Detailed component design

Fault tolerance: This system likely fits easily onto one or two physical machines. However, this constitutes a single point of failure. In order to prevent data loss, this system should be replicated over multiple availability zones.
DB Design: The entire post DB fits into a single redis cache instance easily. However, redis is not durable. All writes to the redis cache should also be asynchronously forwarded to a persistent DB. A Key-Value store or even a simple log should work well here, since the DB will typically be written to, and only read when reloading the cache after an outage. To facilitate replicating the system into multiple datacenters, the writes to one DB instance should be asynchronously forwarded to its replicas.
Unique URL generation: since the system is now distributed geographically, we need to ensure that two pastes are not given the same URL. To achieve this, a URL should in some way contain the unique ID of the web server it was generated on to prevent collisions. This could be achieved by simply appending the Unique ID of the web server onto the end of the URL.