Tesla

Tesla is an HTTP client losely based on Faraday. It embraces the concept of middleware when processing the request/response cycle.

WARNING: Tesla is currently under heavy development, so please don't use it in your production application just yet.

Nevertheless all comments/issues/suggestions are more than welcome - please submit them using GitHub issues, thanks!

Basic usage

# Example get request
response = Tesla.get("http://httpbin.org/ip")
response.status   # => 200
response.body     # => '{\n  "origin": "87.205.72.203"\n}\n'
response.headers  # => %{'Content-Type' => 'application/json' ...}


response = Tesla.get("http://httpbin.org/get", %{a: 1, b: "foo"})
response.url     # => "http://httpbin.org/get?a=1&b=foo"


# Example post request
response = Tesla.post("http://httpbin.org/post", "data")

Installation

Add tesla as dependency in mix.exs

defp deps do
  [{:tesla, "~> 0.1.0"},
   {:ibrowse, github: "cmullaparthi/ibrowse", tag: "v4.1.1"}, # default adapter
   {:exjsx, "~> 3.1.0"}] # for JSON middleware
end

When using ibrowse adapter add it to list of applications in mix.exs

def application do
  [applications: [:ibrowse, ...], ...]
end

Creating API clients

Use Tesla.Builder module to create API wrappers.

For example

defmodule GitHub do
  use Tesla.Builder

  plug Tesla.Middleware.BaseUrl, "https://api.github.com"
  plug Tesla.Middleware.Headers, %{'Authorization' => 'xyz'}
  plug Tesla.Middleware.EncodeJson
  plug Tesla.Middleware.DecodeJson

  adapter Tesla.Adapter.Ibrowse

  def user_repos(login) do
    get("/user/" <> login <> "/repos")
  end
end

Then use it like this:

GitHub.get("/user/teamon/repos")
GitHub.user_repos("teamon")

Adapters

Tesla has support for different adapters that do the actual HTTP request processing.

ibrowse

Tesla has built-in support for ibrowse Erlang HTTP client.

To use it simply include adapter Tesla.Adapter.Ibrowse line in your API client definition.

NOTE: Remember to include ibrowse in applications list.

ibrowse is also the default adapter when using generic Tesla.get(...) etc. methods.

Test / Mock

When testing it might be useful to use simple function as adapter:

defmodule MyApi do
  use Tesla

  adapter fn (env) ->
    case env.url do
      "/"       -> {200, %{}, "home"}
      "/about"  -> {200, %{}, "about us"}
    end
  end
end

Middleware

Basic

• Tesla.Middleware.BaseUrl - set base url for all request
• Tesla.Middleware.Headers - set request headers

JSON

NOTE: requires exjsx as dependency

• Tesla.Middleware.DecodeJson - decode response body as JSON
• Tesla.Middleware.EncodeJson - endode request body as JSON

If you are using different json library writing middleware should be straightforward. See json.ex for implementation.

Dynamic middleware

All methods can take a middleware function as the first parameter. This allow to use convinient syntax for modyfiyng the behaviour in runtime.

Consider the following case: GitHub API can be accessed using OAuth token authorization.

We can't use plug Tesla.Middleware.Headers, %{'Authorization' => 'token here'} since this would be compiled only once and there is no way to insert dynamic user token.

Instead, we can use Tesla.build_client to create a dynamic middleware function:

defmodule GitHub do
  # same as above

  def client(token) do
    Tesla.build_client [
      {Tesla.Middleware.Headers, %{'Authorization' => "token: " <> token }}
    ]
  end
end

and then:

client = GitHub.client(user_token)
client |> GitHub.user_repos("teamon")
client |> GitHub.get("/me")

Writing your own middleware

A Tesla middleware is a module with call/3 function:

defmodule MyMiddleware do
  def call(env, run, options) do
    # ...
  end
end

The arguments are:

• env - Tesla.Env instance
• run - continuation function for the rest of middleware/adapter stack
• options - arguments passed during middleware configuration (plug MyMiddleware, options)

There is no distinction between request and response middleware, it's all about executing run function at the correct time.

For example, z request logger middleware could be implemented like this:

defmodule Tesla.Middleware.RequestLogger do
  def call(env, run, _) do
    IO.inspect env # print request env
    run.(env)
  end
end

and response logger middleware like this:

defmodule Tesla.Middleware.ResponseLogger do
  def call(env, run, _) do
    res = run.(env)
    IO.inspect res # print response env
    res
  end
end

See core.ex and json.ex for more examples.

Asynchronous requests

If adapter supports it, you can make asynchronous requests by passing respond_to: pid option:

Tesla.get("http://example.org", respond_to: self)

receive do
  {:tesla_response, res} -> res.status # => 200
end