Vinh v9n

System Design Cheatsheet

Picking the right architecture = Picking the right battles + Managing trade-offs

Basic Steps

Clarify and agree on the scope of the system

User cases (description of sequences of events that, taken together, lead to a system doing something useful)
- Who is going to use it?
- How are they going to use it?

MongoDB 3.2.x SSL with Letsencrypt

Letsencrypt is an initative which aims to increase the use of encryption for websites. It basically allows people to apply for free certificates provided that they prove the they control the requested domain. We will look at the what is needed to secure your MongoDB installation. For more details on setting up a MongoDB server see MongoDB 3.2.x.

Set the hostname

We sould to set the hostname to match the name of the certificate we are going to optain.

sudo hostname mongo0.example.com

Then update the hostname file to set the server name permanently.

async/await is just the do-notation of the Promise monad

CertSimple just wrote a blog post arguing ES2017's async/await was the best thing to happen with JavaScript. I wholeheartedly agree.

In short, one of the (few?) good things about JavaScript used to be how well it handled asynchronous requests. This was mostly thanks to its Scheme-inherited implementation of functions and closures. That, though, was also one of its worst faults, because it led to the "callback hell", an seemingly unavoidable pattern that made highly asynchronous JS code almost unreadable. Many solutions attempted to solve that, but most failed. Promises almost did it, but failed too. Finally, async/await is here and, combined with Promises, it solves the problem for good. On this post, I'll explain why that is the case and trace a link between promises, async/await, the do-notation and monads.

First, let's illustrate the 3 styles by implementing

Exporting your 2FA tokens from Authy to transfer them into another 2FA application

IMPORTANT - Update regarding deprecation of Authy desktop apps

Past August 2024, Authy stopped supported the desktop version of their apps:
See Authy is shutting down its desktop app | The 2FA app Authy will only be available on Android and iOS starting in August for details.

And indeed, after a while, Authy changed something in their backend which now prevents the old desktop app from logging in. If you are already logged in, then you are in luck, and you can follow the instructions below to export your tokens.

If you are not logged in anymore, but can find a backup of the necessary files, then restore those files, and re-install Authy 2.2.3 following the instructions below, and it should work as expected.

	(ns firstshot.chessknightmove
	(:refer-clojure :exclude [== >= <= > < =])
	(:use clojure.core.logic
	clojure.core.logic.arithmetic))

	(defn knight-moves
	"Returns the available moves for a knight (on a 8x8 grid) given its current position."
	[x y]
	(let [xmax 8 ymax 8]
	(run* [q]

	package main

	import (
	"log"
	"syscall"
	"unsafe"
	)

	var (
	kernel32 = syscall.NewLazyDLL("kernel32.dll")

	/*<?php
	///public class PhpJava { public static void main(String[] args) { System.out.printf("/%s",
	//\u000A\u002F\u002A
	class PhpJava {
	static function main() {
	echo(//\u000A\u002A\u002F
	"Hello World!");
	}}
	//\u000A\u002F\u002A
	PhpJava::main();

	#!/bin/bash
	# TODO: skip tiny files (so small they couldn't be photos)
	# TODO: make sure sym links and other file system oddities are handled
	# TODO: look at paralellization for perf boost

	#
	# Constants
	#
	CHAR_COUNT=12
	BLOCK_COUNT=6

	# Should work on all Debian based distros with systemd; tested on Ubuntu 16.04+.
	# This will by default install all plugins; you can customize this behavior on line 6. Selecting too many plugins can cause issues when downloading.
	# Run as root (or sudo before every line) please. Note this is not designed to be run automatically; I recommend executing this line by line.

	apt install curl
	curl https://getcaddy.com \| bash -s personal dns,docker,dyndns,hook.service,http.authz,http.awses,http.awslambda,http.cache,http.cgi,http.cors,http.datadog,http.expires,http.filemanager,http.filter,http.forwardproxy,http.geoip,http.git,http.gopkg,http.grpc,http.hugo,http.ipfilter,http.jekyll,http.jwt,http.locale,http.login,http.mailout,http.minify,http.nobots,http.prometheus,http.proxyprotocol,http.ratelimit,http.realip,http.reauth,http.restic,http.upload,http.webdav,net,tls.dns.auroradns,tls.dns.azure,tls.dns.cloudflare,tls.dns.cloudxns,tls.dns.digitalocean,tls.dns.dnsimple,tls.dns.dnsmadeeasy,tls.dns.dnspod,tls.dns.dyn,tls.

	unless ARGV.size > 0
	puts " Missing executable file argument"
	puts " Usage (in a Dockerfile)"
	puts " RUN crystal run ./path/to/list-deps.cr -- ./bin/executable"
	exit 1
	end

	executable = File.expand_path(ARGV[0])

	unless File.exists?(executable)