I found this piece of code in a Github PR that aims at adding the Redis Sentinel support to a Redis client. (Redis sentinel is a service that will frequently check the state of multiple redis, and return the (redis) master’s IP.

I’m not an expert in PHP, so, I really don’t understand how it works, even though it seems to work correctly.

    public function masterAddress(): PromiseInterface
    {
        $chain = reject(new RuntimeException('Initial reject promise'));
        foreach ($this->urls as $url) {
            $chain = $chain->then(function ($masterUrl) {
                return $masterUrl;
            }, function () use ($url) {
                return $this->onError($url);
            });
        }

        return $chain;
    }

In this code:

1. $this->urls are the different redis sentinel’s urls
2. $masterAddress (which is returned from the promise) is the master’s url
3. By calling $this->masterAddress() I get the master’s URL

I don’t even understand how it could return an URL since the $url variable isn’t even used, so, how the chain could be anything else than just a chain having a “reject”.