ftgogo – event-driven architecture demonstration application

Introduction

ftgogo (food-to-gogo) is a Golang implementation of the FTGO application described in the book “Microservice Patterns” by Chris Richardson. A library edat was developed to provide for Golang many of the solutions that Eventuate, the framework used by FTGO, provides for Java.

Purpose

This repository exists to demonstrate the patterns and processes involved when constructing a distributed application using event-driven architecture.

Prerequisites

Docker – Everything is built and run from a docker compose environment.

Execution

Open a command prompt and then execute the following docker command

NOTE: The first time you bring everything up the init script for Postgres will run automatically. The services will crash-loop for a bit because of that. Eventually things will stabilize.

Mac and Linux Users

COMPOSE_DOCKER_CLI_BUILD=1 DOCKER_BUILDKIT=1 docker-compose up

Windows Users

set "COMPOSE_DOCKER_CLI_BUILD=1" & set "DOCKER_BUILDKIT=1" & docker-compose up

Use Ctrl-C to stop all services.

Running individual services

Not recommended but each service can be run using go run .. You’ll need to use an .env file or set some environment variables to properly run the service.

Use go run . --help to see what flags and the list of environment variables that can be set.

Application

Services

Several services also have sibling CDC services.

Design

  • Services exist within a “domain” folder. Within that folder you’ll find the following layout.

    /"domain"        - Domain that is a subdomain in the application domain
    |-/cmd           - Parent for servers, cli, and tools that are built using the code in this domain
    | |-/cdc         - CDC (Change Data Capture) server. If the service publishes messages it will also have this
    | |-/service     - Primary domain service
    |-/internal      - Use Golangs special treatment of "internal" to sequester our code from the other services
      |-/adapters    - The "Adapters" from "Ports & Adapters". These are the implementations of domain interfaces
      |-/application - CQRS parent. Processes under this will implement business rules and logic
      | |-/commands  - Application commands. Processes that apply some change to the subdomain
      | |-/queries   - Application queries. Processes that request information from the subdomain
      |-/domain      - The definitions, interfaces, and the domain rules and logic
      |-/sagas       - The definitions for complex multi-service interactions
    
  • The api for each service has been defined in an /"domain"/cmd/service/openapi.yaml file.

    An issue in the openapi library being used prevents the serving the openapi document via HTTP request. A swagger UI may be added sometime later if this is resolved, or a different solution is used.

  • Each “domain” uses go modules for dependency management and has a go.mod file just for it.

  • The services use the following components from edat

    • edat/es – implements event sourcing
    • edat/msg – implements transactional messaging
    • edat/sagas – implements orchestrated sagas
  • Application domain apis, events, commands, and entities are available to all subdomains from the serviceapis folder and its child packages.

DDD / Hexagonal Architecture / Ports & Adapters

The organizational layout of the services is to bring separation to the layers. The /"domain"/cmd/service/application.go file found in all the services is where ports (web handlers, message receivers) are combined with the application commands and queries.

  • The application is connected to the /internal/domain using interfaces backed by real implementations found in the /internal/adapters folder.
  • Commands and queries have no dependencies on any primary ports or on any adapters directly. In fact, they may only receive adapters that implement secondary ports defined by interfaces in the /internal/domain folder.
  • No dependencies exist on any ports or adapters from the code in /internal/domain and this is crucial to having a clean architecture.

Regarding the layout. What I landed on felt alright to work with, felt like it provided the right separations, and felt like it wasn’t an over complication of DDD applied to a folder structure. You be the judge.

CQRS

Each service divides the requests it receives into commands and queries. Using a simple design described here by Three Dots Labs all of our handlers can be setup to use a command or query.

While most application commands in this repository will adhere to the “commands return nothing” rule, some commands return simple scalar values, mostly entity ids, in addition to the error and I am OK with that.

Event Sourcing

Several services use event sourcing and keep track of the changes to aggregates using commands and recorded events. Check out the Order aggregate for an example.

Sagas

The same three sagas found in FTGO have been implemented here in the order-service.

  • CreateOrderSaga – saga responsible for the creation of a new order
  • CancelOrderSaga – saga responsible for the cancelling and releasing of order resources like tickets and accounting reserves
  • ReviseOrderSaga – saga responsible for the processing the changes made to an open order

Event-driven architecture

All inter-service communication is handled asynchronously using messages.

Dual-Writes / Outbox / CDC

An implementation of the outbox pattern is included. It provides the solution to the dual write problem. Any service that publishes messages is actually publishing the message into the database. A CDC sibling service then processes the messages from the database and publishes the message into NATS Streaming. This process provides at-least-once delivery.

Services can be made to publish messages directly without having to use an outbox and CDC. The pattern works best, only?, with backends that provide transactional support.

Other

Metrics/Instrumentation

Prometheus metrics for each service are available at http://localhost:[port]/metrics. The order-service has a few additional counters. See the order-service application for more information.

Mono-repository

This demonstration application is a mono-repository for the Golang services. I chose to use as few additional frameworks as possible so you’ll find there is also quite a bit of shared code in packages under /shared-go

/shared-go is named the way it is because I intended to build one of the services in another language. I didn’t but left the name the way it was.

Type Registration

Commands, Events, Snapshots, and other serializable entities and value objects are registered in groups in each /"domain"/internal/domain/register_types.go and in the child packages of serviceapis. This type registration is a feature of edat/core and is not unique to this application.

The purpose of doing this type registration is to avoid boilerplate marshalling and unmarshalling code for every struct.

Changes from FTGO

I intend for this demonstration to exist as a faithful Golang recreation of the original. If a difference exists either because of opinion or is necessary due of the particulars of Go, I will try my best to include them all here.

Changed

  • I’ve kept most API requests and responses the same “shape” but routes are prefixed with /api and use snake_case instead of camelCase for property names.
  • In FTGO many apis and messages that operated on Tickets used the OrderID as the TicketID. I could have done the same but chose to let the Ticket aggregates use their own IDs. The TicketID was then included in responses and messages where it was needed.
  • Order-History is not using DynamoDB. The purpose of Order-History is to provide a “view” or “query” service and it should demonstrate using infrastructure best suited for that purpose. For now, I’m using Postgres but intend to use Elasticsearch soon.
  • The OrderService->createOrder method I felt was doing too much. The command implementation creates the order like before, but the published entity event that results from that command is now the catalyst for starting the CreateOrderSaga.

Missing

  • Tests. Examples of testing these services. Both Unit and Integration
  • Api-Gateway. I haven’t gotten around to creating the gateway.

Out Of Scope

Just like the original the following are outside the scope of the demonstration.

  • Logins & Authentication
  • Accounts & Authorization
  • AWS/Azure/GCP or any other cloud deployment instructions or scripts
  • Tuning guidance
  • CI/CD guidance
  • Chaos testing – although feel free to terminate a service or cdc process for a bit and see if it breaks (it shouldn’t)

Quick Demo

Postman can be used to load the api collection for an easy demo.

With the application running using one of the commands above you can make the following calls to see the processes, events, and entities involved.

  • Consumer Service: Register Consumer
  • Restaurant Service: Create Restaurant
  • Order: Create Order

Loading FTGOGO.postman_collection.json into Postman will provide pre-built calls with semi-random data that can be used to test the above.

Contributing

Pull requests are welcome. For major changes, please open an issue first to discuss what you would like to change.

Please make sure to update tests as appropriate.

No warranties

From time to time I expect to make improvements that may be breaking. I provide no expectation that local copies of this demonstration application won’t be broken after fetching any new commit(s). If it does fail to run; simply remove the related docker volumes and re-run the demonstration.

GitHub

https://github.com/stackus/ftgogo