#include <db_cxx.h> int DbSequence::get(DbTxn *txnid, int32_t delta, db_seq_t *retp, u_int32_t flags);
The DbSequence::get() method returns the next available element in
the sequence and changes the sequence value by delta. The value of delta must be greater than zero. If there are
enough cached values in the sequence handle then they will be
returned. Otherwise the next value will be fetched from the database
and incremented (decremented) by enough to cover the delta and the next batch of cached values.
For maximum concurrency a non-zero cache size should be specified
prior to opening the sequence handle and
DB_TXN_NOSYNC
should be specified for each DbSequence::get() method call.
By default, sequence ranges do not wrap; to cause the sequence to wrap around the beginning or end of its range, specify the DB_SEQ_WRAP flag to the DbSequence::set_flags() method.
The DbSequence::get() method will return EINVAL if the record in the
database is not a valid sequence record, or the sequence has reached
the beginning or end of its range and is not configured to wrap.
The flags parameter must be set to 0 or by bitwise inclusively OR'ing together one or more of the following values:
DB_TXN_NOSYNC
If the operation is implicitly transaction protected (the txnid argument is NULL but the operation occurs to a transactional database), do not synchronously flush the log when the transaction commits.
If the operation is part of an application-specified transaction, the txnid parameter is a transaction handle returned from DbEnv::txn_begin(); if the operation is part of a Berkeley DB Concurrent Data Store group, the txnid parameter is a handle returned from DbEnv::cdsgroup_begin(); otherwise NULL. If no transaction handle is specified, but the operation occurs in a transactional database, the operation will be implicitly transaction protected. No txnid handle may be specified if the sequence handle was opened with a non-zero cache size.
If the underlying database handle was opened in a transaction, calling
DbSequence::get() may result in changes to the sequence object;
these changes will be automatically committed in a transaction
internal to the Berkeley DB library. If the thread of control calling
DbSequence::get() has an active transaction, which holds locks on
the same database as the one in which the sequence object is stored,
it is possible for a thread of control calling DbSequence::get() to
self-deadlock because the active transaction's locks conflict with the
internal transaction's locks. For this reason, it is often preferable
for sequence objects to be stored in their own database.