Using CLJS and shadow-cljs for serverless DigitalOcean Functions

2023-01-12

DigitalOcean (DO) Functions: "a serverless computing solution that runs on-demand, enabling you to focus on your code, scale instantly with confidence, and save costs by eliminating the need to maintain servers." Since I'm a fanatic, I would like to write some Clojure for my serverless use case. Because DO offers Node as a runtime, we're able to use ClojureScript to write code and deploy it to the serverless, er, server.

Source code for this blog post is available.

You'll need a DO account. Log in, select the appropriate team, then select Functions on the left-hand column. Create a function namespace and you're ready to go.

Next we need the doctl binary. Here's what I did:

brew install doctl
doctl auth init
doctl serverless install
doctl serverless connect

This gets you authenticated with DO so you can deploy from the command line.

npx create-cljs-project do_serverless
cd do_serverless

Now, let's edit the generated shadow.cljs a bit. Add this map as the value under :builds:

{:core {:target :node-script
         :main core/main
         :output-to "packages/do-serverless/core/core.js"}}

Create src/main/core.cljs and define main in it:

(ns core)

(defn main [])

Create packages/do-serverless/core/package.json with this in it:

{
  "name": "core",
  "version": "1.0.0",
  "description": "CLJS on DO!",
  "main": "core.js",
  "dependencies": {
    "source-map-support": "^0.5.21"
  },
  "devDependencies": {}
}

Lastly, create project.yml:

packages:
  - name: do-serverless
    actions:
      - name: core
        runtime: nodejs:default

OK! Let's see where we're at:

shadow-cljs release core
doctl serverless deploy .

Now we can go to the Functions tab on DO's site and run our function by first going to the function namespace, clicking on the name of the function, and hitting Run. I get this error:

2023-01-12T11:14:08.172732642Z stdout: Action entrypoint 'main' is not a function.

What tha...

At this point, I dug around and found that DO maintains a bunch of sample functions. Going to the Node one, we see this:

exports.main = (args) => { ... }

Huh. OK, so let's do that in our example, src/main/core.cljs:

(ns core)

(defn main [& args]
  (println "hello!")
  (println "args: " args))

(set! js/exports.main main)

And re-build and deploy.

2023-01-12T11:22:14.933096349Z stdout: hello!
2023-01-12T11:22:14.933797937Z stdout: args:  nil
2023-01-12T11:22:14.961195498Z stdout: hello!
2023-01-12T11:22:14.982016323Z stdout: args:  (#js {} ... // output truncated

Ok, so when our function executes, our main gets executed twice. I don't know why this happens. If I run our compiled javascript file locally with node, I only see one execution:

$ node packages/do-serverless/core/core.js
hello!
args:  nil

So, OK, some detail that's above my head. My use case for serverless would, uh, not do well with running everything twice. So, what to do?

Well, we know whatever we tell shadow our main is will get run. And we also know whatever we tell DO our main is (the js/exports.main bit) will also run. Well, I only care about the DO side of things!

(ns core)

(defn my-actual-function [& args]
  (println "hello!")
  (println "args: " args))

(defn main [])

(set! js/exports.main my-actual-function)

2023-01-12T11:28:57.786063804Z stdout: hello!
2023-01-12T11:28:57.793552189Z stdout: args:  (#js {} ... // output truncated

Neat!