getnelson / helm

A native Scala client for interacting with Consul

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A native Scala client for interacting with Consul. There is currently one supported client, which uses http4s to make HTTP calls to Consul. Alternative implementations could be added with relative ease by providing an additional free interpreter for the ConsulOp algebra.

Getting Started

Add the following to your build.sbt:

libraryDependencies += "io.getnelson.helm" %% "http4s" % "x.y.z"

Where x.y.z is the desired Helm version. Check for the latest release Maven Central.


Consul operations are specified by the ConsulOp algebra. Two examples are get and set:

import ConsulOp.ConsulOpF

val setAlgebra: ConsulOpF[Unit] = ConsulOp.kvSet(key = "testKey", value = "testValue".getBytes)

val getAlgebra: ConsulOpF[QueryResponse[List[KVGetResult]]] = ConsulOp.kvGet(key = "testKey",
                                                                             recurse = Some(false),
                                                                             datacenter = None,
                                                                             separator = None,
                                                                             index = None,
                                                                             maxWait = None)

These are however just descriptions of what operations the program might perform in the future, just creating these operations does not actually execute the operations. In order to perform the gets and sets, we need to use the http4s interpreter.

The http4s interpreter

First we create an interpreter, which requires an http4s client and a base url for consul:

import scala.concurrent.ExecutionContext
import cats.effect.IO
import helm.http4s._
import org.http4s.Uri.uri
import org.http4s.client.blaze.BlazeClientBuilder

val ec =
implicit val contextShift = IO.contextShift(ec)
// It's better to run everything inside of resource.use() where possible, but this makes for a simpler example
val client = BlazeClientBuilder[IO](ec).resource.allocated.unsafeRunSync()._1

val baseUrl = uri("")

val interpreter = new Http4sConsulClient(baseUrl, client)

Now we can apply commands to our http4s client to get back IOs which actually interact with consul.

import helm._
import cats.effect.IO

val set: IO[Unit]                             =, setAlgebra)
val get: IO[QueryResponse[List[KVGetResult]]] =, getAlgebra)

// actually execute the calls
val setResult: Unit =                             set.unsafeRunSync()
val getResult: QueryResponse[List[KVGetResult]] = get.unsafeRunSync()

Typically, the Helm algebra would be a part of a Coproduct with other algebras in a larger program, so running the IO immediately after is not typical.


Contributions are welcome; particularly to expand the algebra with additional operations that are supported by Consul but not yet supported by Helm.