Eliminating the Bandwidth Bottleneck of Central Query Dispatching Through TCP Connection Hand-Over

Abstract

In scale-out database architectures, client queries must be routed to individual backend database servers for processing. In dynamic database systems, where backend servers join and leave a cluster or data partitions move between servers, clients do not know which server to send queries to. Using a central dispatcher, all queries and responses are routed via a single node. In a system with many high-performance backends, such a central node can become the system bottleneck. This paper compares three different approaches for query dispatching in terms of scaling network throughput and processing flexibility. Off-the-shelf TCP/HTTP load-balancers cannot dispatch individual queries arriving over a single connection to different backend servers, unless they are extended to understand the database wire protocol. For small response sizes up to 4 KB, a purpose-built query dispatcher delivers the highest throughput. For larger responses (i.e., BLOBs or data sets for external analysis), a novel approach for network proxying that transparently maps TCP connections between backend servers performs best.We propose hybrid query dispatching that performs a TCP connection hand-over on demand when returning large database results.