Example Terraform HTTP backend implementation for Node.js. Stores the state in a local object. Not meant for production, but more to show how one would go about implementing the HTTP api for it since it's not well documented.

example-http-backend.tf

terraform {
	backend "http" {
		address = "http://localhost:8080/tfstate"
		lock_address = "http://localhost:8080/tfstate"
		unlock_address = "http://localhost:8080/tfstate"
	}
}

server.js

const URL = require('url');
const bodyParser = require('body-parser');
const express = require('express');
const app = express();

app.use(bodyParser.json());
app.use(bodyParser.text());

app.use((req, res, next) => {
	req.uri = URL.parse(req.url, true);
	req.uri
	next();
});

app.use((req, res, next) => {
	console.error('\x1b[47m\n\x1b[m');
	console.error('------------');
	console.error('%s %s', req.method, req.url);
	console.error('');
	console.error('HEADERS');
	console.error('');
	console.error(req.headers);
	console.error('');
	console.error('BODY');
	console.error('');
	console.error(req.body);
	console.error('------------');
	next();
});

let locked = null;
let state = null;

app.use('/tfstate', (req, res) => {
	switch (req.method) {
	case 'LOCK':
		if (locked) {
			console.error('FAIL: already locked');
			return res.status(409).end('nice try, buckaroo - I already seen ye');
		}

		console.error('OK: lock');
		locked = req.body.ID;
		return res.status(200).end('as you wisharoo, buckaroo');

	case 'UNLOCK':
		if (locked && req.body.ID !== locked) {
			console.error('FAIL: lock conflict (wrong ID)');
			return res.status(409).end('uhwaiit, who are you again?');
		}

		console.error('OK: unlock');
		locked = null;
		return res.status(200).end('mischief managed, buckaroo');

	case 'POST':
		if (!locked || (locked && locked === req.uri.query['ID'])) {
			console.error('OK: store');
			state = req.body;
			return res.status(200).end('gimme dat JSON, buckaroo');
		}

		console.error('FAIL: locked');
		return res.status(423).end('not today, karen');

	case 'GET':
		console.error('OK: give state');
		return res.status(200).end(state !== null && typeof state === 'object' ? JSON.stringify(state) : state || '');

	default:
		console.error('FAIL: unknown method');
		return res.status(405).end('wat?');
	}
});

app.get('/health', (req, res) => res.status(200).end('ok'));

app.listen(8080);

Note that you have to check if the state is an object if you're using an automatic body parser since some old versions of Terraform's state were plaintext.