SOA Metric: Consumer-to-Service Ratio

The next time you're asked to provide insight into how your SOA initiative is moving along, try using the Consumer-to-Service ratio.

This is a useful metric to track primarily because each reuse averts the costs of developing, operating, and maintaining a new single-purpose service. The idea is that as the number of consumers increases for a service, the return on the costs—both initial and ongoing—that is attributed to that service increases.

However, use caution when you report this metric. As the number of consumers increases for a service, the reliance on that service increases, which exposes the enterprise to an increased consequence of failure. Because scalability is the result of design, implementation, and infrastructure choices and investments, as more consumers begin to employ and rely on a service, the workload of that service increases and could potentially move beyond the limits that the service was expecting—thus, increasing the likelihood of failure. Finally, with consumers converging into a single point, the ability of a service to mature over time becomes increasingly more challenging and costly, as any change to the service requires an increased investment in impact analysis, regression testing, and coordination across the enterprise.

I also bring this out in the September 2009 edition of Microsoft's The Enterprise Journal (http://msdn.microsoft.com/en-us/architecture/aa699435.aspx)