Transactions are needed to coordinate changes of state across multiple clients and services. The Jini transaction model uses a simple model of transactions, with details of semantics left to the clients and services. The Jini distribution supplies a transaction manager that can be used.
Transactions are a neccessary part of many distributed operations. Frequently two or more objects may need to synchronize changes of state so that they all occur, or none occur. This happens in situations such as control of ownership, where one party has to give up ownership at the ``same'' time as another asserts ownership. What has to be avoided is only one party performing the action, which could result in either no owners or two owners.
The theory of transactions often talks about the ``ACID'' properties:
The practice of transactions is that they use the two-phase commit protocol. This requires that participants in a transaction be asked to ``vote'' on a transaction. If all agree to go ahead, then the transaction ``commits'', which is binding on all the participants. If any ``abort'' during this voting stage then it forces abortion of the transaction on all participants.
Jini has adopted the syntax of the two-phase commit method. It is up to the clients and services within a transaction to observe the ACID properties if they desire. Jini essentially supplies the mechanism of two-phase commit, and leaves the policy of meaning to the participants in a transaction.
Restricting Jini transactions to a two-phase commit model without associating
a particular semantics to this means that a transaction can be represented in
a simple way, just as a long identifier. This identifier is
obtained from a transaction manager, and will uniquely label the transaction
to that manager. (It is not guaranteed unique between
managers, though - unlike service ID's.)
All participants in the transaction communicate with the
transaction manager, using this identifier to label which transaction they
belong to.
The participants in a transaction may disappear; the transaction manager may
disappear. So transactions are managed by a lease, which will expire unless
it is renewed. When a transaction manager is asked for a new transaction it
returns a TransactionManager.Created object,
containing the transaction identifier and lease:
public interface TransactionManager {
public static class Created {
public final long id;
public final Lease lease;
}
...
}
Created object may be passed around between participants in the
lease. One of them will need to look after lease renewals. All the participants
will use the transaction identifier in communication with the transaction
manager.
A transaction manager looks after the two-phase commit protocol for all the
participants in a transaction. It is responsible for creating a new
transaction by its create() method. Any of the participants
may force the transaction to abort by abort(), or can force
it to the two-phase commit stage by calling commit().
public interface TransactionManager {
Created create(long leaseFor) throws ...;
void join(long id, TransactionParticipant part,
long crashCount) throws ...;
void commit(long id) throws ...;
void abort(long id) throws ...;
...
}
When a participant joins a transaction, it registers a listener of type
TransactionParticipant. If any participant calls commit(),
the transaction manager starts the voting process using all of these listeners.
If all of these are prepared to commit, then the manager moves all of these
listeners to the commit stage. Alternatively, any of the participants may call
abort(), which forces all of the listeners to abort.
When an object becomes a participant listener in a transaction, it allows the transaction manager to call various methods
public interface TransactionParticipant ... {
int prepare(TransactionManager mgr, long id) throws ...;
void commit(TransactionManager mgr, long id) throws ...;
void abort(TransactionManager mgr, long id) throws ...;
int prepareAndCommit(TransactionManager mgr, long id) throws ...;
}
The ``normal'' mode of operation (that is, when nothing goes wrong with
the transaction) is for a call to be made on the transaction manager
to commit. It then enters the two-phase commit stage where it asks
each participant listener to firstly prepare() and then
to either commit() or abort().
Mahalo is a transaction manager supplied by Sun as part of the Jini
distribution. It can be used without change. It runs as a Jini service,
like reggie.
Just like all Jini services, it has two parts: the part that runs as
a server, needing its own set of class files in mahalo.jar,
and the set of class files that need to be available to clients in
mahalo-dl.jar. It also needs a security policy, an HTTP server
and log files. It can be started using a command line such as
java -Djava.security.policy=policy.all \
-Dcom.sun.jini.mahalo.managerName=TransactionManager \
-jar /home/jan/tmpdir/jini1_0/lib/mahalo.jar \
http://`hostname`:8080/mahalo-dl.jar \
/home/jan/projects/jini/doc/policy.all \
/tmp/mahalo_log public &
The classic use of transactions is to handle money transfers between accounts. In this there are two accounts, one of which is debited and the other credited. This is not too exciting as an example, so we shall try a more complex situation. A service may decide to charge for its use. If a client decides this cost is reasonable, it will first credit the service and then request that the service be performed. The actual accounts will be managed by an accounts service, which will need to be informed of the credits and debits that occur. A simple accounts model is that the service gets, say, a customer ID from the client, and passes its own ID and the customer ID to the accounts service which manages both accounts. Simple, prone to all sorts of e-commerce issues that we have no intention of going into, and similar to the way credit cards work!
Figure 16.1 shows the messages in a normal sequence diagram.
The client makes a call getCost() to the service, and receives
the cost in return. It then makes another call credit()
on the service, which makes a call creditDebit() on the accounts
before returning. The client then makes a final call requestService()
on the service and gets back a result.
There are a number of problems with this that can benefit by use of a transaction
model. The steps of credit() and creditDebit() should
certainly be performed either both together or not at all. But in addition there is
the issue of the quality of the service: suppose the client is not happy with
the results from the service. It would like to reclaim its money, or better yet,
not spend it in the first case! So if we include the delivery of the service in
the transaction then there is the opportunity to for the client to abort the
transaction before it is committed.
Figure 16.2 shows the
larger set of messages in the sequence diagram for ``normal'' execution.
As before, the client requests the cost from the service.
After getting this, it asks the transaction manager to create a transaction
and receives the transaction id. It then joins the transaction itself.
When it asks the service to credit an amount, the service also joins the
transaction. The service then asks the account to creditDebit()
the amount, and as part of this the account also joins the transaction.
The client then requests the service and gets the result. If all is fine,
it then asks the transaction manager to commit(), which triggers
the prepare and commit phase. The transaction manager asks each participant
to prepare() and if it gets satisfactory replies from each
it then asks each one to commit().
The points of failure in this transaction include
NotEnoughPayment exception - joining and aborting
is used for illustrating transaction possibilities.)
The service is a version of the familiar file classifier that requires a payment
before it will divulge the MIME type for a file name. A bit unrealistic, perhaps,
but that doesn't matter for purposes here. There will be an interface
PayableFileClassifier, which extends the FileClassifier
interface. We will also make it extend the Payable interface, just in
case we want to charge for other services. In line with other interfaces, we shall
extend this to a RemotePayableFileClassifier, and then implement this
by a PayableFileClassifierImpl.
The PayableFileClassifierImpl can use the implementation of the
rmi.FileClassifierImpl, so we shall make it extend this class.
We also want it to be a participant in a transaction, so it must implement the
TransactionParticipant interface. This leads to the
inheritance diagram of figure 16.3,
which isn't really as complex as it looks.
The new elements in this hierarchy are firstly the interface Payable
package common;
import java.io.Serializable;
import net.jini.core.transaction.server.TransactionManager;
/**
* Payable.java
*/
public interface Payable extends Serializable {
void credit(long amount, long accountID,
TransactionManager mgr,
long transactionID)
throws java.rmi.RemoteException;
long getCost() throws java.rmi.RemoteException;
} // Payable
PayableFileClassifier. This will
be used by the client to search for the service
package common;
/**
* PayableFileClassifier.java
*/
public interface PayableFileClassifier extends FileClassifier, Payable {
} // PayableFileClassifier
package txn;
import common.PayableFileClassifier;
import java.rmi.Remote;
/**
* RemotePayableFileClassifier.java
*/
public interface RemotePayableFileClassifier extends PayableFileClassifier, Remote {
} // RemotePayableFileClasssifier
The implementation of this joins the transaction, finds an Accounts service
from a known location (using unicast lookup),
registers the money transfer, and then performs the service. There is no real state
information kept by this implementation which is altered by the transaction.
When asked to prepare() by the transaction manager it can just return
NOTCHANGED. If there was state, the prepare() and
commit() methods would have more content.
The prepareAndCommit() method may be called by a transaction manager
as an optimisation, and the version given in this example follows the
specification given in the Jini Transaction document.
package txn;
import common.MIMEType;
import common.Accounts;
import rmi.FileClassifierImpl;
//import common.PayableFileClassifier;
//import common.Payable;
import net.jini.core.transaction.server.TransactionManager;
import net.jini.core.transaction.server.TransactionParticipant;
import net.jini.core.transaction.server.TransactionConstants;
import net.jini.core.transaction.UnknownTransactionException;
import net.jini.core.transaction.CannotJoinException;
import net.jini.core.transaction.CannotAbortException;
import net.jini.core.transaction.server.CrashCountException;
import net.jini.core.lookup.ServiceTemplate;
import net.jini.core.lookup.ServiceRegistrar;
import net.jini.core.discovery.LookupLocator;
import java.rmi.RemoteException;
import java.rmi.RMISecurityManager;
/**
* PayableFileClassifierImpl.java
*/
public class PayableFileClassifierImpl extends FileClassifierImpl
implements RemotePayableFileClassifier, TransactionParticipant {
protected TransactionManager mgr = null;
protected Accounts accts = null;
protected long crashCount = 0; // ???
protected long cost = 10;
protected final long myID = 54321;
public PayableFileClassifierImpl() throws java.rmi.RemoteException {
super();
System.setSecurityManager(new RMISecurityManager());
}
public void credit(long amount, long accountID,
TransactionManager mgr,
long transactionID) {
System.out.println("crediting");
this.mgr = mgr;
// before findAccounts
System.out.println("Joining txn");
try {
mgr.join(transactionID, this, crashCount);
} catch(UnknownTransactionException e) {
e.printStackTrace();
} catch(CannotJoinException e) {
e.printStackTrace();
} catch(CrashCountException e) {
e.printStackTrace();
} catch(RemoteException e) {
e.printStackTrace();
}
System.out.println("Joined txn");
findAccounts();
if (accts == null) {
try {
mgr.abort(transactionID);
} catch(UnknownTransactionException e) {
e.printStackTrace();
} catch(CannotAbortException e) {
e.printStackTrace();
} catch(RemoteException e) {
e.printStackTrace();
}
}
try {
accts.creditDebit(amount, accountID, myID,
transactionID, mgr);
} catch(java.rmi.RemoteException e) {
e.printStackTrace();
}
}
public long getCost() {
return cost;
}
protected void findAccounts() {
// find a known account service
LookupLocator lookup = null;
ServiceRegistrar registrar = null;
try {
lookup = new LookupLocator("jini://localhost");
} catch(java.net.MalformedURLException e) {
System.err.println("Lookup failed: " + e.toString());
System.exit(1);
}
try {
registrar = lookup.getRegistrar();
} catch (java.io.IOException e) {
System.err.println("Registrar search failed: " + e.toString());
System.exit(1);
} catch (java.lang.ClassNotFoundException e) {
System.err.println("Registrar search failed: " + e.toString());
System.exit(1);
}
System.out.println("Registrar found");
Class[] classes = new Class[] {Accounts.class};
ServiceTemplate template = new ServiceTemplate(null, classes,
null);
try {
accts = (Accounts) registrar.lookup(template);
} catch(java.rmi.RemoteException e) {
System.exit(2);
}
}
public MIMEType getMIMEType(String fileName) throws RemoteException {
if (mgr == null) {
// don't process the request
return null;
}
return super.getMIMEType(fileName);
}
public int prepare(TransactionManager mgr, long id) {
System.out.println("Preparing...");
return TransactionConstants.PREPARED;
}
public void commit(TransactionManager mgr, long id) {
System.out.println("committing");
}
public void abort(TransactionManager mgr, long id) {
System.out.println("aborting");
}
public int prepareAndCommit(TransactionManager mgr, long id) {
int result = prepare(mgr, id);
if (result == TransactionConstants.PREPARED) {
commit(mgr, id);
result = TransactionConstants.COMMITTED;
}
return result;
}
} // PayableFileClassifierImpl
We shall assume that all accounts in this example are managed by a single
Accounts service, that knows about all accounts by a long
identifier. These should be stored in permanent form, and there should be proper
crash recovery mechanisms, etc. For simplicity, we shall
just have a hash table of accounts, with uncommitted transactions kept in a
``pending'' list. When commitment occurs, the pending transaction takes place.
Figure 16.4 shows the accounts class diagram.
Accounts interface is
/**
* Accounts.java
*/
package common;
import net.jini.core.transaction.server.TransactionManager;
public interface Accounts {
void creditDebit(long amount, long creditorID,
long debitorID, long transactionID,
TransactionManager tm)
throws java.rmi.RemoteException;
} // Accounts
/**
* AccountsImpl.java
*/
package txn;
// import common.Accounts;
import net.jini.core.transaction.server.TransactionManager;
import net.jini.core.transaction.server.TransactionParticipant;
import net.jini.core.transaction.server.TransactionConstants;
import java.rmi.server.UnicastRemoteObject;
import java.util.Hashtable;
// import java.rmi.RMISecurityManager;
// debug
import net.jini.core.lookup.ServiceTemplate;
import net.jini.core.lookup.ServiceRegistrar;
import net.jini.core.discovery.LookupLocator;
// end debug
public class AccountsImpl extends UnicastRemoteObject
implements RemoteAccounts, TransactionParticipant, java.io.Serializable {
protected long crashCount = 0; // value??
protected Hashtable accountBalances = new Hashtable();
protected Hashtable pendingCreditDebit = new Hashtable();
public AccountsImpl() throws java.rmi.RemoteException {
// System.setSecurityManager(new RMISecurityManager());
}
public void creditDebit(long amount, long creditorID,
long debitorID, long transactionID,
TransactionManager mgr) {
// Ensure stub class is loaded by getting its class object.
// It has to be loaded from the same place as this object
java.rmi.Remote stub = null;
try {
stub = toStub(this);
} catch(Exception e) {
System.out.println("To stub failed");
e.printStackTrace();
}
System.out.println("To stub found");
String annote = java.rmi.server.RMIClassLoader.getClassAnnotation(stub.getClass());
System.out.println("from " + annote);
try {
Class cl = java.rmi.server.RMIClassLoader.loadClass(annote, "txn.AccountsImpl_Stub");
} catch(Exception e) {
System.out.println("To stub class failed");
e.printStackTrace();
}
System.out.println("To stub class ok");
// mgr = findManager();
try {
System.out.println("Trying to join");
mgr.join(transactionID, this, crashCount);
} catch(net.jini.core.transaction.UnknownTransactionException e) {
e.printStackTrace();
} catch(java.rmi.RemoteException e) {
e.printStackTrace();
} catch(net.jini.core.transaction.server.CrashCountException e) {
e.printStackTrace();
} catch(net.jini.core.transaction.CannotJoinException e) {
e.printStackTrace();
}
System.out.println("joined");
pendingCreditDebit.put(new TransactionPair(mgr,
transactionID),
new CreditDebit(amount, creditorID,
debitorID));
}
// findmanager debug hack
protected TransactionManager findManager() {
// find a known account service
LookupLocator lookup = null;
ServiceRegistrar registrar = null;
TransactionManager mgr = null;
try {
lookup = new LookupLocator("jini://localhost");
} catch(java.net.MalformedURLException e) {
System.err.println("Lookup failed: " + e.toString());
System.exit(1);
}
try {
registrar = lookup.getRegistrar();
} catch (java.io.IOException e) {
System.err.println("Registrar search failed: " + e.toString());
System.exit(1);
} catch (java.lang.ClassNotFoundException e) {
System.err.println("Registrar search failed: " + e.toString());
System.exit(1);
}
System.out.println("Registrar found");
Class[] classes = new Class[] {TransactionManager.class};
ServiceTemplate template = new ServiceTemplate(null, classes,
null);
try {
mgr = (TransactionManager) registrar.lookup(template);
} catch(java.rmi.RemoteException e) {
System.exit(2);
}
return mgr;
}
public int prepare(TransactionManager mgr, long id) {
System.out.println("Preparing...");
return TransactionConstants.PREPARED;
}
public void commit(TransactionManager mgr, long id) {
System.out.println("committing");
}
public void abort(TransactionManager mgr, long id) {
System.out.println("aborting");
}
public int prepareAndCommit(TransactionManager mgr, long id) {
int result = prepare(mgr, id);
if (result == TransactionConstants.PREPARED) {
commit(mgr, id);
result = TransactionConstants.COMMITTED;
}
return result;
}
class CreditDebit {
long amount;
long creditorID;
long debitorID;
CreditDebit(long a, long c, long d) {
amount = a;
creditorID = c;
debitorID = d;
}
}
class TransactionPair {
TransactionPair(TransactionManager mgr, long id) {
}
}
} // AccountsImpl
The final component in this is the client which starts the transaction.
The simplest code for this would just use the blocking lookup()
method of ClientLookupManager to find first the service and
then the transaction manager. We use the longer way to show various other
ways of doing things. This implementation uses
a nested class which extends Thread. Because of this, it cannot
extend UnicastRemoteObject, and so is not automatically exported.
In order to export itself, it has to call
UnicastRemoteObject.exportObject). This must be done before the
call to join the transaction, which expects a remote object.
package client;
import common.PayableFileClassifier;
import common.MIMEType;
import java.rmi.RMISecurityManager;
import net.jini.discovery.LookupDiscovery;
import net.jini.discovery.DiscoveryListener;
import net.jini.discovery.DiscoveryEvent;
import net.jini.core.lookup.ServiceRegistrar;
import net.jini.core.lookup.ServiceTemplate;
import net.jini.core.transaction.server.TransactionManager;
import net.jini.core.transaction.server.TransactionConstants;
import net.jini.core.transaction.server.TransactionParticipant;
// import com.sun.jini.lease.LeaseRenewalManager;
import net.jini.lease.LeaseRenewalManager;
import net.jini.core.lease.Lease;
import net.jini.lookup.entry.Name;
import net.jini.core.entry.Entry;
import java.rmi.RemoteException;
import java.rmi.server.UnicastRemoteObject;
/**
* TestTxn.java
*/
public class TestTxn implements DiscoveryListener {
PayableFileClassifier classifier = null;
TransactionManager mgr = null;
long myClientID; // my account id
public static void main(String argv[]) {
new TestTxn();
// stay around long enough to receive replies
try {
Thread.currentThread().sleep(100000L);
} catch(java.lang.InterruptedException e) {
// do nothing
}
}
public TestTxn() {
System.setSecurityManager(new RMISecurityManager());
LookupDiscovery discover = null;
try {
discover = new LookupDiscovery(LookupDiscovery.ALL_GROUPS);
} catch(Exception e) {
System.err.println(e.toString());
System.exit(1);
}
discover.addDiscoveryListener(this);
}
public void discovered(DiscoveryEvent evt) {
ServiceRegistrar[] registrars = evt.getRegistrars();
for (int n = 0; n < registrars.length; n++) {
System.out.println("Service found");
ServiceRegistrar registrar = registrars[n];
new LookupThread(registrar).start();
}
// System.exit(0);
}
public void discarded(DiscoveryEvent evt) {
// empty
}
public class LookupThread extends Thread implements TransactionParticipant, java.io.Serializable {
ServiceRegistrar registrar;
long crashCount = 0; // ???
LookupThread(ServiceRegistrar registrar) {
this.registrar = registrar;
}
public void run() {
long cost = 0;
// try to find a classifier if we haven't already got one
if (classifier == null) {
System.out.println("Searching for classifier");
Class[] classes = new Class[] {PayableFileClassifier.class};
ServiceTemplate template = new ServiceTemplate(null, classes,
null);
try {
Object obj = registrar.lookup(template);
System.out.println(obj.getClass().toString());
Class cls = obj.getClass();
Class[] clss = cls.getInterfaces();
for (int n = 0; n < clss.length; n++) {
System.out.println(clss[n].toString());
}
classifier = (PayableFileClassifier) registrar.lookup(template);
} catch(java.rmi.RemoteException e) {
e.printStackTrace();
System.exit(2);
}
if (classifier == null) {
System.out.println("Classifier null");
} else {
System.out.println("Getting cost");
try {
cost = classifier.getCost();
} catch(java.rmi.RemoteException e) {
e.printStackTrace();
}
if (cost > 20) {
System.out.println("Costs too much: " + cost);
classifier = null;
}
}
}
// try to find a transaction manager if we haven't already got one
if (mgr == null) {
System.out.println("Searching for txnmgr");
Class[] classes = new Class[] {TransactionManager.class};
ServiceTemplate template = new ServiceTemplate(null, classes,
null);
/*
Entry[] entries = {new Name("TransactionManager")};
ServiceTemplate template = new ServiceTemplate(null, null,
entries);
*/
try {
mgr = (TransactionManager) registrar.lookup(template);
} catch(java.rmi.RemoteException e) {
e.printStackTrace();
System.exit(2);
}
if (mgr == null) {
System.out.println("Manager null");
return;
}
}
if (classifier != null && mgr != null) {
System.out.println("Found both");
TransactionManager.Created tcs = null;
System.out.println("Creating transaction");
try {
tcs = mgr.create(Lease.FOREVER);
} catch(java.rmi.RemoteException e) {
mgr = null;
return;
} catch(net.jini.core.lease.LeaseDeniedException e) {
mgr = null;
return;
}
long transactionID = tcs.id;
// join in ourselves
System.out.println("Joining transaction");
// but first, export ourselves since we don't extend UnicastRemoteObject
try {
UnicastRemoteObject.exportObject(this);
} catch(RemoteException e) {
e.printStackTrace();
}
try {
mgr.join(transactionID, this, crashCount);
} catch(net.jini.core.transaction.UnknownTransactionException e) {
e.printStackTrace();
} catch(java.rmi.RemoteException e) {
e.printStackTrace();
} catch(net.jini.core.transaction.server.CrashCountException e) {
e.printStackTrace();
} catch(net.jini.core.transaction.CannotJoinException e) {
e.printStackTrace();
}
new LeaseRenewalManager().renewUntil(tcs.lease,
Lease.FOREVER,
null);
System.out.println("crediting...");
try {
classifier.credit(cost, myClientID,
mgr, transactionID);
} catch(Exception e) {
System.err.println(e.toString());
}
System.out.println("classifying...");
MIMEType type = null;
try {
type = classifier.getMIMEType("file1.txt");
} catch(java.rmi.RemoteException e) {
System.err.println(e.toString());
}
// if we get a good result, commit, else abort
if (type != null) {
System.out.println("Type is " + type.toString());
System.out.println("Calling commit");
// new CommitThread(mgr, transactionID).run();
try {
System.out.println("mgr state " + mgr.getState(transactionID));
mgr.commit(transactionID);
} catch(Exception e) {
e.printStackTrace();
}
} else {
try {
mgr.abort(transactionID);
} catch(java.rmi.RemoteException e) {
} catch(net.jini.core.transaction.CannotAbortException e) {
} catch( net.jini.core.transaction.UnknownTransactionException e) {
}
}
}
}
public int prepare(TransactionManager mgr, long id) {
System.out.println("Preparing...");
return TransactionConstants.PREPARED;
}
public void commit(TransactionManager mgr, long id) {
System.out.println("committing");
}
public void abort(TransactionManager mgr, long id) {
System.out.println("aborting");
}
public int prepareAndCommit(TransactionManager mgr, long id) {
int result = prepare(mgr, id);
if (result == TransactionConstants.PREPARED) {
commit(mgr, id);
result = TransactionConstants.COMMITTED;
}
return result;
}
} // LookupThread
class CommitThread extends Thread {
TransactionManager mgr;
long transactionID;
public CommitThread(TransactionManager m, long id) {
mgr = m;
transactionID = id;
try {
Thread.sleep(1000);
} catch(Exception e) {
}
}
public void run() {
try {
mgr.abort(transactionID);
} catch(Exception e) {
e.printStackTrace();
}
}
} // CommitThread
} // TestTxn
Transactions are needed to coordinate changes of state across multiple clients and services. The Jini transaction model uses a simple model of transactions, with details of semantics left to the clients and services. The Jini distribution supplies a transaction manager that can be used.
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