Story

I have 3 memcached servers running where I shutdown the one or the other to investigate how PHP-memcached behaves upon a server not beeing reachable.

I have defined 4 servers in PHP, 1 to simulate a server that is mostly offline (spare server). When I shutdown 1 server (=> 2 are still online), the third ->get() gives me a result.

When I shutdown one more server (=> 1 is still online), it won’t find objects pushed to that last server.

Sample output

First run, 3 of 4 servers up:

Entity not found in cache on 1st try: NOT FOUND
Entity not found in cache on 2nd try: NOT FOUND
Entity not found in cache on 3rd try: NOT FOUND
Entity not found in cache on 4th try: NOT FOUND

Second run, 3 of 4 servers up:

Entity found in Cache: SUCCESS

Third run, 2 of 4 servers up:

Entity not found in cache on 1st try: CONNECTION FAILURE
Entity not found in cache on 2nd try: SERVER IS MARKED DEAD
Entity not found in cache on 3rd try: NOT FOUND
Entity not found in cache on 4th try: NOT FOUND

Fourth run, 1 of 4 servers up:

Entity not found in cache on 1st try: CONNECTION FAILURE
Entity not found in cache on 2nd try: SERVER IS MARKED DEAD
Entity not found in cache on 3rd try: CONNECTION FAILURE
Entity not found in cache on 4th try: SERVER IS MARKED DEAD

Although there is one server left online and I do push my object to memcached everytime it does not find any in cache, it is not able to find the key anymore.

I think it should also work with only a single server left.

Can you explain this behaviour to me?

It looks like it is not possible to implement something that is safe even when I shutdown 19 of 20 servers.

Sidequestion: libketama is not really maintained anymore, is it still good to use it? The logic behind the lib was rather good and is also used in the varnish caching server.

Appendix

My Script:

<?php
require_once 'CachableEntity.php';
require_once 'TestEntity.php';

echo PHP_EOL;

$cache = new Memcached();
$cache->setOption(Memcached::OPT_LIBKETAMA_COMPATIBLE, true);
$cache->setOption(Memcached::OPT_DISTRIBUTION, Memcached::DISTRIBUTION_CONSISTENT);
$cache->setOption(Memcached::OPT_SERVER_FAILURE_LIMIT, 1);
$cache->setOption(Memcached::OPT_REMOVE_FAILED_SERVERS, true);
$cache->setOption(Memcached::OPT_AUTO_EJECT_HOSTS, true);

$cache->setOption(Memcached::OPT_TCP_NODELAY, true);
//$cache->setOption(Memcached::OPT_RETRY_TIMEOUT, 10);

$cache->addServers([
    ['localhost', '11212'],
    ['localhost', '11213'],
    ['localhost', '11214'],
    ['localhost', '11215'], // always offline
]);


$entityId = '/test/test/article_123456789.test';

$entity = new TestEntity($entityId);

$found = false;

$cacheKey = $entity->getCacheKey();

$cacheResult = $cache->get($cacheKey);
if (empty($cacheResult)) {
    echo 'Entity not found in cache on 1st try: ' . $cache->getResultMessage(), PHP_EOL;
    
    $cacheResult = $cache->get($cacheKey);
    if (empty($cacheResult)) {
        echo 'Entity not found in cache on 2nd try: ' . $cache->getResultMessage(), PHP_EOL;
        
        $cacheResult = $cache->get($cacheKey);
        if (empty($cacheResult)) {
            echo 'Entity not found in cache on 3rd try: ' . $cache->getResultMessage(), PHP_EOL;
            
            $cacheResult = $cache->get($cacheKey);
            if (empty($cacheResult)) {
                echo 'Entity not found in cache on 4th try: ' . $cache->getResultMessage(), PHP_EOL;
                
                $entity
                    ->setTitle('TEST')
                    ->setText('Hellow w0rld. Lorem Orem Rem Em M IpsuM')
                    ->setUrl('http://www.google.com/content-123456789.html');
                
                $cache->set($cacheKey, $entity->serialize(), 120);
            }
        }
        else { $found = true; }
    }
    else { $found = true; }
}
else { $found = true; }


if ($found === true) {
    echo 'Entity found in Cache: ' . $cache->getResultMessage(), PHP_EOL;
    $entity->unserialize($cacheResult);
    echo 'Title: ' . $entity->getTitle(), PHP_EOL;
}

echo PHP_EOL;