Version 1.3.0
Multithreading

All WT_CONNECTION methods are thread safe, and WT_CONNECTION handles can be shared between threads.

Applications typically open a single connection to each database, per process.

WT_SESSION and WT_CURSOR methods are not thread safe and WT_SESSION and WT_CURSOR handles cannot be accessed concurrently by multiple threads. Applications typically open one WT_SESSION handle for each thread accessing a database, and then one or more WT_CURSOR handles within the session.

WT_SESSION and WT_CURSOR methods may be accessed by different threads serially (for example, a pool of threads managed by the application with a set of shared session or cursor handles). There is no thread-local state in WiredTiger, but no built-in synchronization of session or cursor handles, either, so if multiple threads access a session or cursor handle, access must be serialized by the application.

Code samples

The code below is taken from the complete example program ex_thread.c.

This is an example of a thread entry point. A new session is opened for the thread and used for all operations within that thread.

void *
scan_thread(void *arg)
{
WT_SESSION *session;
WT_CURSOR *cursor;
const char *key, *value;
int ret;
ret = conn->open_session(conn, NULL, NULL, &session);
ret = session->open_cursor(session, "table:access",
NULL, NULL, &cursor);
/* Show all records. */
while ((ret = cursor->next(cursor)) == 0) {
ret = cursor->get_key(cursor, &key);
ret = cursor->get_value(cursor, &value);
printf("Got record: %s : %s\n", key, value);
}
return (arg);
}

Here is the main function that starts the threads. It opens a single connection, shared between the threads, and closes the connection after waiting for all of the threads to exit.

int
main(void)
{
WT_SESSION *session;
WT_CURSOR *cursor;
pthread_t threads[NUM_THREADS];
int i, ret;
if ((ret = wiredtiger_open(home, NULL,
"create", &conn)) != 0)
fprintf(stderr, "Error connecting to %s: %s\n",
home, wiredtiger_strerror(ret));
/* Note: further error checking omitted for clarity. */
ret = conn->open_session(conn, NULL, NULL, &session);
ret = session->create(session, "table:access",
"key_format=S,value_format=S");
ret = session->open_cursor(session, "table:access", NULL,
"overwrite", &cursor);
cursor->set_key(cursor, "key1");
cursor->set_value(cursor, "value1");
ret = cursor->insert(cursor);
ret = session->close(session, NULL);
for (i = 0; i < NUM_THREADS; i++)
ret = pthread_create(&threads[i], NULL, scan_thread, NULL);
for (i = 0; i < NUM_THREADS; i++)
ret = pthread_join(threads[i], NULL);
ret = conn->close(conn, NULL);
return (ret);
}