# Multi-Tenant Applications With EF Core > One application serves multiple customers, while keeping their data isolated. You can approach multi-tenancy with a single database and logical isolation, or multiple databases and physical isolation. I'll show you how to implement both with EF Core. Published: 2023-05-20. Author: Milan Jovanović. Canonical: https://milanjovanovic.tech/blog/multi-tenant-applications-with-ef-core A multi-tenant application serves many customers from one deployment while keeping each tenant's data isolated. You get there with a single database and logical isolation, or a database per tenant and physical isolation. In EF Core, a global query filter on a `TenantId` column applies the tenant filter to every query for you. Most software applications today are built around the concept of **multi-tenancy**. One application serves multiple customers, while keeping their data **isolated**. You can approach **multi-tenancy** in two ways: - **Single database** and **logical isolation** of tenants - **Multiple databases** and **physical isolation** of tenants Which option you decide to use will depend mostly on your requirements. Some industries like healthcare require a high degree of **data isolation**, and using a **database per tenant** is a must. So how are we going to **implement multi-tenancy** support with **EF Core**? We can use **Query Filters** to apply a **tenant filter** to all database queries. Implement it once and you can almost forget about it. Let's see what are some of the problems we need to solve. ## How To Use EF Core Query Filters If you want an in-depth dive into **Query filters**, take a look at the newsletter issue where I talked about [**using query filters with EF Core.**](https://milanjovanovic.tech/blog/how-to-use-global-query-filters-in-ef-core) Here's a quick refresher on **Query filters**: - Configure the **query filter** by calling `HasQueryFilter` for your entity - **EF** will apply it to all queries for that entity - You can turn it off with `IgnoreQueryFilters` - Only **one** query filter **per entity** is allowed And here's a simple example: ```csharp modelBuilder .Entity() .HasQueryFilter(order => !order.IsDeleted); ``` All queries to the `Order` table will include an `IsDeleted = FALSE` condition. ## Single Database Multi-Tenancy With EF Core You will need two things to **implement multi-tenancy** on a **single database**: - A way to know **who** the **current tenant** is - A way to **filter** the **data** for that **tenant** only The typical approach for **multi-tenancy** on a **single database** is having a `TenantId` column in your tables. And then filtering on that column when querying the database. You can use the **Query filters** feature in **EF Core** to apply a **global filter** for some entity. Inside of the `OnModelCreating` method we configure the query filter on the `Order` entity: ```csharp public class OrdersDbContext : DbContext { private readonly string _tenantId; public OrdersDbContext( DbContextOptions options, TenantProvider tenantProvider) : base(options) { _tenantId = tenantProvider.TenantId; } protected override void OnModelCreating(ModelBuilder modelBuilder) { modelBuilder .Entity .HasQueryFilter(o => o.TenantId == _tenantId); } } ``` We're using the `TenantProvider` class to get the **current tenant** value. Here's what the `TenantProvider` implementation looks like: ```csharp public sealed class TenantProvider { private const string TenantIdHeaderName = "X-TenantId"; private readonly IHttpContextAccessor _httpContextAccessor; public TenantProvider(IHttpContextAccessor httpContextAccessor) { _httpContextAccessor = httpContextAccessor; } public string TenantId => _httpContextAccessor .HttpContext .Request .Headers[TenantIdHeaderName]; } ``` The `TenantId` is coming from the HTTP request header in this example. A few other options to get the `TenantId` are: - Query string - `api/orders?tenantId=example-tenant-id` - JWT Claim - API Key If you want a more **secure implementation** you should go with [**JWT Claims**](https://milanjovanovic.tech/blog/jwt-authentication-aspnetcore) or [**API Keys**](https://milanjovanovic.tech/blog/how-to-implement-api-key-authentication-in-aspnet-core) to provide the `TenantId` value. ## Separate Databases Multi-Tenancy With EF Core What if we want to **isolate** each tenant to a **separate database**? Here are the changes we need to make: - Applying different **connection string per tenant** - Resolving the connection string for each tenant _somehow_ You can't use **Query filters** here, since we are working with different databases. So you will need to store the **tenant information** and **connection strings** somewhere. A simple example would be store them in the **application settings**: ```json "Tenants": { { "Id": "tenant-1", "ConnectionString": "Host=tenant1.db;Database=tenant1" }, { "Id": "tenant-2", "ConnectionString": "Host=tenant2.db;Database=tenant2" } } ``` You can then register an `IOptions` instance with a list of `Tenant` objects. And we need to slightly modify the `TenantProvider` class to return a **connection string** for the current tenant: ```csharp public sealed class TenantProvider { private const string TenantIdHeaderName = "X-TenantId"; private readonly IHttpContextAccessor _httpContextAccessor; private readonly TenantSettings _tenantSettings; public TenantProvider( IHttpContextAccessor httpContextAccessor, IOptions tenantsOptions) { _httpContextAccessor = httpContextAccessor; _tenants = tenantsOptions.Value; } public string TenantId => _httpContextAccessor .HttpContext .Request .Headers[TenantIdHeaderName]; public string GetConnectionString() { return _tenantSettings.Tenants.Single(t => t.Id == TenantId); } } ``` And the last part is registering your `DbContext` to **dynamically resolve** the **connection string** for the current **tenant**. ```csharp builder.Services.AddDbContext((sp, o) => { var tenantProvider = sp.GetRequiredService(); var connectionString = tenantProvider.GetConnectionString(); o.UseSqlServer(connectionString); }); ``` On every request, we create a new `OrdersDbContext` and connect to the appropriate database for that tenant. You should definitely consider storing the tenant **connection strings** in a **secure** place like **Azure Key Vault**. ## Closing Thoughts I hope you now have a better understanding of how to build a **multi-tenant system** with **EF Core**. I showed you the bare bones implementation, which you can improve to make it more robust and secure. Building **multi-tenant systems** isn't easy, but when you understand the basic principles it shouldn't be too difficult either. That's all for this week. See you next Saturday. --- ## Frequently asked questions ### What is a multi-tenant application? A multi-tenant application is one application that serves multiple customers (tenants) while keeping their data isolated. Each tenant only sees its own data, even though everyone shares the same application. ### What are the two approaches to multi-tenancy? A single database with logical isolation of tenants, or multiple databases with physical isolation. The choice depends on your requirements: some industries like healthcare require a high degree of data isolation, where a database per tenant is a must. ### How do you implement multi-tenancy with EF Core on a single database? Add a TenantId column to your tables and apply an EF Core global query filter with HasQueryFilter comparing TenantId to the current tenant. A TenantProvider resolves the current tenant per request, and EF applies the filter to every query for that entity automatically. ### How do you resolve the current tenant in an ASP.NET Core request? Common options are an HTTP request header, a query string parameter, a JWT claim, or an API key. For a more secure implementation, prefer JWT claims or API keys over headers and query strings. ### How do you implement a database per tenant with EF Core? Query filters don't work across databases, so you store a connection string per tenant, resolve it from the current tenant on each request, and register the DbContext to use that connection string dynamically. Store the connection strings somewhere secure, like Azure Key Vault. ### What are the limitations of EF Core query filters? Only one query filter is allowed per entity, and the filter applies to all queries for that entity unless you explicitly disable it with IgnoreQueryFilters. You configure a filter by calling HasQueryFilter in your model configuration.