Why Large Cache Systems Need Routing Layers

In a large cache system, performance bottlenecks often arise not from the cache load itself but from the overhead of managing numerous TCP connections, especially when many application instances maintain their own connections to cache nodes, leading to CPU resource consumption on tasks like TLS negotiation and connection tracking. Introducing a routing layer between clients and cache nodes can alleviate these issues by multiplexing connections, thus reducing the number of direct connections each client has to make with cache nodes and minimizing CPU overhead on connection management. By transitioning from a direct client-to-cluster architecture to a hub-and-spoke model, routing layers centralize and manage connections more efficiently, allowing backend nodes to focus resources on request processing rather than infrastructure overhead. Advanced implementations, like those by Twitter and Facebook, have demonstrated the value of this approach by pushing connection complexity out of cache nodes, although the quality of execution is crucial, as poor implementations can introduce new bottlenecks. Moreover, routing layers can also cache frequently accessed data, absorbing significant traffic and reducing backend load, leading to improved latency performance. While routing layers offer substantial benefits in terms of connection management and load distribution, they introduce additional complexity that requires careful monitoring and engineering expertise to ensure that they do not themselves become performance bottlenecks.