3708 cards

How to run a small web server (one-liner)

Sometimes you just want to have a small web server that serves files to test something.

Serve the current directory

On Ruby 1.9.2+ you can do the following ("." for current directory).

ruby -run -ehttpd . -p8000

Python 2.x offers a similar way.

python -m SimpleHTTPServer 8000 .

This is the same way with Python 3.x

python -m http.server

In both cases your web server is single-threaded and will block when large files are being downloaded from you.

WEBrick also offers [a simple way](https://stackoverflow.com/quest…

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Sidekiq: Problems and Troubleshooting

When using Sidekiq in your application, you must write thread-safe code.

This wiki page also lists gems that are known to be unsafe on threaded applications.
When adding a gem that will also be used by a Sidekiq worker, make sure to confirm it's thread-safe.


I18n fallback locales

When you need to create a locale for a language variant (like Austrian for German), you probably don't want to duplicate your entire de.yml file only to change a few minor exceptions for our Austrian friends.

Luckily, the I18n gem used by Rails has a fallback feature where you can make one locale file fall back to another if no translation is available.

In the example above you would have a config/locales/de_DE.yml:

    # hundreds of translations here

… and another …

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How to view a file from another branch

Just run git show branch:file. Examples:

git show HEAD~:bin/command
git show origin/master:../lib/version.rb

Rbenv: Alias a Ruby version

For newer Ubuntu versions we currently need to install the patch level version 1.8.7-p375, otherwise the dev dependencies from openssl will cause the installation to fail.

For a project that specifies the Ruby version 1.8.7 in the .ruby-version the rbenv autoswitch will not work. You have several options how you can solve this problem:

  • Install rbenv-aliases, which will alias your Ruby 1.8.7-p375 …

Fixing flaky integration tests

This card shows basic techniques for fixing a flaky integration test suite that sometimes passes and sometimes fails. "Integration test" is a test script that remote-controls a web browser with tools like Selenium WebDriver.

Although the examples in this card use Cucumber and Selenium, the techniques are applicable to all languages and testing tools.

Why tests are flaky

Your tests probably look like this:

When I click on A
When I click on B
When I click on C
Then I should see effects of C

A test like this works fine most of t…


Icon font vertical alignment in Windows

I had an issue with icons from an icon font aligning differently on Linux, iOS and Windows (seemingly browser-independent). With vertical-align:middle, they aligned properly on Linux, iOS and macOS, whereas with a vertical-align of -18%, it looked good on Windows and iOS, but not Linux.

Further investigation showed that not only icons, but also normal capital letters aligned differently. No setting of vertical-align could fix this, neither top, bottom, middle, nor additional paddings or margins. It seems like browsers take the…


Heads up: JavaScript does not like big numbers

In a JavaScript console, type this:

> 9112347935156469760


This occurs because JavaScript uses double precision floats to store numbers.

So according to IEEE floating point definition only numbers between -(2^53 - 1) (-9007199254740991) and 2^53 - 1 (9007199254740991) can safely be represented in JavaScript.

Note that ECMAScript 6 will probably also offer [Number.MAX_SAFE_INTEGER](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference…

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Running Rspec examples by name, or: Running a single shared example

When an Rspec example fails, I usually investigate by running that example again using rspec <file:line>. However, this does not work with shared examples, since Rspec doesn't know in which context the shared example should be run.

But there is a different way: You can run the shared example using the -e, --example option. It takes a string value and runs all scenarios containing that substring in their full description.

This allows you to run a single uniquely named example, all examples with
similar names, all the examples in a u…


Rails and Postgres: How to test if your index is used as expected

This is a small example on how you can check if your Postgres index can be used by a specific query in you Rails application. For more complex execution plans it might still be a good idea to use the same path of proof.

1. Identify the query your application produces

puts User.order(:last_name, :created_at).to_sql
# => SELECT "users".* FROM "users" ORDER BY "users"."last_name" ASC, "users"."created_at" ASC

2. Add an index in your migration and migrate

add_index :users, [:last_name, :created_at]

3. Test the…


Differences between transactions and locking

Web applications can be used by multiple users at the same time. A typical application server like Passenger has multiple worker processes for a single app. In a distributed deployment setup like we use at makandra you will even have multiple application servers, each with their own worker pool.

This means that your code needs to deal with concurrent data access. The two main tools we use to cope with concurrency are database transactions and distributed locks. These two are not interchangeable. You ca…

Rspec 3: what to do when `describe` is undefined

In Rspec 3 the DSL like describe is exposed globally by default. Therefore it is not necessary to write Rspec.describe.

However, there is a config option to disable this beavior, which also disables the old should-syntax:

RSpec.configure do |config|

In order to still use describe without Rspec, you can set expose_dsl_globally = true after disable_monkey_patching!:

RSpec.configure do |con…

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Rails: Overwriting default accessors

All columns of a model's database table are automagically available through accessors on the Active Record object.

When you need to specialize this behavior, you may override the default accessors (using the same name as the attribute) and simply call the original implementation with a modified value. Example:

class Poet < ApplicationRecord

  def name=(value)


Note that you can also avoid the original setter and directly read/write from/to the instance's attribute storage. However th…


Configuring RubyGems to not install documentation by default

When installing gems, a lot of time is spent building locally installed documentation that you probably never use.

We recommend you disable documentation generation for gem install by default.
Note that Bundler won't install documentation, so this advice applies only when installing gems manually.

If you don't already have it, create a ~/.gemrc file. The gemrc is a Yaml file, so add the following line to add default switches to the gem command.

gem: --no-document

(If you do n…


XHR is not JSON

When a Rails controller action should handle both HTML and JSON responses, do not use request.xhr? to decide that. Use respond_to.

I've too often seen code like this:

def show
  # ...
  if request.xhr?
    render json: @user.as_json
     # renders default HTML view

This is just plain wrong. Web browsers often fetch JSON via XHR, but they (should) also send the correct Accept HTTP header to tell the server the data they expect to receive.

If you say request.xhr? as a means for "…

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Linux Performance Analysis in 60,000 Milliseconds

You login to a Linux server with a performance issue: what do you check in the first minute?

dmesg | tail
vmstat 1
mpstat -P ALL 1
pidstat 1
iostat -xz 1
free -m
sar -n DEV 1
sar -n TCP,ETCP 1
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MySQL will not use indexes if you query the wrong data type

When MySQL refuses to use your index, you can do a number of things wrong. One of them might be conditions with improper data types.

An example

For example, let's assume you have a users table with an email field (varchar) which is indexed.

MySQL will use the index when your query is well-formed:

mysql> EXPLAIN SELECT * FROM users WHERE email = 'foo@example.com';
| id | select_type | table | type  | poss...

Understanding grid sizes of (SVG) icons

A primer on vector graphics

For rastered image formats like JPG or PNG, each pixel is basically drawn on a fixed size canvas. To display such an image in a different size (say: 1.5 times larger than original), the renderer (your Browser / GPU / Monitor) needs to interpolate the color values of missing pixels. The image will appear slightly blurred.

This is different for vector graphics like the SVG (Scalable Vector Graphics) format. You can imagine SVG files as XML file contai…

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