Use the cache, Luke, Part 1: from memcached to Membase memcached buckets

I start with a quote:

Matt Ingenthron said internally at Membase Inc they view Memcached as a rabbit. It is fast, but it is pretty dumb and procreates quickly. Before you know it, it will be running wild all over your system.

But this post isn’t about switching from a volatile cache to a persistent solution. It is about removing the dumb part from the memcached setup.

We started with memcached as this is the first step. The setup had its quirks since AWS EC2 doesn’t provide by default a fixed addressing method while the memcached client from PHP still has issues with the timeouts. Therefore, the fallback was the plain memcache client.

The fixed addressing issue was resolved by deploying Elastic IPs with a little trick for the internal network, as explained by Eric Hammond. This might be unfeasible for large enough deployments, but it wasn’t our case. Amazon introduced ElastiCache since then which removes this limitation, but having a bunch of t1.micros with reservation is still way much cheaper. Which makes me wonder why they won’t introduce machine addresses which internally resolve as internal address. They have this technology for a lot of their services, but it is simply unavailable for plain EC2 instances.

Back to the memcached issues. Having a Membase cluster that provides a memcached bucket is a nice drop-in replacement, if you lower a little bit your memory allocation. Membase over memcached still has some overhead as its services tend to occupy more RAM. The great thing is that the cluster requires fewer machines with fixed addressing. We use a couple for high availability reasons, but this is not the rule. The rest have the EC2 provided dynamic addresses. If a machine happens to go down, another one can take up its place.

But there still is the client issue. memcached for PHP is dumb. memcache for PHP is even dumber. None of these can actually speak the Membase goodies. This is the part where Moxi (Memcached Proxy) kicks in. For memcached buckets, Moxi can discover the newly added machines to the Membase cluster without doing any client configuration. Without any Moxi server configuration as the config is streamed to the servers via the machines that have the fixed addresses. With plain memcached, every time there was a change, we needed to deploy the application. The memcached cluster was basically nullified till it was refilled. Doesn’t happen with Moxi + Membase. Since there no “smart client” for PHP which includes the Moxi logic, we use client side Moxi in order to reduce the network round-trips. There still is a local communication over the loopback interface, but the latency is far smaller than doing server-side Moxi. Basically the memcache for PHP client connects to 127.0.0.1:11211 aka where Moxi lives, then the request hits the appropriate Membase server that holds our cached data. It also uses the binary protocol and SASL authentication which is unsupported by the memcache for PHP client.

The last of the goodies about the Membase cluster: it actually has an interface. I may not be an UI fan, I live most of my time in /bin/bash, but I am a stats junkie. The Membase web console can give you realtime info about how the cluster is doing. With plain memcached you’re left in the dust with wrapping up your own interface or calling stats over plain TCP. Which is so wrong at so many levels.

PS: v2.0 will be called Couchbase for political reasons. But currently the stable release is still called Membase.

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