Click here to close now.

Welcome!

Java Authors: Pat Romanski, ScriptRock Blog, Liz McMillan, Carmen Gonzalez, Elizabeth White

Related Topics: Java, Websphere, Weblogic

Java: 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
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 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...
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.
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...
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.
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.
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.
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.
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 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 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.
@ThingsExpo has been named the Top 5 Most Influential M2M Brand by Onalytica in the ‘Machine to Machine: Top 100 Influencers and Brands.' Onalytica analyzed the online debate on M2M by looking at over 85,000 tweets to provide the most influential individuals and brands that drive the discussion. According to Onalytica the "analysis showed a very engaged community with a lot of interactive tweets. The M2M discussion seems to be more fragmented and driven by some of the major brands present in the M2M space. This really allows some room for influential individuals to create more high value inter...
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 Akana, formerly SOA Software, has been named “Bronze 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. Akana’s comprehensive suite of API Management, API Security, Integrated SOA Governance, and Cloud Integration solutions helps businesses accelerate digital transformation by securely extending their reach across multiple channels – mobile, cloud and Internet of Things. Akana enables enterprises to share data as APIs, connect and integrate applications, drive part...