Load balancers need in-band feedback control.

Server load balancers (LBs) are critical components of interactive services, routing client requests to servers in a pool. LBs improve service performance and increase availability by spreading the request load evenly across servers. It is time to rethink what LBs can do for applications. As application compute becomes increasingly granular (e.g., microservices), request-processing latencies at servers will be ever more impacted by software and system variability at small time scales (e.g., 100 μ s-1ms). Beyond balancing load, we argue that LBs must actively optimize application response time, by adapting request-routing to quickly-varying server performance. Specifically, we advocate for in-band feedback control : LBs should adapt the request-routing policy using purely local observations of server performance, derived from requests traversing the LB. A key challenge to designing such feedback controllers is that high-speed LBs only see the requests, not the responses. We present the design of an LB that adapts to a server latency inflation of 1 ms and reduces tail latencies in milliseconds, while observing only client-to-server traffic.