DPL Transaction Example
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## DPL Transaction Example

The following Java code provides a fully functional example of a multi-threaded transactional JE application using the DPL. This example is nearly identical to the example provided in the previous section, except that it uses an entity class and entity store to manage its data.

As is the case with the previous examples, this example opens an environment and then an entity store. It then creates 5 threads, each of which writes 500 records to the database. The primary key for these writes are based on pre-determined integers, while the data is randomly generated data. This means that the actual data is arbitrary and therefore uninteresting; we picked it only because it requires minimum code to implement and therefore will stay out of the way of the main points of this example.

Each thread writes 10 records under a single transaction before committing and writing another 10 (this is repeated 50 times). At the end of each transaction, but before committing, each thread calls a function that uses a cursor to read every record in the database. We do this in order to make some points about database reads in a transactional environment.

Of course, each writer thread performs deadlock detection as described in this manual. In addition, normal recovery is performed when the environment is opened.

To implement this example, we need three classes:

- `TxnGuide.java`

  This is the main class for the application. It performs environment and store management, spawns threads, and creates the data that is placed in the database. See TxnGuide.java for implementation details.

- `StoreWriter.java`

  This class extends `java.lang.Thread`, and as such it is our thread implementation. It is responsible for actually reading and writing store. It also performs all of our transaction management. See StoreWriter.java for implementation details.

- `PayloadDataEntity.java`

  This is an entity class used to encapsulate several data fields. See PayloadDataEntity.java for implementation details.

### TxnGuide.java

The main class in our example application is used to open and close our environment and store. It also spawns all the threads that we need. We start with the normal series of Java package and import statements, followed by our class declaration:

    // File TxnGuideDPL.java

    package persist.txn;

    import com.sleepycat.je.DatabaseConfig;
    import com.sleepycat.je.DatabaseException;

    import com.sleepycat.je.Environment;
    import com.sleepycat.je.EnvironmentConfig;

    import com.sleepycat.persist.EntityStore;
    import com.sleepycat.persist.StoreConfig;

    import java.io.File;

    public class TxnGuideDPL {

Next we declare our class' private data members. Mostly these are used for constants such as the name of the database that we are opening and the number of threads that we are spawning. However, we also declare our environment and database handles here.

        private static String myEnvPath = "./";
        private static String storeName = "exampleStore";

        // Handles
        private static EntityStore myStore = null;
                    private static Environment myEnv = null;
        private static final int NUMTHREADS = 5;

Next, we implement our `usage()` method. This application optionally accepts a single command line argument which is used to identify the environment home directory.

        private static void usage() {
            System.out.println("TxnGuideDPL [-h <env directory>]");
            System.exit(-1);
        }

Now we implement our `main()` method. This method simply calls the methods to parse the command line arguments and open the environment and store. It also creates and then joins the store writer threads.

        public static void main(String args[]) {
            try {
                // Parse the arguments list
                parseArgs(args);
                // Open the environment and store
                openEnv();

                // Start the threads
                StoreWriter[] threadArray;
                threadArray = new StoreWriter[NUMTHREADS];
                for (int i = 0; i < NUMTHREADS; i++) {
                    threadArray[i] = new StoreWriter(myEnv, myStore);
                    threadArray[i].start();
                }

                for (int i = 0; i < NUMTHREADS; i++) {
                    threadArray[i].join();
                }
            } catch (Exception e) {
                System.err.println("TxnGuideDPL: " + e.toString());
                e.printStackTrace();
            } finally {
                closeEnv();
            }
            System.out.println("All done.");
        }

Next we implement `openEnv()`. This method is used to open the environment and then an entity store in that environment. Along the way, we make sure that the transactional subsystem is correctly initialized.

        private static void openEnv() throws DatabaseException {
            System.out.println("opening env and store");

            // Set up the environment.
            EnvironmentConfig myEnvConfig = new EnvironmentConfig();
            myEnvConfig.setAllowCreate(true);
            myEnvConfig.setTransactional(true);
            //  Environment handles are free-threaded by default in JE,
            // so we do not have to do anything to cause the
            // environment handle to be free-threaded.

            // Set up the entity store
            StoreConfig myStoreConfig = new StoreConfig();
            myStoreConfig.setAllowCreate(true);
            myStoreConfig.setTransactional(true);

            // Open the environment
            myEnv = new Environment(new File(myEnvPath),    // Env home
                                        myEnvConfig);

            // Open the store
            myStore = new EntityStore(myEnv, storeName, myStoreConfig);
        }

Finally, we implement the methods used to close our environment and databases, parse the command line arguments, and provide our class constructor. This is fairly standard code and it is mostly uninteresting from the perspective of this manual. We include it here purely for the purpose of completeness.

        private static void closeEnv() {
            System.out.println("Closing env and store");
            if (myStore != null ) {
                try {
                    myStore.close();
                } catch (DatabaseException e) {
                    System.err.println("closeEnv: myStore: " +
                        e.toString());
                    e.printStackTrace();
                }
            }

            if (myEnv != null ) {
                try {
                    myEnv.close();
                } catch (DatabaseException e) {
                    System.err.println("closeEnv: " + e.toString());
                    e.printStackTrace();
                }
            }
        }

        private TxnGuideDPL() {}

        private static void parseArgs(String args[]) {
            int nArgs = args.length;
            for(int i = 0; i < args.length; ++i) {
                if (args[i].startsWith("-")) {
                    switch(args[i].charAt(1)) {
                        case 'h':
                            if (i < nArgs - 1) {
                                myEnvPath = new String(args[++i]);
                            }
                        break;
                        default:
                            usage();
                    }
                }
            }
        }
    }

### PayloadDataEntity.java

Before we show the implementation of the store writer thread, we need to show the class that we will be placing into the store. This class is fairly minimal. It simply allows you to store and retrieve an `int`, a `String`, and a `double`. The `int` is our primary key.

    package persist.txn;
    import com.sleepycat.persist.model.Entity;
    import com.sleepycat.persist.model.PrimaryKey;
    import static com.sleepycat.persist.model.Relationship.*;

    @Entity
    public class PayloadDataEntity {
        @PrimaryKey
        private int oID;

        private String threadName;

        private double doubleData;

        PayloadDataEntity() {}

        public double getDoubleData() { return doubleData; }
        public int getID() { return oID; }
        public String getThreadName() { return threadName; }

        public void setDoubleData(double dd) { doubleData = dd; }
        public void setID(int id) { oID = id; }
        public void setThreadName(String tn) { threadName = tn; }

    }

### StoreWriter.java

`StoreWriter.java` provides the implementation for our entity store writer thread. It is responsible for:

- All transaction management.

- Responding to deadlock exceptions.

- Providing data to be stored in the entity store.

- Writing the data to the store.

In order to show off some of the ACID properties provided by JE's transactional support, `StoreWriter.java` does some things in a less efficient way than you would probably decide to use in a true production application. First, it groups 10 database writes together in a single transaction when you could just as easily perform one write for each transaction. If you did this, you could use auto commit for the individual database writes, which means your code would be slightly simpler and you would run a much smaller chance of encountering blocked and deadlocked operations. However, by doing things this way, we are able to show transactional atomicity, as well as deadlock handling.

To begin, we provide the usual package and import statements, and we declare our class:

    package persist.txn;

    import com.sleepycat.je.CursorConfig;
    import com.sleepycat.je.DatabaseException;
    import com.sleepycat.je.DeadlockException;
    import com.sleepycat.je.Environment;
    import com.sleepycat.je.Transaction;

    import com.sleepycat.persist.EntityCursor;
    import com.sleepycat.persist.EntityStore;
    import com.sleepycat.persist.PrimaryIndex;

    import java.util.Iterator;
    import java.util.Random;
    import java.io.UnsupportedEncodingException;

    public class StoreWriter extends Thread
    {

Next we declare our private data members. Notice that we get handles for the environment and the entity store. The random number generator that we instantiate is used to generate unique data for storage in the database. Finally, the `MAX_RETRY` variable is used to define how many times we will retry a transaction in the face of a deadlock.

        private EntityStore myStore = null;
        private Environment myEnv = null;
        private PrimaryIndex<Integer,PayloadDataEntity> pdIndex;
        private Random generator = new Random();
        private boolean passTxn = false;

        private static final int MAX_RETRY = 20;

Next we implement our class constructor. The most interesting thing about our constructor is that we use it to obtain our entity class's primary index.

        // Constructor. Get our handles from here
        StoreWriter(Environment env, EntityStore store)

            throws DatabaseException {
            myStore = store;
            myEnv = env;

            // Open the data accessor. This is used to store persistent
            // objects.
            pdIndex = myStore.getPrimaryIndex(Integer.class,
                        PayloadDataEntity.class);
        }

Now we implement our thread's `run()` method. This is the method that is run when `StoreWriter` threads are started in the main program (see TxnGuide.java).

        // Thread method that writes a series of records
        // to the database using transaction protection.
        // Deadlock handling is demonstrated here.
        public void run () {

The first thing we do is get a `null` transaction handle before going into our main loop. We also begin the top transaction loop here that causes our application to perform 50 transactions.

            Transaction txn = null;

            // Perform 50 transactions
            for (int i=0; i<50; i++) {

Next we declare a `retry` variable. This is used to determine whether a deadlock should result in our retrying the operation. We also declare a `retry_count` variable that is used to make sure we do not retry a transaction forever in the unlikely event that the thread is unable to ever get a necessary lock. (The only thing that might cause this is if some other thread dies while holding an important lock. This is the only code that we have to guard against that because the simplicity of this application makes it highly unlikely that it will ever occur.)

               boolean retry = true;
               int retry_count = 0;
               // while loop is used for deadlock retries
               while (retry) {

Now we go into the `try` block that we use for deadlock detection. We also begin our transaction here.

                    // try block used for deadlock detection and
                    // general exception handling
                    try {

                        // Get a transaction
                        txn = myEnv.beginTransaction(null, null);

Now we write 10 objects under the transaction that we have just begun. By combining multiple writes together under a single transaction, we increase the likelihood that a deadlock will occur. Normally, you want to reduce the potential for a deadlock and in this case the way to do that is to perform a single write per transaction. In other words, we should be using auto commit to write to our database for this workload.

However, we want to show deadlock handling and by performing multiple writes per transaction we can actually observe deadlocks occurring. We also want to underscore the idea that you can combing multiple database operations together in a single atomic unit of work. So for our example, we do the (slightly) wrong thing.

                        // Write 10 PayloadDataEntity objects to the
                        // store for each transaction
                        for (int j = 0; j < 10; j++) {
                            // Instantiate an object
                            PayloadDataEntity pd = new PayloadDataEntity();

                            // Set the Object ID. This is used as the
                            // primary key.
                            pd.setID(i + j);

                            // The thread name is used as a secondary key, and
                            // it is retrieved by this class's getName()
                            // method.
                            pd.setThreadName(getName());

                            // The last bit of data that we use is a double
                            // that we generate randomly. This data is not
                            // indexed.
                            pd.setDoubleData(generator.nextDouble());

                            // Do the put
                            pdIndex.put(txn, pd);
                        }

Having completed the inner database write loop, we could simply commit the transaction and continue on to the next block of 10 writes. However, we want to first illustrate a few points about transactional processing so instead we call our `countObjects()` method before calling the transaction commit. `countObjects()` uses a cursor to read every object in the entity store and return a count of the number of objects that it found.

Because `countObjects()` reads every object in the store, if used incorrectly the thread will self-deadlock. The writer thread has just written 500 objects to the database, but because the transaction used for that write has not yet been committed, each of those 500 objects are still locked by the thread's transaction. If we then simply run a non-transactional cursor over the store from within the same thread that has locked those 500 objects, the cursor will block when it tries to read one of those transactional protected records. The thread immediately stops operation at that point while the cursor waits for the read lock it has requested. Because that read lock will never be released (the thread can never make any forward progress), this represents a self-deadlock for the thread.

There are three ways to prevent this self-deadlock:

1. We can move the call to `countObjects()` to a point after the thread's transaction has committed.

2. We can allow `countObjects()` to operate under the same transaction as all of the writes were performed.

3. We can reduce our isolation guarantee for the application by allowing uncommitted reads.

For this example, we choose to use option 3 (uncommitted reads) to avoid the deadlock. This means that we have to open our cursor handle so that it knows to perform uncommitted reads.

                        // commit
                        System.out.println(getName() + " : committing txn : " + i);
                        System.out.println(getName() + " : Found " +
                            countObjects(txn) + " objects in the store.");

Having performed this somewhat inelegant counting of the objects in the database, we can now commit the transaction.

                        try {
                            txn.commit();
                            txn = null;
                        } catch (DatabaseException e) {
                            System.err.println("Error on txn commit: " +
                                e.toString());
                        }
                        retry = false;

If all goes well with the commit, we are done and we can move on to the next batch of 10 objects to add to the store. However, in the event of an error, we must handle our exceptions correctly. The first of these is a deadlock exception. In the event of a deadlock, we want to abort and retry the transaction, provided that we have not already exceeded our retry limit for this transaction.

                    } catch (DeadlockException de) {
                        System.out.println("################# " + getName() +
                            " : caught deadlock");
                        // retry if necessary
                        if (retry_count < MAX_RETRY) {
                            System.err.println(getName() +
                                " : Retrying operation.");
                            retry = true;
                            retry_count++;
                        } else {
                            System.err.println(getName() +
                                " : out of retries. Giving up.");
                            retry = false;
                        }

In the event of a standard, non-specific database exception, we simply log the exception and then give up (the transaction is not retried).

                    } catch (DatabaseException e) {
                        // abort and don't retry
                        retry = false;
                        System.err.println(getName() +
                            " : caught exception: " + e.toString());
                        e.printStackTrace();

And, finally, we always abort the transaction if the transaction handle is not null. Note that immediately after committing our transaction, we set the transaction handle to null to guard against aborting a transaction that has already been committed.

                    } finally {
                        if (txn != null) {
                            try {
                                txn.abort();
                            } catch (Exception e) {
                                System.err.println("Error aborting txn: " +
                                    e.toString());
                                e.printStackTrace();
                            }
                        }
                    }
                }
            }
        }

The final piece of our `StoreWriter` class is the `countObjects()` implementation. Notice how in this example we open the cursor such that it performs uncommitted reads:

        // A method that counts every object in the store.

        private int countObjects(Transaction txn)  throws DatabaseException {
            int count = 0;

            CursorConfig cc = new CursorConfig();
            // This is ignored if the store is not opened with uncommitted read
            // support.
            cc.setReadUncommitted(true);
            EntityCursor<PayloadDataEntity> cursor = pdIndex.entities(txn, cc);

            try {
                for (PayloadDataEntity pdi : cursor) {
                        count++;
                }
            } finally {
                if (cursor != null) {
                    cursor.close();
                }
            }

            return count;

        }
    }

This completes our transactional example. If you would like to experiment with this code, you can find the example in the following location in your JE distribution:

    JE_HOME/examples/persist/txn

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