Silo, Pool, and Bridge Models - SaaS Lens

This whitepaper contains additional guidance not found in the SaaS Lens of the AWS Well-Architected Tool.

Silo, Pool, and Bridge Models

SaaS applications can be built with a variety of different architectural models. Regulatory, competitive, strategic, cost efficiency, and market considerations all have some influence on the shape of your SaaS architecture. At the same time, there are strategies and patterns that are applied when defining the footprint of a SaaS application. These patterns fall into one of three categories—silo, bridge, and pool.

The silo model refers to an architecture where tenants are provided dedicated resources. Imagine, for example, as SaaS environment where each tenant of your system has a fully independent infrastructure stack. Or, perhaps each tenant of your system has a separate database. When some or all of a tenant’s resources are deployed in this dedicated fashion, we refer to this as a silo model. It’s important to note that—even though the silo has dedicated resources—a silo environment still relies on a shared identity, onboarding, and operational experience where all tenants are managed and deployed via a shared construct. This differentiates SaaS from a managed service model where customers might be running separate versions of your product with separate onboarding, management, and operational experiences.

In contrast, the pool model of SaaS refers to a scenario where tenants share resources. This is the more classic notion of multi-tenancy where tenants rely on shared, scalable infrastructure to achieve economies of scale, manageability, agility, and so on. These shared resources can apply to some or all of the elements of your SaaS architecture, including compute, storage, messaging, etc.

The final pattern is the bridge model. Bridge is meant to acknowledge the reality that SaaS businesses aren’t always exclusively silo or pool. Instead, many systems have a mixed mode where some of the system is implemented in a silo model and some is in a pooled model. For example, some microservices in your architecture might be implemented with silo and others might use pool. The regulatory profile of a service’s data and its noisy neighbor attributes might steer a microservice to a silo model. Meanwhile the agility, access patterns, and cost profile of another microservice could tip it toward a pool model.