We consider the problem of tracking with small relative error an integer function f(n) defined by a distributed update stream f'(n) in the distributed monitoring model. In this model, there are k sites over which the updates f'(n) are distributed, and they must communicate with a central coordinator to maintain an estimate of f(n).

A quantile summary is a data structure that approximates to epsilon-relative error the order statistics of a much larger underlying dataset. In this paper we develop a randomized online quantile summary for the cash register data input model and comparison data domain model that uses O((1/epsilon) log(1/epsilon)) words of memory. This improves upon the previous best upper bound of O((1/epsilon) (log(1/epsilon))^(3/2)) by Agarwal et al. (PODS 2012). Further, by a lower bound of Hung and Ting (FAW 2010) no deterministic summary for the comparison model can outperform our randomized summary in terms of space complexity. Lastly, our summary has the nice property that O((1/epsilon) log(1/epsilon)) words suffice to ensure that the success probability is 1 - exp(-poly(1/epsilon)).