Reference application developed in the Functional event-driven architecture: Powered by Scala 3 book.

• Web App
  • ScalaJS
• Overview
• Requirements
• Services
  • Lib
  • Domain
  • Core
  • Feed
  • Forecasts
  • Processor
  • Snapshots
  • Alerts
  • WS Server
  • Tracing
  • Tests
  • X Demo
  • X QA
• Monitoring
• Topic compaction

The web application allows users to subscribe/unsubscribe to/from symbol alerts such as EURUSD, which are emitted in real-time via Web Sockets.

It is written in Elm and can be built as follows.

$ cd web-app && nix-build
$ xdg-open result/index.html # or specify browser

There's also a shell.nix handy for local development.

$ cd web-app && nix-shell
$ elm make src/Main.elm --output=Main.js
$ xdg-open index.html # or specify browser

If Nix is not your jam, you can install Elm by following the official instructions and then compile as usual.

$ cd web-app
$ elm make src/Main.elm --output=Main.js
$ xdg-open index.html # or specify browser

There is also a replica of the Elm application written in Scala using the Tyrian framework. You can run it via Nix as follows (it requires flakes).

$ cd modules/ws-client && nix develop
$ yarn start
yarn run v1.22.17
warning package.json: No license field
parcel index.html --no-cache --dist-dir dist --log-level info
Server running at http://localhost:1234
✨ Built in 1.82s

Or without Nix, you need to run these commands before (requires yarn and parcel).

$ yarn install
$ yarn build

Here's an overview of all the components of the system.

• Dotted lines: Pulsar messages such as commands and events.
• Bold lines: read and writes from / to external components (Redis, Postgres, etc).

The back-end application is structured as a mono-repo, and it requires both Apache Pulsar and Redis up and running. To make things easier, you can use the provided docker-compose.yml file.

Note: The docker-compose file depends on declared services to be published on the local docker server. All docker builds are handled within the build.sbt using sbt-native-packager. To build all of the service images, run sbt docker:publishLocal.

$ docker-compose up -d pulsar redis

To run the Kafka Demo (see more below), only Zookeeper and Kafka are needed.

$ docker-compose -f kafka.yml up

If we don't specify any arguments, then all the containers will be started, including all our services (except feed), Prometheus, Grafana, and Pulsar Manager.

$ docker-compose up
Creating network "trading_app" with the default driver
Creating trading_pulsar_1 ... done
Creating trading_redis_1  ... done
Creating trading_ws-server_1      ... done
Creating trading_pulsar-manager_1 ... done
Creating trading_alerts_1         ... done
Creating trading_processor_1      ... done
Creating trading_snapshots_1      ... done
Creating trading_forecasts_1      ... done
Creating trading_tracing_1        ... done
Creating trading_prometheus_1     ... done
Creating trading_grafana_1        ... done

It is recommended to run the feed service directly from sbt whenever necessary, which publishes random data to the topics where other services are consuming messages from.

The back-end application consists of 9 modules, from which 5 are deployable applications, and 3 are just shared modules. There's also a demo module and a web application.

modules
├── alerts
├── core
├── domain
├── feed
├── forecasts
├── it
├── lib
├── processor
├── snapshots
├── tracing
├── ws-client
├── ws-server
└── x-demo

Capability traits such as Logger, Time, GenUUID, and potential library abstractions such as Consumer and Producer, which abstract over different implementations such as Kafka and Pulsar.

Commands, events, state, and all business-related data modeling.

Core functionality that needs to be shared across different modules such as snapshots, AppTopic, and TradeEngine.

Generates random TradeCommands and ForecastCommands followed by publishing them to the corresponding topics. In the absence of real input data, this random feed puts the entire system to work.

Registers new authors and forecasts, while calculating the author's reputation.

The brain of the trading application. It consumes TradeCommands, processes them to generate a TradeState and emitting TradeEvents via the trading-events topic.

It consumes TradeEvents and recreates the TradeState that is persisted as a snapshot, running as a single instance in fail-over mode.

The alerts engine consumes TradeEvents and emits Alert messages such as Buy, StrongBuy or Sell via the trading-alerts topic, according to the configured parameters.

It consumes Alert messages and sends them over Web Sockets whenever there's an active subscription for the alert.

A decentralized application that hooks up on multiple topics and creates traces via the Open Tracing protocol, using the Natchez library and Honeycomb.

All unit tests can be executed via sbt test. There's also a small suite of integration tests that can be executed via sbt it/test (it requires Redis to be up).

It contains all the standalone examples shown in the book. It also showcases both KafkaDemo and MemDemo programs that use the same Consumer and Producer abstractions defined in the lib module.

It contains the smokey project that models the smoke test for trading.

JVM stats are provided for every service via Prometheus and Grafana.

Two Pulsar topics can be compacted to speed-up reads on startup, corresponding to Alert and TradeEvent.Switch.

To compact a topic on demand (useful for manual testing), run these commands.

$ docker-compose exec pulsar bin/pulsar-admin topics compact persistent://public/default/trading-alerts
Topic compaction requested for persistent://public/default/trading-alerts.
$ docker-compose exec pulsar bin/pulsar-admin topics compact persistent://public/default/trading-switch-events
Topic compaction requested for persistent://public/default/trading-switch-events

In production, one would configure topic compaction to be triggered automatically at the namespace level when certain threshold is reached. For example, to trigger compaction when the backlog reaches 10MB:

$ docker-compose exec pulsar bin/pulsar-admin namespaces set-compaction-threshold --threshold 10M public/default