Highly available HTTP request routing and DNS using Cassandra

Cassandra is mostly known for its I/O scalability, but its shared-nothing, highly available foundation is equally useful for applications requiring multi-datacenter distribution and constant uptime. It’s actually quite easy to build and manage HA services by punting the real HA problems to Cassandra’s battle-tested replication and sharding implementation. In this session, we’ll explore Pantheon’s edge routing layer, including:

• How application servers register their presence in Cassandra with a Twisted Python-based REST API. We’ll test updating routes using using cURL.

• Using Twisted Python to expose the Cassandra route mappings via DNS. This stack will provide us with a distributed, highly available UDP DNS cluster in about 30 lines of Python code back-ending to Cassandra. We’ll then read routes using the “dig” shell client.

• Using node.js as an HTTP reverse proxy to look up, cache (in local Redis), and use (with load balancing) routes discovered by reading the “A” records found in DNS. This part tops off the stack and allows us to distribute HTTP requests among application servers registered in Cassandra.

Since this is a case study, we’ll also consider how Pantheon manages the DNS/Cassandra layer to maintain full availability (read and write) during deployments and data center interlink failures.