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Consistent Hashing with Bounded Loads

Mikkel Thorup
Proceedings of the Twenty-Ninth Annual ACM-SIAM Symposium on Discrete Algorithms (2018), pp. 587-604


Designing algorithms for balanced allocation of clients to servers in dynamic settings is a challenging problem for a variety of reasons. Both servers and clients may be added and/or removed from the system periodically, and the main objectives of allocation algorithms are: the uniformity of the allocation, and the number of moves after adding or removing a server or a client. The most popular solution for our dynamic settings is Consistent Hashing. However, the load balancing of consistent hashing is no better than a random assignment of clients to servers, so with n of each, we expect many servers to be overloaded with Θ(logn/loglogn) clients. In this paper, with n clients and n servers, we get a guaranteed max-load of 2 while only moving an expected constant number of clients for each update. We take an arbitrary user specified balancing parameter c=1+ϵ>1. With m balls and n bins in the system, we want no load above ⌈cm/n⌉. Meanwhile we want to bound the expected number of balls that have to be moved when a ball or server is added or removed. Compared with general lower bounds without capacity constraints, we show that in our algorithm when a ball or bin is inserted or deleted, the expected number of balls that have to be moved is increased only by a multiplicative factor O(1ϵ2) for ϵ≤1 (Theorem 4) and by a factor 1+O(logcc) for ϵ≥1 (Theorem 3). Technically, the latter bound is the most challenging to prove. It implies that we for superconstant c only pay a negligible cost in extra moves. We also get the same bounds for the simpler problem where we instead of a user specified balancing parameter have a fixed bin capacity C for all bins.