Chapter 3. Transaction Basics
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## Chapter 3. Transaction Basics

Once you have enabled transactions for your environment and your databases, you can use them to protect your database operations. You do this by acquiring a transaction handle and then using that handle for any database operation that you want to participate in that transaction.

You obtain a transaction handle using the `DbEnv::txn_begin()` method.

Once you have completed all of the operations that you want to include in the transaction, you must commit the transaction using the `DbTxn::commit()` method.

If, for any reason, you want to abandon the transaction, you abort it using `DbTxn::abort()`.

Any transaction handle that has been committed or aborted can no longer be used by your application.

Finally, you must make sure that all transaction handles are either committed or aborted before closing your databases and environment.

### Note

If you only want to transaction protect a single database write operation, you can use auto commit to perform the transaction administration. When you use auto commit, you do not need an explicit transaction handle. See Auto Commit for more information.

For example, the following example opens a transactional-enabled environment and database, obtains a transaction handle, and then performs a write operation under its protection. In the event of any failure in the write operation, the transaction is aborted and the database is left in a state as if no operations had ever been attempted in the first place.

    #include "db_cxx.h"

    ...

    int main(void)
    {
        u_int32_t env_flags = DB_CREATE     |  // If the environment does not
                                               // exist, create it.
                              DB_INIT_LOCK  |  // Initialize locking
                              DB_INIT_LOG   |  // Initialize logging
                              DB_INIT_MPOOL |  // Initialize the cache
                              DB_INIT_TXN;     // Initialize transactions

        u_int32_t db_flags = DB_CREATE | DB_AUTO_COMMIT;
        Db *dbp = NULL;
        const char *file_name = "mydb.db";
        const char *keystr ="thekey";
        const char *datastr = "thedata";

        std::string envHome("/export1/testEnv");
        DbEnv myEnv(0);

        try {

            myEnv.open(envHome.c_str(), env_flags, 0);
            dbp = new Db(&myEnv, 0);

            // Open the database. Note that we are using auto commit for
            // the open, so the database is able to support transactions.
            dbp->open(NULL,       // Txn pointer
                      file_name,  // File name
                      NULL,       // Logical db name
                      DB_BTREE,   // Database type (using btree)
                      db_flags,   // Open flags
                      0);         // File mode. Using defaults

            Dbt key, data;
            key.set_data(keystr);
            key.set_size((strlen(keystr) + 1) * sizeof(char));
            key.set_data(datastr);
            key.set_size((strlen(datastr) + 1) * sizeof(char));

            DbTxn *txn = NULL;
            myEnv.txn_begin(NULL, &txn, 0);
            try {
                db->put(txn, &key, &data, 0);
                txn->commit(0);
            } catch (DbException &e) {
                std::cerr << "Error in transaction: "
                           << e.what() << std::endl;
                txn->abort();
            }

        } catch(DbException &e) {
            std::cerr << "Error opening database and environment: "
                      << file_name << ", "
                      << envHome << std::endl;
            std::cerr << e.what() << std::endl;
        }

        try {
            if (dbp != NULL)
                dbp->close(0);
            myEnv.close(0);
        } catch(DbException &e) {
            std::cerr << "Error closing database and environment: "
                      << file_name << ", "
                      << envHome << std::endl;
            std::cerr << e.what() << std::endl;
            return (EXIT_FAILURE);
        }

        return (EXIT_SUCCESS);
    }

## Committing a Transaction

In order to fully understand what is happening when you commit a transaction, you must first understand a little about what JE is doing with the logging subsystem. Logging causes all database write operations to be identified in logs, and by default these logs are backed by files on disk. These logs are used to restore your databases in the event of a system or application failure, so by performing logging, JE ensures the integrity of your data.

Moreover, JE performs write-ahead logging. This means that information is written to the logs before the actual database is changed. This means that all write activity performed under the protection of the transaction is noted in the log before the transaction is committed. Be aware, however, that database maintains logs in-memory. If you are backing your logs on disk, the log information will eventually be written to the log files, but while the transaction is on-going the log data may be held only in memory.

When you commit a transaction, the following occurs:

- Any log information held in memory is (by default) synchronously written to disk. Note that this requirement can be relaxed, depending on the type of commit you perform. See Non-Durable Transactions for more information. Also, if you are maintaining your logs entirely in-memory, then this step will of course not be taken. To configure your logging system for in-memory usage, see Configuring In-Memory Logging.

- A commit record is written to the log files. This indicates that the modifications made by the transaction are now permanent.

- All locks held by the transaction are released. This means that read operations performed by other transactions or threads of control can now see the modifications without resorting to uncommitted reads (see Reading Uncommitted Data for more information).

To commit a transaction, you simply call `DbTxn::commit()`.

Notice that committing a transaction does not necessarily cause data modified in your memory cache to be written to the files backing your databases on disk. Dirtied database pages are written for a number of reasons, but a transactional commit is not one of them. The following are the things that can cause a dirtied database page to be written to the backing database file:

- Checkpoints.

  Checkpoints cause all dirtied pages currently existing in the cache to be written to disk, and a checkpoint record is then written to the logs. You can run checkpoints explicitly. For more information on checkpoints, see Checkpoints.

- Cache is full.

  If the in-memory cache fills up, then dirtied pages might be written to disk in order to free up space for other pages that your application needs to use. Note that if dirtied pages are written to the database files, then any log records that describe how those pages were dirtied are written to disk before the database pages are written.

Be aware that because your transaction commit caused database modifications recorded in your logs to be forced to disk, your modifications are by default "persistent" in that they can be recovered in the event of an application or system failure. However, recovery time is gated by how much data has been modified since the last checkpoint, so for applications that perform a lot of writes, you may want to run a checkpoint with some frequency.

Note that once you have committed a transaction, the transaction handle that you used for the transaction is no longer valid. To perform database activities under the control of a new transaction, you must obtain a fresh transaction handle.

### Non-Durable Transactions

As previously noted, by default transaction commits are durable because they cause the modifications performed under the transaction to be synchronously recorded in your on-disk log files. However, it is possible to use non-durable transactions.

You may want non-durable transactions for performance reasons. For example, you might be using transactions simply for the isolation guarantee. In this case, you might not want a durability guarantee and so you may want to prevent the disk I/O that normally accompanies a transaction commit.

There are several ways to remove the durability guarantee for your transactions:

- Specify `DB_TXN_NOSYNC` using the `DbEnv::set_flags()` method. This causes JE to not synchronously force any log data to disk upon transaction commit. That is, the modifications are held entirely in the in-memory cache and the logging information is not forced to the filesystem for long-term storage. Note, however, that the logging data will eventually make it to the filesystem (assuming no application or OS crashes) as a part of JE's management of its logging buffers and/or cache.

  This form of a commit provides a weak durability guarantee because data loss can occur due to an application or OS crash.

  This behavior is specified on a per-environment handle basis. In order for your application to exhibit consistent behavior, you need to specify this flag for all of the environment handles used in your application.

  You can achieve this behavior on a transaction by transaction basis by specifying `DB_TXN_NOSYNC` to the `DbTxn::commit()` method.

- Specify `DB_TXN_WRITE_NOSYNC` using the `DbEnv::set_flags()` method. This causes logging data to be synchronously written to the OS's file system buffers upon transaction commit. The data will eventually be written to disk, but this occurs when the operating system chooses to schedule the activity; the transaction commit can complete successfully before this disk I/O is performed by the OS.

  This form of commit protects you against application crashes, but not against OS crashes. This method offers less room for the possibility of data loss than does `DB_TXN_NOSYNC`.

  This behavior is specified on a per-environment handle basis. In order for your application to exhibit consistent behavior, you need to specify this flag for all of the environment handles used in your application.

- Maintain your logs entirely in-memory. In this case, your logs are never written to disk. The result is that you lose all durability guarantees. See Configuring In-Memory Logging for more information.

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