{@code EntityIndex} objects are thread-safe. Multiple threads may safely * call the methods of a shared {@code EntityIndex} object.
* *An index is conceptually a map. {key:value} mappings are * stored in the index and accessed by key. In fact, for interoperability with * other libraries that use the standard Java {@link Map} or {@link SortedMap} * interfaces, an {@code EntityIndex} may be accessed via these standard * interfaces by calling the {@link #map} or {@link #sortedMap} methods.
* *{@code EntityIndex} is an interface that is implemented by several * classes in this package for different purposes. Depending on the context, * the key type (K) and value type (V) of the index take on different meanings. * The different classes that implement {@code EntityIndex} are:
*In all cases, the index key type (K) is a primary or secondary key class. * The index value type (V) is an entity class in all cases except for a {@link * SecondaryIndex#keysIndex}, when it is a primary key class.
* *In the following example, a {@code Employee} entity with a {@code * MANY_TO_ONE} secondary key is defined.
* *
* {@literal @Entity}
* class Employee {
*
* {@literal @PrimaryKey}
* long id;
*
* {@literal @SecondaryKey(relate=MANY_TO_ONE)}
* String department;
*
* String name;
*
* private Employee() {}
* }
*
* Consider that we have stored the entities below:
* *| Entities | ||
|---|---|---|
| ID | Department | Name |
| 1 | Engineering | Jane Smith |
| 2 | Sales | Joan Smith |
| 3 | Engineering | John Smith |
| 4 | Sales | Jim Smith |
{@link PrimaryIndex} maps primary keys to entities:
* *
* {@code PrimaryIndex} primaryIndex =
* store.getPrimaryIndex(Long.class, Employee.class);
*
* | primaryIndex | |||
|---|---|---|---|
| Primary Key | Entity | ||
| 1 | 1 | Engineering | Jane Smith |
| 2 | 2 | Sales | Joan Smith |
| 3 | 3 | Engineering | John Smith |
| 4 | 4 | Sales | Jim Smith |
{@link SecondaryIndex} maps secondary keys to entities:
* *
* {@code SecondaryIndex} secondaryIndex =
* store.getSecondaryIndex(primaryIndex, String.class, "department");
*
* | secondaryIndex | |||
|---|---|---|---|
| Secondary Key | Entity | ||
| Engineering | 1 | Engineering | Jane Smith |
| Engineering | 3 | Engineering | John Smith |
| Sales | 2 | Sales | Joan Smith |
| Sales | 4 | Sales | Jim Smith |
{@link SecondaryIndex#keysIndex} maps secondary keys to primary * keys:
* *
* {@code EntityIndex} keysIndex = secondaryIndex.keysIndex();
*
* | keysIndex | |||
|---|---|---|---|
| Secondary Key | Primary Key | ||
| Engineering | 1 | ||
| Engineering | 3 | ||
| Sales | 2 | ||
| Sales | 4 | ||
{@link SecondaryIndex#subIndex} maps primary keys to entities, for the * subset of entities having a specified secondary key:
* *
* {@code EntityIndex} subIndex = secondaryIndex.subIndex("Engineering");
*
* | subIndex | |||
|---|---|---|---|
| Primary Key | Entity | ||
| 1 | 1 | Engineering | Jane Smith |
| 3 | 3 | Engineering | John Smith |
An {@code EntityIndex} provides a variety of methods for retrieving * entities from an index. It also provides methods for deleting entities. * However, it does not provide methods for inserting and updating. To insert * and update entities, use the {@link PrimaryIndex#put} family of methods in * the {@link PrimaryIndex} class.
* *An {@code EntityIndex} supports two mechanisms for retrieving * entities:
*Using the example entities above, calling {@link #get} on the primary * index will always return the employee with the given ID, or null if no such * ID exists. But calling {@link #get} on the secondary index will retrieve * the first employee in the given department, which may not be very * useful:
* *
* Employee emp = primaryIndex.get(1); // Returns by unique ID
* emp = secondaryIndex.get("Engineering"); // Returns first in department
*
* Using a cursor, you can iterate through all duplicates in the secondary * index:
* *
* {@code EntityCursor} cursor = secondaryIndex.entities();
* try {
* for (Employee entity : cursor) {
* if (entity.department.equals("Engineering")) {
* // Do something with the entity...
* }
* }
* } finally {
* cursor.close();
* }
*
* But for a large database it is much more efficient to iterate over only * those entities with the secondary key you're searching for. This could be * done by restricting a cursor to a range of keys:
* *
* {@code EntityCursor} cursor =
* secondaryIndex.entities("Engineering", true, "Engineering", true);
* try {
* for (Employee entity : cursor) {
* // Do something with the entity...
* }
* } finally {
* cursor.close();
* }
*
* However, when you are interested only in the entities with a particular * secondary key value, it is more convenient to use a sub-index:
* *
* {@code EntityIndex} subIndex = secondaryIndex.subIndex("Engineering");
* {@code EntityCursor} cursor = subIndex.entities();
* try {
* for (Employee entity : cursor) {
* // Do something with the entity...
* }
* } finally {
* cursor.close();
* }
*
* In addition to being more convenient than a cursor range, a sub-index * allows retrieving by primary key:
* ** Employee emp = subIndex.get(1);* *
When using a sub-index, all operations performed on the sub-index are * restricted to the single key that was specified when the sub-index was * created. For example, the following returns null because employee 2 is not * in the Engineering department and therefore is not part of the * sub-index:
* ** Employee emp = subIndex.get(2);* *
For more information on using cursors and cursor ranges, see {@link * EntityCursor}.
* *Note that when using an index, keys and values are stored and retrieved * by value not by reference. In other words, if an entity object is stored * and then retrieved, or retrieved twice, each object will be a separate * instance. For example, in the code below the assertion will always * fail.
** MyKey key = ...; * MyEntity entity1 = index.get(key); * MyEntity entity2 = index.get(key); * assert entity1 == entity2; // always fails! ** *
Any type of index may be used to delete entities with a specified key by * calling {@link #delete}. The important thing to keep in mind is that * all entities with the specified key are deleted. In a primary index, * at most a single entity is deleted:
* ** primaryIndex.delete(1); // Deletes a single employee by unique ID* *
But in a secondary index, multiple entities may be deleted:
* *
* secondaryIndex.delete("Engineering"); // Deletes all Engineering employees
*
* This begs this question: How can a single entity be deleted without * knowing its primary key? The answer is to use cursors. After locating an * entity using a cursor, the entity can be deleted by calling {@link * EntityCursor#delete}.
* *Transactions can be used to provide standard ACID (Atomicity, * Consistency, Integrity and Durability) guarantees when retrieving, storing * and deleting entities. This section provides a brief overview of how to use * transactions with the Direct Persistence Layer. For more information on * using transactions, see Writing * Transactional Applications.
* *Transactions may be used only with a transactional {@link EntityStore}, * which is one for which {@link StoreConfig#setTransactional * StoreConfig.setTransactional(true)} has been called. Likewise, a * transactional store may only be used with a transactional {@link * Environment}, which is one for which {@link * EnvironmentConfig#setTransactional EnvironmentConfig.setTransactional(true)} * has been called. For example:
* *
* EnvironmentConfig envConfig = new EnvironmentConfig();
* envConfig.setTransactional(true);
* envConfig.setAllowCreate(true);
* Environment env = new Environment(new File("/my/data"), envConfig);
*
* StoreConfig storeConfig = new StoreConfig();
* storeConfig.setTransactional(true);
* storeConfig.setAllowCreate(true);
* EntityStore store = new EntityStore(env, "myStore", storeConfig);
*
* Transactions are represented by {@link Transaction} objects, which are * part of the {@link com.sleepycat.db Base API}. Transactions are created * using the {@link Environment#beginTransaction Environment.beginTransaction} * method.
* *A transaction will include all operations for which the transaction * object is passed as a method argument. All retrieval, storage and deletion * methods have an optional {@link Transaction} parameter for this purpose. * When a transaction is passed to a method that opens a cursor, all retrieval, * storage and deletion operations performed using that cursor will be included * in the transaction.
* *A transaction may be committed by calling {@link Transaction#commit} or * aborted by calling {@link Transaction#abort}. For example, two employees * may be deleted atomically with a transaction; other words, either both are * deleted or neither is deleted:
* *
* Transaction txn = env.beginTransaction(null, null);
* try {
* primaryIndex.delete(txn, 1);
* primaryIndex.delete(txn, 2);
* txn.commit();
* txn = null;
* } finally {
* if (txn != null) {
* txn.abort();
* }
* }
*
* WARNING: Transactions must always be committed or aborted to
* prevent resource leaks which could lead to the index becoming unusable or
* cause an OutOfMemoryError. To ensure that a transaction is
* aborted in the face of exceptions, call {@link Transaction#abort} in a
* finally block.
For a transactional store, storage and deletion operations are always * transaction protected, whether or not a transaction is explicitly used. A * null transaction argument means to perform the operation using auto-commit, * or the implied thread transaction if an XAEnvironment is being used. A * transaction is automatically started as part of the operation and is * automatically committed if the operation completes successfully. The * transaction is automatically aborted if an exception occurs during the * operation, and the exception is re-thrown to the caller. For example, each * employee is deleted using a an auto-commit transaction below, but it is * possible that employee 1 will be deleted and employee 2 will not be deleted, * if an error or crash occurs while deleting employee 2:
* ** primaryIndex.delete(null, 1); * primaryIndex.delete(null, 2);* *
When retrieving entities, a null transaction argument means to perform * the operation non-transactionally. The operation is performed outside the * scope of any transaction, without providing transactional ACID guarantees. * If an implied thread transaction is present (i.e. if an XAEnvironment is * being used), that transaction is used. When a non-transactional store is * used, transactional ACID guarantees are also not provided.
* *For non-transactional and auto-commit usage, overloaded signatures for * retrieval, storage and deletion methods are provided to avoid having to pass * a null transaction argument. For example, {@link #delete} may be called * instead of {@link #delete(Transaction,Object)}. For example, the following * code is equivalent to the code above where null was passed for the * transaction:
* ** primaryIndex.delete(1); * primaryIndex.delete(2);* *
For retrieval methods the overloaded signatures also include an optional * {@link LockMode} parameter, and overloaded signatures for opening cursors * include an optional {@link CursorConfig} parameter. These parameters are * described further below in the Locking and Lock Modes section.
* *There are two special consideration when using cursors with transactions. * First, for a transactional store, a non-null transaction must be passed to * methods that open a cursor if that cursor will be used to delete or update * entities. Cursors do not perform auto-commit when a null transaction is * explicitly passed or implied by the method signature. For example, the * following code will throw {@link DatabaseException} when the {@link * EntityCursor#delete} method is called:
* *
* // Does not work with a transactional store!
* {@code EntityCursor} cursor = primaryIndex.entities();
* try {
* for (Employee entity : cursor) {
* cursor.delete(); // Will throw DatabaseException.
* }
* } finally {
* cursor.close();
* }
*
* Instead, the {@link #entities(Transaction,CursorConfig)} signature must * be used and a non-null transaction must be passed:
* *
* {@code EntityCursor} cursor = primaryIndex.entities(txn, null);
* try {
* for (Employee entity : cursor) {
* cursor.delete();
* }
* } finally {
* cursor.close();
* }
*
* The second consideration is that error handling is more complex when * using both transactions and cursors, for the following reasons:
*For example:
* *
* Transaction txn = env.beginTransaction(null, null);
* {@code EntityCursor} cursor = null;
* try {
* cursor = primaryIndex.entities(txn, null);
* for (Employee entity : cursor) {
* cursor.delete();
* }
* cursor.close();
* cursor = null;
* txn.commit();
* txn = null;
* } finally {
* if (cursor != null) {
* cursor.close();
* }
* if (txn != null) {
* txn.abort();
* }
* }
*
* This section provides a brief overview of locking and describes how lock * modes are used with the Direct Persistence Layer. For more information on * locking, see Writing * Transactional Applications.
* *When using transactions, locks are normally acquired on each entity that * is retrieved or stored. The locks are used to isolate one transaction from * another. Locks are normally released only when the transaction is committed * or aborted.
* *When not using transactions, locks are also normally acquired on each * entity that is retrieved or stored. However, these locks are released when * the operation is complete. When using cursors, in order to provide * cursor stability locks are held until the cursor is moved to a * different entity or closed.
* *This default locking behavior provides full transactional ACID guarantees * and cursor stability. However, application performance can sometimes be * improved by compromising these guarantees. As described in Writing * Transactional Applications, the {@link LockMode} and {@link * CursorConfig} parameters are two of the mechanisms that can be used to make * compromises.
* *For example, imagine that you need an approximate count of all entities * matching certain criterion, and it is acceptable for entities to be changed * by other threads or other transactions while performing this query. {@link * LockMode#READ_UNCOMMITTED} can be used to perform the retrievals without * acquiring any locks. This reduces memory consumption, does less processing, * and improves concurrency.
* *
* {@code EntityCursor} cursor = primaryIndex.entities(txn, null);
* try {
* Employee entity;
* while ((entity = cursor.next(LockMode.READ_UNCOMMITTED)) != null) {
* // Examine the entity and accumulate totals...
* }
* } finally {
* cursor.close();
* }
*
* The {@link LockMode} parameter specifies locking behavior on a * per-operation basis. If null or {@link LockMode#DEFAULT} is specified, the * default lock mode is used.
* *It is also possible to specify the default locking behavior for a cursor * using {@link CursorConfig}. The example below is equivalent to the example * above:
* *
* CursorConfig config = new CursorConfig();
* config.setReadUncommitted(true);
* {@code EntityCursor} cursor = primaryIndex.entities(txn, config);
* try {
* Employee entity;
* while ((entity = cursor.next()) != null) {
* // Examine the entity and accumulate totals...
* }
* } finally {
* cursor.close();
* }
*
*
* The use of other lock modes, cursor configuration, and transaction * configuration are discussed in Writing * Transactional Applications.
* *Lock conflict handling is another important topic discussed in Writing * Transactional Applications. To go along with that material, here we * show a lock conflict handling loop in the context of the Direct Persistence * Layer. The example below shows deleting all entities in a primary index in * a single transaction. If a lock conflict occurs, the transaction is aborted * and the operation is retried.
* * *
* void doTransaction(final Environment env,
* final {@code PrimaryIndex} primaryIndex,
* final int maxTries)
* throws DatabaseException {
*
* boolean success = false;
* long sleepMillis = 0;
* for (int i = 0; i < maxTries; i++) {
* // Sleep before retrying.
* if (sleepMillis != 0) {
* Thread.sleep(sleepMillis);
* sleepMillis = 0;
* }
* Transaction txn = null;
* try {
* txn = env.beginTransaction(null, null);
* final {@code EntityCursor} cursor =
* primaryIndex.entities(txn, null);
* try {
* // INSERT APP-SPECIFIC CODE HERE:
* // Perform read and write operations, for example:
* for (Employee entity : cursor) {
* cursor.delete();
* }
* } finally {
* cursor.close();
* }
* txn.commit();
* success = true;
* return;
* } catch (DeadlockException e) {
* sleepMillis = LOCK_CONFLICT_RETRY_SEC * 1000;
* continue;
* } finally {
* if (!success) {
* if (txn != null) {
* txn.abort();
* }
* }
* }
* }
* // INSERT APP-SPECIFIC CODE HERE:
* // Transaction failed, despite retries.
* // Take some app-specific course of action.
* }
*
* Each Direct Persistence Layer index is associated with an underlying * {@link Database} or {@link SecondaryDatabase} defined in the {@link * com.sleepycat.db Base API}. At this level, an index is a Btree managed by * the Berkeley DB Java Edition transactional storage engine. Although you may * never need to work at the {@code Base API} level, keep in mind that some * types of performance tuning can be done by configuring the underlying * databases. See the {@link EntityStore} class for more information on * database and sequence configuration.
* *If you wish to access an index using the {@code Base API}, you may call * the {@link PrimaryIndex#getDatabase} or {@link SecondaryIndex#getDatabase} * method to get the underlying database. To translate between entity or key * objects and {@link DatabaseEntry} objects at this level, use the bindings * returned by {@link PrimaryIndex#getEntityBinding}, {@link * PrimaryIndex#getKeyBinding}, and {@link SecondaryIndex#getKeyBinding}.
* * @author Mark Hayes */ public interface EntityIndexThe operation will not be transaction protected, and {@link * LockMode#DEFAULT} is used implicitly.
* * * @param key the key to search for. * * @return whether the key exists in the index. * * * @throws DatabaseException the base class for all BDB exceptions. */ boolean contains(K key) throws DatabaseException; /** * Checks for existence of a key in this index. * * * @param txn the transaction used to protect this operation, or null * if the operation should not be transaction protected. * * @param key the key to search for. * * @param lockMode the lock mode to use for this operation, or null to * use {@link LockMode#DEFAULT}. * * @return whether the key exists in the index. * * * @throws DatabaseException the base class for all BDB exceptions. */ boolean contains(Transaction txn, K key, LockMode lockMode) throws DatabaseException; /** * Gets an entity via a key of this index. * *The operation will not be transaction protected, and {@link * LockMode#DEFAULT} is used implicitly.
* * @param key the key to search for. * * @return the value mapped to the given key, or null if the key is not * present in the index. * * * @throws DatabaseException the base class for all BDB exceptions. */ V get(K key) throws DatabaseException; /** * Gets an entity via a key of this index. * * @param txn the transaction used to protect this operation, or null * if the operation should not be transaction protected. * * @param key the key to search for. * * @param lockMode the lock mode to use for this operation, or null to * use {@link LockMode#DEFAULT}. * * @return the value mapped to the given key, or null if the key is not * present in the index. * * * @throws DatabaseException the base class for all BDB exceptions. */ V get(Transaction txn, K key, LockMode lockMode) throws DatabaseException; /** * Returns a non-transactional count of the entities in this index. * *This operation is faster than obtaining a count by scanning the index * manually, and will not perturb the current contents of the cache. * However, the count is not guaranteed to be accurate if there are * concurrent updates.
* * @return the number of entities in this index. * * * @throws DatabaseException the base class for all BDB exceptions. */ long count() throws DatabaseException; /** * Deletes all entities with a given index key. * *Auto-commit is used implicitly if the store is transactional.
* * @param key the key to search for. * * @return whether any entities were deleted. * * * @throws DatabaseException the base class for all BDB exceptions. */ boolean delete(K key) throws DatabaseException; /** * Deletes all entities with a given index key. * * @param txn the transaction used to protect this operation, null to use * auto-commit, or null if the store is non-transactional. * * @param key the key to search for. * * @return whether any entities were deleted. * * * @throws DatabaseException the base class for all BDB exceptions. */ boolean delete(Transaction txn, K key) throws DatabaseException; /** * Opens a cursor for traversing all keys in this index. * *The operations performed with the cursor will not be transaction * protected, and {@link CursorConfig#DEFAULT} is used implicitly. If the * store is transactional, the cursor may not be used to update or delete * entities.
* * * @return the cursor. * * @throws DatabaseException the base class for all BDB exceptions. */ EntityCursorThe operations performed with the cursor will not be transaction * protected, and {@link CursorConfig#DEFAULT} is used implicitly. If the * store is transactional, the cursor may not be used to update or delete * entities.
* * @return the cursor. * * @throws DatabaseException the base class for all BDB exceptions. */ EntityCursorThe operations performed with the cursor will not be transaction * protected, and {@link CursorConfig#DEFAULT} is used implicitly. If the * store is transactional, the cursor may not be used to update or delete * entities.
* * * @param fromKey is the lower bound of the key range, or null if the range * has no lower bound. * * @param fromInclusive is true if keys greater than or equal to fromKey * should be included in the key range, or false if only keys greater than * fromKey should be included. * * @param toKey is the upper bound of the key range, or null if the range * has no upper bound. * * @param toInclusive is true if keys less than or equal to toKey should be * included in the key range, or false if only keys less than toKey should * be included. * * @return the cursor. * * @throws DatabaseException the base class for all BDB exceptions. */ EntityCursorThe operations performed with the cursor will not be transaction * protected, and {@link CursorConfig#DEFAULT} is used implicitly. If the * store is transactional, the cursor may not be used to update or delete * entities.
* * @param fromKey is the lower bound of the key range, or null if the range * has no lower bound. * * @param fromInclusive is true if keys greater than or equal to fromKey * should be included in the key range, or false if only keys greater than * fromKey should be included. * * @param toKey is the upper bound of the key range, or null if the range * has no upper bound. * * @param toInclusive is true if keys less than or equal to toKey should be * included in the key range, or false if only keys less than toKey should * be included. * * @return the cursor. * * @throws DatabaseException the base class for all BDB exceptions. */ EntityCursor