We now use the methods described above to add partial support to the simple_txn example that we presented in Transactional Application. That is, in this section we will:

Note that when we are done with this section, we will be only partially ready to run the application. Some critical pieces will be missing; specifically, we will not yet be handling the differences between a master and a replica. (We do that in the next chapter).

Also, note that in the following code fragments, additions and changes to the code are marked in bold.

To begin, we copy the simple_txn code to a new file called rep_mgr.c. We then make the corresponding change to the program name.

/*
 * File: rep_mgr.c
 */

#include <stdlib.h>
#include <string.h>
#ifndef _WIN32
#include <unistd.h>
#endif

#include <db.h>

#ifdef _WIN32
extern int getopt(int, char * const *, const char *);
#endif

#define CACHESIZE   (10 * 1024 * 1024)
#define DATABASE    "quote.db"

const char *progname = "rep_mgr";

int create_env(const char *, DB_ENV **);
int env_init(DB_ENV *, const char *);
int doloop (DB_ENV *);
int print_stocks(DBC *); 

Next we update our usage function. The application will continue to accept the -h parameter so that we can identify the environment home directory used by this application. However, we also add the

/* Usage function */
static void
usage()
{
    fprintf(stderr, "usage: %s ", progname);
    fprintf(stderr, "[-h home] -m host:port [-o host:port]\n");
    fprintf(stderr, "\t\t[-p priority]\n");
    fprintf(stderr, "where:\n");
    fprintf(stderr, "\t-h identifies the environment home directory\n");
    fprintf(stderr, "\t-o identifies another site participating in "); 
    fprintf(stderr, "this replication group\n");
    fprintf(stderr, "\t-m identifies the host and port used by this ");
    fprintf(stderr, "site (required).\n");
    fprintf(stderr, "\t-n identifies the number of sites in this ");
    fprintf(stderr, "replication group (required).\n");
    fprintf(stderr, "\t-p identifies the election priority used by ");
    fprintf(stderr, "this replica.\n");
    exit(EXIT_FAILURE);
} 

Now we can begin working on our main() function. We begin by adding a couple of variables that we will use to collect TCP/IP host and port information. We also declare a couple of flags that we use to make sure some required information is provided to this application.

int
main(int argc, char *argv[])
{
    extern char *optarg;
    DB_ENV *dbenv;
    const char *home;
    char ch, *host, *portstr;
    int ret, local_is_set, totalsites;
    u_int32_t port;

    dbenv = NULL;

    ret = local_is_set = totalsites = 0;
    home = NULL; 

At this time we can create our environment handle and configure it exactly as we did for simple_txn. The only thing that we will do differently here is that we will set a priority, arbitrarily picked to be 100, so that we can be sure the environment has a priority other than 0 (the default value). This ensures that the environment can become a master via an election.

    if ((ret = create_env(progname, &dbenv)) != 0)
            goto err;

    /* Default priority is 100 */
    dbenv->rep_set_priority(dbenv, 100);   

Now we collect our command line arguments. As we do so, we will configure host and port information as required, and we will configure the application's election priority if necessary.

    /* Collect the command line options */
    while ((ch = getopt(argc, argv, "h:m:o:p:")) != EOF)
        switch (ch) {
        case 'h':
            home = optarg;
            break;
        /* Set the host and port used by this environment */
        case 'm':
            host = strtok(optarg, ":");
            if ((portstr = strtok(NULL, ":")) == NULL) {
                fprintf(stderr, "Bad host specification.\n");
                goto err;
            }
            port = (unsigned short)atoi(portstr);
            if (dbenv->repmgr_set_local_site(dbenv, host, port, 0) != 0) {
                fprintf(stderr,
                    "Could not set local address %s.\n", host);
                goto err;
            }
            local_is_set = 1;
            break;
        /* Identify another site in the replication group */
        case 'o':
            host = strtok(optarg, ":");
            if ((portstr = strtok(NULL, ":")) == NULL) {
                fprintf(stderr, "Bad host specification.\n");
                goto err;
            }
            port = (unsigned short)atoi(portstr);
            if (dbenv->repmgr_add_remote_site(dbenv, host, port, 0) != 0) {
                fprintf(stderr,
                    "Could not add site %s.\n", host);
                goto err;
            }
            break;
        /* Set the number of sites in this replication group */
        case 'n':
            totalsites = atoi(optarg);
            if ((ret = dbenv->rep_set_nsites(dbenv, totalsites)) != 0)
                dbenv->err(dbenv, ret, "set_nsites");
            break;
        /* Set this replica's election priority */
        case 'p':
            dbenv->rep_set_priority(dbenv, atoi(optarg));
            break; 
        case '?':
        default:
            usage();
        }

    /* Error check command line. */
    if (home == NULL || !local_is_set || !totalsites)
        usage(); 

Having done that, we can call env_init(), which we use to open our environment handle. Note that this function changes slightly for this update (see below).

    if ((ret = env_init(dbenv, home)) != 0)
        goto err; 

Finally, we start replication before we go into the doloop() function (where we perform all our database access).

    if ((ret = dbenv->repmgr_start(dbenv, 3, DB_REP_ELECTION)) != 0)
        goto err; 

    if ((ret = doloop(dbenv)) != 0) {
        dbenv->err(dbenv, ret, "Application failed");
        goto err;
    }

err: if (dbenv != NULL)
        (void)dbenv->close(dbenv, 0);

    return (ret);
} 

Beyond that, the rest of our application remains the same for now, with the exception of the env_init() function, which we use to actually open our environment handle. The flags we use to open the environment are slightly different for a replicated application than they are for a non-replicated application. Namely, replication requires the DB_INIT_REP flag.

Also, because we are using the replication framework, we must prepare our environment for threaded usage. For this reason, we also need the DB_THREAD flag.

 int
env_init(DB_ENV *dbenv, const char *home)
{
    u_int32_t flags;
    int ret;

    (void)dbenv->set_cachesize(dbenv, 0, CACHESIZE, 0);
    (void)dbenv->set_flags(dbenv, DB_TXN_NOSYNC, 1);

    flags = DB_CREATE | 
            DB_INIT_LOCK | 
            DB_INIT_LOG | 
            DB_INIT_MPOOL |
            DB_INIT_TXN | 
            DB_INIT_REP |
            DB_THREAD |
            DB_RECOVER;
    if ((ret = dbenv->open(dbenv, home, flags, 0)) != 0)
        dbenv->err(dbenv, ret, "can't open environment");
    return (ret);
}

This completes our replication updates for the moment. We are not as yet ready to actually run this program; there remains a few critical pieces left to add to it. However, the work that we performed in this section represents a solid foundation for the remainder of our replication work.