Graph Reconstruction in the Congested Clique

Abstract

The congested clique model is a message-passing model of distributed computation where the underlying communication network is the complete graph of n nodes. In this paper we consider the situation where the joint input to the nodes is an nnode labeled graph G, i.e., the local input received by each node is the indicator function of its neighborhood in G. Nodes execute an algorithm, communicating with each other in synchronous rounds and their goal is to compute some function that depends on G. In every round, each of the n nodes may send up to n − 1 different b-bit messages through each of its n − 1 communication links. We denote by R the number of rounds of the algorithm. The product Rb, that is, the total number of bits received by a node through one link, is the cost of the algorithm.The most difficult problem we could attempt to solve is the reconstruction problem, where nodes are asked to recover all the edges of the input graph G. Formally, given a class of graphs G, the problem is defined as follows: if G / ∈ G, then every node must reject; on the other hand, if G ∈ G, then every node must end up, after the R rounds, knowing all the edges of G. It is not difficult to see that the cost Rb of any algorithm that solves this problem (even with public coins) is at least Ω(log |G n |/n), where G n is the subclass of all n-node labeled graphs in G. In this paper we prove that previous bound is tight and that it is possible to achieve it with only R = 2 rounds. More precisely, we exhibit (i) a one-round algorithm that achieves this bound for hereditary graph classes; and (ii) a two-round algorithm that achieves this bound for arbitrary graph classes. Later, we show that the bound Ω(log |G n |/n) cannot be achieved in one-round for arbitrary graph classes, and we give tight algorithms for that case.From (i) we recover all known results concerning the reconstruction of graph classes in one round and bandwidth O(log n): forests, planar graphs, cographs, etc. But we also get new one-round algorithms for other hereditary graph classes such as unit disc graphs, interval graphs, etc. From (ii), we can conclude that any problem restricted to a class of graphs of size 2 O(n log n) can be solved in the congested clique model in two rounds, with bandwidth O(log n). Moreover, our general two-round algorithm is valid for any set of labeled graphs, not only for graph classes (which are sets of labeled graphs closed under isomorphims).