The Rabin-Karp algorithm

I was doing a little studying on algorithms when I stumbled into what looked to me as a pretty simple question: “Find the first occurrence of a string inside another string”. This can be simply achieved with two nested loops and a worst case scenario performance of O(nm) or O(n^2) if m’s size is relative to n.

Here is an implementation that uses two loops:

function findString(s1, s2) {
  // We only need to loop until s2 doesn't fit anymore
  var loopLimit = s1.length - s2.length;
  for (var i = 0; i < = loopLimit; i++) {
    for (var j = 0; j < s2.length; j++) {
      // As soon as we find a mismatch we abort the loop
      if (s1[i + j] !== s2[j]) {

    // If j reached the size of s2, it means all letters
    // in s2 matched. We have succeeded!
    if (j === s2.length) {
      return true;

  return false;

findString('aaaaaaaaaa', 'aaaab'); // false
findString('aaaaaaaaab', 'aaaab'); // true
findString('aaaaaaaaab', 'ab'); // true

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Load testing a Rails app with Vegeta

I’m building a very simple app using Rails. While looking for guidance for preparing it for production, I found a lot of articles suggesting to put Nginx in front of it. After talking to some people they explained some reasons why this is suggested:

  • Ngingx can serve static assets – This appears to be the greatest and clearer advantage. You can configure Ngingx to directly serve static assets without having to hit Rails at all. This is very good because every request that comes to Rails will block all other request because Ruby is single threaded
  • Nginx can do caching for you – Nginx can cache some of the static assets, which would give them a performance boost
  • Nginx is multithreaded – Nginx can serve multiple static assets at the same time Rails is serving requests

These are definitely advantages (specially the first one), but having Nginx in front of my server also adds complexity to my deployment. To figure out if the added complexity worth it, I decided to run some load tests. Here I will explain how I did it and what were the results.

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Update let’s encrypt certificate without restarting your server

I started using HTTPS in my blog a few months ago and today came the time to renew my certificate. I thought I had automated the process correctly but it turns out for my configuration I have to take some extra steps.

In my previous post I suggested using this command:

21 7,19 * * * /home/user/certbot-auto renew --quiet --no-self-upgrade

But it tries to spin a server in port 80, and I’m already using port 80 for my blog, so the server fails to start.

There is another approach that allows you to renew your certificate without having to free port 80. It works by writing a file to a folder in your webroot and having let’s encrypt server read that file. This sounds pretty straight forward but it was actually a little tricky for me, since I’m using docker.

My blog runs WordPress inside a docker container. Inside the docker container the webroot is /var/www/html and this folder contains all wordpress files. I can’t write directly to this folder because it is inside the docker container, so I had to use a volume. I also can’t mount the whole /var/www/html folder because there are already files in that location inside the container. To make it work I had to mount to /var/www/html/.well-known, which is the folder certbot-auto creates.

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Securing your network with iptables

There comes a time on every system administrator’s life when they need to start being a little more conscious about security. That time has finally come for me.

I have a couple of servers in DigitalOcean where I run various sites and services. Some of these need to communicate with each other to do their job, for example, this blog runs in a server with Apache and PHP and communicates with another server that is running a MySQL database.

This is all good, but one of the most important rules of security is to only allow access to resources on a per-need basis. What this means is that from a security standpoint, nobody should be able to access a resource unless explicitly allowed. This rule applies to almost all scenarios that require some kind of access control and is a good idea to follow it whenever possible.

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Simple strategy for MySQL backups

I now have a good amount of data in my blog that I would be very sad if I lost. As a precautionary measure I decided to build a little system that will backup my data regularly so I’m prepared in case of a disaster.

The strategy

The strategy is going to be very simple. I’m going to create a user in my database that has read permissions on the tables I want to backup. This user will run mysqldump from a different machine and will save the backups there. I will create a cron job that will do this once a day.

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Free centralized log management with Loggly

I’m looking for a centralized log management system that I can plug into some of my hobby projects and while I was about to spin up my ELK server (Elasticsearch, Logstash, Kibana) I found that Loggly has a free tier. I have used Loggly before and it is pretty good so I decided to give it a try.

Before we start setting things up in Loggly, we need to decide which logs we want to send. Here are a few that apply for me:
– Apache logs for (Running inside docker container)
– MySQL logs (Running inside Digital Ocean droplet)
– Cron logs (Also inside Digital Ocean droplet)

Before we start configuring our system we need to create a Loggly account.

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House painting problem

The question

There are a row of houses, each house can be painted with one of three colors red, blue or green. The cost of painting each house with a certain color is different. You have to paint all the houses such that no two adjacent houses have the same color. You have to paint the houses with minimum cost. How would you do it?

Note: Painting house-1 with red costs different from painting house-2 with red. The costs are different for each house and each color.

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Host your Docker images for free with

I’m slowly incrementing the number of projects I host in my personal servers and as the number increases I find the need to standardize the way I deploy each service. Currently each service has a different way of running and I have to try to remember how to do it each time I have an update. As one of the steps to a more streamlined deploy process I decided for each service to have a production ready image hosted in a Docker registry. The deploy will then just be a matter of downloading and running the image in the production machine (not perfect, but a step forward).

My first idea was to host a Docker registry myself, but luckily I found a service that offers 20 private repositories for free. To start using, you just need to register for the basic plan and create a new repo.

To push images you can use the command line. Start by logging in:

docker login --username=username

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SSL termination on load-balanced wordpress

Previously I wrote a post explaining how to do SSL termination with Ha-proxy. It seemed to be working fine, but it was giving me problems about mixed content when loading my blog.

What was happening was that my blog was being served on, but all the JS, CSS and links where being returned in http. This actually makes a lot of sense because the load balancer is requesting content using http and then forwarding this content to the browser.

Once the problem is understood, the solution is just a matter of finding out how to tell wordpress to render https content when Ha-proxy receives an https request. A way to do this is by sending a header to wordpress when the request came on port 443. We can do this in haproxy.cfg:

frontend https-in
        bind *:443 ssl crt /certs/ncona.pem
        reqadd X-Forwarded-Proto:\ https

        acl ncona-web-frontend hdr(host) -i

        use_backend ncona-web if ncona-web-frontend

The reqadd instruction will add a header to the request being sent to the backend. Now we can inspect for this header in wp-config.php:

if ($_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https') {
  $_SERVER['HTTPS'] = 'on';
  $_SERVER['SERVER_PORT'] = 443;

This solved the problem and I can finally serve my blog with https.

SSL termination with HA-Proxy

SSL termination refers to the process of terminating the encrypted connection at the load balancer and handling all internal traffic in an unencrypted way. This means that traffic between your load balancer and internal services (or between internal services) will not be encrypted, so you should make sure your network is secure. If you have your own data center, you can trust your network, otherwise you should set up a VPN so traffic can’t be sniffed.

Terminating SSL at the load balancer has a few advantages:

  • Single place responsible of managing encryption and decryption of traffic
  • Centralized place to store certificates
  • The load balancer can analyze the traffic and take special actions based on this
  • The load balancer can modify the request and response if necessary

A somewhat common scenario of wanting the load balancer to modify the request is adding headers to HTTP requests. More specifically, it is common to have the load balancer add a X-Forwarded-For header, which includes the IP address where the request originated. Without this header, all requests would look like they originated in the load balancer.

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