Transactions
Sophia supports fast optimistic single-statement and multi-statement transactions. Transactions are completely isolated from each other under Serializable Snapshot isolation (SSI).
Single-statement transactions
Single-statement transactions are automatically processed when sp_set(), sp_delete(), sp_upsert(), sp_get() are used on a database object.
As a part of a transactional statement a key-value document must be prepared using sp_document() method.
First argument of sp_document() method must be a database object.
Object must be prepared by setting key and value fields, where value is optional. It is important that while setting key and value fields, only pointers are copied. Real data copies only during first operation.
Prepared document is automatically freed on commit.
void *db = sp_getobject(env, "db.test");
void *o = sp_document(db);
sp_setstring(o, "key", "hello", 0);
sp_setstring(o, "value", "world", 0));
sp_set(db, o); /* transaction */
o = sp_document(db);
sp_set(o, "key", "hello", 0);
sp_delete(db, o);
sp_get(database) method returns a document that is semantically equal to sp_document(database), but is read-only.
Example:
void *o = sp_document(db);
sp_set(o, "key", "hello", 0);
void *result = sp_get(db, o);
if (result) {
int valuesize;
char *value = sp_getstring(result, "value", &valuesize);
printf("%s\n", value);
sp_destroy(result);
}
Multi-statement transactions
Multi-statement transaction is automatically processed when sp_set(), sp_delete(), sp_upsert(), sp_get() are used on a transactional object.
The sp_begin() function is used to start a multi-statement transaction.
During transaction, no updates are written to the database files until a sp_commit() is called. On commit, all modifications that were made are written to the log file in a single batch.
To discard any changes made during transaction operation, sp_destroy() function should be used. No nested transactions are supported.
There are no limit on a number of concurrent transactions. Any number of databases can be involved in a multi-statement transaction.
void *a = sp_getobject(env, "db.database_a");
void *b = sp_getobject(env, "db.database_b");
char key[] = "hello";
char value[] = "world";
/* begin a transaction */
void *transaction = sp_begin(env);
void *o = sp_document(a);
sp_setstring(o, "key", key, sizeof(key));
sp_setstring(o, "value", value, sizeof(value));
sp_set(transaction, o);
o = sp_document(b);
sp_setstring(o, "key", key, sizeof(key));
sp_setstring(o, "value", value, sizeof(value));
sp_set(transaction, o);
/* complete */
sp_commit(transaction);
A transactional status should be checked (both for single and multi-statement):
int status = sp_commit(transaction);
switch (status) {
case -1: /* error */
case 0: /* ok */
case 1: /* rollback */
case 2: /* lock */
}
Rollback status means that transaction has been rollbacked by another concurrent transaction. Lock status means that transaction is not finished and waiting for concurrent transaction to complete. In that case commit should be retried later or transaction can be rollbacked. Any error happened during multi-statement transaction does not rollback a transaction.