A Elm client for the NATS protocol
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This library is a pure Elm implementation of the NATS client protocol on top a websocket.

It works by adding new Nats.Cmd and Nats.Sub to TEA, and merging them into classic Cmd and Sub in update and subscriptions.

It clearly is an alpha version, which means the API is likely to change.


  1. A running gnatsd server
  2. A running nats-websocket-gw, probably with the '--no-origin-check' for tests. For production use, I recommend embedding nats-websocket-gw as a library and not use the binary directly.


NATS subscriptions handling is a stateful business, which is all done by the Nats module. You need to wire it into your application.

  1. Import the Nats module

    import Nats
    import Nats.Protocol
    import Nats.Cmd as NatsCmd
    import Nats.Sub as NatsSub
  2. Add a state to your top-level model, or at least one that contains all the components that uses the Nats API.

    type alias Model =
        { nats: Nats.State Msg
        -- ...
  3. Add a NatsMsg tag to your Msg type:

    type Msg
        = DoSomething
        | NatsMsg Nats.Msg
  4. Initialize the State with the websocket url. If working locally with nats-websocket-gw, it should be "ws://localhost:8910/nats":

    init =
        { nats = Nats.init NatsMsg "WEBSOCKET_URL"

    If you have credentials or a client name to send as CONNECT options, use setAuthToken, setUserPass and/or setName:

    init =
        { nats =
            Nats.init NatsMsg "WEBSOCKET_URL"
                |> Nats.setAuthToken "A token"
                |> Nats.setName "My client name"
  5. Define the top-level subscription and nats subscriptions:

    subscriptions model =
        Nats.listen model.state
    natsSubscriptions model =
  6. Define a "mergeNats" function that can post-process your model and nats subscriptions and commands to give you a final state and commands:

    mergeNats : ( Model, NatsCmd.Cmd Msg, Cmd Msg ) -> ( Model, Cmd Msg )
    mergeNats ( model, natsCmd, cmd ) =
            ( natsState, extraCmd ) =
                Nats.merge model.nats (natsSubscriptions model) natsCmd
            { model
                | nats = natsState
                ! [ cmd, extraCmd ]
  7. Change your update function to apply mergeNats after the classic 'case msg of', which now returns ( Model, NatsCmd.Cmd Msg, Cmd Msg) instead of ( Model, Cmd Msg ):

    update : Msg -> Model -> ( Model, Cmd Msg )
    update msg model =
            (case msg of
                NoOp ->
                    ( model, NatsCmd.none, Cmd.none )
                NatsMsg natsMsg ->
                        ( nats, natsCmd ) =
                            Nats.update natsMsg model.nats
                        ( { model
                            | nats = nats
                        , NatsCmd.none
                        , natsCmd



In update, use publish to generate the right Cmd:

    ( model
    , Nats.publish "subject1" "Hello world!"
    , Cmd.none


  1. Define a Msg tag for receiving messages

    type Msg
        = ReceiveSubject1 Nats.Protocol.Message
  2. Add the subscriptions to the "natsSubscriptions" function:

    natsSubscriptions model =
        Nats.subscribe "subject1" ReceiveSubject1

Multiple subscriptions to the same subject

natsSubscriptions are recognized by their subject only, which means several subscriptions on the same subject will collide and only one of them will be honored.

To avoid this problem, Elm-Nats allow you to tag the subscriptions subject by adding a '#' followed by any string you like, so it can sort them out.

Only the part before the '#' will be sent in the SUB command to the NATS server.

In the following example, both subscriptions are on the "subject1" subject:

natsSubscriptions model =
        [ Nats.subscribe "subject1#1" ReceiveSubject1
        , Nats.subscribe "subject1#2" ReceiveAlsoSubject1

See the SubComp in example, it can define any number of subscriptions on the same subject.


  1. Define a Msg tag for receiving the response

    type Msg
        = ReceiveResponse (Result Nats.Errors.Timeout Nats.Protocol.Message)
  2. Send a request from your update function

    ( model
    , Nats.request "subject1" ReceiveResponse
    , Cmd.none