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. Multi-threaded programs must wait for all other threads to exit before closing the WT_CONNECTION handle because that will implicitly close all other handles. Alternatively, a thread can set an event handler using WT_CONNECTION::open_session to be notified when a WT_CONNECTION close is in progress.

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, and 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.

static WT_THREAD_RET
scan_thread(void *conn_arg)
{
    WT_CONNECTION *conn;
    WT_CURSOR *cursor;
    WT_SESSION *session;
    int ret;
    const char *key, *value;
 
    conn = conn_arg;
    error_check(conn->open_session(conn, NULL, NULL, &session));
    error_check(session->open_cursor(session, "table:access", NULL, NULL, &cursor));
 
    /* Show all records. */
    while ((ret = cursor->next(cursor)) == 0) {
        error_check(cursor->get_key(cursor, &key));
        error_check(cursor->get_value(cursor, &value));
 
        printf("Got record: %s : %s\n", key, value);
    }
    if (ret != WT_NOTFOUND)
        fprintf(stderr, "WT_CURSOR.next: %s\n", session->strerror(session, ret));
 
    return (WT_THREAD_RET_VALUE);
}

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(int argc, char *argv[])
{
    WT_CONNECTION *conn;
    WT_SESSION *session;
    WT_CURSOR *cursor;
    wt_thread_t threads[NUM_THREADS];
    int i;
 
    home = example_setup(argc, argv);
 
    error_check(wiredtiger_open(home, NULL, "create", &conn));
 
    error_check(conn->open_session(conn, NULL, NULL, &session));
    error_check(session->create(session, "table:access", "key_format=S,value_format=S"));
    error_check(session->open_cursor(session, "table:access", NULL, "overwrite", &cursor));
    cursor->set_key(cursor, "key1");
    cursor->set_value(cursor, "value1");
    error_check(cursor->insert(cursor));
    error_check(session->close(session, NULL));
 
    for (i = 0; i < NUM_THREADS; i++)
        error_check(__wt_thread_create(NULL, &threads[i], scan_thread, conn));
 
    for (i = 0; i < NUM_THREADS; i++)
        error_check(__wt_thread_join(NULL, &threads[i]));
 
    error_check(conn->close(conn, NULL));
 
    return (EXIT_SUCCESS);
}