BDB concurrent access

This is a brief introduction of howto  use BDB concurrently, and also give a sample code snippet.

Usage key points:

1. In order to support concurrentBDB access, BDB environment (DbEnv) must be used.

a)  DbEnv must be open() with DB_INIT_LOCKflags.

b)  In fact, DB_INIT_CDB useimplicit lock strategy for multiple read /single write access mode, that means lock subsystem is not enabled, so mutex/lock cannot be explicitly used in this mode.

c)  Please refer to: http://docs.oracle.com/cd/E17076_02/html/api_reference/C/envopen.html#envopen_DB_INIT_CDB

2. mutex: mutex object is validonly within a process, i.e., mutex cannot be used cross process.

3. lock: lock object is valid forboth cross threads and cross processes.

4. BDB concurrent cannot supportwhen database is resident on a NFS.

a)  Please refer to: http://www.oracle.com/technetwork/database/berkeleydb/db-faq-095848.html

Code Sample: use a mutex

int main() {    /** initialize DbEnv */    DbEnv * env = new DbEnv(0);    env->open("data", DB_CREATE | DB_INIT_LOCK | DB_INIT_MPOOL, 0644);    /** initialize mutex object */    db_mutex_t mutex;    env->mutex_alloc(DB_MUTEX_SELF_BLOCK, &mutex);    /** lock mutex */    env->mutex_lock(mutex);            /** unlock mutex */    env->mutex_unlock(mutex);    /** release mutex */    env->mutex_free(mutex);}

Code Sample: use a lock

int main() {    /** initialize DbEnv */    DbEnv * env = new DbEnv(0);    env->open("data", DB_CREATE | DB_INIT_LOCK | DB_INIT_MPOOL, 0644);    /** initialize lock object */    u_int32_t lockid;    env->lock_id(&lockid);    DbLock lock;    db_lockmode_t lock_type = DB_LOCK_WRITE;    char objbuf[1024];    sprintf(objbuf, "testlock");    Dbt dbt(objbuf, (u_int32_t)strlen(objbuf));    /** hold lock */    env->lock_get(lockid, 0, &dbt, lock_type, &lock);    /** release lock */    env->lock_put(&lock);    /** release lockid */    env->lock_id_free(lockid);}