In a software system, it is inevitable that components will occasionally fail to produce acceptable results and become unavailable to serve its clients. Replication is the technique often used to increase the availability of a system. But replication introduces the issue of inconsistency among the replicas. TACT is a middleware toolkit for building services that can make a controlled and systematic tradeoff between the availability and the data consistency in the system. This paper presents a tradeoff model TOM that gives various properties and constraints that apply to a tradeoff situation. Using this model we show that the tradeoff in TACT is essentially 4 dimensional rather than just 2. The capacity and demand of the system adds additional dimensions to the basic availability and consistency tradeoff. We also show how TOM can be used to fulfill the QoS goals of the system with dynamically changing load and failure characteristics.