Connection Factory: Should it be made thread-safe?
Hi,
I have a web app - running on Tomcat 6, and Struts 1.3.
It uses DAO for communication between the business logic classes and the Database. The database connections are retrieved from a connection pool managed by Tomcat. There is a ConnectionFactory object - a singleton - that each DAO requests a connection from. The question is, should the method ConnectionFactory.getConnection() be synchronized? If it is no, is it possible for two DAO's to get the same DB Connection from it?
Thanks.
there is no need to make it thread safe bacause it is alredy managed by uising cnnection pooling.yes according to your requirement you can configure pool size in tomcat and all the server like glassfish.............
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Hello,
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has anybody ever tried to use a native library from
JNI, when the library (Windows DLL) is not thread safe?
Now we want many Java clients.
That would mean each client makes its calls
to the library in its own thread. Because the library
is not thread safe, this would cause problems.Right. And therefore you have to encapsulate the DLL behind a properly synchronized interface class.
Now the details of how you have to do that depends: (a) does the DLL contain state information other than TLS? (b) do you know which methods are not thread-safe?
Depending on (a), (b) two extremes are both possible:
One extreme would be to get an instance of the interface to the DLL from a factory method you'll have to write, where the factory method will block until it can give you "the DLL". Every client thread would obtain "the DLL", then use it, then release it. That would make the whole thing a "client-driven" "dedicated" server. If a client forgets to release the DLL, everybody else is going to be locked out. :-(
The other extreme would be just to mirror the DLL methods, and mark the relevant ones as synchronized. That should be doable if (a) is false, and (b) is true.
(2)
Now we discussed to load the library several times -
separately for each client (for each thread).
Is this possible at all? How can we do that?
And do you think we can solve the problem in this
way?The DLL is going to be mapped into the process address space on first usage. More Java threads just means adding more references to the same DLL instance.
That would not result in thread-safe behavior. -
Thread safe RMI.
My RMI server provides clients with the ability to CRUD data but in order to manage concurrency I did the following.
1stly I would like to understand correctly the issues of RMI server objects....
It is my understanding that RMI server can cater for eg 100 clients by spawning 100 threads at random for each client. This process is not managed (thread wise) and the result is that 20 clients wishing to update record A can do so at will? Various steps can be taken from what I gather...
a) Synchronise (expensive)
b) implement a lock manager.
The step I have taken is include a lock manager and I have ensured that all locking an unlocking occur in a singleton server object that manages all sensitive data CRUD operations. Now I use a lock manager but I would like to know what happens if for eg the 1st RMI client dies and has its thread blocking
all other threads from locking the same record? Does the thread die with the client or is there a counter measure for this? The obvious answer that comes to mind is Object.wait() inside the lock manager?
The reason I ask was that because all of the locking occurs in a single method call in the Network
Server's JVM, so is there a need to worry about the RMI connection dying during the middle the locking operation.
Edited by: Yucca on May 23, 2009 8:14 PM/*
* @(#)LockManager.java
* Version 1.0.0
* 27/03/2009
import java.util.HashMap;
import java.util.Map;
import java.util.logging.Logger;
import java.util.ResourceBundle;
class LockManager {
* The <code>Map</code> containing all the record number keys of currently
* locked records that pair with the cookie value assigned to them when they
* are initially locked.
private static Map<Integer, Long> currentlyLockedMap =
new HashMap<Integer, Long>();
private static final Logger LOG = Logger.getLogger("project.db");
* Locks a record so that it can only be updated or deleted by this client.
* Returned value is a <code>long</code> that is the cookie value that must
* be used when the record is unlocked, updated, or deleted.
* If the specified record is already locked by a different client, then the
* current thread gives up the CPU and consumes no CPU cycles until the
* record is unlocked.
* @param recNo the assigned primary key of the record
* to be locked for an operation.
* @param data a <code>Data</code> instance used to
* check if the record exists before
* attempting the lock operation.
* @return A <code>long</code> containing the
* cookie value that must be used when the
* record is unlocked.
* @throws RecordNotFoundException if specified record does not exist or if
* specified record is marked as deleted
* in the database file.
long lock(int recNo, DB data) throws RecordNotFoundException {
LOG.entering(this.getClass().getName(), "lock", recNo);
synchronized (currentlyLockedMap) {
try {
while (currentlyLockedMap.containsKey(recNo)
&& currentlyLockedMap.get(recNo)
!= Thread.currentThread().getId()) {
currentlyLockedMap.wait();
// Check if record exists.
data.read(recNo);
long cookie = Thread.currentThread().getId();
currentlyLockedMap.put(recNo, cookie);
LOG.fine("Thread " + Thread.currentThread().getName()
+ "got Lock for " + recNo);
LOG.fine("Locked record count = " + currentlyLockedMap.size());
LOG.exiting(this.getClass().getName(), "lock", true);
return cookie;
} catch (InterruptedException ie) {
throw new SystemException("Unable to lock", ie);
* Releases the lock on a record. The cookie must be the cookie returned
* when the record was locked.
* @param recNo the assigned primary key of the record
* to be unlocked after an operation.
* @param cookie the cookie returned when the record was
* locked.
* @throws RecordNotFoundException if specified record does not exist or if
* specified record is marked as deleted
* in the database file.
* @throws SecurityException if the record is locked with a cookie
* other than cookie.
void unlock(int recNo, long cookie) throws RecordNotFoundException,
SecurityException {
LOG.entering(this.getClass().getName(), "unlock",
new Object[] { recNo, cookie });
synchronized (currentlyLockedMap) {
checkLock(recNo, cookie);
currentlyLockedMap.remove(recNo);
LOG.fine("released lock for " + recNo);
currentlyLockedMap.notifyAll();
LOG.exiting(this.getClass().getName(), "unlock");
* Checks if the given record is locked before doing any modification or
* unlocking for the record.
* <p/>
* Checks the <code>Map</code> of record number keys and cookie values
* to see if it contains the given record number.
* @param recNo the assigned primary key of the record
* to be checked if it is locked.
* @param lockCookie the cookie returned when the record was
* initially locked.
* @throws SecurityException if no lock exists for the record or if
* the record has been locked with a
* different cookie.
void checkLock(int recNo, long lockCookie) throws SecurityException {
LOG.entering(this.getClass().getName(), "checkLock",
new Object[] { recNo, lockCookie });
// Check if record has been locked
if (!currentlyLockedMap.containsKey(recNo)) {
throw new SecurityException(ResourceBundle.getBundle(
"resources.ErrorMessageBundle").getString(
"lockNotAppliedKey"));
// Check if record has been locked by different cookie.
if (currentlyLockedMap.get(recNo) != lockCookie) {
throw new SecurityException(ResourceBundle.getBundle(
"resources.ErrorMessageBundle").getString(
"notLockOwnerKey"));
LOG.exiting(this.getClass().getName(), "checkLock");
}Edited by: Yucca on May 23, 2009 8:16 PM
Edited by: Yucca on May 23, 2009 8:18 PMIt is my understanding that RMI server can cater for eg 100 clients by spawning 100 threads at random for each client.No. It spawns a new thread for every new connection. At the client end, RMI makes a new connection for every call unless it can find an idle connection to the same host that is less than 15 seconds old. So if the client is doing concurrent calls there will be concurrent threads at the server for that client.
This process is not managed (thread wise)'Managed' meaning what?
the result is that 20 clients wishing to update record A can do so at will?The result is that an RMI server is not thread safe unless you make it so.
a) Synchronise (expensive)Compared to a database update the cost of synchronization is trivial.
b) implement a lock manager. The database should already have one of those, and so does java.util.concurrent. Don't write your own. Personally I would just syncrhonize around the database calls.
what happens if for eg the 1st RMI client dies and has its thread(a) the client doesn't have a thread, see above. The call has a thread.
(b) at the server, the call will execute, regardless of the state of the client, until it is time to write the result back to the client, at which point the write will encounter an IOException of some description and the thread will exit.
blocking all other threads from locking the same record?That can't happen unless you foul up your concurrency management.
Does the thread die with the clientIt dies with the call.
is there a need to worry about the RMI connection dying during the middle the locking operation.The server JVM won't notice until it is time to write the call result back, see above. -
Coherence SimpleParser class is not thread safe?
Coherense has very convinent XML utility class, which we use it a lot within our Coherence related applications.
But we encounter some mysterious lock up (maybe deadlock?) issue and identified that it might be that the com.tangosol.run.xml.SimpleParser class is not thread safe.
We are using tomcat 6 and spring 2.0.6.
One of the webapp has 2 bean which implements InitializingBean interface.
Bean A's afterPropertiesSet() method will use com.tangosol.run.xml.XmlHelper.loadXml method to parse a XML file.
Bean B's afterPropertiesSet() method will acts as a tcp extend client and retrieve some data from a coherence cluster. Which I believe coherence will also use it's XML utility class when parsing the configuration files.
We encounter tomcat lockup (which never finish startup webapp deployment porcess) randomly like 1 out of 2 or 3 tries.
Use jconsole and connect to tomcat we can see that the main thread stuck in SimpleParser class. Here is the thread dump.
Name: main
State: RUNNABLE
Total blocked: 156 Total waited: 0
Stack trace:
com.tangosol.run.xml.SimpleParser.instantiateDocument(SimpleParser.java:150)
com.tangosol.run.xml.SimpleParser.parseXml(SimpleParser.java:115)
- locked com.tangosol.run.xml.SimpleParser@f10c77
com.tangosol.run.xml.SimpleParser.parseXml(SimpleParser.java:71)
com.tangosol.run.xml.SimpleParser.parseXml(SimpleParser.java:84)
com.tangosol.run.xml.XmlHelper.loadXml(XmlHelper.java:109)
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1201)
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1171)
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:425)
org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:251)
org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:156)
- locked java.util.concurrent.ConcurrentHashMap@dee55c
org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:248)
org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:160)
org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:287)
org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:352)
- locked java.lang.Object@d21555
org.springframework.web.context.ContextLoader.createWebApplicationContext(ContextLoader.java:244)
org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:187)
org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:49)
org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:3830)
org.apache.catalina.core.StandardContext.start(StandardContext.java:4337)
- locked org.apache.catalina.core.StandardContext@1c64ed8
org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:791)
- locked java.util.HashMap@76a6d9
org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:771)
org.apache.catalina.core.StandardHost.addChild(StandardHost.java:525)
org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:825)
org.apache.catalina.startup.HostConfig.deployWARs(HostConfig.java:714)
org.apache.catalina.startup.HostConfig.deployApps(HostConfig.java:490)
org.apache.catalina.startup.HostConfig.start(HostConfig.java:1138)
org.apache.catalina.startup.HostConfig.lifecycleEvent(HostConfig.java:311)
org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:117)
org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1053)
- locked org.apache.catalina.core.StandardHost@1c42c4b
org.apache.catalina.core.StandardHost.start(StandardHost.java:719)
- locked org.apache.catalina.core.StandardHost@1c42c4b
org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1045)
- locked org.apache.catalina.core.StandardEngine@37fd24
org.apache.catalina.core.StandardEngine.start(StandardEngine.java:443)
org.apache.catalina.core.StandardService.start(StandardService.java:516)
- locked org.apache.catalina.core.StandardEngine@37fd24
org.apache.catalina.core.StandardServer.start(StandardServer.java:710)
- locked [Lorg.apache.catalina.Service;@1cc55fb
org.apache.catalina.startup.Catalina.start(Catalina.java:566)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
java.lang.reflect.Method.invoke(Unknown Source)
org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:288)
org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:413)
After we add the depends-on tag to enforce bean B wait on bean A to finish initialization, we no longer encounter the lockup during tomcat startup.
We suspect that maybe SimpleParser class is not thread safe and will cause potential deadlock issue.
Edited by: user639604 on Jun 22, 2009 10:36 AMWhile it doesn't show up as deadlock, I believe it probably is, as evidenced by these two threads:
"Timer-0" prio=10 tid=0xcb9a2800 nid=0x454b in Object.wait() [0xcb6e0000..0xcb6e10a0]
java.lang.Thread.State: RUNNABLE
at com.tangosol.run.xml.SimpleParser.instantiateDocument(SimpleParser.java:150)
at com.tangosol.run.xml.SimpleParser.parseXml(SimpleParser.java:115)
- locked <0xf44e52f0> (a com.tangosol.run.xml.SimpleParser)
at com.tangosol.run.xml.SimpleParser.parseXml(SimpleParser.java:71)
at com.tangosol.run.xml.SimpleParser.parseXml(SimpleParser.java:99)
at com.tangosol.run.xml.XmlHelper.loadXml(XmlHelper.java:129)
at com.tangosol.run.xml.XmlHelper.loadXml(XmlHelper.java:95)
at com.tangosol.run.xml.XmlHelper.loadXml(XmlHelper.java:72)
at com.tangosol.util.ExternalizableHelper.<clinit>(ExternalizableHelper.java:4466)
at com.evidentsoft.opcache.coherence.OPCacheCoherenceStorage.retrieve(OPCacheCoherenceStorage.java:341)
at com.evidentsoft.opcache.coherence.OPCacheCoherenceStorage.retrieve(OPCacheCoherenceStorage.java:420)
at com.evidentsoft.opcache.OPCacheManager.find(OPCacheManager.java:68)
at com.evidentsoft.logserver.coherence.ClusterDetector.detectNewClusters(ClusterDetector.java:97)
at com.evidentsoft.logserver.coherence.ClusterDetector.access$000(ClusterDetector.java:19)
at com.evidentsoft.logserver.coherence.ClusterDetector$1.run(ClusterDetector.java:67)
at java.util.TimerThread.mainLoop(Unknown Source)
at java.util.TimerThread.run(Unknown Source)
"main" prio=10 tid=0x08059000 nid=0x4539 in Object.wait() [0xf7fd0000..0xf7fd11f8]
java.lang.Thread.State: RUNNABLE
at com.tangosol.run.xml.SimpleParser.instantiateDocument(SimpleParser.java:150)
at com.tangosol.run.xml.SimpleParser.parseXml(SimpleParser.java:115)
- locked <0xf44ecd90> (a com.tangosol.run.xml.SimpleParser)
at com.tangosol.run.xml.SimpleParser.parseXml(SimpleParser.java:71)
at com.tangosol.run.xml.SimpleParser.parseXml(SimpleParser.java:84)
at com.tangosol.run.xml.XmlHelper.loadXml(XmlHelper.java:109)
at com.evidentsoft.coherence.util.ClusterConfigurator.generateConfigFile(ClusterConfigurator.java:319)
at com.evidentsoft.coherence.util.ClusterConfiguratorProxy.afterPropertiesSet(ClusterConfiguratorProxy.java:51)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1201)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1171)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:425)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:251)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:156)
- locked <0xd65efb88> (a java.util.concurrent.ConcurrentHashMap)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:248)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:160)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:287)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:352)
- locked <0xd65efc28> (a java.lang.Object)
at org.springframework.web.context.ContextLoader.createWebApplicationContext(ContextLoader.java:244)
at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:187)
at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:49)
at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:3830)
at org.apache.catalina.core.StandardContext.start(StandardContext.java:4337)
- locked <0xd6092f60> (a org.apache.catalina.core.StandardContext)
at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:791)
- locked <0xd54ff278> (a java.util.HashMap)
at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:771)
at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:525)
at org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:825)
at org.apache.catalina.startup.HostConfig.deployWARs(HostConfig.java:714)
at org.apache.catalina.startup.HostConfig.deployApps(HostConfig.java:490)
at org.apache.catalina.startup.HostConfig.start(HostConfig.java:1138)
at org.apache.catalina.startup.HostConfig.lifecycleEvent(HostConfig.java:311)
at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:117)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1053)
- locked <0xd54ff1e8> (a org.apache.catalina.core.StandardHost)
at org.apache.catalina.core.StandardHost.start(StandardHost.java:719)
- locked <0xd54ff1e8> (a org.apache.catalina.core.StandardHost)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1045)
- locked <0xd4fa60b8> (a org.apache.catalina.core.StandardEngine)
at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:443)
at org.apache.catalina.core.StandardService.start(StandardService.java:516)
- locked <0xd4fa60b8> (a org.apache.catalina.core.StandardEngine)
at org.apache.catalina.core.StandardServer.start(StandardServer.java:710)
- locked <0xd4f17ea0> (a [Lorg.apache.catalina.Service;)
at org.apache.catalina.startup.Catalina.start(Catalina.java:566)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:288)
at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:413)The reason it isn't showing up as a deadlock in the thread dump is that the ExternalizableHelper static initializer isn't completing, so the other thread (blocking it) is waiting indefinitely on that class to become available.
Peace,
Cameron Purdy | Oracle Coherence -
ODBC SQLGetData not thread-safe when retrieving lob
Hi MaxDB developpers,
we are in the process of migrating our solution from SapDb 7.4.03.32 to MaxDb 7.6.03.7. We use the ODBC driver on windows, from multi-threaded applications.
We encountered bugs in the ODBC driver 7.4.03.32 and made our own fixes since we had the sources. I checked if these problems were fixed in 7.6.03.7 and they are allmost addressed, but one:
when two threads use two different stmt from the same dbc and call simultaneously SQLGetData to retrieve a LONG column, a global variable not protected by a critical section is changed and the application crashes. The variable in cause is dbc_block_ptr->esqblk.sqlca.sqlrap->rasqlldp which is set by pa30bpcruntime and reset by pa30apcruntime during the call to apegetl. Calls to apegetl are protected by PA09ENTERASYNCFUNCTION except in SQLGetData, when it calls pa60MoveLongPos or pa60MoveLong.
Since MaxDB is a critical feature of our application, we would like to know when this bug can be fixed by SAP. Or maybe could we get access to the sources of sqlod32w.dll 7.6.03.7 to fix it ourselves ?
Thanks,
GuillaumeHello Guillaume
Regarding the threaded access to SQLGetData. Of course, it is possible to manage the syncronization as you proposed. However, I'm still not sure, whether this really solves general problems.
The point is, that the MaxDB-ODBC driver is thread safe for concurrent connections, but not for concurrent statement processing within a single connection. Therefore I would like to ask you how your application accesses data via SQLGetData (due to connections and threads), and what amount of data is usually transfered.
Regards Thomas -
Is abap thread safe? Some question in Singleton pattern in ABAP
Hi Grus,
I have a very basic question but really make me headache...
Recently I am learning the design pattern in ABAP and as you know in JAVA there is a key word "Synchronized" to keep thread safe. But in ABAP, I didn't find any key words like that. So does that mean ABAP is always a single thread language? And I found that there is a way looks like "CALL FUNCTION Remotefunction STARTING NEW TASK Taskname DESTINATION dest" to make multi-thread works. As you can see it use the destination, so does that mean actually the function module is always executed in a remote system, and in every system, it is always single thread?
Could you help me on the question? Thanks a lot, grus
And here comes up to my mind another question...It's a little bit mad but I think it may works....What if I set every attribute and method as static in the singleton class...Since as you can see it is already a singleton so every attribute in it should be only one piece. So then I don't even need to implement a get_instance( ) method to return the instance. Just call "class_name=>some_method( )" directly then singleton is achieved...What do you think?
BR,
SteveSteve,
I've the same question, few days ago I tried to use the singleton in ABAP. In Java programming is possible to use the same reference in two sessions or programs, sharing attributes, methods and all data, but I could not do in ABAP.
In my test I created a program with one global class using the singleton pattern, so I expected that when I run my program and see the reference returned after the get_instance method it should be equal to the same program run in another session, but the ABAP will create a new reference and instantiate again.
So I deduced that the only way to share it between sessions in ABAP is using the ABAP Shared Memory Objects.
I can be wrong, but I think that ABAP use a thread by user session (Each window) and we can't change it.
Best regards. -
hello all,
please help me by answering these questions?
singleton patterns calls for creation of a single class that can be shared between classes. since one class has been created
Can singletons be used in a multithreaded environment? is singleton thread safe?
Are threading problems a consequence of the pattern or programming language?
thank you very much,
HiruHi,
Before explaining whether a singleton is thread safe
e i want to explaing what is thread safe here
When multiple threads execute a single instance of a
program and therefore share memory, multiple threads
could possibly be attempting to read and write to the
same place in memory.If we have a multithreaded
program, we will have multiple threads processing the
same instance.What happens when Thread-A examines
instance variable x? Notice how Thread-B has just
incremented instance variable x. The problem here is
Thread-A has written to the instance variable x and
is not expecting that value to change unless Thread-A
explicitly does so. Unfortunately Thread-B is
thinking the same thing regarding itself; the only
problem is they share the same variable.
First method to make a program threadsafe-: Avoidance
To ensure we have our own unique variable instance
for each thread, we simply move the declaration of
the variable from within the class to within the
method using it. We have now changed our variable
from an instance variable to a local variable. The
difference is that, for each call to the method, a
new variable is created; therefore, each thread has
its own variable. Before, when the variable was an
instance variable, the variable was shared for all
threads processing that class instance. The following
thread-safe code has a subtle, yet important,
difference.
second defense:Partial synchronization
Thread synchronization is an important technique to
know, but not one you want to throw at a solution
unless required. Anytime you synchronize blocks of
code, you introduce bottlenecks into your system.
When you synchronize a code block, you tell the JVM
that only one thread may be within this synchronized
block of code at a given moment. If we run a
multithreaded application and a thread runs into a
synchronized code block being executed by another
thread, the second thread must wait until the first
thread exits that block.
It is important to accurately identify which code
block truly needs to be synchronized and to
synchronize as little as possible. In our example, we
assume that making our instance variable a local
variable is not an option.
Third Defence:--Whole synchronization
Here u should implement an interface which make the
whole class a thread safe on or synchronized
Thre views are made by Phillip Bridgham, M.S., is a
technologist for Comtech Integrated Systems.I'm
inspired by this and posting the same hereWas there a point in all of that? The posted Singleton is thread-safe. Furthermore, some of that was misleading. A local variable is only duplicated if the method is synchronized, a premise I did not see established. Also, it is misleading to say that only one Thread can be in a synchronized block of code at any time because the same block may be using different monitors at runtime, in which case two threads could be in the same block of code at the same time.
private Object lock;
public void doSomething() {
lock = new Object();
synchronized(lock) {
// Do something.
}It is not guaranteed that only one Thread can enter that synchronized block because every Thread that calls doSomething() will end up synchronizing on another monitor.
This is a trivial example and obviously something no competent developer would do, but it illustrates that the statement assumes premises that I have not seen established. It would be more accurate to say that only one Thread can enter a synchronized block so long as it uses the same monitor.
It's also not noted in the discussion of local variables vs instance variables that what he says only applies to primitives. When it comes to actual Objects, just because the variable holding the reference is unique to each Thread does not make use of it thread-safe if it points to an Object to which references are held elsewhere. -
<snip>
This (as you probably know) is due to the fact that the code provided by
most of the DB vendors is not thread safe.
<snip>
Sean's comment, above, speaks to an issue that is causing some concern
within my (large - 180 projects in development) organisation.
May I please ask the forum if there are others out there who have an
understanding of / concern with this "problem"?
My perception (quite possibly flawed) of the "problem" is that the RDBMS
cannot multi-thread data access objects. So we find ourselves in a situation
where we can achieve scaleability in just about all other
performance-sensitive areas of a system's technical architecture (we're
using DCE -- but have found that Encina is not advisable except where there
is a true requirement for heterogeneous distributed 2 phase commit, which we
don't often see.....) but when we go to hit on the RDBMS, we go back to good
ole single-threading.
In certain circumstances, this shortfall of RDBMS technology -- I won't
mention any names, of course, but the initials are "Oracle" -- seems to be
hindering our achievement of a desired technical architecture.
Is this a "Pro*C / PL/SQL stored procedures" problem or is it something that
is in the RDBMSs' DNA?
How can we get around it?
Any comments?
Regards
JonJon
I agree. But it is the best solution within the constraints of existing
technology. At least we don't use a process per client. 10 replicated
copies of a service could service the needs of 100 clients.
Eric
>
At 13:15 6/09/96 EST, you wrote:
Eric
Thanks for your response. Yep. I realise that the issue I've presented is
clearly not something that Forte causes or is responsible for in any way.
Forte can, as you've pointed out, actually help in this area. But I don't
think that getting Forte to spawn another instance of a data access server
is really the best solution. Ie, that's not what we tend to have in mind
when we think about "scaleability". The best solution is -- perhaps -- to
get the RDBMS people to thread-safe all code and libraries. I have pointedly
asked Oracle for a position on this -- and got the usual blank stare.
I also considered whether people might get upset at me for posting what is
clearly a non-Forte-specifc question in a Forte forum. But then I went ahead
and did it anyway. Justification being (assumption follows) that the kind of
people who hang out on the Forte forum may tend to be more
architecture-oriented than your run of the mill VB / SQL*Net / PL/SQL stored
procedures kinda guy/gal, and may be using or considering Forte (plug for
Forte follows) precisely because it clearly enables a superior architecture.
Should proably post to the comp.database.oracle forum, but I just don't know
them as well.
Regards
Jon
From: Eric Gold
To: McLeod, Jon
Cc: [email protected]
Subject: Re: RDBMS non-thread-safe issue
Date: Friday, 6 September 1996 11:21AM
Jon
In response to this message.....read below...
Sean's comment, above, speaks to an issue that is causing some concern
within my (large - 180 projects in development) organisation.
May I please ask the forum if there are others out there who have an
understanding of / concern with this "problem"?
My perception (quite possibly flawed) of the "problem" is that the RDBMS
cannot multi-thread data access objects. So we find ourselves in a situation
where we can achieve scaleability in just about all other
performance-sensitive areas of a system's technical architecture (we're
using DCE -- but have found that Encina is not advisable except where there
is a true requirement for heterogeneous distributed 2 phase commit, which we
don't often see.....) but when we go to hit on the RDBMS, we go back to good
ole single-threading.
In certain circumstances, this shortfall of RDBMS technology -- I won't
mention any names, of course, but the initials are "Oracle" -- seems to be
hindering our achievement of a desired technical architecture.
Is this a "Pro*C / PL/SQL stored procedures" problem or is it something that
is in the RDBMSs' DNA?
How can we get around it?
Any comments?Jon,
Go ahead ask the Forum any questions you want. This "problem"
is not a problem in Forte. What we allow you to do is "replicate" your
data access services so that each one runs inside its own
process. Each one of these processes (aka partitions) has its
own connection to the database. The routing to the replicated
partitions is transparent to the clients. Clients send a
message like "DatabaseService.GetCustomer()" and then the Forte
router sees which replicated copy of the service is not currently
processing a request and routes it to that free replicate. You
can dynamically increase or decrease the number of replicated
copies of the service easily.
We call this feature "load balancing" in Forte. It is achieved
by checking a box in the data access service object definition.
You can dynamically increase/decrease the number of replicates
and also dynamically move replicates to other nodes in the environment.
This approach assumes that you are using application driven
security and not database security. Each replicated copy
of the service is using the same generic username/password
to connect to the database.
I am forwarding this answer to forte-users because others
might not completely understand this feature.
Eric
Eric Gold
Technical Director
Forte Australia
Voice: 61-2-9926-1403
Fax: 61-2-9926-1401
Eric Gold
Technical Director
Forte Australia
Voice: 61-2-9926-1403
Fax: 61-2-9926-1401 -
PopUpManager + Concurrency = NOT Thread-Safe
Is there any way to make the PopUpManager not throw evil
messages like this:
RangeError: Error #2006: The supplied index is out of bounds.
at flash.display::DisplayObjectContainer/addChildAt()
at mx.managers::SystemManager/
http://www.adobe.com/2006/flex/mx/internal::rawChildren_addChildAt()
at mx.managers::SystemManager/addChild()
at mx.managers::PopUpManagerImpl/addPopUp()
at mx.managers::PopUpManager$/addPopUp()
at mx.controls::Menu/show()
at jseamless/::openRightClick()
at Function/
http://adobe.com/AS3/2006/builtin::apply()
at ()
at
flash.external::ExternalInterface$/flash.external:ExternalInterface::_callIn()
When a call is made concurrently. I think I can resolve the
issue if I can replace the PopUpManager's implementation. Can
someone tell me how to do that? I'd like to write a thread-safe
version of the PopUpManager to handle concurrent calls to
it....right now it just dies showing the error above.This should be addressed in the 9.2.0.1 Oracle JDBC release (pehaps also in earlier releases).
Don't know why the original bug is still open (it likely got re-filed and fixed).
-- Ekkehard -
Dear Tuxedo experts,
Is a WTC "Tuxedo Connection Factory" thread-safe? I don't find it in the dev documentation?
+// Get context+
javax.naming.Context myContext = new InitialContext();
+// Begin Transaction+
TuxedoConnectionFactory tuxConFactory = (TuxedoConnectionFactory)
ctxt.lookup("tuxedo.services.TuxedoConnection");
Is it safe to call tuxConFactory.getTuxedoConnection() from a multi-threaded environment?
RegardsHi,
Yes, all of WTC is thread safe. In fact, I'm not sure how it couldn't be.
Regards,
Todd Little
Oracle Tuxedo Chief Architect -
I have heard a lot of folks talking about a class that is "Thread Safe".
I am assuming that this means the obect can be accessed by more than one thread at a time. I do not know what the concept really means in practice.
My question is, how can I tell if something is thread safe, what do I need to look out for?Basically you're looking to avoid unwanted behavior by simultaneous access. Here are a couple examples I've seen:
Assuming myID is a class variable...
public void assignID(Client currentClient) {
myID = (some new random value)
currentClient.ID = myID;
In the above example, what happens if two threads access the method at the same time? The first one starts, assigning a value to the myID variable. Then the second one starts, assigning a value to myID AGAIN. Then when both clients assign their currentClients, they'll both end up with the same ID. To avoid a code block being accessed by two clients simultaneously, use the "synchronized" keyword.
Here's another one. Assume you have a Hashtable:
Enumerator myEnum = myHashtable.keys()
while (myEnum.hasMoreElements()) {
Object myKey = myEnum.nextElement();
myHashtable.remove(myKey);
This should not work in Java (and I know it doesn't in C# which I'm currrently being forced to use at work) because you're iterating through the keys of a Hashtable. But while you're trying to go linearly from key to key, you're also removing elements of the Hashtable. You're trying to operate on the object twice and the object doesn't allow it.
There are a couple ideas. I wrote a database pool once and had to synchronize methods so two clients didn't try to execute a query through the same Connection, etc.
Michael Bishop -
Problem with transacted JMS connection factory and transaction timeouts
We encountered an interesting problem using transacted JMS connection factories.
An EJB starts a container managed transaction and tries to validate a credit card
before creating some information to a database for the user, in case of success
an SMS is sent to the user via the transacted JMS queue. If the credit card authentications
duration is about the same as the transactions timeout (in this case the default
30 seconds) sometimes the database inserts is committed but the JMS insert is
rollbacked. How can this be?
If the authorization duration is much longer than 30 seconds everything works
fine (both database and JMS inserts rollbacked), the same is true if a rollback
is insured by calling EJBContext.setRollbackOnly(). The problem thus occurs only
if the duration is approximately the same as the transaction timeout, it appears
that the database insert is not timeouted but the JMS insert is. How can this
be if they are both participating in the same transaction.
The JMSConnectionFactory used is a Connection factory with XA-enabled. The result
is the same also with the default "javax.jms.QueueConnectionFactory" and if we
configure our own factory with user transactions enabled.
Any help appreciated!
Tomas Granö wrote:
> We encountered an interesting problem using transacted JMS connection factories.
> An EJB starts a container managed transaction and tries to validate a credit card
> before creating some information to a database for the user, in case of success
> an SMS is sent to the user via the transacted JMS queue. If the credit card authentications
> duration is about the same as the transactions timeout (in this case the default
> 30 seconds) sometimes the database inserts is committed but the JMS insert is
> rollbacked. How can this be?
It should not be.
>
> If the authorization duration is much longer than 30 seconds everything works
> fine (both database and JMS inserts rollbacked), the same is true if a rollback
> is insured by calling EJBContext.setRollbackOnly(). The problem thus occurs only
> if the duration is approximately the same as the transaction timeout, it appears
> that the database insert is not timeouted but the JMS insert is. How can this
> be if they are both participating in the same transaction.
>
> The JMSConnectionFactory used is a Connection factory with XA-enabled. The result
> is the same also with the default "javax.jms.QueueConnectionFactory" and if we
> configure our own factory with user transactions enabled.
>
> Any help appreciated!
Make sure that your session is not "transacted". In other words,
the first parameter to createSession() must be false. There is an
unfortunate name re-use here. If a session is "transacted", it
maintains an independent "inner transaction" independent of the
outer transaction. From the above description, it seems unlikely
that your application has this wrong, as you say that
"setRollbackOnly" works - but please check anyway.
Make sure that you are using a true XA capable driver and database
(XA "emulation" may not suffice)
Beyond the above, I do not see what can be going wrong. You
may want to try posting to the transactions and jdbc newsgroups. Note
that JMS is appears to be exhibiting the correct behavior, but the
JDBC operation is not. The JDBC operation appears to have
its timeout independent of the transaction monitor's timeout.
Tom
-
JMS proxy and XA connection factory
Hi all,
I would like to ask you what is the best practice to adopt about the scenario described below.
A JMS proxy retrieves a message and processes it.
Any error could occurr during processing it and, in case of errors, the JMS proxy error handler publishes the message on ad hoc recovery destination D.
(later another app will check failed messages for fixing & republishing them into ALSB).
In that scenario we have the JMS proxy and the jms business service that, in case of errors, publishes on destination D.
Should the JMS proxy and the business service use both a XA connection factory in order to perform all the above steps in one transaction ?
Otherwise if the business service itself fails publishing on destination D, the message retrieved by the JMS proxy is lost and not re-delivered to the JMS proxy.
Or XA connection factory is not needed and could I use
Routing Options with exactly once or both are needed ?
Thanks
ferpHi all,
I did some tests and follow what I've achieved.
Scenario 1.
- Precondition
-- A JMS proxy with XA factory + Error Destination (MyRecoveryQueue)
-- A business service BS with XA factory that publishes into MyOutboundQueue
-- No "exactly once" routing option used calling BS
-- an error is forced in the pipeline
- Flow
-- proxy retrieves a message and try to publish it using BS
-- an error is forced in the pipeline:
--- transaction is rolled back, message redelivered to proxy
--- the message is posted to the error destination after all the retries failed
Scenario 2.
- Precondition
-- As 1. but no error is forced in the pipeline
-- the MyOutboundQueue destination queue is paused
- Flow
-- proxy retrieves a message and try to publish it using BS
-- BS fails to publish it on MyOutboundQueue (because it is paused)
--- transaction is rolled back, message redelivered to proxy
--- the message is posted to the error destination after all the retries failed
Scenario 2A.
- Precondition
-- As 2. with MyOutboundQueue paused and resumed
- Flow:
-- proxy retrieves a message and try to publish it using BS
-- BS fails to publish it on MyOutboundQueue (because it is paused)
--- transaction is rolled back, message redelivered to proxy
--- before all the retries failed the queue is resumed
--- BS succeeds to publish the message
Scenario 3.
- Precondition
-- A JMS proxy with NO XA factory + Error Destination
-- A business service BS with NO XA factory
-- No "exactly once" routing option used calling BS
-- an error is forced in the pipeline
- Flow
-- proxy retrieves a message and try to publish it using BS
-- an error is forced in the pipeline:
--- transaction is NOT rolled back, message NOT redelivered to proxy
-- so
--- no message delivered to MyOutboundQueue2 destination
--- no message delivered to MyRecoveryQueue2 destination
--- message consumed from MyQueue2 and now is lost!
To publish message into MyRecoveryQueue2 a proxy error handler has to be added and here added explicit publish to MyRecoveryQueue2.
But naturally in that case if in the proxy error handler, the explicit publish fails or any error occurrs no message is delivered to recovery queue.
So if I'm not using an XAFactory the message is auto-acknowledged as soon as it is read and I've to use XA factory if I want the message to be put back in the queue in case of errors and the retry to happen.
So both my proxy service and business service use an XA factory.
Regards
ferp
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