Documentation of 'org.apache.derby.impl.sql.catalog.SequenceUpdater' Java class
SequenceUpdater
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org.apache.derby.impl.sql.catalog

Class SequenceUpdater

  • All Implemented Interfaces:
    Cacheable
    Direct Known Subclasses:
    SequenceUpdater.BulkInsertUpdater, SequenceUpdater.SyssequenceUpdater

    public abstract class SequenceUpdater
extends java.lang.Object
implements Cacheable

    An object cached in the data dictionary which manages new values for sequences. Note that this class must be public and have a 0-arg constructor in order to satisfy the Cacheable contract.

    This is the abstract superclass of specific implementations for specific sequences. For instance, one subclass handles the ANSI/ISO sequences stored in SYSSEQUENCES. Another subclass could handle the sequences stored in Derby's identity columns.

    This class does a couple tricky things:

    • It pre-allocates a range of values from a sequence so that we don't have to change the on-disk value every time we get the next value for a sequence.
    • When updating the on-disk value, we use a subtransaction of the user's execution transaction. If the special transaction cannot do its work immediately, without waiting for a lock, then a TOO MUCH CONTENTION error is raised. It is believed that this can only happen if someone holds locks on SYSSEQUENCES, either via sequence DDL or a scan of the catalog. The TOO MUCH CONTENTION error tells the user to not scan SYSSEQUENCES directly, but to instead use the SYSCS_UTIL.SYSCS_PEEK_AT_SEQUENCE() if the user needs the current value of the sequence generator.

    Here is the algorithm pursued when the caller asks for the next number in a sequence:

    • We try to get the next number from a cache of pre-allocated numbers. The endpoint (last number in the pre-allocated range) was previously recorded in the catalog row which describes this sequence. If we are successful in getting the next number, we return it and all is well.
    • Otherwise, we must allocate a new range by updating the catalog row. We should not be in contention with another connection because the update method is synchronized.

    • Constructor Detail

      • SequenceUpdater

        public SequenceUpdater()
        No-arg constructor to satisfy the Cacheable contract

    • Method Detail

      • clean

        public void clean(boolean forRemove)
           throws StandardException
        Description copied from interface: Cacheable
        Clean the object. It is up to the object to ensure synchronization of the isDirty() and clean() method calls.
        If forRemove is true then the object is being removed due to an explict remove request, in this case the cache manager will have called this method regardless of the state of the isDirty()
        If an exception is thrown the object must be left in the clean state.
        MT - thread safe - Can be called at any time by the cache manager, it is the responsibility of the object implementing Cacheable to ensure any users of the object do not conflict with the clean call.
        Specified by:
        clean in interface Cacheable
        Throws:
        StandardException - Standard Derby error policy.

      • isDirty

        public boolean isDirty()
        Description copied from interface: Cacheable
        Returns true of the object is dirty. May be called when the object is kept or unkept.
        MT - thread safe
        Specified by:
        isDirty in interface Cacheable

      • getIdentity

        public java.lang.Object getIdentity()
        Description copied from interface: Cacheable
        Get the identity of this object.
        MT - thread safe.
        Specified by:
        getIdentity in interface Cacheable

      • clearIdentity

        public void clearIdentity()
        Description copied from interface: Cacheable
        Put the object into the No Identity state.
        MT - single thread required - Method must only be called be cache manager and the cache manager will guarantee only one thread can be calling it.
        Specified by:
        clearIdentity in interface Cacheable

      • createIdentity

        public Cacheable createIdentity(java.lang.Object key,
                                java.lang.Object createParameter)
                         throws StandardException
        Description copied from interface: Cacheable
        Create a new item.

        Create a new item and set the identity of the object to represent it. The object will be in the No Identity state, ie. it will have just been created or clearIdentity() was just called.
        The object must copy the information out of key, not just store a reference to key if the key is not immutable. After this call the expression getIdentity().equals(key) must return true.

        If the class of the object needs to change (e.g. to support a different format) then the object should create a new object, call its initParameter() with the parameters the original object was called with, set its identity and return a reference to it. The cache manager will discard the reference to the old object.
        If an exception is thrown the object must be left in the no-identity state.
        MT - single thread required - Method must only be called be cache manager and the cache manager will guarantee only one thread can be calling it.

        Specified by:
        createIdentity in interface Cacheable
        Returns:
        an object reference if the object can take on the identity, null otherwise.
        Throws:
        StandardException - If forCreate is true and the object cannot be created.
        See Also:
        CacheManager.create(java.lang.Object, java.lang.Object)

      • setIdentity

        public Cacheable setIdentity(java.lang.Object key)
                      throws StandardException
        Description copied from interface: Cacheable
        Set the identity of the object.

        Set the identity of the object to represent an item that already exists, e.g. an existing container. The object will be in the No Identity state, ie. it will have just been created or clearIdentity() was just called.
        The object must copy the information out of key, not just store a reference to key. After this call the expression getIdentity().equals(key) must return true.
        If the class of the object needs to change (e.g. to support a different format) then the object should create a new object, call its initParameter() with the parameters the original object was called with, set its identity and return a reference to it. The cache manager will discard the reference to the old object.
        If an exception is thrown the object must be left in the no-identity state.
        MT - single thread required - Method must only be called be cache manager and the cache manager will guarantee only one thread can be calling it.

        Specified by:
        setIdentity in interface Cacheable
        Returns:
        an object reference if the object can take on the identity, null otherwise.
        Throws:
        StandardException - Thrown on error
        See Also:
        Cacheable.setIdentity(java.lang.Object)

      • reset

        public void reset(java.lang.Long newValue)
           throws StandardException

        Reset the sequence generator to a new start value. This is used by the special bulk-insert optimization in InsertResultSet.

        Throws:
        StandardException

      • getCurrentValueAndAdvance

        public void getCurrentValueAndAdvance(NumberDataValue returnValue)
                               throws StandardException

        Get the next sequence number managed by this generator and advance the number. Could raise an exception if the legal range is exhausted and wrap-around is not allowed. Only one thread at a time is allowed through here. We do not want a race between the two calls to the sequence generator: getCurrentValueAndAdvance() and allocateNewRange().

        Parameters:
        returnValue - This value is stuffed with the new sequence number.
        Throws:
        StandardException

      • peekAtCurrentValue

        public java.lang.Long peekAtCurrentValue()
                                  throws StandardException

        Get the current value of the sequence generator without advancing it. May return null if the generator is exhausted.

        Throws:
        StandardException

      • updateCurrentValueOnDisk

        public boolean updateCurrentValueOnDisk(java.lang.Long oldValue,
                                        java.lang.Long newValue)
                                 throws StandardException

        Update the value on disk. Does its work in a subtransaction of the user's execution transaction. If that fails, raises a TOO MUCH CONTENTION exception.

        Returns:
        Returns true if the value was successfully updated, false if we lost a race with another session.
        Throws:
        StandardException
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