Methods, systems, and apparatus, including computer programs encoded on computer storage media, for clustering data points. One of the methods includes maintaining data representing a respective ordered tuple of skeleton data points for each of a plurality of clusters. One or more intersecting clusters are determined for a new data point. An updated tuple of skeleton data points is generated for an updated cluster by selecting updated skeleton data points, including selecting the new data point or an existing jth skeleton data point of one of the one or more intersecting clusters according to which random value, of the jth random value for the new data point or the random value for the jth existing skeleton data point, is closest to a limiting value. The new data point is then assigned to the updated cluster.