Vert.x 4: How to build a reactive RESTful Web Service

Image for post
Image for post

Eclipse Vert.x is a tool-​kit for build­ing re­ac­tive ap­pli­ca­tions on the JVM. Re­ac­tive ap­pli­ca­tions are both scal­able as work­loads grow and re­silient when fail­ures arise, making them ideal for low-latency and high-throughput use cases. Its main advantage resides in the way it approaches multi-threading. Threads can live within a sin­gle process, per­form con­cur­rent work and share the same mem­ory space. However, as work­load grows, the OS ker­nel struggles when there is too much con­text switch­ing work with in-flight re­quests. Because of that, some threads are blocked while they are wait­ing on I/O op­er­a­tions to com­plete while others are ready to han­dle I/O re­sults. Vert.x takes a different approach to this problem by using event loops:

Image for post
Image for post
Event loop

In­stead of block­ing a thread when an I/O op­er­a­tion oc­curs, it moves on to the next event (which is ready to progress) and re­sumes the ini­tial event when it’s the appropriate callback’s turn in the queue. As long as the event loop is not blocked, there’s a significant performance improvement, making Vert.x one of the fastest in the Java ecosystem.

What’s new in Vert.x 4?

Vert.x is backed by a large ecosys­tem of re­ac­tive mod­ules that are useful to writ­e mod­ern applications: a com­pre­hen­sive web stack, re­ac­tive data­base dri­vers, mes­sag­ing, event streams, clus­ter­ing, met­rics, dis­trib­uted trac­ing and more. This release adds a significant number of new features and improvements on top of that:

  • Futurisation — Implementation of fu­ture/call­back hy­brid model;
  • Monitoring — Vert.x Trac­ing sup­ports both Open­trac­ing and Zip­kin. It also com­ple­ments Vert.x Met­rics;
  • Reactive SQL clients — High-​performance re­ac­tive SQL clients, fully in­te­grated with Vert.x Met­rics and Vert.x Trac­ing;
  • Reactive Redis client — The re­vamped client API now sup­ports all Redis con­nec­tion modes with ex­ten­si­ble sup­port for Redis com­mands;
  • SQL Templating — SQL Client Tem­plates is a li­brary de­signed to fa­cil­i­tate build­ing SQL queries;
  • Web Validation — Ex­ten­si­ble sync/async HTTP re­quest val­ida­tor pro­vid­ing a DSL to de­scribe ex­pected HTTP re­quests;
  • Web OpenAPI — New sup­port for Con­tract Dri­ven de­vel­op­ment based on Ope­nAPI;
  • Authentication and Authorization — A set of new mod­ules are now avail­able (e.g. vertx-auth-ldap, vertx-auth-sql & vertx-auth-webauth) and improvements were made to older modules (e.g. vertx-auth-oauth).

Building our reactive RESTful Web Service

There’s a good amount of documentation on how to use Vert.x and its modules. However, most of the code examples are focused on a specific feature and presented as a single class Java application (usually MainVerticle). Personally, I feel that it doesn’t represent the full reach of its capabilities, so I decided to prepare an application that implements a fully-reactive web service capable of communicating with a relational database (e.g. PostgreSQL) and providing a CRUD API while addressing some production-ready requirements such as metrics, logging, health checks, database migrations, tests and so on. The codebase is organized in traditional N-Tier architecture and it uses some of the latest features such as SQL Templating and Web Validation.

Image for post
Image for post
Codebase package structure


A router takes an HTTP request, finds the first matching route for that request and proxies the request to that route. Each route can have multiple handlers and each handler should be responsible for a different requirement. You can use this layer to define your API endpoints, your API versioning and even enable request validation. Below you’ll find several routes versioned by its URI path using three handlers: a Logger handler, a Web Validation handler and a custom business logic handler.


As I mentioned before, handlers should be used for different requirements. Vert.x provides many of them out of the box. Logger handlers are used to log requests information, the new Web Validation handlers are used to perform request schema validation, Error handlers are used to customize error messages, the list goes on. You can also implement your own handlers, just like the one that you can find below. This handler provides all the operations necessary by the routes that were defined previously. They extract information from the request (query parameters, path parameters and body) initially and return a JSON response with an appropriate HTTP status code at the end, leaving all the business logic to the service layer.


Services are a common abstraction to represent middleware. They are used to process data using some business logic and can be very useful to establish a set of available operations that coordinate the application’s response. Our service layer is able to create, read, update and delete data. It’s responsible for transactional control, for providing connections to the repository layer, for transforming entities into DTOs and for implementing specific features such as pagination. It uses the Reactive PostgreSQL client, which is non-blocking, allowing it to handle many database connections with a single thread.


Repositories are classes that encapsulate the logic required to access data sources. They centralize common data access functionality, providing better maintainability and decoupling the infrastructure or technology used to access databases from the domain model layer. Below you can find multiple SQL statements as well as the new SQL templating, which can be very useful to map structured data to our entities.


Vert.x comes with a scalable concurrency model out of the box that you can use to save time writing your own. It shares similarities with the Actor model especially with respect to concurrency, scaling and deployment strategies. Instead of using actors, it uses verticles. There are two types of verticles:

  • Standard — Standard verticles are assigned an event loop thread when they are created and the start method is called with that event loop. This means you can write all the code in your application as single-threaded and let Vert.x worry about the threading and scaling;
  • Worker — A worker verticle is just like a standard verticle, but it’s executed using a thread from the worker thread pool. They are designed for calling blocking code.

Although the usage of verticles is entirely optional, it’s generally a good idea to do it concurrency-wise. Below you’ll find a verticle describing how to run an HTTP server with all of your configurations.

Database migrations

Database management should be composed of incremental and reversible changes allowing us to control our relational database schema versions. By using a schema migration tool (e.g. Flyway) you should have the possibility to define your schema evolution and seed its initial data programmatically. This requirement has a blocking nature, so you should deploy a worker verticle.

Image for post
Image for post

Health checks

Health check endpoints enable us to periodically test the health of our service. Sometimes, applications transition to broken states and cannot recover except by being restarted. Other times applications are temporarily unable to serve traffic (e.g. an application might need to load large data during startup) and, in such case, it shouldn’t restart the application and/or allow requests either. Providing liveness and readiness probes, respectively, allows us to detect and mitigate these situations. Below you’ll find a way to express the current state of the application and the information on whether a connection to the database can be established.

Image for post
Image for post


Application monitoring provides detailed observability into the performance, availability and user experience of applications and their supporting infrastructure. By gathering statistics from the HTTP server, database, API or any existing module you’re better prepared to recover from failures and also able to get insights into what is happening inside the application. Vert.x provides a convenient integration with Micrometer whose data can become available through an endpoint that is scraped periodically by Prometheus and consequently by Grafana in order to produce proper dashboards.

Image for post
Image for post


Finally, it’s very important to check whether the actual software matches the expected requirements and/or create an automated way of identifying errors, gaps or missing requirements. There are many types of tests: unit tests, integration tests, component tests, end-to-end tests and so on. Component tests are interesting in the way they allow us to test our web service using the consumer perspective (e.g. API) as the main driver. At the same time, it allows us to test the interaction of the web service with the database, all as one unit. The main challenge is to ensure that the local environment is the same as the production environment. For that reason, you could use Testcontainers in order to mimic PostgreSQL or any other technology.

Final thoughts

Mod­ern ker­nels have very good sched­ulers, but we can­not ex­pect them to deal with 50k threads as eas­ily as they would do with 5k. It’s important to recognize that threads aren’t cheap: cre­at­ing a thread takes a few milliseconds and consumes about 1MB of mem­ory. Vert.x approach addresses these issues and provides a rich ecosystem that allows developers to build highly scalable and performant applications. There’s just one caveat: code that runs on event loops should not per­form block­ing I/O or lengthy pro­cess­ing. But don’t worry if you have such code: Vert.x has worker threads and APIs to process events back on the event loop. You can find the codebase and all the necessary configurations on the following repository.



Software Engineer

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store