Skip to content
Cratis

Select MongoDB databases by tenant

Cratis Arc selects a MongoDB database using the current tenant: default tenants use the configured database name, and named tenants use a tenant-specific suffix. This separates each named tenant’s collections into its own database without changing the code that requests a collection.

Establish the tenant context before resolving a database or collection. Both are scoped services, so a singleton must not capture them; otherwise it can keep using the database selected for an earlier scope. Database naming does not authorize access. Tenant resolution and access checks must prevent unauthorized tenant selection. See the tenancy overview for that setup, or tenancy end to end to compare MongoDB with EF Core and Chronicle.

With the configured database name app, the default resolver chooses:

Current tenant Database
TenantId.NotSet app
TenantId.Default app
A named tenant with value north app+north

tenantId.IsDefault covers both default aliases. Preserve that behavior in a custom resolver so default-tenant reads still reach the database where their data is stored. Choosing a different name can return empty results without an error if that database does not exist.

Custom Database Resolvers

Section titled “Custom Database Resolvers”

Implement IMongoDatabaseNameResolver if your existing databases use a different naming pattern. Apply naming changes as a data migration decision: a resolver selects a database name but does not move existing data.

Custom Resolver Example

Section titled “Custom Resolver Example”

This resolver uses a tenant prefix instead of Arc’s suffix. A named tenant north with base database app resolves to north_app; both default aliases still resolve to app because the branch checks IsDefault.

using Cratis.Arc.MongoDB;
using Cratis.Arc.Tenancy;
using Microsoft.Extensions.Options;


public class CustomMongoDatabaseNameResolver(
    ITenantIdAccessor tenantIdAccessor,
    IOptions<MongoDBOptions> options) : IMongoDatabaseNameResolver
{
    public string Resolve()
    {
        var baseName = options.Value.Database;
        var tenantId = tenantIdAccessor.Current;


        return tenantId.IsDefault
            ? baseName
            : $"{tenantId.Value}_{baseName}";
    }
}

Registering the Resolver

Section titled “Registering the Resolver”

Register the resolver in the MongoDB host configuration before resolving databases or collections:

builder.UseCratisMongoDB(configureMongoDB: mongodb =>
{
    mongodb.WithDatabaseResolver<CustomMongoDatabaseNameResolver>();
});

When to Use a Custom Resolver

Section titled “When to Use a Custom Resolver”

Use a custom resolver to match existing database names or include an environment or region in the name. Check the result for both default aliases and a named tenant before directing application traffic to the new layout.

The Neo4j integration names graph databases per event store and namespace by a different, shared algorithm.

A name resolver only changes the database name; it does not configure sharding or change the server or credentials. For tenant-specific servers or credentials, continue with connection resolution.

Community: Discord