|| Checking for direct PDF access through Ovid
Vector clocks have been used to capture causal dependencies between processes in distributed computing systems. Vector clocks are not suitable for mobile computing systems due to (i) lack of scalability: its size is equal to the number of nodes, and (ii) its inability to cope with fluctuations in the number of nodes. This paper presents two efficient alternatives to vector clock, namely, sets of dependency sequences, and hierarchical clock. Both the alternatives are scalable and are immune to fluctuations in the number of nodes in the system.