A sample project of a microservice using http4s, doobie, and circe.
The microservice allows CRUD of todo items with a description and an importance (high, medium, low).
The end points are:
| Method | Url | Description |
|---|---|---|
| GET | /todos | Returns all todos. |
| GET | /todos/{id} | Returns the todo for the specified id, 404 when no todo present with this id. |
| POST | /todos | Creates a todo, give as body JSON with the description and importance, returns a 201 with the created todo. |
| PUT | /todos/{id} | Updates an existing todo, give as body JSON with the description and importance, returns a 200 with the updated todo when a todo is present with the specified id, 404 otherwise. |
| DELETE | /todos/{id} | Deletes the todo with the specified todo, 404 when no todo present with this id. |
Here are some examples on how to use the microservice with curl, assuming it runs on the default port 8080:
Create a todo:
curl -X POST --header "Content-Type: application/json" --data '{"description": "my todo", "importance": "high"}' http://localhost:8080/todos
Get all todos:
curl http://localhost:8080/todos
Get a single todo (assuming the id of the todo is 1):
curl http://localhost:8080/todos/1
Update a todo (assuming the id of the todo is 1):
curl -X PUT --header "Content-Type: application/json" --data '{"description": "my todo", "importance": "low"}' http://localhost:8080/todos/1
Delete a todo (assuming the id of the todo is 1):
curl -X DELETE http://localhost:8080/todos/1
http4s is used as the HTTP layer. http4s provides streaming and functional HTTP for Scala. This example project uses cats-effect, but is possible to use http4s with another effect monad.
By using an effect monad, side effects are postponed until the last moment.
http4s uses fs2 for streaming. This allows to return streams in the HTTP layer so the response doesn't need to be generated in memory before sending it to the client.
In the example project this is done for the GET /todos endpoint.
doobie is used to connect to the database. This is a pure functional JDBC layer for Scala. This example project uses cats-effect in combination with doobie, but doobie can use another effect monad.
Because both http4s and doobie use an effect monad, the combination is still pure and functional.
circe is the recommended JSON library to use with http4s. It provides automatic derivation of JSON Encoders and Decoders.
pureconfig is used to read the configuration file application.conf.
This library allows reading a configuration into well typed objects.
h2 is used as a database. This is an in memory database, so when stopping the application, the state of the microservice is lost.
Using Flyway the database migrations are performed when starting the server.
This example project contains both unit tests, which mock the repository that accesses the database, and integration tests that use the http4s HTTP client to perform actual requests.
You can run the microservice with sbt run. By default it listens to port number 8080, you can change
this in the application.conf.