MongoDB replica set administration using the PHP driver

rs_failover.php

<?php

/**
 * Testing replica set step down and node removal
 *
 * Assumes a three-member replica set is running:
 *
 *   mongod --port 2000 --dbpath /data/rs0-db0 --replSet rs0
 *   mongod --port 2001 --dbpath /data/rs0-db1 --replSet rs0
 *   mongod --port 2002 --dbpath /data/rs0-db2 --replSet rs0
 */
$m = new MongoClient('mongodb://honeydew:2000,honeydew:2001,honeydew:2002', array('replicaSet' => 'rs0'));

for (;;) {
    $rs = $m->admin->command(array('isMaster'=>1));

    printf("RS members: %s; primary: %s\n", implode(',', $rs['hosts']), isset($rs['primary']) ? $rs['primary'] : 'not found');

    if (1 === count($rs['hosts'])) {
        printf("RS has only one member; aborting…\n");
        break;
    }

    if (isset($rs['primary'])) {
        stepDownAndRemove($m, $rs['primary']);
    }

    printf("Sleeping for five seconds…\n");
    sleep(5);
}

/**
 * Step down the given old primary and remove it from the replica set
 *
 * @param MongoClient $m
 * @param string $oldPrimary
 */
function stepDownAndRemove(MongoClient $m, $oldPrimary)
{
    printf("Stepping down %s…\n", $oldPrimary);
    rsStepDown($m);

    printf("Sleeping for five seconds…\n");
    sleep(5);

    printf("Polling for new primary…\n");
    for ($i = 0; $oldPrimary === ($newPrimary = rsPollForPrimary($m)); $i++) {
        /* If the previous step down request was too soon after an failover, it
         * may not have been honored. Retry every five polls.
         */
        if (0 == $i % 5) {
            printf("Retry: stepping down %s…\n", $oldPrimary);
            rsStepDown($m);
        }

        // Sleep to prevent spamming of isMaster queries
        sleep(1);
    }

    printf("Found new primary %s; removing old primary %s…\n", $newPrimary, $oldPrimary);
    rsRemove($m, $oldPrimary);
}

/**
 * Poll the server with isMaster queries until a primary is found
 *
 * @param MongoClient $m
 */
function rsPollForPrimary(MongoClient $m)
{
    for (;;) {
        try {
            $rs = $m->admin->command(array('isMaster' => 1));
        } catch (MongoConnectionException $e) {
            // Driver may not find a candidate if an election is in progress
        }

        if (isset($rs['primary'])) {
            return $rs['primary'];
        }

        sleep(1);
    }
}

/**
 * Ported from the rs.stepDown() helper function in the MongoDB shell
 *
 * @param MongoClient $m
 * @param integer $sec
 */
function rsStepDown(MongoClient $m, $sec = 60)
{
    try {
        $m->admin->command(array('replSetStepDown' => $sec));
    } catch (MongoCursorException $e) {
        // Driver will drop the connection
    }
}

/**
 * Ported from the rs.remove() helper function in the MongoDB shell
 *
 * @param MongoClient $m
 * @param string $host
 */
function rsRemove(MongoClient $m, $host)
{
    $collection = $m->selectCollection('local', 'system.replset');

    if (1 !== $collection->count()) {
        throw new UnexpectedValueException('local.system.replset has unexpected contents');
    }

    $config = $collection->findOne();

    if (empty($config)) {
        throw new UnexpectedValueException('no config object retrievable from local.system.replset');
    }

    $config['version']++;

    foreach ($config['members'] as $i => $_) {
        if ($host === $config['members'][$i]['host']) {
            array_splice($config['members'], $i, 1);

            try {
                $m->admin->command(array('replSetReconfig' => $config));
            } catch (MongoCursorException $e) {
                // Driver will drop the connection
            }

            return;
        }
    }

    throw new UnexpectedValueException(sprintf("couldn't find %s in %s", $host, json_encode($config['members'])));
}

Console log:

$ php rs_remove.php 
RS members: honeydew:2000,honeydew:2002,honeydew:2001; primary: honeydew:2000
Stepping down honeydew:2000…
Sleeping for five seconds…
Polling for new primary…
Found new primary honeydew:2002; removing old primary honeydew:2000…
Sleeping for five seconds…
RS members: honeydew:2002,honeydew:2001; primary: honeydew:2002
Stepping down honeydew:2002…
Sleeping for five seconds…
Polling for new primary…
Retry: stepping down honeydew:2002…
Found new primary honeydew:2001; removing old primary honeydew:2002…
Sleeping for five seconds…
RS members: honeydew:2001; primary: honeydew:2001
RS has only one member; aborting…