Neo4j and Emacs

Neo4j is one of the most powerful graph database. However, support for Neo4j in Emacs is still limited. Luckily, with the help of comint-mode, we can easily create a custom inferior shell for executing query and retrieving the result from a running Neo4j instance. This article from Mastering Emacs illustrates how to write your own command interpreter in Emacs. Based on that guide, I have developed a new package for Emacs to simplify the steps of composing and testing cypher query command. This post will summarize my experience and my setup to interact with Neo4j from Emacs.

Cypher mode

First, of course you need a major mode for displaying and editing the cypher query command. You can easily install it using package.el. The mode will automatically associate any files with .cyp and .cypher extension so you don’t need to do any thing after installing it. It also supports basic indentation beside the must-have syntax highlighting.

Firewall on Linux server

I’m working on a project that uses Clojure. Usually, for these kind of project, I will open up an nREPL server for inspecting the web app while fixing bug. However, the problem is that the nREPL server seems to allow all kind of connections, from the local connections to the external ones without prompting for password. I also have a Neo4j instance running the graph database used for the website on another port and sometimes I need access to its web interface to look up the data inside the graph. That leads me to the need of setting up a firewall on my VPS to block all untrusted connections.

iptables seems to be the most popular firewall tool on Linux server out there. However, working with the complex iptables table rules through the command line can be a bit struggling. Fortunately, there is ferm, an utility tool that helps you maintain complex firewalls, without having the trouble to rewrite the complex rules over and over again. It allows the entire firewall rule set to be stored in a separate file, and to be loaded with one command.

Organize ferm with Ansible

Usually, the Ansible config for each project consists of some roles that is reusable among projects and the custom tasks for that project (as I mentioned before in this post Vagrant and Ansible - Organize for reusability). In order to apply that structure, we need to an Ansible role for installing ferm and generate a default config file for it. That config file should have some directives for including the per project config file. In each project, there will be some Ansible tasks in each project for defining particular firewall rule for that project.

Update: this method is outdated again. The new solution is presented here Using Gulp with Browserify and Watchify - Update Nov 2015

Old method

In my previous 2014-08-06-using-watchify-with-gulp-for-fast-browserify-build.md, I have demonstrated how to use Browserify and Watchify in Gulp to automate the build process. The steps are to create a browserify bundle and return that bundle with a bunch of pipe inside a gulp task like this

var b = browserify({
  cache: {},
  packageCache: {},
  fullPaths: true
});
b = watchify(b);
b.add('./main.js');
return b.bundle()
    .pipe(uglify())
    .pipe(gulp.dest(dest));

We have to manually add the main.js file into Browserify so it will become ugly and complex when you have multiple bundles to build, not just one main.js file. It would be much better if we can do something like this, passing the source files as a glob as we usually do with Gulp

return gulp.src(source)
    .pipe(buildBrowserify)
    .pipe(uglify())
    .pipe(gulp.dest(dest));

In this post, I will illustrate how to create that buildBrowserify function.

Vagrant and Ansible

Recently, I have started a personal project built with Clojure, a website for managing family records and visualizing pedigree tree. The problem is that I need an automation tool for setting up the development environment and deploying my website to the server. And yes, Vagrant is one of the best solution out there. I have used Vagrant with Chef before and found that Chef is a bit complex and requires installation of Ruby and Chef on the server before you can deploying anything.

After looking at all of Vagrant’s Provisioning solution, I decided to give Ansible a try because it is simple and operate over SSH, which means I need to install nearly nothing on the server to use it (of course you still can Ansible on server and do a local Provision there, but it’s only one more command to type).

This blog post is the summary of my experience with Vagrant and Ansible, how I set up my development environment and how can I re-use the code for other types of project. The sample project can be found at https://github.com/tmtxt/clojure-web-skeleton. Before you come to the next part, take a look at the basic usage of Vagrant and Ansible.

Basic Structure

A project with Vagrant and Ansible will look like this

├── Vagrantfile
├── ansible
│   ├── group_vars
│   │   └── all
│   ├── main.yml
│   ├── roles
│   │   ├── apt
│   │   ├── emacs
│   │   ├── git
│   │   ├── nvm
│   │   ├── oraclejdk
│   │   ├── postgres
│   │   ├── virtualenv
│   │   └── zsh
│   └── templates
│       ├── db_config.clj
│       └── system_config.clj
├── project.clj
├── src
└── static

PostgreSQL official documentation is one good resource for researching PostgreSQL features. However, the documentation on its home page are exported to PDF format only, which make it hard to read on other devices since it has no text-reflowable feature. Luckily, Peter Eisentraut made a small commit that add epub target to the documentation build to export the epub file.

Here are the building instruction and the download links to the epub file that I have built before

Image Blend modes and HTML Canvas

Image Blend modes are the methods used to determine how 2 image layers are blended (mixed) to each other. As the digital images are stored in pixel, which are represented by numerical values, there are a large number of possible ways for blending based on the mathematical functions. With the help of Canvas API, now we can easily retrieve the images, export all the pixels on the image, apply blending effect, calculate to get the new blended pixels, export and display the new image on the web or save to server.

In this post, I will demonstrate some basic steps for applying Image Blend modes using HTML canvas API.

The Logic

Wikipedia already listed some popular blending methods here Blend Modes.

Assume that we have two image layers, top layer and bottom layer. For each loop, a is the value of a color channel in the underlying layer and b is that of the corresponding channel of the upper layer, we got the function for calculating the new pixel ƒ(a, b).

The steps for generating the new blended image from the two images are

• First, retrieve those two images
• Create two separate canvases with the same size and draw those two images into the corresponding canvas.
• Get all the pixel data from the two canvases.
• Loop through each pixel, apply the blending function to create a new pixel and store it inside an array
• Create a new canvas with the same size, use the blended pixel data to draw the blended image on that new canvas
• Export the canvas to image, file to save to server or to local computer.

Note: I mentioned that we need to create two canvases with the same size. However, that is just to make it easy for the demonstration purpose. You can still generate two canvases with different size, but you will need to change the calculation function a bit.

If you have not known what Conkeror is, you probably miss one of the greatest web browser in your life. Read this post for an introduction of how impressive Conkeror is Conkeror and How it changed the way I surf the web.

Overview

One of the most interesting feature that Conkeror brings from Emacs is the Browser Object. If you are coming from an Emacs background, you are not unfamiliar with universal-arguments, which can modify the function’s behaviour once it is called before activating the function. Browser Objects in Conkeror brings that idea from Emacs to the web environment but with better approach. Browser Objects in Conkeror can represent practically any type of data (e.g image, hyperlinks, frame,…).

Quote from Conkeror Wiki page

In the web environment, there are many data types which one wants to manipulate. Text, hyperlinks, images, form fields, to name just a few. Without some kind of abstraction layer to unify the interface to all these types of objects, it would be necessary to have a combinatorially large number of commands. For instance, you would need separate commands for follow-link, follow-image, and follow-frame. With browser objects, you have just one command, follow, which can operate on many types of data.

On my Mac, I always have 2 different partitions, one for installing the system and the other one for my home directory. Today, I’ve just re-installed the whole system and of course I have to specify the home path after the installation. However, after changing the home directory, many of my applications cannot work properly. I have checked and recognize that the $HOME variable in the shell now has a trailing slash (/Volumes/tmtxt/ instead of /Volumes/tmtxt).

That was so annoying and took me hours to investigate. Finally I found the problem. It comes from one of my carelessness when I change the home directory location.

Yes, I typed the path manually… How stupid I was. Why not just use the Choose button right next there :LOL:

Just change it to another folder, restart the computer and change back to the right home folder. Everything will be fixed automatically.

Most of us split Emacs horizontally by 2 windows so that we can easily edit and compare multi documents at the same time. However, the problem is that the two windows usually have the same size and sometimes difficult for you to work with files with long lines. Golden Ratio Mode is a small but useful package for Emacs that will automatically resize the focused window by golden ratio.

The window that has the main focus will have the perfect size for editing, while the ones that are not being actively edited will be re-sized to a smaller size that doesn’t get in the way, but at the same time will be readable enough to know it’s content.

You can install it using package.el. After finishing installation, just add this to your init.el file

(require 'golden-ratio)
(golden-ratio-mode 1)

Introduction

Emacs’s SQLi mode can be a better replacement for the default terminal database client (psql, sqlite3,…). I prefer SQLi mode because it supports syntax highlighting (and also Postgres keyword) and allows me to flexibly choose which region in the buffer to send to the shell to execute.

SQLi is integrated by default in Emacs. The following interpreters are supported

• psql by PostgreSQL
• mysql by MySQL
• sqlite or sqlite3 for SQLite
• solsql by Solid
• SQL*Plus by Oracle
• dbaccess by Informix
• isql by SyBase
• sql by Ingres
• osql by MS SQL Server
• isql by Interbase
• db2 by DB2 (IBM)
• inl by RELEX

Make sure that the client you need to use can be located inside the Emacs’s PATH. You can install exec-path-from-shell package using package.el for Emacs to auto import the PATH from your default shell.

If you are using Mac OS, I have written a post about PostgreSQL installation and configuration steps on Mac here: Install and Config PostgreSQL on Mac.