- Why Transactions?
- Terminology
- Application Structure
- Opening the Environment
- Opening the Databases
- Recoverability and Deadlock Avoidance
- Atomicity
- Repeatable Reads
- Transactional Cursors
- Nested Transactions
- Environment Infrastructure
- Deadlock Detection
- Performing Checkpoints
- Database and Log File Archival Procedures
- Log File Removal
- Recovery Procedures
- Recovery and Filesystem Operations
- Berkeley DB Recoverability
- Transaction Throughput
This chapter is from the book
This chapter is from the book
This chapter is from the book
Opening the Environment
Creating transaction-protected applications using the Berkeley DB library is quite easy. Applications first use DBENV[Right Arrow]open to initialize the database environment. Transaction-protected applications normally require all four Berkeley DB subsystems, so the DB_INIT_MPOOL, DB_INIT_LOCK, DB_INIT_LOG, and DB_INIT_TXN flags should be specified.
Once the application has called DBENV[Right Arrow]open, it opens its databases within the environment. Once the databases are opened, the application makes changes to the databases inside of transactions. Each set of changes that entails a unit of work should be surrounded by the appropriate txn_begin, txn_commit, and txn_abort calls. The Berkeley DB access methods will make the appropriate calls into the Locking, Logging and Memory Pool subsystems in order to guarantee transaction semantics. When the application is ready to exit, all outstanding transactions should have been committed or aborted.
Databases accessed by a transaction must not be closed during the transaction. Once all outstanding transactions are finished, all open Berkeley DB files should be closed. When the Berkeley DB database files have been closed, the environment should be closed by calling DBENV[Right Arrow]close.
The following code fragment creates the database environment directory and then opens the environment, running recovery. Our DB_ENV database environment handle is declared to be free-threaded using the DB_THREAD flag, and so may be used by any number of threads that we may subsequently create.
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <pthread.h>
#include <stdarg.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <db.h>
#define ENV_DIRECTORY "TXNAPP"
void env_dir_create(void);
void env_open(DB_ENV **);
int
main(int argc, char *argv)
{
extern char *optarg;
extern int optind;
DB *db_cats, *db_color, *db_fruit;
DB_ENV *dbenv;
pthread_t ptid;
int ch;
while ((ch = getopt(argc, argv, "")) != EOF)
switch (ch) {
case '?':
default:
usage();
}
argc -= optind;
argv += optind;
env_dir_create();
env_open(&dbenv);
return (0);
}
void
env_dir_create()
{
struct stat sb;
/*
* If the directory exists, we're done. We do not further check
* the type of the file, DB will fail appropriately if it's the
* wrong type.
*/
if (stat(ENV_DIRECTORY, &sb) == 0)
return;
/* Create the directory, read/write/access owner only. */
if (mkdir(ENV_DIRECTORY, S_IRWXU) != 0) {
fprintf(stderr,
"txnapp: mkdir: %s: %s\n", ENV_DIRECTORY, strerror(errno));
exit (1);
}
}
void
env_open(DB_ENV **dbenvp)
{
DB_ENV *dbenv;
int ret;
/* Create the environment handle. */
if ((ret = db_env_create(&dbenv, 0)) != 0) {
fprintf(stderr,
"txnapp: db_env_create: %s\n", db_strerror(ret));
exit (1);
}
/* Set up error handling. */
dbenv_set_errpfx(dbenv, "txnapp");
/*
* Open a transactional environment:
* create if it doesn't exist
* free-threaded handle
* run recovery
* read/write owner only
*/
if ((ret = dbenv_open(dbenv, ENV_DIRECTORY,
DB_CREATE | DB_INIT_LOCK | DB_INIT_LOG |
DB_INIT_MPOOL | DB_INIT_TXN | DB_RECOVER | DB_THREAD,
S_IRUSR | S_IWUSR)) != 0) {
dbenv_err(dbenv, ret, "dbenv_open: %s", ENV_DIRECTORY);
exit (1);
}
*dbenvp = dbenv;
}
After running this initial program, we can use the db_stat utility to display the contents of the environment directory:
prompt> db_stat -e -h TXNAPP 3.2.1 Environment version. 120897 Magic number. 0 Panic value. 1 References. 6 Locks granted without waiting. 0 Locks granted after waiting. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Mpool Region: 4. 264KB Size (270336 bytes). -1 Segment ID. 1 Locks granted without waiting. 0 Locks granted after waiting. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Log Region: 3. 96KB Size (98304 bytes). -1 Segment ID. 3 Locks granted without waiting. 0 Locks granted after waiting. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Lock Region: 2. 240KB Size (245760 bytes). -1 Segment ID. 1 Locks granted without waiting. 0 Locks granted after waiting. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Txn Region: 5. 8KB Size (8192 bytes). -1 Segment ID. 1 Locks granted without waiting. 0 Locks granted after waiting.