Constructing with Nix on Replit

As of Could perhaps merely 2021, Replit helps all programming languages thru the vitality of Nix. However Nix can discontinue pretty quite so a lot of extra than valid enable us to use original languages. On this text, we will duvet numerous different use-cases for Nix on Replit, alongside side:

  • Environment up a production-grade web stack, with a database and improve for numerous web servers.
  • Running third-occasion packages in Replit.
  • Playing DOOM in a repl.

What’s Nix?

Nix is a tool designed for managing packages and system configurations. It has some similarities to kit managers that you just might maybe also beget feeble within the past, resembling Homebrew on macOS, APT on Debian-based mostly entirely mostly Linux distributions, Python’s pip, or Node.js’s NPM. In case you beget no longer feeble a kit manager ahead of, or no longer it’s in general an app retailer.

While you occur to set up a kit with a frequent kit manager – divulge you engage to beget to set up a browser love Firefox – it will download some recordsdata, unpack them in various areas for your system, and speed some configuration scripts. This can additionally set up all the additional packages and libraries Firefox needs to speed. Many of those will beget dependencies themselves, and so it will set up those too. Within the kill, placing in a single kit can require many extra installations and result in frequent changes to your system.

In most cases, packages shall be incompatible with every other, attributable to counting on different variations of the same dependency. As an instance, App #1 depends on libxyz 1.4 and App #2 depends on libxyz 1.5. Installing App #2 upgrades libxyz and breaks App #1. Downgrading libxyz fixes App #1 but breaks App #2.

Here’s known as dependency hell, and or no longer it’s one in every of the issues Nix was once designed to resolve. Whereas a frequent kit manager might maybe well also set up Vim to /usr/bin/vim, nix will set up it to a itemizing that looks love this: 

/nix/retailer/-vim-

The itemizing title has three parts: the kit title, the model, and a hash of the total kit info, resembling configuration alternatives. Two packages on two different programs with the same hash shall be similar, but two packages with the same title and model but different hashes will beget dinky differences. Thanks to this, we are in a position to set up numerous variations of the same kit, neatly fixing our App #1 and App #2 dependency hell.

Whereas a kit manager love APT might maybe well also scatter the contents of an put in program all the diagram in which thru /bin, /etc, /usr, and other directories that require special privileges to write down to, Nix retains the total lot in /nix/retailer. This lets us securely set up packages as a non-privileged particular person.

Applications in Nix are constructed the use of derivations, which that you just might maybe also call to mind as put scripts. All derivations are written within the Nix language, a purposeful programming language, a lot like Haskell or F#. In case you beget no longer feeble a purposeful language ahead of, basically the most classic yelp to adore is that there might be no power bid, i.e. that you just might maybe also’t present an explanation for variables outdoor of functions. Helpful languages are peaceable of functions that decide some input and assemble some output. Whenever a goal is accomplished with a given input, it will return the same output. This requires a obvious methodology to frequent crucial programming languages but enhances predictability and reproducibility of output, two very appropriate qualities for a put system.

To learn extra about Nix, take a look at out the next sources:

How will we use Nix on Replit?

Every repl you label is backed by a Docker container. One thing you discontinue from your repl, from working code to executing instructions within the shell, will occur within the context of the runner particular person in this container. For causes of security, this particular person would not beget root privileges, and attributable to this truth cannot set up packages the use of a frequent kit manager love APT. However the runner in a Nix repl can set up packages with Nix.

This opens up a colossal array of chances. In outdated tutorials, we beget centered on the use of repls to speed custom code in various languages. The Replit Database makes it capacity to label applications with power storage, but Nix permits us to use a frequent DBMS love MySQL or Postgres. We are in a position to additionally set up webservers and even graphical packages love Inkscape and LibreOffice.

Basically, Nix turns our repl proper into a fully fledged server. Within the next few sections, we will duvet one of the principal principal issues we are in a position to discontinue which ability that.

Deploying a production web stack on Replit with Nix

The first yelp we will put with Nix is a production web stack with the next parts:

  • Python Flask because the applying server.
  • Waitress because the Web Server Gateway Interface.
  • NGINX because the web server.
  • Postgres because the SQL database.

Why Waitress?

In case you’ve gotten got constructed the rest with Python and Flask, resembling one in every of our outdated tutorials, that you just might maybe also beget feeble Flask’s constructing server to work alongside with your application. While this server is colossal for constructing and debugging, or no longer it’s optimised for a single particular person and will get pretty slack if numerous particular person tries to use it.

Flask development server

We are in a position to improve the speed of our Flask apps by the use of a production-grade WSGI server as an alternative of Flask’s default, resembling Gunicorn or Waitress.

Why NGINX?

What if we desire to host extra than valid a Flask app? Shall we divulge an e-commerce place at www.example.com. The important thing retailer application is powered by Flask, but our advertising and marketing division would engage to launch up a weblog at www.example.com/weblog. We might maybe well also put working a blog performance into our e-commerce place, then all another time it could well be mighty sooner and simpler to use a separate application, resembling WordPress or Ghost.

Here’s where a fully featured web server, resembling NGINX or Apache, is available in. These web servers might maybe well also additionally be configured to again numerous different applications and disclose material directories at different areas on one or extra domains. They’re additionally mighty sooner at serving static disclose material than even a production-grade WSGI server, so even single-app deployments decide pleasure within the use of them.

Why Postgres?

Now we beget feeble Replit’s Database for power storage in numerous outdated tutorials. While or no longer it’s easy to use in supported languages love Python, it would not beget the vitality and suppleness of a mainstream SQL database, and we cannot continue the use of it if we ever tear our code out of Replit. Postgres is a popular SQL database feeble by many, from little startups to tech giants love Apple, Reddit and Spotify. We are in a position to use it too if we set up it on Nix.

Repl overview

Now we beget made a Nix repl containing the production web stack available here: https://replit.com/@ritza/nix-template

Birth it now, or fork it to your profile, and we will tear over how it works. Plan certain that the repl’s config recordsdata are exhibiting.

Show configuration

The first file we will watch at is replit.nix. Here’s the deplorable Nix file that tells our repl what packages to set up. Within the default Nix repl, it looks love this: 

{ pkgs }: {
    deps = [
        pkgs.cowsay
    ];
}

The first line, { pkgs }: , defines an anonymous goal that takes a single argument, pkgs. After we speed our repl, this goal shall be known as and its contents accomplished. On this case, its contents is a list of packages to set up, one merchandise long. Therefore, all this goal does is set up cowsay, a program that prints an ASCII cow.

Cowsay

Against this, the replit.nix file in our production web stack repl is extra delicate. It looks love this: 

{ pkgs }: 
let

    nginxModified = pkgs.nginx.overrideAttrs (oldAttrs: rec {
        configureFlags = oldAttrs.configureFlags ++ [
            "--http-client-body-temp-path=/home/runner/REPL-NAME-HERE/cache/client_body"
            "--http-proxy-temp-path=/home/runner/REPL-NAME-HERE/cache/proxy"
            "--http-fastcgi-temp-path=/home/runner/REPL-NAME-HERE/cache/fastcgi"
            "--http-uwsgi-temp-path=/home/runner/REPL-NAME-HERE/cache/uwsgi"
            "--http-scgi-temp-path=/home/runner/REPL-NAME-HERE/cache/scgi"
         ];
    });

in {
    deps = [
        nginxModified
        pkgs.python39
        pkgs.python39Packages.flask
        pkgs.python39Packages.waitress
        pkgs.postgresql
        pkgs.python39Packages.psycopg2
    ];

}

You ought to recognise some similarities between this code and the default replit.nix. We’re silent defining an anonymous goal that takes pkgs, but now we’re placing in numerous kit.

All of the packages in deps are straight from Nix’s kit repository, other than for nginxModified. We desire to put some changes to nginx to salvage it to speed in our repl. Nix’s language and system configuration skills put that mighty extra efficient to discontinue than if we had been the use of a obvious kit manager that did not improve recompiling packages.

Nix’s let ... in { ... } relief watch over structure is feeble when we desire to make clear native variables feeble in a given goal. We present an explanation for the variables after let after which use them after in. Let’s decide a more in-depth watch on the definition of nginxModified: 

    nginxModified = pkgs.nginx.overrideAttrs (oldAttrs: rec {
        configureFlags = oldAttrs.configureFlags ++ [
            "--http-client-body-temp-path=/home/runner/REPL-NAME-HERE/cache/client_body"
            "--http-proxy-temp-path=/home/runner/REPL-NAME-HERE/cache/proxy"
            "--http-fastcgi-temp-path=/home/runner/REPL-NAME-HERE/cache/fastcgi"
            "--http-uwsgi-temp-path=/home/runner/REPL-NAME-HERE/cache/uwsgi"
            "--http-scgi-temp-path=/home/runner/REPL-NAME-HERE/cache/scgi"
         ];
    });

Here we’re taking pkgs.nginx and calling overrideAttrs to commerce the configuration flags that are space when compiling NGINX. We desire to add a few flags that commerce the paths NGINX makes use of to paths that are accessible in our repl. Exhibit that we beget created the total directories within the anticipated areas.

The derivation that runs when we set up pkgs.nginx might maybe well also additionally be came upon here. Our model will discontinue the same issues, but with a few extra items in configureFlags.

That is it for replit.nix. Now let’s decide a watch at .replit. This file defines what record will salvage accomplished when we click on the “Flee” button, and what custom atmosphere variables shall be available to our repl.

speed = "sh launch up.sh"

[env]
PGDATA = "/house/runner/${REPL_SLUG}/records"

We will discontinue the shell script launch up.sh when we press “Flee”, and we beget defined PGDATA, an atmosphere variable Postgres makes use of to stumble on its records itemizing. Let’s watch at launch up.sh subsequent: 

# launch up Postgres
pg_ctl cease

initdb
cp postgresql.conf.tpl records/postgresql.conf

socker_dir="/house/runner/${REPL_SLUG}/postgres"

sed -i "s/replace_unix_dir/${socker_dir}/" records/postgresql.conf

pg_ctl -l /house/runner/${REPL_SLUG}/postgresql.log launch up

createdb -h 127.0.0.1
psql -h 127.0.0.1 -c "label database appdb;"

# launch up nginx
pkill nginx

nginx -e /house/runner/$REPL_SLUG/logs/error.log -c /house/runner/$REPL_SLUG/nginx.conf

# launch up Flask app
python principal.py

In yelp, we launch up Postgres, then NGINX, after which our Python Flask application.

Our Postgres code first stops any present cases of Postgres, then calls initdb, which is ready to label a original database on the itemizing specified in $PGDATA if none exists. We then replica our Postgres configuration file into our records itemizing and use sed to beget in its unix_socket_directories value, one more itemizing we beget to commerce to salvage issues working in a repl.

The file postgressql.conf.tpl is long and largely unimportant. The very top fragment of that file that shall be related for frequent use is the next traces under the heading “Connection Settings”: 



listen_addresses = '127.0.0.1'      
                    
                    
                    
port = 5432

These traces space the database to hear on the native loopback on TCP port 5432. This can enable other packages in our repl to join to it over the network, with out having it uncovered to the web or other repls. Here’s related for the next few traces of our script: 

pg_ctl -l /house/runner/${REPL_SLUG}/postgresql.log launch up

createdb -h 127.0.0.1
psql -h 127.0.0.1 -c "label database appdb;"

The first line begins our database, and the closing two label a Postgres instance usable by runner and within that, a database named appdb. Both of those traces will fail on subsequent runs of launch up.sh (if the database has already been created), so we have not got to difficulty about overwriting our database every time we speed our repl.

The code for starting NGINX is extra efficient: 


pkill nginx

nginx -e /house/runner/$REPL_SLUG/logs/error.log -c /house/runner/$REPL_SLUG/nginx.conf

First, we rupture any present nginx processes, after which we launch up NGINX, telling it to write down errors to logs/error.log and use the configuration file nginx.conf. Treasure postgresql.conf, this configuration file is basically unimportant for frequent use. The next changes beget been fabricated from Nix’s default NGINX configuration file:

  1. The chance pid specifies a repl-accessible PID file space: 

     pid        /house/runner/REPL-NAME-HERE/logs/nginx.pid;

  2. The chance access_log within the http block specifies a repl-accessible salvage entry to log file space: 

     access_log  /house/runner/REPL-NAME-HERE/logs/salvage entry to.log;

  3. The server block has been modified to host our Python server, as detailed below.

         server {
         hear       8080;
         server_name  localhost;

         

         

         space / {
             proxy_pass   http://127.0.0.1: 8181;
         }

         

         
         
         error_page   500 502 503 504  /50x.html;
         space = /50x.html {
             root   html;
         }

     }

In NGINX, server blocks are what you utilize to space up web sites on particular person domains. Every area (e.g. example.com) or subdomain (e.g. weblog.example.com) will beget its receive server block. To label a server block that can present an explanation for what our repl hosts, we use the next NGINX directives: 

    hear  8080
    server_name  localhost

Here we beget space NGINX as a lot as speed our server on TCP port 8080, as per Replit’s web hosting pointers. Within our server block, we are in a position to beget one or extra space blocks. These repeat NGINX what disclose material to host at different URLs. We are in a position to use the proxy_pass directive to again the contents of one more webserver working in our repl, or the root directive to again static recordsdata. That that you just can well learn extra about configuring NGINX within the unswerving NGINX Beginner’s Manual.

The closing yelp we discontinue in launch up.sh is launch up up our Python Flask server: 


python principal.py

The code for principal.py ought to take a study acquainted whenever you occur to’ve gotten got feeble Flask ahead of: 

from flask import Flask
from waitress import again
import psycopg2

app = Flask(__name__)

@app.route("/")
def index():

    connection = psycopg2.join(
        host="127.0.0.1",
        database="appdb")

    cursor = connection.cursor()
    cursor.discontinue('SELECT model()')
    db_version = cursor.fetchone()
    cursor.discontinuance()

    return f"Hello from Python! PostgreSQL database model: {db_version}"

#app.speed(host='127.0.0.1', port=8181) # dev server
again(app, host='127.0.0.1', port=8181, url_scheme='https') # production server

On this code, we beget created a Flask application that connects to our Postgres database and is served on the native loopback deal with at TCP port 8181 by the Waitress WSGI server.

In case you speed the repl now, that you just might maybe watch a web page exhibiting model info about our PostgreSQL database.

From this deplorable, that you just might maybe also put and configure a production-gripping web application. Are attempting the next suggestions:

  • Put into effect the code from one in every of our outdated Flask-based mostly entirely mostly tutorials, resembling this PDF file generator, this technical competition place, or this PDF retailer. Ogle whenever you occur to might maybe well also adapt the disclose material to use Postgres in preference to the Replit Database.
  • Put into effect your receive Flask web application, the use of Postgres as a database.
  • Add a 2nd application listening on a obvious loopback port and available from a obvious URL. This might maybe be your receive Python or Node.js mission, or a deployment of open-source tool resembling Ghost or WordPress.

Just a few other issues that you just might maybe also discontinue with Nix and Replit

We began with a complex example to present you a opinion of the vitality and capacity of Nix, but there are a few other issues that you just might maybe also discontinue with it that put no longer require the same amount of space up.

Jupyter Notebook

That that you just can well speed a Jupyter Notebook in a Nix repl. Add pkgs.jupyter to the dep. checklist in replit.nix, space speed in .replit to sh launch up.sh, and label a launch up.sh script with the next contents: 

mkdir records
jupyter notebook --ip 0.0.0.0 --port 8080 --notebook-dir /house/runner/$REPL_SLUG/records

Notebooks are web-based mostly entirely mostly interactive constructing environments that enable you to combine runnable code, text notes, mathematical equations, and charts and graphs. They’re commonly feeble by records scientists.

https://replit.com/@ritza/nix-jupyter

Jupyter notebook

VSCode Server

That that you just can well speed a Visual Studio Code Server, which is ready to enable you to use a non-public, customised model of the popular text editor from wherever, merely by navigating to your repl’s URL.

https://replit.com/@ritza/nix-vscode-server

VS Code server

Various web stacks

In bid of NGINX, that you just might maybe also use Apache, and as a replacement of Postgres, that you just might maybe also are attempting MySQL or maybe a NoSQL database love MongoDB. And naturally, that you just might maybe also use valid about any mainstream programming language to write down your web application code as a replacement of Python, resembling JavaScript, Ruby or Java.

Other programming languages

That that you just can well space up and use a programming language that is rarely any longer formally supported by Replit, resembling Racket (nixpkgs.racket), Prolog (nixpkgs.swiProlog), or even COBOL (nixpkgs.gnu-cobol).

https://replit.com/@ritza/nix-cobol

DOSBox

We are in a position to salvage the popular MS-DOS emulator DOSBox working in a Nix repl by valid placing within the lawful kit and working it. From there, we are in a position to speed any DOS program and use it in our repl’s VNC window.

https://replit.com/@ritza/nix-dosbox

Here is DOOM in a repl:

Doom

Other DOS packages that you just might maybe also are attempting:

  • Liero, a keyboard-sharing worm combating game.
  • WordStar 4.0, the notice processor George RR Martin makes use of to write down the Game of Thrones e-book series.

Total pointers for constructing with Nix repls

As we beget viewed above, Nix permits us to use repls for extra than valid writing and testing code, but some packages require an even amount of configuration to salvage working. Listed here are a few general pointers for getting packages working in Nix repls:

  • First, explore the kit you engage to beget on the Nix kit search web place and add it to deps in replit.nix. In case you don’t watch any errors within the console after working your repl, it doubtlessly labored. Some packages must be manually began by specifying a speed record in .replit, whereas others will launch up robotically.
  • Many of the set up errors that you just might maybe come upon will repeat to file paths that put no longer exist for your repl, or that you just don’t favor permission to salvage entry to. These can on the total be fixed whenever you occur to might maybe well also configure your kit to explore those recordsdata for your repl’s file checklist as a replacement, which is hosted on disk at /house/runner/REPL-NAME-HERE/. Depending on the kit, that you just might maybe also presumably discontinue this with a custom configuration file, love we did with Postgres, or that you just might maybe also beget to alter the methodology Nix installs it, as we did with NGINX.
  • That that you just can well peep recordsdata and directories that put no longer seem like visible for your repl’s filepane from the shell, the use of authentic Unix instructions love ls, cd and cat. This entails recordsdata and directories within the Nix retailer, at /nix/retailer. This can commonly be precious for debugging.
  • That that you just can well peep a list of working processes with the shell record ps aux, and discontinuance them with pkill .
  • Loading the Nix atmosphere will every on occasion decide a really long time, particularly whenever you occur to might maybe well also beget custom derivations.
  • In case you is also organising a web server, consult with our steerage on deploying HTTP servers.

As a closing reminder, guarantee to abide by Replit’s Terms of Provider when the use of Nix repls, and steer clear of placing in packages that use excessive system sources (resembling cryptocurrency miners) or are intentionally malicious.

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