.Net Async APIs

Async/Await Keywords

async: Marks a method as asynchronous.

await: Pauses the execution of the method until the awaited task completes, freeing up the thread to handle other work.

Tasks (Task and Task<T>):

Task: Represents an asynchronous operation that does not return a value.

Task<T>: Represents an asynchronous operation that returns a value of type T.

Threading in Async:

Asynchronous APIs do not create new threads; they use the existing thread pool efficiently.

For I/O-bound operations, the thread is freed while waiting for the I/O to complete.

Scenarios for Async APIs:

I/O-Bound Work: APIs like HttpClient.GetAsync, DbContext.ToListAsync.

CPU-Bound Work: Parallel processing using Task.Run

Why Use Async APIs?

1. Better Scalability:

   • Async APIs allow servers to handle more requests by freeing up threads when waiting for I/O-bound operations.

2. Improved Responsiveness:

   • For UI applications, async prevents the UI from freezing during long operations.

3. Efficient Resource Usage:

• Threads are used efficiently, minimizing CPU time and context switching.

Best Practices

1. Always use async/await for I/O-bound operations like database calls or HTTP requests.
2. Propagate Task rather than blocking calls.
3. Ensure proper exception handling with try-catch.
4. Avoid unnecessary use of Task.Run for operations that are already asynchronous.

Generate Models from SQL Server using Entity Framework Core

To generate models from SQL Server database tables using Entity Framework (EF) in .NET, you can follow the Database-First approach with Entity Framework Core. Here's a step-by-step guide:

Steps to Generate Models from SQL Server Tables in EF Core:

1. Install Entity Framework Core NuGet Packages:

   Open the Package Manager Console or NuGet Package Manager in Visual Studio, and install the following packages:

   • For SQL Server support:

     mathematica
     Install-Package Microsoft.EntityFrameworkCore.SqlServer  

   • For tools to scaffold the database:

     mathematica
     Install-Package Microsoft.EntityFrameworkCore.Tools  

2. Add Connection String in appsettings.json:

   In the appsettings.json file, add your SQL Server connection string:

   json
   {    "ConnectionStrings": {      "DefaultConnection": "Server=your_server;Database=your_database;User Id=your_username;Password=your_password;"    }  }  

3. Scaffold the Models Using Database-First Approach:

   In the Package Manager Console, run the following command to scaffold the models and DbContext based on your SQL Server database:

   bash
   Scaffold-DbContext "Your_Connection_String" Microsoft.EntityFrameworkCore.SqlServer -OutputDir Models  

   Replace "Your_Connection_String" with the actual connection string or the name from appsettings.json. For example:

   bash
   Scaffold-DbContext "Server=your_server;Database=your_database;User Id=your_username;Password=your_password;" Microsoft.EntityFrameworkCore.SqlServer -OutputDir Models  

   This command will generate the entity classes (models) corresponding to your database tables and a DbContext class in the Models folder.

   • Additional Options:
     • -Tables: Scaffold specific tables.
     • -Schemas: Include specific schemas.
     • -Context: Set a specific name for the DbContext class.
     • -Force: Overwrite existing files.

   Example of scaffolding specific tables:

   bash
   Scaffold-DbContext "Your_Connection_String" Microsoft.EntityFrameworkCore.SqlServer -OutputDir Models -Tables Table1,Table2  

4. Use the Generated DbContext:

   Once the models are generated, you can use the DbContext class to interact with the database in your code.

   In your Startup.cs or Program.cs (for .NET 6+), add the DbContext service:

   csharp
   public class Startup  {      public void ConfigureServices(IServiceCollection services)      {          services.AddDbContext<YourDbContext>(options =>              options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));      }  }  

   Replace YourDbContext with the name of the generated context.

5. Use the Models in Your Code:

   Now, you can use the DbContext to query and save data. For example:

   csharp
   public class YourService  {      private readonly YourDbContext _context;        public YourService(YourDbContext context)      {          _context = context;      }        public async Task<List<YourEntity>> GetAllEntities()      {          return await _context.YourEntities.ToListAsync();      }  }  

That's it! You've now generated models from SQL Server tables using Entity Framework Core in .NET.

Multi Tenant Single Database Architecture

Architecture Overview

1. Azure SQL Server (Single Database, Multi-Tenant)

   • Tenant Isolation: Utilize row-based data partitioning with a TenantId field in all tables to ensure each tenant's data is separated.
   • Elastic Pool: For scalability, consider Azure SQL Elastic Pools, which help optimize cost and performance.
   • Security: Implement role-based access control (RBAC) and encryption (Azure Transparent Data Encryption - TDE) to ensure data protection.

2. .NET Middleware (Microservices or Web API)

   • ASP.NET Core Web API: Use ASP.NET Core to build RESTful services.
   • Multi-Tenancy: Implement middleware in the API to handle tenant context, which extracts tenant information from the request (e.g., from headers, subdomain, or JWT token).
     • Use a TenantResolver service to identify tenant context.
     • Apply filters for tenant-based data isolation in data queries.
   • Authentication & Authorization: Use Azure AD B2C or IdentityServer4 for user authentication and JWT tokens to secure API endpoints.
   • Order Management: The API should handle stock order creation, updates, cancellations, and queries.
   • Microservices (optional): Consider breaking down the system into microservices (e.g., OrderService, TradeService, UserService) to increase scalability.

3. Redis Cache

   • Caching Strategy: Use Redis for caching frequently accessed data like tenant metadata, stock prices, order histories, and trade status.
   • Session Management: Store session data (if needed) in Redis for quick retrieval, minimizing database round-trips.
   • Caching API Responses: Cache expensive database queries (e.g., stock prices or order histories) to improve performance.
   • Tenant Isolation in Cache: Use a prefix for each tenant in Redis keys to ensure separation.

4. Azure Components

   • Azure App Service: Deploy the .NET API on Azure App Service for easy scaling and management.
   • Azure Key Vault: Securely store credentials, API keys, and connection strings.
   • Azure Blob Storage: For storing non-relational data like trade receipts, logs, etc.
   • Azure Monitor & Application Insights: Track application health, performance metrics, and logging.

Data Flow Example

1. User Authentication: The client (mobile/web app) sends a login request; the .NET Middleware validates the credentials using Azure AD B2C.
2. Tenant Context Setup: After successful authentication, a JWT token containing the TenantId is issued. All subsequent API calls include this token.
3. Order Processing:
   • Client sends a stock trade order request (buy/sell).
   • The API validates the request, ensures the user has permission (based on TenantId), and checks stock availability.
   • The order is processed and stored in the SQL Server with the TenantId.
   • A success response is returned, and the result is cached in Redis for fast retrieval.
4. Caching:
   • Redis stores stock prices, order statuses, and user session data for faster access.
   • When a stock price or order status is updated, the Redis cache is invalidated and refreshed.

Scaling Considerations

• Horizontal Scaling: Use Azure App Service Autoscaling to handle traffic spikes.
• SQL Performance: Regularly optimize SQL queries and use indexes to maintain performance at scale.
• Redis Scaling: Use Azure Redis Cache for distributed caching across instances.
• Multi-Tenant Strategy: If needed, evolve the system to shard tenants across multiple databases as the user base grows.

Strategies to handle Tenant upgrades and new Schema changes

Handling database upgrades and schema changes in a multi-tenant system is critical to ensure minimal downtime, consistent data integrity, and support for different versions of the schema across tenants. Here's a structured approach to manage this process:

1. Database Versioning

Each schema should have a version number to keep track of changes. Use a version control system for database schemas, just like for application code. Tools like Flyway or Liquibase can be used to track and apply schema changes consistently across tenants.

Versioning Strategy:

• Schema Version Table: Create a table in each tenant's database or schema that tracks the current schema version.

  sql
  CREATE TABLE SchemaVersion (    VersionNumber INT,    AppliedAt DATETIME  );  

• Baseline Version: Assign a baseline version when onboarding a tenant, so each tenant's schema can be upgraded from its specific starting point.

2. Schema Change Tools

Use automated database migration tools to handle schema changes, allowing you to version, apply, and roll back changes safely.

• Flyway: A popular choice for managing SQL-based schema migrations. It applies schema updates based on versioned migration scripts.
  • Each migration is written as a SQL or Java file (e.g., V1.1__Add_Column.sql).
  • Flyway automatically checks the database schema version and applies the necessary migrations sequentially.
• Liquibase: Another option that offers more control over schema updates, including rollback scripts. Liquibase can manage schema changes declaratively through XML, YAML, JSON, or SQL files.

3. Upgrade Process for Multi-Tenant Systems

Approach 1: Rolling Schema Updates

This approach allows tenants to remain operational during schema upgrades. Each tenant's schema is updated independently, minimizing downtime and allowing for gradual upgrades.

Steps:

1. Prepare Backward-Compatible Changes:

   • When adding new columns or tables, ensure that they do not affect the current functionality.
   • Ensure new features are toggled off (using feature flags) until all tenants are upgraded.

2. Apply Schema Migrations Tenant-by-Tenant:

   • Use the schema version table to check the current version for each tenant.
   • Apply the migration scripts based on the tenant's current version. The migration tool (e.g., Flyway) will ensure each tenant's schema is updated sequentially.
   • You can run migrations for each tenant separately to avoid locking issues or heavy load on the database.

   Example Flyway Command:

   bash
   flyway -url=jdbc:sqlserver://yourserver;databaseName=tenant_db -schemas=tenant1_schema -user=user -password=password migrate  

3. Test and Verify:

   • After upgrading the schema for each tenant, verify that the schema matches the expected version.
   • Perform validation using database health checks or by testing key queries to ensure data integrity.

4. Feature Enablement:

   • Once all tenants have been upgraded to the required version, enable the new features via feature flags.
   • Ensure backward compatibility by maintaining support for both old and new versions until all tenants have migrated.

Approach 2: Blue-Green Deployment for Databases

For large schema changes that might affect all tenants simultaneously, a blue-green deployment strategy can be applied to the database:

• Blue Environment: The existing schema that all tenants are using.
• Green Environment: The updated schema version. New code is deployed against this version.

Steps:

1. Prepare Green Schema: Set up the new schema (Green) in a parallel environment. Apply the necessary schema changes without affecting the live environment.

2. Dual Writes: Implement dual-write logic in your application, where data is written to both the old and new schemas during the transition period. This allows the system to maintain consistency while tenants transition to the new schema.

3. Switch Traffic: Once the new schema is verified, redirect tenants to the green environment with the updated schema.

4. Roll Back: If issues are found in the Green environment, you can immediately roll back to the Blue environment.

Approach 3: Tenant-by-Tenant Phased Upgrades

If you have many tenants, you can phase upgrades over time by upgrading a small batch of tenants in each phase. This allows you to test the schema changes on a subset of tenants before rolling them out more broadly.

Steps:

1. Select a Batch of Tenants: Begin with a few non-critical or test tenants.
2. Apply Schema Changes: Use Flyway/Liquibase to apply schema updates only to the selected batch.
3. Monitor and Test: Closely monitor application performance and test the system for that batch.
4. Roll Out to More Tenants: Once verified, repeat the process for other tenant groups until all are updated.

4. Backward Compatibility and Zero-Downtime Deployments

Always ensure that database schema changes are backward-compatible with the current version of your application to avoid breaking tenants that have not yet been upgraded.

Best Practices:

• Additive Changes First: Start by adding new columns or tables without modifying or removing existing structures. Existing queries and services should continue to work without disruption.

  • Example: When adding a new column, set default values or allow nulls to avoid breaking old code.

  sql
  ALTER TABLE Orders ADD COLUMN NewStatus NVARCHAR(100) DEFAULT 'Pending';  

• Deprecation Process: Only remove old fields or tables after all tenants have migrated and the system is no longer using those columns.

• Data Migration: If data needs to be transformed or moved as part of the schema change, use background jobs to migrate data progressively, reducing downtime.

  • Example: When splitting a column into two, add the new columns first, copy the data over, then gradually switch to using the new columns in code.

5. Handling Schema Conflicts and Rollbacks

In a multi-tenant system, it's crucial to be prepared for potential schema conflicts and the need for rollbacks.

Schema Conflict Handling:

• Tenant-Specific Logic: Sometimes, one tenant may require a schema change that conflicts with another tenant's schema version. Ensure that schema upgrades are tenant-specific when necessary, and that one tenant's schema change doesn't affect others.

• Test on Staging Environments: Before applying schema updates, test the changes on a staging environment with data similar to each tenant's schema and version. This ensures that you catch schema conflicts early.

Rollback Strategy:

• Database Snapshots: Take snapshots or backups before applying schema changes. In case of failure, you can restore the schema to its previous version.

• Migration Rollback Scripts: For every migration, write a corresponding rollback script that can revert the schema to its previous state. Liquibase has built-in rollback capabilities, while Flyway requires writing explicit rollback scripts.

6. Database Upgrade Automation Using CI/CD

Automating the schema upgrade process using CI/CD pipelines ensures consistency, minimizes errors, and reduces downtime.

• Azure DevOps Pipelines: Integrate schema migration tools (Flyway, Liquibase) into your CI/CD pipelines. This ensures that every database change is automatically applied as part of the deployment.

• Automated Testing: Include database migration tests in your pipeline to ensure that the schema changes work as expected before being deployed to production.

Example YAML for an Azure DevOps pipeline integrating Flyway:

yaml
trigger:    branches:      include:        - main    jobs:  - job: DatabaseMigrations    steps:      - task: UseFlyway@2        inputs:          flywayCommand: 'migrate'          databaseUrl: '$(DB_URL)'          username: '$(DB_USER)'          password: '$(DB_PASSWORD)'  

Conclusion

Handling database upgrades and schema changes in a multi-tenant system involves careful planning to ensure backward compatibility, automation through migration tools like Flyway or Liquibase, and the use of strategies like rolling updates or blue-green deployments. By using these techniques, you can ensure minimal disruption to tenants while continuously evolving your system's database structure.

Strategies to handle Multi Tenant Customizations and different schemas

Handling per-tenant customizations and schema differences in a multi-tenant order management system requires flexibility at both the application and database levels. Here's how you can address both:

1. Per-Tenant Customizations

Each tenant might have unique requirements, such as specific business rules, UI customizations, or custom workflows. These customizations should be handled without compromising the integrity of the core system.

Approaches:

• Feature Flags: Implement feature flags to enable or disable specific features for each tenant. Services like Azure App Configuration or LaunchDarkly can manage these flags, allowing you to toggle features without redeploying the codebase.

• Custom Business Logic per Tenant: You can introduce tenant-specific logic in your microservices by utilizing configuration-based strategies. For instance, if Tenant A has different tax calculation rules than Tenant B, you can define these rules in a configuration database or use rule engines like Drools for complex business logic.

• UI Customizations: If tenants need custom branding or workflows, you can:

  • Use Dynamic CSS/Theming for different layouts, colors, and branding based on the tenant.
  • Implement Dynamic Component Rendering: Based on the tenant's configuration, the frontend will render different components or workflows.

• Pluggable Services/Modules: For larger customizations, you can structure your microservices in a modular way, where certain microservices or plugins can be tenant-specific and only loaded if needed.

2. Handling Different Schemas for Each Tenant

In a multi-tenant architecture, tenants might require not only data separation but also different database schemas. There are several strategies to manage schema differences:

Approaches:

• Schema-per-Tenant:

  • How it Works: Each tenant has a separate schema within the same database. You can customize the schema structure (tables, columns) per tenant while sharing the same database server and instance.
  • Pros: Provides flexibility in customizing the database schema for each tenant. Ideal if tenants have varying requirements for data structure.
  • Cons: Complexity in management increases as the number of tenants grows, especially in terms of migrations, backups, and updates.

• Database-per-Tenant:

  • How it Works: Each tenant has an entirely separate database. The schema can vary as needed for each tenant, and you can optimize the database configuration independently.
  • Pros: Offers complete isolation of tenant data and schema. Simplifies tenant-specific schema changes and offers strong data security and isolation.
  • Cons: Can become expensive with more tenants. Managing backups, updates, and monitoring for each database can be complex.

• Shared Database with Flexible Schema:

  • How it Works: In this model, all tenants share the same database, but each tenant's schema can be extended by adding columns or additional metadata fields (for example, using JSON columns to store custom data). You can use polymorphic tables or partitioned tables based on tenants.
  • Pros: Easy to scale since you're using a single database. Ideal for scenarios where tenants mostly share the same schema but have slight variations.
  • Cons: Limited flexibility if schema differences are significant.

3. Implementing Schema Differences

Depending on the approach chosen, the implementation of schema variations can be handled by:

• Schema-per-Tenant (Single Database):

  • You can dynamically switch the schema in your queries based on the tenant's context. This is supported by most ORMs like Entity Framework in .NET, where you can dynamically switch the schema or connection string based on the tenant.

    Example:

    csharp
    public DbContext GetDbContext(string tenantId)  {      var tenantSchema = GetTenantSchema(tenantId); // Fetch tenant-specific schema      var optionsBuilder = new DbContextOptionsBuilder<MyDbContext>();      optionsBuilder.UseSqlServer($"Server=myServer;Database=myDb;Schema={tenantSchema};...");      return new MyDbContext(optionsBuilder.Options);  }  

• Database-per-Tenant:

  • Store the connection strings for each tenant in a configuration store (e.g., Azure Key Vault, App Configuration). Based on the incoming request, the system switches to the appropriate database.

    Example:

    csharp
    public DbContext GetDbContext(string tenantId)  {      var connectionString = GetTenantConnectionString(tenantId); // Fetch tenant-specific connection string      var optionsBuilder = new DbContextOptionsBuilder<MyDbContext>();      optionsBuilder.UseSqlServer(connectionString);      return new MyDbContext(optionsBuilder.Options);  }  

• Shared Database with Flexible Schema:

  • Store tenant-specific fields as JSON or extra columns in the database, and use Entity Framework's HasJsonConversion() or equivalent methods to map them.
  • Introduce versioning in the database schema to track different schema versions used by different tenants.

4. Tenant Identification and Routing

Regardless of schema or database strategy, each incoming request must be associated with the correct tenant. This can be done through:

• Request Headers: Include the tenant ID in the request header and intercept it in middleware.
• Subdomains: Map each tenant to a unique subdomain (e.g., tenant1.myapp.com, tenant2.myapp.com) and use middleware to resolve the tenant.
• JWT Tokens: Embed the tenant ID within JWT tokens that are used for authentication, allowing easy identification across services.

5. Database Versioning and Migrations

When you have different schemas for tenants, you must manage schema migrations carefully:

• Version Control: Each tenant's schema should have a version number to track upgrades. Use a tool like Flyway or Liquibase to manage migrations per tenant.

• Rolling Updates: When rolling out new features or schema updates, ensure that the migrations can be applied gradually across tenants without disrupting service for other tenants.

6. Deployment Considerations in Azure

• Azure SQL Elastic Pool: If using a database-per-tenant model, Azure SQL Elastic Pool can help optimize resource usage by sharing resources among databases while keeping them isolated.
• Azure API Management: Route API requests to the appropriate microservices based on the tenant's needs, using a tenant ID for service discovery.
• Azure Cosmos DB: If using Cosmos DB, you can use collections for schema flexibility, as it supports multiple schema versions within the same collection.

Conclusion

Handling per-tenant customizations and schema differences requires a combination of flexible architecture, database strategy, and dynamic service behavior. By choosing the right multi-tenancy model and adopting dynamic configurations, you can meet varying tenant needs while ensuring the system remains scalable and maintainable.

Design Multi-tenant .Net Microservices

To design a multi-tenant order management system using .NET microservices deployed in Azure, the system should support multiple tenants (clients) while ensuring data security, scalability, and maintainability. Here's an outline of how to approach this architecture:

1. Architecture Overview

• Microservices: Implement each core functionality as a separate microservice, such as User Management, Order Processing, Inventory Management, and Payment Processing.
• API Gateway: Use Azure API Management as the gateway to route requests to the appropriate microservices, providing authentication, logging, rate limiting, and more.
• Multi-Tenancy Model: You can implement multi-tenancy either by:
  • Database-per-tenant: Each tenant gets a separate database, providing strong isolation.
  • Shared database with tenant-specific schemas: A single database where each tenant's data resides in separate schemas.
  • Shared database with a tenant ID column: All tenants share the same database and schema, but each record is tagged with a tenant ID.
• Azure Kubernetes Service (AKS): Use AKS to manage and deploy the microservices, ensuring scalability and management through containerized applications.

2. Key Components

• Order Service: Manages stock order creation, updates, and cancellations.
• User Service: Handles user authentication and role-based access control (RBAC), allowing tenants to manage their own users and permissions.
• Inventory Service: Tracks stock availability and updates based on placed orders.
• Payment Service: Processes payments, integrates with payment gateways, and manages invoices.
• Notification Service: Sends real-time notifications to tenants about order statuses, stock updates, etc.
• Audit and Logging Service: Logs user actions, transaction details, and order history to provide traceability.

3. Azure Components

• Azure API Management: Acts as the API gateway, handling security, routing, rate limiting, and caching.
• Azure Kubernetes Service (AKS): Manages the lifecycle of containerized microservices, allowing auto-scaling, rolling updates, and monitoring.
• Azure Service Bus: Provides messaging between microservices for decoupled communication, handling event-driven operations like order completion notifications.
• Azure SQL Database / Cosmos DB: Stores data based on the selected multi-tenancy model. Azure SQL is suited for relational data, while Cosmos DB offers multi-region replication and scalability.
• Azure Active Directory (Azure AD): For user authentication, including support for multi-tenant authentication scenarios using OAuth2 and OpenID Connect.
• Azure Monitor: To provide logging, metrics, and real-time monitoring for the health of the microservices.

4. Data Flow

• API Gateway receives a request for placing an order.
• The Order Service validates the order and communicates with the Inventory Service to ensure stock availability.
• The Payment Service handles payment processing and, upon success, triggers the Order Service to finalize the order.
• The Notification Service informs the tenant's users of the order status in real time.

5. Security Considerations

• Authentication and Authorization: Use Azure AD for authentication and RBAC for multi-tenant access control.
• Data Encryption: Ensure that data is encrypted both in transit (SSL) and at rest.
• Tenant Isolation: Depending on the chosen multi-tenancy model, enforce strong isolation between tenant data.

6. Scalability and Performance

• Horizontal Scaling: AKS can be set to auto-scale based on traffic, allowing the system to handle increased load.
• Caching: Use Azure Redis Cache to store frequently accessed data, reducing load on the database.
• Sharding: If using a shared database model, consider sharding data to enhance performance across tenants.

7. Monitoring and Maintenance

• Use Azure Monitor and Application Insights for real-time monitoring, diagnostics, and telemetry collection.
• Implement CI/CD pipelines using Azure DevOps or GitHub Actions to automate deployment and updates of the microservices.

This architecture provides a scalable, secure, and multi-tenant capable stock order management system using Azure and .NET microservices.

java.lang.NoSuchFieldError: Class com.sun.tools.javac.tree.JCTree$JCImport

SpringBoot Error: java.lang.NoSuchFieldError: Class com.sun.tools.javac.tree.JCTree$JCImport does not have member field 'com.sun.tools.javac.tree.JCTree qualid'

Resolution:  Check Springboot version in pom.xml.  The minimal Spring Boot version is 3.1.4.