Monday, October 7, 2019

Software Transactional Memory with Quarkus

We have recently contributed a quarkus extension called quarkus-narayana-stm which simplifies the use of STM in your microservices.

It will be available in the 0.24.0 quarkus release. If you would like to experiment with it before this release then you can either take one of the nightly builds or you can build it locally by git cloning the quarkus repo and then run the build. This will add the io.quarkus:quarkus-narayana-stm:999-SNAPSHOT maven dependency to your local maven repository and you may then get started by following the guide. There is also a quickstart that provides a worked example of how to use it in your microservices. The example shows how to manage concurrent accesses to a single counter. More sophisticated usage patterns are the norm but this simple example does give a flavour of how easy it is to manage concurrency with the Narayana STM implementation.

Wednesday, September 18, 2019

Heuristic exceptions

A transaction is finished either with commit or rollback. But have you considered that the third transaction outcome is <<unspecified>>? This third type of outcome may occur when the participant does not follow the coordinator's orders and makes a decision on the contrary. Then the outcome of the whole transaction could be inconsistent. Some participants could follow the coordinator guidelines to commit while the disobedient participant rolls-back. In such a case, the coordinator cannot report back to the application that "the work was successfully finished". From the perspective of the outside observer, the consistency is damaged. The coordinator itself cannot do much more. It followed the rules of the protocol but the participants disobeyed. Such transaction can only be marked with <<unspecified>> result &mdash which is known as heuristic outcome.
The resolution of that outcome requires a third-party intervention. Saying differently somebody has to go and verify the state of data and make corrections.

XA specification

In scope of this article we talk about the two phase commit protocol and how the XA specification uses it.
XA specification takes the two-phase commit, the abstractly defined consensus protocol, and carries it down to the ground of implementation. It defines rules for communication amongst parties, prescribes state model and assesses outcomes, exceptions etc.
Where the 2PC talks about coordinator and participants the XA specification define a model where the coordinator is represented by a transaction manager (TM), the participant is modelled as the resource manager (RM) and the employer of them is an application program which talks to both of them.
The transaction manager drives a global transaction which enlists resource managers as participants responsible for part of work. For the work managed by RM is used term transaction branch. The resource manager is represented normally by a Java code that implements the JTA API while it uses some internal mechanism to call the underlying data resource. The resource manager could be represented by a JDBC driver and then underlying resource could be the PostgreSQL database running in some distinct datacenter.

JTA Specification

The Java Transaction API is a projection of XA specification to the language of Java. In general, it's a high-level API that strives to provide a comprehensible tool for handling transactional code based on the XA specification.

Several purposes of JTA

The JTA is meant to be used first by an application developer. In terms of XA specification, it would be the application program. Here we have the UserTransaction interface. It gives the chance to begin and commit the transaction. In terms of XA specification, it's a representation of the global transaction. In a newer version (from version 1.2) JTA defines the annotations like Transactional That gives the user chance to control the application scope declaratively. As you can notice neither with the UserTransaction nor with he @Transactional annotation you can't do much more than to define where the transaction begins and where it ends. How to put a participant into the transaction? That limited capability is because developer is anticipated running the application in a managed environment. It could be for example Java EE application server or a different container.

The container is a second consumer of the JTA API. JTA gives the entry point to the world of transaction management. The container uses the interface TransactionManager &mdash which provides ability of managing transaction scope but also gives access to the Transaction object itself. Transaction is used to enlist the resource (the participant) to the global transaction. As the resource for enlistment is used the XAResource interface in JTA. The XAResource is managed by the transaction manager and is operated by the resource manager. Then container may arrange Synchronizations are callbacks called at the start and the end of the 2PC processing (used e.g. by JPA).

The third perspective where JTA participates in is communication with resource managers. The API defines the class XAResource (represents a participant resource to be enlisted to the global transaction), XAException (represents an error state defined in the XA specification) and Xid. The Xid represents an identifer which is unique for each transaction branch and consists of two parts — first an unique identifier of the global transaction and second an unique identifier of the resource manager. If you want to see how the transaction manager uses these classes to communicate with the resource manager take a look at the example from documentation of SQL Server.

Note:If you study the JTA API by looking into the javadoc and strolling the package summary then you can wonder about some other classes which were not mentioned here. Part of the javax.transaction package are interfaces used exclusively by JTS (transaction services running with ORB). That's mitigated by the fact the Java SE 11 removed support of ORB and those classes were removed from JDK as well.
Plus the JTA classses are now (from Java SE 11) split over Java SE and Java EE bundles as the package javax.transaction.xa is solely part of the Java SE while javax.transaction belongs to the Jakarta EE API.

Type of failures

Now when we have talked the model let's see the failure states. First, it's needed to realize that a failure in the application does not mean failure from the protocol perspective. If there is trouble in the application code or a network is transiently not available such occurrences can lead to rollback. It's a failure from the perspective of the application but for the transaction manager the rollback is just another valid state to shift to.
Even if the whole transaction manager crashes (e.g. the underlying JVM is killed) the system still should maintain data consistency and the transaction is recovered when the transaction manager (or rather recovery manager) comes back to life.

What's the trouble for the protocol is an unexpected behaviour of the resource manager (or the backed resource). We can track basically two incident types. A heuristic outcome where the resource deliberately decides to process some action (which is different from transaction manager decision). Or a heuristic outcome caused by a bug in the code (either of the transaction manager or the resource manager).

Let's discuss some examples for these types.
The deliberate decision could be a situation where the transaction manager calls the prepare on the database. The database confirms the prepare — it promises to finish the transaction branch with commit. But unfortunately the transaction manager crashes and nobody comes to restart it for a long time. The database decides that it's pretty long time to hold resources and delaying other transactions to proceed. Thus it decides to commit the work. The processing in the database may continue from that time. But later the transaction manager is restarted and it tries to commit all other branches belonging to the global transaction (let's say e.g. a JMS broker). That resource responds with an error and the transaction manager decides to rollback the whole global transaction. Now it accesses the database with the request for the rollback. But the database already committed its branch — the heuristic outcome just occurred.

An example for the bug in the code could be the PostgreSQL database driver. The driver was returning a wrong error code in case of intermittent connection failure. The XA specification defines that in such case the XAException should be thrown and it has to carry one of the following error codes — either the XAException.XAER_RMFAIL or the XAException.XA_RETRY. But the JDBC driver was returning XAException.XAER_RMERR. Such error code means that an irrecoverable error occurred. It makes the transaction manager think there is no way of automatic recovery and it switches the state of such transaction to heuristic immediately.

Heuristic exceptions

As the last part of this article we take a look on the heuristic outcomes of the transaction. The heuristics is represented with an exception being thrown. The exception reports the reason of the failure. It does so with error code or with type of class.

There are two main types of exception classes. First type is the XAException. This one is part of the communication contract between the transaction manager and the resource manager. It should not happen for the application code to obtain this type of exception. But for sure you can observe the XAException in the container log. It shows that there happened an error during transaction processing.

The second type is represented with multiple classes named Heuristic*Exception. These are exceptions that application code works with. They are thrown from the UserTransaction methods and they are checked.

Heuristic outcome with XAResource

The XAException reports reason of failure with the use of error codes. XA specification defines the meaning. It depends on the context in which it's used. For example the code XAException.XA_RETRY could be used for reporting error from commit with meaning to retry the commit action. But on the other hand it's not permitted to be used as an error code for the one-phase commit.

Then where are those heuristic states? Let's check what could happen when the transaction manager calls the XAResource calls of prepare, commit and rollback.
If the prepare is called then there is not many chances that heuristic occurs. At that time no promise from the resource is placed and the work can be easily roll-back or the worse timed-out. The only occurrence that can bring the system to the heuristic state is if the resource manager returns undefined code for this phase. But that is cause the most probably only by a bug in the implementation. Consult the XA specification which are those.
The more interesting are the commit and rollback calls. The commit and rollback are (or could be) called after the prepare. Heuristic exception means that the resource promised to commit (he acknowledge the prepare call) but it does not wait for transaction manager to command it for the next action and it finished the transaction branch on its own. The error codes are those with prefix XA_HEUR*. The decision on its own does not mean an error for the protocol in all cases.

Let's talk about rollback now. The global transaction was successfully prepared but the transaction manager decided at the end to roll-back it. It calls the rollback to the XAResource. The error XAException.XA_HEURRB announces that the resource manager decided to roll-back the transaction branch prior it was asked for it by the transaction manager. But as the transaction manager decided to go for the roll-back too the heuristic outcome followed the decision.
The XAException.XA_HEURCOM means that all work represented by the transaction branch was committed (at time the rollback is executed on the XAResource). That's bad from the data consistency as some other transaction branches could be already rolled-back.
To explain the meaning of the XAException.XA_HEURMIX it's needed to mention that the transaction branch could consist of several "local transactions". For example, PostgreSQL JDBC driver starts a database transaction to insert data to the database. Later (still in the scope of the same global transaction) it decides to update the data. It starts another database transaction. The transaction manager is clever enough to join those two database transactions which belong to the same database resource (controlled by the same resource manager) under the one transaction branch. It's good as it could reduce the communication overhead. So the XA_HEURMIX says that part of workload involved in the transaction branch was committed and the other part was rolled-back.
The XAException.XA_HEURHAZ says that the resource manager made a decision on its own but it's not capable to say what was the result of such an independent decision.

The most interesting part is the commit call. First it uses the XA_HEUR* exceptions in the same meaning as the rollback call and all what is said in the previous paragraph pays for the commit too. But up to that there are three new error codes. They do not contain word HEUR but in result they mean it. Those are XAER_RMERR which announces that an unspecified error happened during the currently executing commit call. But instead of committing the resource manager had just rolled-back the transaction branch. That means we are in the same state as with the XA_HEURRB The XAER_NOTA says that resource manager does not know anything about this transaction branch. That means the resource manager lost the notion about it and it either commits it or rolled-back it or it may do an arbitrary one in the future. That means we are in the same state as with the XA_HEURHAZ. The last one is the XAER_PROTO which says that the commit was called in a wrong context — for example it was called without the prepare being invoked before. This seems being similar to XAER_NOTA and thus have the same impact as the XA_HEURRB.

Heuristic outcome with the "application exceptions"

For the "application exceptions" it could be considered easier. The heuristic exceptions can be thrown only from the commit call (see UserTransaction javadoc). The UserTransaction gives chance to finish the transaction with commit or roll-back. The roll-back means that transaction branches should be aborted and all work discarded. When UserTransaction.rollback() is called the resource manager had not promised succesful outcome yet. The time the rollback is called is time when all transaction processing data is available only in memory. Thus resource manager has nothing onto decide differently. If there is some trouble the the other types of exceptions are thrown — like IllegalStateException or SystemException (see the UserTransaction javadoc).
But with the UserTransaction.commit it is different. This call means that two-phase commit protocol is to be started and XAResource.prepare/commit/rollback calls are involved. Here the JTA uses the checked exceptions to inform the application to handle the trouble. The application checked exceptions are RollbackException,HeuristicMixedException,HeuristicRollbackException. The RollbackException is not a heuristic exception (at least by name) but still. That exception informs that even the code asked for commit all work (in all transaction branches) was undone by the rollback. The HeuristicMixedException means that some transaction branches were committed and others were rolled-back. This is exception thrown for example if during the commit phase of 2PC. One of the XAResource.commit calls returns XAException.XA_HEURRB (aka it was rolled-back) while the others were succesfully committed.
The HeuristicRollbackException has the same final outcome from the global transaction perspective as the RollbackException. It only emphasizes that the fact that the roll-back was deliberately chosen by all the resources prior to the commit was executed by the transaction manager. In comparison, the RollbackException means that the transaction manager was just trying to commit all resources but during the process of committing trouble occurred and all the work was rolled-back (all resources rolled-back). To be perfectly honest I'm not sure I can't see a real difference between these two.

We talked about all exceptions defined at the UserTransaction.commit definition so we are done. Oh wait, we are not!
There is one more exception defined in the javax.transaction package. It's the HeuristicCommitException. The HeuristicCommitException is not defined at the UserTransaction.commit as even all resources would idependently decide to commit the global transaction result is still just committed. Which is intended as UserTransaction.commit is called. Then what is the purpose of it then?
We need to look into the implementation. It's used at calls of commit and rollback at a subordinate transaction. The subordinate transaction is a transaction which is driven by a parent transaction. The parent transaction (named as top-level as well) manages the subordinate and decides the overall outcome.
When the subordinate transaction is commanded it reports the outcome back to the top-level one. It's a similar relation as the XAResource has to the global transaction. Because the subordinate transaction needs to report heuristic decisions back from the commit and rollback calls the HeuristicCommitException serves for cases when subordinate transaction decided to commit prior the top-level transaction commanded for a final action.

NOTE: Don't interchange the subordinate transaction for the nested transaction. If the nested transaction is aborted the upper transaction can continue in processing and it can finish with commit at the end (but if the top-level transaction rolls-back the nested transaction has to roll-back as well).
The subordinate transaction is a composite part of the top-level transaction. If the subordinate transaction aborts the top-level one aborts as well.

Summary

That's all. Hopefully, you understand a bit more on the meaning of the heuristic outcomes for the XA and JTA specifications. And for sure you won't be writing code like

try {
  UserTransaction.begin();
  ...
  UserTransaction.commit();
} catch (Throwable t) {
  // some strange error happened so we print it to the log
  t.printStackTrace();
}

Wednesday, June 26, 2019

Expiry scanners and object store in Narayana

What are the expiry scanners?

The expiry scanner serves for garbage collection of aged transaction records in Narayana.
Before elaborating on that statement let's first find out why is such functionality needed.

Narayana object store and transaction records

Narayana creates persistent records when process transactions. These records are saved to the transaction log called Narayana object store. The records are utilized during transaction recovery when a failure of a transaction happens. Usual reasons for the transaction failure is a crash of the JVM or a network connection issue or an internal error on the remote participant. The records are created during the processing of transactions. Then they are removed immediately after the transaction successfully finishes (regardless of the transaction outcome – commit or rollback). That implies that the Narayana log contains only the records of the currently active transactions and the failed ones. The records on active transactions are expected to be removed when the transaction finishes. The records on failed transactions are stored until the time they are recovered – finished by periodic recovery – or by the time they are resolved by human intervention.
...or by the time they are garbage collected by the expiry scanner.

Narayana stores transaction record in a hierarchical structure. The hierarchy location depends on the type of record. The object store could be stored on the hard drive – either as a directory structure, or in the journal store (the implementation which is used is created by ActiveMQ Artemis project), or it can be placed to the database via JDBC connection.

NOTE: Narayana object store saves data about transaction processing, but the same storage is used to persist other runtime data which is expected to survive the crash of the JVM.

Object store records for JTA and JTS

Transaction processing records are stored differently independence whether JTA or JTS mode is used. The JTA runs the transactions inside the same JVM. While JTS is designed to support distributed transactions. When JTS is used, the components of the transaction manager are not coupled inside the same JVM. The components communicate with each other via messages, regardless the components run within the same JVM or as different processes or on different nodes. JTS mode saves more transaction processing data to object store than the JTA alternative.

For standard transaction processing the JTA starts with the enlisting participant under the global transaction. Then two-phase commit starts and prepare is called at each participant. When the prepare 2PC phase ends, the record informing about the success of the phase is stored under the object store. After this point, the transaction is predetermined to commit (until that point the rollback would be processed in case of the failure, see presumed rollback). The 2PC commit phase is processed by calling commit on each participant. After this phase ends the record is deleted from the object store.
The prepare "tombstone record" informs about the success of the phase but contains information on successfully prepared participants which were part of the transaction.
 
This is how the transaction object storage looks like after the prepare was successfully processed. The type which represents the JTA tombstone record is StateManager/BasicAction/TwoPhaseCoordinator/AtomiAction.
data/tx-object-store/
ShadowNoFileLockStore
└── defaultStore
   ├── EISNAME
   │   ├── 0_ffff0a000007_6d753eda_5d0f2fd1_34
   │   └── 0_ffff0a000007_6d753eda_5d0f2fd1_3a
   └── StateManager
       └── BasicAction
           └── TwoPhaseCoordinator
               └── AtomicAction
                   └── 0_ffff0a000007_6d753eda_5d0f2fd1_29
In the case of the JTS, the processing runs mostly the same way. But one difference is that the JTS saves more setup data (created once during initialization of transaction manager, see FactoryContact, RecoveryCoordinator). Then the second difference to JTA is that the JTS stores the information about each prepared participant separately for JTS the participants are separate entities and each of them handles the persistence on his own. Because of that, a "prepare record" is created for each participant separately (see Mark's clarification below in comments).  When XAResource.prepare is called there is created a record type CosTransactions/XAResourceRecord. When the XAResource.commit is called then the record is deleted. After the 2PC prepare is successfully finished the record StateManager/BasicAction/TwoPhaseCoordinator/ArjunaTransactionImple is created and is removed when the 2PC commit phase is finished. The record ArjunaTransactionImple is the prepare "tombstone record" for JTS.
Take a look at how the object store with two participants and finished 2PC prepare phase looks like
data/tx-object-store/
ShadowNoFileLockStore
└── defaultStore
   ├── CosTransactions
   │   └── XAResourceRecord
   │       ├── 0_ffff0a000007_-55aeb984_5d0f33c3_4b
   │       └── 0_ffff0a000007_-55aeb984_5d0f33c3_50
   ├── Recovery
   │   └── FactoryContact
   │       └── 0_ffff0a000007_-55aeb984_5d0f33c3_15
   ├── RecoveryCoordinator
   │   └── 0_ffff52e38d0c_c91_4140398c_0
   └── StateManager
       └── BasicAction
           └── TwoPhaseCoordinator
               └── ArjunaTransactionImple
                   └── 0_ffff0a000007_-55aeb984_5d0f33c3_41

Now, what about the failures?

When the JVM crashes, network error or another transaction error happens the transaction manager stops to process the current transaction. Depending on the type of failure it either abandons the state and passes responsibility to finish the transaction to the periodic recovery manager. That's the case e.g. for the "clean" failures – the JVM crash or the network crash. The periodic recovery starts processing when the system is restarted and/or it periodically retries to connect to the participants to finish the transaction.
Continuing with the object store example above. JVM crashes and further restarts make that periodic recovery to observe the 2PC prepare was finished – there is the AtomicAction/ArjunaTransactionImple record in the object store. The recovery manager lists the participants (represented with XAResources) which were part of the transaction and it tries to commit them.

ARJUNA016037: Could not find new XAResource to use for recovering non-serializable XAResource

Let me make a quick side note to one interesting point in the processing. Interesting at least from the Narayana perspective.
If you are using Narayana transaction manager for some time you are well familiar with the log error message:

[com.arjuna.ats.jta] (Periodic Recovery) ARJUNA016037: Could not find new XAResource to use for recovering non-serializable XAResource XAResourceRecord

This warning means: There was a successful prepared transaction as we can observe the record in the object store. But periodic recovery manager is not capable to find out what is the counterparty participant – e.g. what database or JMS broker the record belongs to.
This situation happens when the failure (JVM crash) happens in a specific time. That's time just after XAResource.commit is called. It makes the participant (the remote side - e.g. the database) to remove its knowledge about the transaction from its resource local storage. But at that particular point in time, the transaction record was not yet removed from the Narayana object store.
The JVM crash happened so after the application restarts the periodic recovery can observe a record in the object store. It tries to match such record to the information obtained from the participant's resource local storage (uses XAResource.recover call).
 
As the participant's resource local storage was cleaned there is no information obtained. Now the periodic recovery does see any directly matching information to its record in the object store.
From that said, we can see the periodic recovery complains that there is a participant record which does not contain "connection data" as it's non-serializable. And there is no matching record at the participant's resource local storage.

NOTE: One possibility to get rid of the warning in the log would be to serialize all the information about the participant (serializing the XAResource). Such serialized participants provide an easy way for the periodic recovery manager to directly call methods on the un-serialized instance (XAResource.recover). But it would mean to serialize e.g. the JDBC connection which is hardly possible.

The description above explains the JTA behaviour. In the case of the JTS, if the transaction manager found a record in the object store which does not match any participant's resource local storage info then the object store record is considered as assumed completed. Such consideration means changing the type of record in the object store. Changing the type means moving the record to a different place in the hierarchical structure of the object store. When the record is moved to an unknown place for the periodic recovery it stops to consider it as a problematic one and it stops to print out warnings to the application log. The record is then saved under ArjunaTransactionImple/AssumedCompleteServerTransaction in the hierarchical structure.
This conversion of the in-doubt record to the assumed completed one happens by default after 3 cycles of recovery. Changing the number of cycles could be done by providing system property -DJTSEnvironmentBean.commitedTransactionRetryLimit=…

The ARJUNA016037 the warning was a topic in various discussions

The warning is shown again and again in the application log. It's shown each time the periodic recovery is running – as it informs there is a record and I don't know what to do with that.

NOTE: The periodic recovery runs by default every 2 minutes.

Now, what we can do with that?


Fortunately, there is an enhancement of the recovery processing in the Narayana for some time already. When the participant driver (ie. resource manager "deployed" in the same JVM) implements the Narayna SPI XAResourceWrapper it provides the information what resource is the owner of the participant record. Narayana periodic recovery is then capable to deduce if the orphaned object store record belongs to the particular participant's resource local storage. Then it can assume that the participant committed already its work. Narayana can update its own object store and periodic recovery stops to show the warnings.
An example of the usage of the SPI is in the Active MQ Artemis RA.

Transaction processing failures

Back to the transaction processing failures (JVM crash, network failure, internal participant error).
As mentioned the "clean failures" can be automatically handled by the periodic recovery. But the "clean" failures are not the only ones you can experience. The XA protocol permits a heuristic failure. Those are failures which occurs when the participant does not follow the XA protocol. Such failures are not automatically recoverable by periodic recovery. Human intervention is needed.
 
Such failures occur mostly because of an internal error at the remote participant. An example of such failure could be that the transaction manager commands the resource to commit with XAResource.commit call. But the resource manager responds that it already rolled-back the resource transaction arbitrarily. In such a case, Narayana saves this unexpected state into the object store. The transaction is marked having the heuristic outcome. And the periodic recovery observes the heuristic record in the object store and informs about it during each cycle.
Now, it's the responsibility of the administrator to get an understanding of the transaction state and handle it.
But if he does not process such a transaction for a very long time then...

Expiry scanners

...then we are back at the track to the expiry scanners.
What does mean that a record stays in the object for a very long time?

The "very long time" is by default 12 hours for Narayana. It's the default time after when the garbage collection process starts. This garbage collection is the responsibility of the expiry scanners. The purpose is cleaning the object store from the long staying records. When there is a record left in the heuristic state for 12 hours in the object store or there is a record without the matching participant's resource local storage info in the object store then the expiry scanner handles it. The purpose of such handling causes is the periodic recovery stops to observe the existence of such in-doubt participant and subsequently to stop complaining about the existence of the record.

Handling a record means moving a record to a different place (changing the type of the record and placing the record to a different place in the hierarchical structure) or removing the record completely from the object store.

Available implementations of the expiry scanner

For the JTA transaction types, there are following expiry scanners available in Narayana
  • AtomicActionExpiryScanner : moving records representing the prepared transaction (AtomicAction) to the inferior hierarchy place named /Expired.
  • ExpiredTransactionStatusManagerScanner : removing records about connection setup for the status manager. This record is not connected with transaction processing and represents Narayana runtime data.

For the JTS transaction types, there are following expiry scanners available in Narayana
  • ExpiredToplevelScanner Removing ArjunaTransactionImple/AssumedCompleteTransaction record from the object store. The AssumedCompleteTransaction originates from the type ArjunaTransactionImple and is moved to the assumed type by the JTS periodic recovery processing.
  • ExpiredServerScanner Removing ArjunaTransactionImple/AssumedCompleteServerTransaction record from the object store. The AssumedCompleteServerTransaction originates from the type ArjunaTransactionImple/ServerTransaction/JCA and is moved to the assumed type by the JTS periodic recovery processing.
  • ExpiredContactScanner : Scanner removes the records which let the recovery manager know what Narayana instance belongs to which JVM. This record is not connected with transaction processing and represents Narayana runtime data.

Setup of expiry scanners classes

As explained elsewhere Narayana can be set up either with system properties passed directly to the Java program or defined in the file descriptor jbossts-properties.xml. If you run the WildFly application server the system properties can be defined at the command line with -D… when starting application server with standalone.sh/bat script. Or they can be persistently added into the bin/standalone.conf config file.
The class names of the expiry scanners that will be active after Narayana initialization can be defined by property com.arjuna.ats.arjuna.common.RecoveryEnvironmentBean.expiryScannerClassNames or RecoveryEnvironmentBean.expiryScannerClassNames (named differently, doing the same service). The property then contains the fully qualified class names of implementation of ExpiryScanner interface. The class names are separated with space or an empty line.
An example of such settings could be seen at Narayana quickstarts. Or when it should be defined directly here it's
-DRecoveryEnvironmentBean.expiryScannerClassNames="com.arjuna.ats.internal.arjuna.recovery.ExpiredTransactionStatusManagerScanner com.arjuna.ats.internal.arjuna.recovery.AtomicActionExpiryScanner"

NOTE: when you configure the WildFly app server then you are allowed to use only the shortened property name of -DRecoveryEnvironmentBean.expiryScannerClassNames=…. The longer variant does not work because of the way the issue WFLY-951 was implemented.

NOTE2: when you are running the WildFly app server then the expired scanners enabled by default could be observed by looking into the source code at ArjunaRecoveryManagerService (consider variants for JTA and JTS modes).

Setup of expiry scanners interval

To configure the time interval after the "orphaned" record is handled as the expired one you can use the property the property with the name com.arjuna.ats.arjuna.common.RecoveryEnvironmentBean.expiryScanInterval or RecoveryEnvironmentBean.expiryScanInterval. The value could be a positive whole number. Such number defines that the records expire after that number of hours. If you define the value as a negative whole number then the first run of the expire scanner run skipped. Next run of the expire scanner expires the records after that (positive) number of hours. If you define the value to be 0 then records are never handled by expiry scanners.


That's all in terms of this article. Feel free to ask a question here or at our forum at https://developer.jboss.org/en/jbosstm.

Monday, April 29, 2019

JTA and CDI integration

The Narayana release 5.9.5.Final comes with few nice CDI functionality enhancements. This blogpost introduces these changes while placing them to the context of the JTA and CDI integration, particularly with focus to Weld.

TL;DR

The fastest way to find out the way of using the JTA with the CDI is walking through the Narayana CDI quickstart.

JTA and CDI specifications

JTA version 1.2 was published in 2013. The version introduced the integration of JTA with CDI. The specification came with the definition of annotations javax.transaction.Transactional and javax.transaction.TransactionScoped. Those two provide a way for transaction boundary definition and for handling application data bounded to the transaction.

Narayana, as the implementation of the JTA specification, provides those capabilities in the CDI maven module.
Here we come with the maven coordinates:
<groupId>org.jboss.narayana.jta</groupId>
<artifactId>cdi</artifactId>

The module brings Narayana CDI extension to the user's project. The extension installs interceptors which manage transactional boundaries for method invocation annotated with @Transactional. Then the extension defines a transaction scope declared with the @TransactionScoped annotation.

On top of the functionality defined in the JTA specification, it's the CDI specification which defines some more transaction-related features. They are the transactional observer methods and the definition of the javax.transaction.UserTransaction built-in bean.

Let's summarize what that all means in practice.

@Transactional

With the use of the @Transactional annotation, transaction boundary could be controlled declaratively. The use of the annotation is really similar to the container-managed transactions in EJB.

When the annotation is used for a bean or a method the Narayana CDI extension (CDI interceptor is used) verifies the existence of the transaction context when the method is called. Based on the value of the value parameter an appropriate action is taken. The value is defined from enumeration Transactional.TxType
For example when @Transactional(Transactional.TxType.REQUIRES_NEW) is used on the method then on the start of its execution a new transaction is started. If the incoming method call contains an existing transaction it's suspended during the method execution and then resumed after it finishes. For details about the other Transactional.TxType values consider the javadoc documentation.

NOTE: be aware of the fact that for the CDI container can intercept the method call the CDI managed instance has to be used. For example, when you want to use the capability for calling an inner bean you must use the injection of the bean itself.

@RequestScope
public class MyCDIBean {
  @Inject
  MyCDIBean myBean;

  @Transactional(TxType.REQUIRED)
  public void mainMethod() {
    // CDI container does not wrap the invocation
    // no new transaction is started
    innerFunctionality();

    // CDI container starts a new transaction
    // the method uses TxType.REQUIRES_NEW and is called from the CDI bean
    myBean.innerFunctionality();
  }

  @Transactional(TxType.REQUIRES_NEW)
  private void innerFunctionality() {
    // some business logic
  }
}
  
>

@TransactionScoped

@TransactionScoped brings an additional scope type in addition to the standard built-in ones. A bean annotated with the @TransactionScoped, when injected, lives in the scope of the currently active transaction. The bean remains bound to the transaction even when it is suspended. On resuming the transaction the scoped data are available again. If a user tries to access the bean out of the scope of the active transaction the javax.enterprise.context.ContextNotActiveException is thrown.

Built-in UserTransaction bean

The CDI specification declares that the Java EE container has to provide a bean for the UserTransaction can be @Injected. Notice that the standalone CDI container has no obligation to provide such bean. The availability is expected for the Java EE container. In Weld, the integration for the Java EE container is provided through the SPI interface TransactionServices.

If somebody wants to use the Weld integrated with Narayana JTA implementation in a standalone application he needs to implement this SPI interface (see more below).

Transaction observer methods

The feature of the transaction observer methods allows defining an observer with the definition of the during parameter at @Observes annotation. During takes a value from the TransactionPhase enumeration. The during value defines when the event will be delivered to the observer. The event is fired during transaction processing in the business logic but then the delivery is deferred until transaction got status defined by the during parameter.
The during parameter can obtain values BEFORE_COMPLETION, AFTER_COMPLETION, AFTER_FAILURE, AFTER_SUCCESS. Using value IN_PROGRESS means the event is delivered to observer immediately when it's fired. It behaves like there is no during parameter used.

The implementation is based on the registration of the transaction synchronization. When the event is fired there is registered a special new synchronization which is invoked by the transaction manager afterwards. The registered CDI synchronization code then manages to launch the observer method to deliver the event.

For the during parameter working and for the events being deferred Weld requires integration through the TransactionServices SPI. The interface defines a method which provides makes for Weld possible to register the transaction synchronization. If the integration with the TransactionServices is not provided then the user can still use the during parameter in his code. But(!) no matter what TransactionPhase value is used the event is not deferred but it's immediately delivered to the observer. The behaviour is the same as when the IN_PROGRESS value is used.

Maybe it could be fine to clarify who fires the event. The event is fired by the user code. For example, take a look at the example in the Weld documentation. The user code injects an event and fires it when considers it necessary.

@Inject @Any Event productEvent;
...
public void persist(Product product) {
  em.persist(product);
  productEvent.select(new AnnotationLiteral(){}).fire(product);
}
The observer is defined in the standard way and using during for the event delivery to be deferred until the time the transaction is finished with success.
void addProduct(@Observes(during = AFTER_SUCCESS) @Created Product product) {
...
}

A bit more about TransactionServices: Weld and JTA integration

As said for the integration of the Weld CDI to JTA it's needed to implement the TransactionServices SPI interface. The interface gives the Weld the chance to gain the UserTransaction thus the built-in bean can provide it when it's @Injected. It provides the way to register transaction synchronization for an event could be deferred until particular transaction status occurs. Up to that, it demands the implementation of the method isTransactionActive. The TransactionScoped is active only when there is some active transaction. This way the Weld is able to obtain the transaction activity state.

Regarding the implementation, you can look at how the interface TransactionServices is implemented in WildFly or in the more standalone way for SmallRye Context Propagation.

A new Narayana CDI features

Narayana brings two new CDI JTA integration capabilities, up to those described above.

The first enhancement is the addition of the transactional scope events. Up to now, Narayana did not fire the scope events for the @TransactionScoped. From now there is fired the scope events automatically by Narayana. The user can observe the initialization and the destruction of the transaction scope. The code for the observer could be like

void transactionScopeActivated(
  @Observes @Initialized(TransactionScoped.class) final Transaction event,
  final BeanManager beanManager) {
...
}
The event payload for the @Initialized is the javax.transaction.Transaction, for the @Destroyed is just the java.lang.Object (when the transaction scope is destroyed there is no active transaction anymore).
As the Narayana implements the CDI in version 1.2 in these days there is not fired an event for @BeforeDestroy. That scope event was introduced in the CDI version 2.0.

The second enhancement is the addition of two built-in beans which can be @Injected in the user code. Those are beans TransctionManager and TransactionSynchronizationRegistry.

The implementation gives priority to the JNDI binding. If there is bound TransactionManager/TransactionSynchronizationRegistry in the JNDI then such instance is returned at the injection point.
If the user defines his own CDI bean or a CDI producer which provides an instance of those two classes then such instance is grabbed for the injection.
As the last resort, the default Narayana implementation of both classes is used. You can consider the TransactionManagerImple and the TransactionSynchronizationRegistryImple to be used.

Using the transactional CDI extension

The easiest way to check the integration in the action is to run our JTA standalone quickstart. You can observe the implementation of the Weld SPI interface TransactionServices. You can check the use of the observers, both the transaction observer methods and the transactional scoped observers. Up to that, you can see the use of the transaction scope and use of the injection for the TransactionManager.

Acknowledgement

Big thanks to Laird Nelson who contributed the new CDI functionality enhancements to Narayana.
And secondly thanks to Matěj Novotný. for his help in understanding the CDI topic.