• Introduction
• Problem Description
• Application Architecture
• Solution Design
• Development Environment
• Opensource Tools / Technologies
• WebApi Endpoints
• Solution Structure
• Interaction between the components
• Scheduled background task with Cron expression
• Communication between the microservices
• How to run the application
• Console App : Gateway Client
• Final Thoughts

Simple Transaction: Microservices Sample Architecture for .NET Core Application

This is second part of Simple Transaction: Sample .Net Core application, a continuation to demonstrate how to build a command-driven / Messaging based solution using .Net Core. The sample application which was used in previous article is extended with few additional microservices.

The Previous Sample implements microservices for a simple automated banking feature like Balance, Deposit, Withdraw in ASP.NET Core Web API with C#.NET, Entity Framework and SQL Server. This second part is about design a queue based messaging solution to plug-in a feature to generate monthly account statement through background service and store it in No-SQL Db (MongoDB) which is then accessed through a separate microservice.

Generating reports like account statements for the users on the fly from the transactional data avaialble in the sql server would cause performance issue in a real time system due to the huge amount of data.

A better way to handle this complex problem is to seperate the statement generation process (Write / Command) and accessing (Read / Query) the data. This sample will show you how to seperate the command reponsibility through background service and Query data through a seperate microservice which solves the performance problem and makes the system capable of handling large data by allowing us to scale the services independently depending on the load.

The below shown is the updated version of the architecture diagram. The highlighted part is the newly added microservice to access the account statement. This "statement" service uses MongoDb as persistent store and Redis Cache as temporary store where the data is loaded into cache from MongoDb for a period of time and it's retrived from cache on subsequent access.

The below diagram shows the high level design of the background implmentation that separate the Command responsibility from Querying data. It has three core components that seperates the write operation using the Command / Message processing and read operation using Query data.

• Publisher API / Scheduler Service
• Receiver Service
• Statement API

This microservice publishes a command into the message bus (Message Queue) when monthly account statement need to be generated for a given month. This service has a automated scheduler to trigger the statement generation process on a scheduled time and also exposes WebApi endpoints to manually trigger the process.

Receiver service is a listener, listens to message queue and consumes the incoming message and process it. Receiver communicates with other depedent service to get the data and generates user's monthly account statement for a given month and stores the document in MongoDB.

This microservice exposes a WepApi endpoint that communicates with the Redis Cache and MongoDb through data repository to access the account statement.

• .Net Core 2.2 SDK
• Visual Studio .Net 2017

• Mongo DB as Persistent Store
• Redis Cache as Provisional Store
• RabbitMQ as Message Broker
• EasyNetQ as RabbitMQ Client
• NCrontab for time-based scheduling
• .Net Core IHostedService as background service

1. Route: "api/publish/statement" [HttpPost] - To publish a message in to queue to trigger the background process. Defaults to previous month.
2. Route: "api/publish/statement/{month}" [HttpPost] - To publish a message in to queue to trigger the background process.
3. Route: "api/publish/statement/{month}/accountnumbers" [HttpPost] - To publish a message in to queue to trigger the background process for the given list of account numbers.

1. Route: "api/statement/{month}" [HttpGet] - To access the account statement of authenticated user for a given month

1. Route: "statement/{month}" [HttpGet] - To access the account statement of authenticated user.

The highlighted part in the below diagram shows the new services / components added to the sample.

The below diagram shows the interaction between the components / objects in the background "Receiver" service.

• Message Queues : Background service both "Publisher" and "Receiver" works with two different queues. One of them is "Trigger" Queue. The initial trigger / command from the scheduler is send to "Trigger" Queue which is processed by the "Receiver" service. The second Queue in the workflow is the "Statement" queue which receives input from the "Trigger" processor and processed by "Statement" processor.
• Message Types: The background service understands Messages of Type ICommand which is defined in "Publisher.Framework". Two message types are defined to work with two different queues, "TriggerMessage" and "StatementMessage".
• Publisher: Sends "Trigger" message into "Trigger" queue on a scheduled time by the scheduler or on demand when API endpoint is called.
• Receiver: Consumes the "Trigger" message and prepares a "Statement" Message for the list of user accounts and publishes a "Statement" message into next queue for each user account. "Receiver" service then consumes the "Statement" message and calls the processor.
• Processor: Contains the processing logic to prepare the account statement.
• Document Repository: Contains the data logic to save the document to MongoDb.

A cron expression is a format to specify a time based schedule to execute a task. NCrontab is the Nuget package included in the "Publisher.Service" to parse the expression and determine the next run. The service is configured to trigger the background process at the start of every month to generate the account statement for the previous month.

Reference: Run scheduled background tasks in ASP.NET Core

There's a need for the microservices to communicate with each other to let other services know when changes happen or to get dependent information. Here, in our sample , both Synchronous and Asynchronous mechanism are used to establish communication between microservices.

Message based asynchronous communication is used to trigger a event. It's a point-to-point communication with a single receiver. This means when a message is published to the queue, there's a single receiver that consumes the message for processing. The below diagram show the communication between "Publisher / Scheduler" and "Receiver through message bus"

The background service "Receiver" communicates with dependent services (Identity and Transaction) through Http request to get the actual data for background processing.

Follow the same instruction that was given in the previous article to run the application. In addition to that, the newly added services (Publisher, Receiver and Statement API) should also be up and running.

You need to have the setup of MongoDB and Redis Cache either in your local environment or in the cloud. Following that, You can update the MongoDB and Redis Cache connection setup in the appsettings.json file of "Receiver.Service" and "Statement.Service" accordingly.

For the database changes, refer to database project included in this version of code which has the updated script.

Not necessary that you need to have two no-sql db for a single microservice to improve the performance of a system. In this case, just the mongoDb should do the need but included redis cache to show an alternate way of acheiving better performance for frequently accessed data.