON's 18th International Cloud Expo®, which will take place on June 7-9, 2016, at the Javits Center in New York City, NY. Supermicro (NASDAQ: SMCI), the leading innovator in high-performance, high-efficiency server technology, is a premier provider of advanced server Building Block Solutions® for Data Center, Cloud Computing, Enterprise IT, Hadoop/Big Data, HPC and ...
18th Cloud Expo, taking place June 7-9, 2016, at the Javits Center in New York City, NY, will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud strategy. Meanwhile, 94% of enterprises are using some...
SoftLayer operates a global cloud infrastructure platform built for Internet scale. With a global footprint of data centers and network points of presence, SoftLayer provides infrastructure as a service to leading-edge customers ranging from Web startups to global enterprises. SoftLayer's modular architecture, full-featured API, and sophisticated automation provide unparalleled performance and control. Its flexible unified platform seamlessly spans physical and virtual devices linked via a world...
SYS-CON Events announced today that BMC Software has been named "Siver Sponsor" of SYS-CON's 18th Cloud Expo, which will take place on June 7-9, 2015 at the Javits Center in New York, New York. BMC is a global leader in innovative software solutions that help businesses transform into digital enterprises for the ultimate competitive advantage. BMC Digital Enterprise Management is a set of innovative IT solutions designed to make digital business fast, seamless, and optimized from mainframe to mo...
"What we see what happens when you have a completely networked society and the potential to now drive the value creation and the collaboration and the ecosystems that are possible when you start to be able to connect people and industries together in ways that have never been possible before," explained Esmeralda Swartz, VP of Marketing Enterprise & Cloud at Ericsson, in this SYS-CON.tv interview at @ThingsExpo, held November 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA.
Companies can harness IoT and predictive analytics to sustain business continuity; predict and manage site performance during emergencies; minimize expensive reactive maintenance; and forecast equipment and maintenance budgets and expenditures. Providing cost-effective, uninterrupted service is challenging, particularly for organizations with geographically dispersed operations.
Cloud computing delivers on-demand resources that provide businesses with flexibility and cost-savings. The challenge in moving workloads to the cloud has been the cost and complexity of ensuring the initial and ongoing security and regulatory (PCI, HIPAA, FFIEC) compliance across private and public clouds. Manual security compliance is slow, prone to human error, and represents over 50% of the cost of managing cloud applications. Determining how to automate cloud security compliance is critical...
The Internet of Things (IoT) is growing rapidly by extending current technologies, products and networks. By 2020, Cisco estimates there will be 50 billion connected devices. Gartner has forecast revenues of over $300 billion, just to IoT suppliers. Now is the time to figure out how you’ll make money – not just create innovative products. With hundreds of new products and companies jumping into the IoT fray every month, there’s no shortage of innovation. Despite this, McKinsey/VisionMobile data...
The IoTs will challenge the status quo of how IT and development organizations operate. Or will it? Certainly the fog layer of IoT requires special insights about data ontology, security and transactional integrity. But the developmental challenges are the same: People, Process and Platform. In his session at @ThingsExpo, Craig Sproule, CEO of Metavine, will demonstrate how to move beyond today's coding paradigm and share the must-have mindsets for removing complexity from the development proc...
SYS-CON Events announced today TechTarget has been named “Media Sponsor” of SYS-CON's 18th International Cloud Expo, which will take place on June 7–9, 2016, at the Javits Center in New York City, NY, and the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. TechTarget is the Web’s leading destination for serious technology buyers researching and making enterprise technology decisions. Its extensive global networ...
SYS-CON Events announced today that MangoApps will exhibit at SYS-CON's 18th International Cloud Expo®, which will take place on June 7-9, 2016, at the Javits Center in New York City, NY. MangoApps provides modern company intranets and team collaboration software, allowing workers to stay connected and productive from anywhere in the world and from any device. For more information, please visit https://www.mangoapps.com/.
SYS-CON Events announced today that IBM Cloud Data Services has been named “Bronze Sponsor” of SYS-CON's 18th Cloud Expo, which will take place on June 7-9, 2016, at the Javits Center in New York City, NY. IBM Cloud Data Services offers a portfolio of integrated, best-of-breed cloud data services for developers focused on mobile computing and analytics use cases.
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 18th International Cloud Expo, which will take place on June 7–9, 2016, at the Javits Center in New York City, NY, and the 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...
The essence of data analysis involves setting up data pipelines that consist of several operations that are chained together – starting from data collection, data quality checks, data integration, data analysis and data visualization (including the setting up of interaction paths in that visualization). In our opinion, the challenges stem from the technology diversity at each stage of the data pipeline as well as the lack of process around the analysis.
SYS-CON Events announced today that Alert Logic, Inc., the leading provider of Security-as-a-Service solutions for the cloud, will exhibit at SYS-CON's 18th International Cloud Expo®, which will take place on June 7-9, 2016, at the Javits Center in New York City, NY. Alert Logic, Inc., provides Security-as-a-Service for on-premises, cloud, and hybrid infrastructures, delivering deep security insight and continuous protection for customers at a lower cost than traditional security solutions. Ful...
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...
In his session at 18th Cloud Expo, Bruce Swann, Senior Product Marketing Manager at Adobe, will discuss how the Adobe Marketing Cloud can help marketers embrace opportunities for personalized, relevant and real-time customer engagement across offline (direct mail, point of sale, call center) and digital (email, website, SMS, mobile apps, social networks, connected objects). Bruce Swann has more than 15 years of experience working with digital marketing disciplines like web analytics, social med...
Designing IoT applications is complex, but deploying them in a scalable fashion is even more complex. A scalable, API first IaaS cloud is a good start, but in order to understand the various components specific to deploying IoT applications, one needs to understand the architecture of these applications and figure out how to scale these components independently. In his session at @ThingsExpo, Nara Rajagopalan is CEO of Accelerite, will discuss the fundamental architecture of IoT applications, ...
SYS-CON Events announced today that Tintri Inc., a leading producer of VM-aware storage (VAS) for virtualization and cloud environments, will exhibit at the 18th International CloudExpo®, which will take place on June 7-9, 2016, at the Javits Center in New York City, New York, and the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA.
SYS-CON Events announced today that ContentMX, the marketing technology and services company with a singular mission to increase engagement and drive more conversations for enterprise, channel and SMB technology marketers, has been named “Sponsor & Exhibitor Lounge Sponsor” of SYS-CON's 18th Cloud Expo, which will take place on June 7-9, 2016, at the Javits Center in New York City, New York. “CloudExpo is a great opportunity to start a conversation with new prospects, but what happens after the...