Multi-tenancy – learn C# (K4, lesson 50)

What you learn in this lesson

tell the tenancy models apart: fully shared, vertically and horizontally partitioned, one instance per customer with deployment stamps

recognize noisy neighbors and say which model contains them and how

keep the tenant ID in every key, every query, every grain and every read model, even where the model doesn't require it

build row-level security in Azure SQL as a second safeguard and prove it with a test

design a control plane for creating, updating and removing offices, and an update process in rings

compare models for 3, 30 and 300 offices by isolation, operations and cost per office, and decide in an ADR

Since lesson 4.1.2, "several offices in one Kladde" has been Q1, at the very top of your quality goals. Since module 4.3, every key has carried a tenant ID, and since lesson 4.7.3 the sign-in admits people from several Entra directories, one per office. One question has remained open, and it is the most expensive architecture decision of this module: How much do the offices share? One Kladde for all, one Kladde per office, or something in between?

This lesson introduces the models, with their consequences for isolation, operations and cost. You build row-level security as a second safeguard, design the control plane that creates, updates and removes offices, and decide the Kladde's model in an ADR. It is built in the final project 4.9.

Tenant and instance

Keep two terms apart in this lesson:

A tenant is an office: a customer of the Kladde, with its people and its data. In the Kladde it is an Entra directory, recognized by the tid (lesson 4.7.3).
An instance (Microsoft also says deployment or stamp) is a set of running infrastructure: a Container Apps environment with its apps and the cluster, a database, a Service Bus namespace, a Key Vault.

How many tenants live on one instance is the tenancy model. For an office to find its instance, every variant with more than one instance needs a mapping tid → instance. It belongs to the control plane, more on that below.

What you learn in this lesson

Tenant and instance

This lesson belongs to course K4