Panda Sky 2.6 - Introducing Self-Assembling Clients For HTTP APIs

“Avengers Assemble!”, but for self-assembling HTTP API clients.

Today I’m excited to introduce the Sky client, making it easy to integrate with any Sky API you build. I’m also announcing Panda Sky v2.6, featuring a couple new command line features to help with testing.

A Self-Assembling API Client

Panda Sky and its accompanying ecosystem are designed to make writing serverless apps painless. The Sky client makes it easy to use a Sky API, integrating with code running in a browser or Node server/lambda.

Interface-First Design

As you may recall, Panda Sky focuses on interface-first design. The API description you write is authoritative. Define it once, and Sky uses it to both orchestrate Cloud resources in your deployment and generate a client that consumes that API.

Here’s a hypothetical blogging app’s API description.

    template: "/"
    description: "Discovery endpoint for the client."
          request: {}
            status: [200]

    template: "/blurbs"
    description: "Virtual collection of blog posts"
          request: {}
            status: [201, 422]

    template: "/blurbs/{key}"
    description: "Individual posts"
          request: {}
            status: [200, 404]
            authorization: ["Bearer"]
            status: [200, 401, 404]

We define a discovery resource that responds with the API description for the client to use. We also define two more resources with HTTP methods that can act upon them. The Sky client parses this to generate functions that issue well-formed HTTP requests. Just give the Sky client the URL of the discovery resource to get started.

import discover from "panda-sky-client"
clientPromise = discover ""

document.addEventListener "DOMContentLoaded", ->
  client = await clientPromise

  document.querySelector "#publishButton"
  .addEventListener "click", (e) ->
    body = document.querySelector("#postTextBox").value
    {key, editKey} = await client.blurbs().post {body}

We import discover from the client and pass it the discovery URL. discover returns a promise that resolves an instantiated client. We setup a button click event listener that uses the client to issue an API request and create a new blog post.

The client generates functions that return Promises. Those promises resolve with response data and reject if the response status code is unexpected (ex, 404 from a GET request). We use the relatively new await operator to allow the promise to resolve asynchronously, making the client functions easy to place into try...catch control structures. Or, you could directly use the Promise .then() / .catch() interface.

For client errors, you may examine the property status to get the error response’s HTTP integer code.

Focus on Resources, Not Building URLs

As a developer implementing against a Sky API, the Sky client shields you from considering URL structure details. What is the path for a given resource? Are there path or querystring parameters, or both?

It shouldn’t matter.

You are writing client code that consumes an API, but your concerns are separate from that API’s implementation details. Opaque URLs allow you to decouple your code from the interface you are consuming, to draw boundaries on your mental model and avoid brittle integrations.

The top-level functions on the instantiated client return resources. The arguments to these functions are the parameters needed to specify a particular resource. In the above example, the blurbs resource didn’t need any, so we invoked it with blurbs(). But to GET a specific blurb resource, we need its key. Place that parameter into the invocation, like this.

do ->
  key = "55d4xOv0JRCxooRnNoHtGQ"
  content = await client.blurb({key}).get()

The Sky client uses the RFC specification for URI template expansion, specifically sections 3.2.2 and 3.2.8, to map the values from the object passed to blurb to the parameters specified in the template.

template: "/blurbs/{key}"

That template could include query string information (3.2.8), or could have a different path structure, like

template: "/blurbs/drafts/{key}{?author,status}"

But you don’t have to worry about anything other than the parameters.

do ->
  key = "55d4xOv0JRCxooRnNoHtGQ"
  author = "David Harper"
  status = 2

  content = await client.blurb({key, author, status}).get()

You have plenty of things to do, so allow the Sky client to handle URL construction for you.

HTTP/2 Comes Standard

The Sky client wraps the Fetch API, so you get HTTP/2 features, like multiplexing, binary encoding efficiency, and header compression; all without any effort from you (Recall that Panda Sky also deploys APIs and custom domains that use HTTP/2 by default).

The Sky client accesses window.fetch automatically in the browser. But if you want to use the Sky client from Node—which is great for writing API tests, for example—you need to provide a Fetch interface when you instanciate the client. We currently recommend the npm package http-h2. You can use it easily like this:

import discover from "panda-sky-client"
import {fetch, disconnectAll} from "fetch-h2"
do ->
  client = await discover url, fetch

In a Node process, you want the function disconnectAll to gracefully close the connections formed by the low-level HTTP/2 client before exiting. In the browser, that connection is managed by the vendor’s implementation.

Functional Reactive Ajax

The history of Ajax in the browser is now almost 2 decades long. jQuery freed us from writing XMLHttpRequest callback boilerplate, and its chaining structure remains influential. But we explored a functional style when we built the Sky client.

While you can use the client with a chain notation:

do ->
  client = await discover endpointURL

  {key, editKey} = await client
  .post body: "Hello World!"

The client is composed entirely of functions, so you can also destructure the constituent functions for reuse or composition.

do ->
  {blurbs, blurb} = await discover endpointURL
  {post} = blurbs()

  {key, editKey} = await post body: "Hello World!"

This gets interesting because functional reactive programming (FRP) uses flows to provide robust data handling in event-based designs. The Sky client’s functions are a perfect fit for that design.

New Commandline Features

Here’s a couple of commandline goodies introduced by Sky v2.6.

Testing in Severless Workflows

I’ve written before about the workflow changes brought about by serverless development. Sky 2.6 introduces the command

$ sky test [env]

Every Sky deployment gets a default URL with AWS Gateway. But, as a human, you consider environments not arbitrary Gateway API IDs. So, if you want to test your staging environment, use:

$ sky test staging

Sky looks up the default endpoint for that environment (if it exists) and feeds it to the command npm test [URL FROM SKY]. From there, you can write whatever script you’d like to issue your tests, with the endpoint URL as a commandline argument.

Sky runs npm test as a child process. The standard error and output streams are both written to the parent’s standard error stream (Sky outputs everything to stderr). The child’s exit code is mapped to the parent’s exit code.

Lookup Your Deployment Endpoints

Sky gives you the ability to easily create new environments during development to suit your needs. Sky 2.6 introduces a way for you to keep tabs on them all.

$ sky list

      https://[API ID].execute-api.[REGION]
production (UPDATE_COMPLETE)
      https://[API ID].execute-api.[REGION]
      https://[API ID].execute-api.[REGION]

Sky goes through all your environments and displays the ones that are deployed. Sky displays their names, CloudFormation status, and default endpoint URL.


The RFC spec for URI template expansion is complex, but Gateway’s basic path templating is limited to what’s defined in 3.2.2. Sky should be able to accept more of the RFC spec and then transform it into a form acceptable to Gateway, possibly using their proxy resource feature.

Also, expect the list command to get more robust. As mixins expand the kinds of resources Sky can deploy, we will need to be able to track them from the commandline.

Final Notes

I’ll keep you posted with news from Sky. Here are some more resources to check out in the meantime.