Configuring a preferred protocol for dual stack clusters (Memcached)

For Memcached clusters you can control the protocol clients will use to connect to the nodes in the cluster with the IP Discovery parameter. The IP Discovery parameter can be set to either IPv4 or IPv6.

The IP discovery parameter controls the IP protocol used in the config get cluster output. Which in turn will determine the IP protocol used by clients which support auto-discovery for ElastiCache for Memcached clusters.

Changing the IP Discovery will not result in any downtime for connected clients. However, the changes will take some time to propagate.

Monitor the output of getAvailableNodeEndPoints for Java and for Php monitor the output of getServerList. Once the output of these functions reports IPs for all of the nodes in the cluster that use the updated protocol, the changes have finished propagating.

Java Example:

MemcachedClient client = new MemcachedClient(new InetSocketAddress("xxxx", 11211));

Class targetProtocolType = Inet6Address.class; // Or Inet4Address.class if you're switching to IPv4

Set<String> nodes;
    
do {
    nodes = client.getAvailableNodeEndPoints().stream().map(NodeEndPoint::getIpAddress).collect(Collectors.toSet());

    Thread.sleep(1000);
} while (!nodes.stream().allMatch(node -> {
            try {
                return finalTargetProtocolType.isInstance(InetAddress.getByName(node));
            } catch (UnknownHostException ignored) {}
            return false;
        }));

Php Example:

$client = new Memcached;
$client->setOption(Memcached::OPT_CLIENT_MODE, Memcached::DYNAMIC_CLIENT_MODE);
$client->addServer("xxxx", 11211);

$nodes = [];
$target_ips_count = 0;
do {
    # The PHP memcached client only updates the server list if the polling interval has expired and a
    # command is sent
    $client->get('test');
 
    $nodes = $client->getServerList();

    sleep(1);
    $target_ips_count = 0;

    // For IPv4 use FILTER_FLAG_IPV4
    $target_ips_count = count(array_filter($nodes, function($node) { return filter_var($node["ipaddress"], FILTER_VALIDATE_IP, FILTER_FLAG_IPV6); }));
 
} while (count($nodes) !== $target_ips_count);

Any existing client connections that were created before the IP Discovery was updated will still be connected using the old protocol. All of the validated clients will automatically reconnect to the cluster using the new IP protocol once the changes are detected in the output of the cluster discovery commands. However, this depends on the implementation of the client.