WT_CONNECTION handles can be shared between threads, and applications generally only open one connection to a given database per process.
All methods on WT_CONNECTION are thread safe.
WT_SESSION and WT_CURSOR handles cannot be shared between threads concurrently: the usual approach is for applications to open one WT_SESSION for each thread that accesses a database. Applications may open multiple WT_CURSOR handles within a session.
Multiple threads must not access a session handle concurrently (including accessing two or more cursor handles in the same session). However, WT_SESSION handles may be accessed by different threads serially (for example, from a pool of threads managed by the application with a set of shared session handles). There is no thread-local state in WiredTiger, but no built-in synchronization of session state either, so if multiple threads access a session handle or dependencies such as cursors, the access must be serialized by the application.
The code below is taken from the complete example program ex_thread.c, available in the source tree as examples/c/\1
.
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); }