Expose a way to inject a start script into adapter-node

[Prinzhorn]

Is your feature request related to a problem? Please describe.

I'm tying to migrate from Sapper to SvelteKit. I'm aware of the discussion in #334 and hooks seem to solve that.

I don't think there is currently a way to have code that runs on startup with the Node adapter. There are things that need to happen once and not for every request. In Sapper my server.js looked like this:

import sirv from 'sirv';
import express from 'express';
import compression from 'compression';
import bodyParser from 'body-parser';
import * as sapper from '@sapper/server';
import { Model } from 'objection';
import knex from './lib/knex.js';
import env from './lib/env.js';

Model.knex(knex); // <==============================================

const app = express();

app.use(compression({ threshold: 0 }));
app.use(sirv('static', { dev: env.isDev }));
app.use(bodyParser.urlencoded({ extended: false }));
app.use(sapper.middleware());

(async () => {
  await knex.migrate.latest({  // <==============================================
    directory: './src/migrations',
  });

  app.listen(env.PORT, (err) => {
    if (err) {
      console.error(err);
    }
  });
})();

The first line I've marked is something that would probably not hurt if done in every hook (setting up my Objection models with the knex instance).

The second line I've marked is running the database migration. In a serverfull environment this is a rather common occurrence. I want to run the migration right before starting the server so that I'm in a consistent state between database and application code. Even if this is scaled across multiple instances it works, since the second time the migrations run it's a NOOP. I honestly have no idea how people do that in a serverless environment with a guarantee that no code is run that expects an outdated schema resulting in possibly undefined behavior.

Describe the solution you'd like

A way to run setup code before the server starts. Maybe an async function setup() {} that is awaited before the server starts? But that can't work with every adapter.

Describe alternatives you've considered

I guess some will argue I should have a second API server separated from SvelteKit and whatnot. But I want less complexity, not more.

How important is this feature to you?

I cannot migrate to SvelteKit, unless I'm missing something.

[bjon]

Why don't not just run it before starting SvelteKit? Like node db-migration.js && node [SvelteKit]?

[Prinzhorn]

Why don't not just run it before starting SvelteKit? Like node db-migration.js && node [SvelteKit]?

But I want less complexity, not more.

I can't be the only one that needs a way to setup the environment for my application and would like a central place for that?

E.g. things like this

// I _think_ the aws-sdk can do this by itself using env variables, so it might be a bad example.
require('aws-sdk').config.update({
	accessKeyId: process.env.AWS_ACCESS_KEY_ID,
	secretAccessKey: process.env.AWS_SECRET_ACCESS_KEY,
	region: 'eu-west-1'
});

or

if (process.env.NODE_ENV !== 'development') {
  // This will make the fonts folder discoverable for Chrome.
  // https://github.com/alixaxel/chrome-aws-lambda/blob/eddc7fbfb2d38f236ea20e2b5861736d3a22783a/source/index.js#L31
  process.env.HOME = '/tmp';
  fs.ensureDirSync('/tmp/.fonts');
  fs.copySync(path.join(__dirname, 'fonts'), '/tmp/.fonts');
}

I could put each of them in a module so importing would execute it once. But where would I import it if there is no central place? I guess I could abuse the hooks.js for that (outside of the actual hook functions), which works for setup code that is synchronous.

I also ran into a case where a module had to be the very first thing that I required (lovell/sharp#860 (comment)) but SvelteKit does not give me that type of flexibility.

Another idea

So what if the node-adapter could have a flag that would make index.js not polka() on it's own, but it exports a single function that I can use? That way I can import index.js, do all the things I want and then start the server.

[Prinzhorn]

So what if the node-adapter could have a flag that would make index.js not polka() on it's own, but it exports a single function that I can use? That way I can import index.js, do all the things I want and then start the server.

Uh, maybe I'll just lazily require('index.js') await import('./index.js') in my own entry script after setup is done 🎉

[Prinzhorn]

Another real-world example: fetching metadata once during startup before launching the server https://docs.digitalocean.com/products/droplets/how-to/provide-user-data/

I'll leave this open to get some more feedback

[mankins]

So what if the node-adapter could have a flag that would make index.js not polka() on it's own, but it exports a single function that I can use? That way I can import index.js, do all the things I want and then start the server.

You could always create your own adapter, using adapter-node as a guide.

I wanted to remove the console.log on server start (and add pino), so made a quick express based adapter that's easy to modify. I agree, it would be nice to have a convention for doing things to the server startup.

[Conduitry]

Does top-level code in hooks.js get run? I would guess it probably would. You could put stuff there that you want to run once on server init and not per request.

[Prinzhorn]

You could always create your own adapter, using adapter-node as a guide.

Thanks for the hint, that sounds reasonable. I'll look into your Express adapter when I start working on this again.

You could put stuff there that you want to run once on server init and not per request.

I'd prefer not to introduce race-conditions and wait for the initialization to succeed before starting the server. I might even need some of the initialization logic to decide on certain parameters for the server. E.g. querying the hostname via cloud metadata service to configure CORS middleware. There are probably countless more use-cases.

[aewing]

I brought this up back in December, and I still think when we load the adapter in kit we should look for and await an init function, then the adapter can use that however it wants to (if at all). In the case of the node adapter, it could execute an async method passed to the adapter from svelte.config.js and we could bootstrap our pools and whatnot there without making things weird for other adapters.

I think it would be cool if the init function could pass back an initial context state for each request.

[aewing]

I don't need this, but I still want it.

[ValeriaVG]

I use a hack-around for my SvelteKit TypeScript Node project:

// Create a promise, therefore start execution
const setup = doSomeAsyncSetup().catch(err=>{
  console.error(err)
  // Exit the app if setup has failed
  process.exit(-1)
})

/** @type {import('@sveltejs/kit').Handle} */
export async function handle({ request, resolve }) {
   // Ensure that the promise is resolved before the first request
   // It'll stay resolved for the time being
    await setup;
    const response = await resolve(request);
    return response
}

Here's how it works:

• App loads hooks and starts executing the promise
• Until setup is complete all the requests are waiting
• If the setup fails - the app exits with an error
• Once setup is complete every request just jumps over await setup as it has already been resolved

Of course, having a setup hook would be cleaner and more straightforward than this. 👍

[dummdidumm]

Related to #1538

[livehtml]

@ValeriaVG big thanks for your suggestion!
Can you please also show example for doSomeAsyncSetup function?