Strategy

Definition

The Strategy pattern defines a family of algorithms, encapsulates each one in a separate class, and makes them interchangeable. Algorithm selection is delegated to configuration or runtime.

Diagram

classDiagram
    class TenantIsolationStrategy {
        SharedDatabase
        SchemaPerTenant
        DatabasePerTenant
    }

    class ITenantIsolationStrategyProvider {
        +CreateDbContext()
    }

    class SharedDatabaseDbContextFactory
    class TenantPerSchemaDbContextFactory
    class TenantPerDatabaseDbContextFactory

    ITenantIsolationStrategyProvider <|.. SharedDatabaseDbContextFactory
    ITenantIsolationStrategyProvider <|.. TenantPerSchemaDbContextFactory
    ITenantIsolationStrategyProvider <|.. TenantPerDatabaseDbContextFactory

    TenantIsolationStrategy ..> ITenantIsolationStrategyProvider : selects

    class IBlobKeyStrategy {
        +BuildObjectKey()
        +ResolveBucketName()
    }

    class PrefixBlobKeyStrategy

    IBlobKeyStrategy <|.. PrefixBlobKeyStrategy

    class IStringEncryptionProvider {
        +EncryptAsync()
        +DecryptAsync()
    }

    class AesStringEncryptionProvider

    IStringEncryptionProvider <|.. AesStringEncryptionProvider

Implementation in Granit

Strategy	Interface	File	Implementations
Tenant isolation	ITenantIsolationStrategyProvider	src/Granit.Persistence/MultiTenancy/	SharedDatabase, TenantPerSchema, TenantPerDatabase
S3 key	IBlobKeyStrategy	src/Granit.BlobStorage/IBlobKeyStrategy.cs	PrefixBlobKeyStrategy
Encryption	IStringEncryptionProvider	src/Granit.Encryption/Providers/	AesStringEncryptionProvider
Tenant resolution	ITenantResolver	src/Granit.MultiTenancy/Resolvers/	HeaderTenantResolver, JwtClaimTenantResolver

Custom variant — Enum-based Selection: TenantIsolationStrategy is an enum used as a selection key. The factory picks the implementation via a switch expression — no reflection or complex configuration.

Rationale

The choice between SharedDatabase, SchemaPerTenant, and DatabasePerTenant has major implications on cost, performance, and security. The Strategy pattern allows changing this decision without modifying application code.

Usage example

// The strategy is selected via appsettings.json (section "Persistence")
// { "Persistence": { "IsolationStrategy": "SchemaPerTenant" } }
services.AddGranitPersistence();

// Application code is identical regardless of the strategy
public sealed class PatientService(AppDbContext db)
{
    public async Task<Patient?> FindAsync(Guid id, CancellationToken cancellationToken)
        => await db.Patients.FindAsync([id], ct);
    // SharedDatabase -> WHERE Id = @id AND TenantId = @tid
    // SchemaPerTenant -> SET search_path TO tenant_xxx; SELECT ... WHERE Id = @id
    // DatabasePerTenant -> Connection to tenant_xxx_db; SELECT ... WHERE Id = @id
}

Further reading

• Strategy — refactoring.guru