Cell Based Single Tenancy

This is part 4 in a series on SaaS Tenancy Models.  Parts 1 , 2 , and 3.

SaaS companies are often approached by potential clients who want their instance to be completely separate from any other client.  Sometimes the request is driven by legal requirements (primarily healthcare and defense), sometimes it is a desire for enhanced security.

Often, running a Multi-Tenant service with a single client will satisfy the client’s needs.  Clients are often willing to pay for the privilege of their account run Single Tenant, making it a potentially lucrative option for a SaaS.

What is a Cell?

A Cell is an independent instance of a SaaS’ software setup.  This is different from having software running in multiple datacenters or even multiple continents.  If the services talk to each other, they are in the same cell regardless of physical location.

Cells can differ with the number and power of servers and databases.  Cells can even have entirely different caching options depending on need.

The 3 most common Cell setups are Production, Staging (or Test), and Local.

Cell Properties

Cell architecture comes with a few distinct properties:

  • Cell structures allow SaaS to grow internationally and offer clients low latency and localized data policies (think GDPR).  Latency from the US to Europe, Asia and South America is noticeable and degrades the client experience.
  • Clients exist in 1 cell at a time.  They can migrate, but they can’t exist in multiple cells.
  • Generally speaking, Cells can not be part of a disaster recovery plan.  Switching clients between Cells usually involves copying the database, and can’t be done if the client’s original Cell is down.

Cell Isolation as a Single Tenant Option

In part 3 I covered the difficulties in operating in a true Single Tenant model at scale.  A Cell with a single client effectively recreates the Single Tenancy experience.

Few clients want this level of isolation, but those that need it are prepared to pay for the extra infrastructure costs of an additional Cell.

Conclusion

For SaaS without global services, a Cell model enables a mix of clients on logically separated Multi-Tenant infrastructure and clients with effectively Single Tenant infrastructure.  This allows the company to pursue clients with Single Tenant needs, and the higher price point they offer.

The catch is that Single Tenant Cells can’t exist in an architecture with global services.  If there is a single service that must have access to all client data, Single Tenant Cells are out.


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