Package io.permazen.kv.simple

Class SimpleKVTransaction

java.lang.Object
io.permazen.kv.AbstractKVStore
io.permazen.kv.simple.SimpleKVTransaction
All Implemented Interfaces:
KVStore, KVTransaction
Direct Known Subclasses:
XMLKVTransaction
public class SimpleKVTransaction extends AbstractKVStore implements KVTransaction
KVTransaction implementation for SimpleKVDatabase.

Locking note: all fields in this class are protected by the Java monitor of the associated SimpleKVDatabase, not the Java monitor of this instance.

  • Constructor Details

  • Method Details

    • getKVDatabase

      public SimpleKVDatabase getKVDatabase()
      Description copied from interface: KVTransaction
      Get the KVDatabase with which this instance is associated.
      Specified by:
      getKVDatabase in interface KVTransaction
      Returns:
      associated database

    • setTimeout

      public void setTimeout(long timeout)
      Description copied from interface: KVTransaction
      Change the timeout for this transaction from its default value (optional operation).
      Specified by:
      setTimeout in interface KVTransaction
      Parameters:
      timeout - transaction timeout in milliseconds, or zero for unlimited

    • watchKey

      public ListenableFuture<Void> watchKey(byte[] key)
      Description copied from interface: KVTransaction
      Watch a key to monitor for changes in its value.

      When this method is invoked, key's current value (if any) as read by this transaction is remembered. The returned Future completes if and when a different value for key is subsequently committed by some transaction, including possibly this one. This includes creation or deletion of the key.

      Key watches outlive the transaction in which they are created, persisting until they complete or are cancel()'ed. When a KVDatabase is KVDatabase.stop()'ed, all outstanding key watches are implicitly cancel()'ed.

      Caveats

      Key watches are not without overhead; applications should avoid overuse. For example, consider creating a single key that is used to consolidate modifications to some set of keys; at the Permazen layer, modification to multiple objects and/or fields can detected and consolidated using an @OnChange method that increments a single Counter field, whose key is then watched (to determine the key corresponding to a Java model object field, use PermazenField.getKey()).

      Conceptually, detection of changes behaves as if by a background thread that periodically creates a new transaction and reads the key's value (the actual implementation will likely be more efficient). This means a change that is quickly reverted could be missed, and that multiple changes could occur before notification. In addition, spurious notifications may occur, where the key's value has not changed.

      A key watch is only guaranteed to be valid if the transaction in which it was created successfully commits. In particular, nothing is specified about how or whether Futures associated with failed transactions complete, so the Futures returned by this method should not be relied on until after a successful commit (perhaps with the help of a transaction callback).

      Key watch support is optional; instances that don't support key watches throw UnsupportedOperationException. Some implementations may only support watching a key that already exists.

      Note: many KVDatabase implementations actually return a ListenableFuture. However, listeners must not perform any long running or blocking operations. Also, because the semantics of RetryKVTransactionException allow for the possibility that the transaction actually did commit, "duplicate" listener notifications could occur.

      Key watch Futures that have not completed yet, but are no longer needed, must be cancel()'ed to avoid memory leaks.

      Key watch support is indepdendent of whether the transaction is read-only.

      Specified by:
      watchKey in interface KVTransaction
      Parameters:
      key - the key to watch
      Returns:
      a Future that returns key when the value associated with key is modified
      See Also:

    • get

      public byte[] get(byte[] key)
      Description copied from interface: KVStore
      Get the value associated with the given key, if any.

      Modifications to the returned byte[] array do not affect this instance.

      Specified by:
      get in interface KVStore
      Overrides:
      get in class AbstractKVStore
      Parameters:
      key - key
      Returns:
      value associated with key, or null if not found

    • getAtLeast

      public KVPair getAtLeast(byte[] min, byte[] max)
      Description copied from interface: KVStore
      Get the key/value pair having the smallest key greater than or equal to the given minimum, if any.

      An optional (exclusive) maximum key may also be specified; if maxKey is null, there is no upper bound; if maxKey <= minKey, null is always returned.

      If keys starting with 0xff are not supported by this instance, and minKey starts with 0xff, then this method returns null.

      Modifications to the returned byte[] arrays do not affect this instance.

      Specified by:
      getAtLeast in interface KVStore
      Overrides:
      getAtLeast in class AbstractKVStore
      Parameters:
      min - minimum key (inclusive), or null for no minimum (get the smallest key)
      max - maximum key (exclusive), or null for no maximum (no upper bound)
      Returns:
      smallest key/value pair with key >= minKey and key < maxKey, or null if none exists

    • getAtMost

      public KVPair getAtMost(byte[] max, byte[] min)
      Description copied from interface: KVStore
      Get the key/value pair having the largest key strictly less than the given maximum, if any.

      An optional (inclusive) minimum key may also be specified; if minKey is null, there is no lower bound (equivalent to a lower bound of the empty byte array); if minKey >= maxKey, null is always returned.

      If keys starting with 0xff are not supported by this instance, and maxKey starts with 0xff, then this method behaves as if maxKey were null.

      Modifications to the returned byte[] arrays do not affect this instance.

      Specified by:
      getAtMost in interface KVStore
      Overrides:
      getAtMost in class AbstractKVStore
      Parameters:
      max - maximum key (exclusive), or null for no maximum (get the largest key)
      min - minimum key (inclusive), or null for no minimum (no lower bound)
      Returns:
      largest key/value pair with key < maxKey and key >= minKey, or null if none exists

    • getRange

      public CloseableIterator<KVPair> getRange(byte[] minKey, byte[] maxKey, boolean reverse)
      Description copied from interface: KVStore
      Iterate the key/value pairs in the specified range. The returned CloseableIterator's remove() method must be supported and should have the same effect as invoking remove() on the corresponding key.

      If keys starting with 0xff are not supported by this instance, and minKey starts with 0xff, then this method returns an empty iteration.

      If keys starting with 0xff are not supported by this instance, and maxKey starts with 0xff, then this method behaves as if maxKey were null.

      The returned CloseableIterator is weakly consistent (see java.util.concurrent). In short, the returned CloseableIterator must not throw ConcurrentModificationException; however, whether or not a "live" CloseableIterator reflects any modifications made after its creation is implementation dependent. Implementations that do make post-creation updates visible in the CloseableIterator, even if the update occurs after some delay, must preserve the order in which the modifications actually occurred.

      The returned CloseableIterator itself is not guaranteed to be thread safe; is should only be used in the thread that created it.

      Invokers of this method are encouraged to close() the returned iterators, though this is not required for correct behavior.

      Modifications to the returned KVPair key and value byte[] arrays do not affect this instance.

      Specified by:
      getRange in interface KVStore
      Parameters:
      minKey - minimum key (inclusive), or null for no minimum (start at the smallest key)
      maxKey - maximum key (exclusive), or null for no maximum (end at the largest key)
      reverse - true to return key/value pairs in reverse order (i.e., keys descending)
      Returns:
      iteration of key/value pairs in the range minKey (inclusive) to maxKey (exclusive)

    • put

      public void put(byte[] key, byte[] value)
      Description copied from interface: KVStore
      Set the value associated with the given key.
      Specified by:
      put in interface KVStore
      Overrides:
      put in class AbstractKVStore
      Parameters:
      key - key
      value - value

    • remove

      public void remove(byte[] key)
      Description copied from interface: KVStore
      Remove the key/value pair with the given key, if it exists.
      Specified by:
      remove in interface KVStore
      Overrides:
      remove in class AbstractKVStore
      Parameters:
      key - key

    • removeRange

      public void removeRange(byte[] minKey, byte[] maxKey)
      Description copied from interface: KVStore
      Remove all key/value pairs whose keys are in a given range.

      The minKey must be less than or equal to maxKey; if they equal (and not null) then nothing happens; if they are both null then all entries are deleted.

      If keys starting with 0xff are not supported by this instance, then:

      • If minKey starts with 0xff, then no change occurs
      • If maxKey starts with 0xff, then this method behaves as if maxKey were null
      Specified by:
      removeRange in interface KVStore
      Overrides:
      removeRange in class AbstractKVStore
      Parameters:
      minKey - minimum key (inclusive), or null for no minimum
      maxKey - maximum key (exclusive), or null for no maximum

    • isReadOnly

      public boolean isReadOnly()
      Description copied from interface: KVTransaction
      Determine whether this transaction is read-only.

      Default is false.

      Specified by:
      isReadOnly in interface KVTransaction
      Returns:
      true if this instance is read-only

    • setReadOnly

      public void setReadOnly(boolean readOnly)
      Description copied from interface: KVTransaction
      Enable or disable read-only mode.

      Read-only transactions allow mutations, but all changes are discarded on KVTransaction.commit().

      Some implementations may impose one or more of the following restrictions on this method:

      • setReadOnly() may only be invoked prior to accessing data;
      • setReadOnly() may only be invoked prior to mutating data; and/or
      • Once set to read-only, a transaction may not be set back to read-write
      If one of the above constraints is violated, an IllegalStateException is thrown.

      Note: for some implementations, the data read from a transaction that is never KVTransaction.commit()'ed is not guaranteed to be up to date, even if that transaction is read-only.

      Default is false.

      Specified by:
      setReadOnly in interface KVTransaction
      Parameters:
      readOnly - read-only setting

    • commit

      public void commit()
      Description copied from interface: KVTransaction
      Commit this transaction.

      Note that if this method throws a RetryKVTransactionException, the transaction was either successfully committed or rolled back. In either case, this instance is no longer usable.

      Note also for some implementations, even read-only transactions must be KVTransaction.commit()'ed in order for the data accessed during the transaction to be guaranteed to be up to date.

      Specified by:
      commit in interface KVTransaction

    • rollback

      public void rollback()
      Description copied from interface: KVTransaction
      Cancel this transaction, if not already canceled.

      After this method returns, this instance is no longer usable.

      Note: for some implementations, rolling back a transaction invalidates guarantees about the the data read during the transaction being up to date, even if the transaction was setReadOnly().

      This method may be invoked at any time, even after a previous invocation of KVTransaction.commit() or KVTransaction.rollback(), in which case the invocation will be ignored. In particular, this method must not throw StaleKVTransactionException.

      Specified by:
      rollback in interface KVTransaction

    • readOnlySnapshot

      public CloseableKVStore readOnlySnapshot()
      Description copied from interface: KVTransaction
      Create a read-only snapshot of the database content represented by this transaction.

      The returned CloseableKVStore should be treated as read-only. It may not actually be read-only, but if it's not, then any changes should have no effect on this instance. The returned CloseableKVStore must be completely independent from this transaction (subsequent changes to either one do not affect the other).

      Note: as with any other information extracted from a KVTransaction, the returned content should not be considered valid until this transaction has been successfully committed.

      The returned CloseableKVStore should be promply close()'d when no longer needed to release any underlying resources. In particular, the caller must ensure that the CloseableKVStore is close()'d even if this transaction's commit fails. This may require adding a transaction synchronization callback, etc.

      This is an optional method; only some underlying key/value store technologies can efficiently support it. Implementations should throw UnsupportedOperationException if not supported.

      Specified by:
      readOnlySnapshot in interface KVTransaction
      Returns:
      independent, read-only copy of this transaction's entire database content

    • finalize

      protected void finalize() throws Throwable
      Ensure transaction is eventually rolled back if leaked due to an application bug.
      Overrides:
      finalize in class Object
      Throws:
      Throwable