Starting

We are going to build a system on top of riak_core, for that we will use some tools and to avoid copy paste and boilerplate we will use a template to get started.

Tools

  • rebar3: The build tool, click on the link to see how to install it.
  • erlang: Our programming language, we assume Erlang version to be at least 17.0

I also assume a unix-like environment with a shell similar to bash or zsh.

Installing the Template

At this point you should have erlang and rebar3 installed, now let’s install the template we are going to use.

mkdir -p ~/.config/rebar3/templates
git clone https://github.com/marianoguerra/rebar3_template_riak_core/ ~/.config/rebar3/templates/rebar3_template_riak_core

We just created the folder ~/.config/rebar3/templates for the templates in case it wasn’t there and cloned our template inside of it.

You can read more about rebar3 templates here.

Creating our Project

Now that we have our tools and our template installed we can start by asking rebar3 to create a new project we will call tanodb using the riak_core template we just installed:

rebar3 new rebar3_riak_core name=tanodb

If it fails saying it can’t find rebar3 check that it’s in your $PATH environment variable.

The output should be something like this:

===> Writing tanodb/apps/tanodb/src/tanodb.app.src
===> Writing tanodb/apps/tanodb/src/tanodb.erl
===> Writing tanodb/apps/tanodb/src/tanodb_app.erl
===> Writing tanodb/apps/tanodb/src/tanodb_sup.erl
===> Writing tanodb/apps/tanodb/src/tanodb_console.erl
===> Writing tanodb/apps/tanodb/src/tanodb_vnode.erl
===> Writing tanodb/rebar.config
===> Writing tanodb/.editorconfig
===> Writing tanodb/.gitignore
===> Writing tanodb/README.rst
===> Writing tanodb/Makefile
===> Writing tanodb/config/nodetool
===> Writing tanodb/config/extended_bin
===> Writing tanodb/config/admin_bin
===> Writing tanodb/config/config.schema
===> Writing tanodb/config/advanced.config
===> Writing tanodb/config/sys.config
===> Writing tanodb/config/vars.config
===> Writing tanodb/config/vars_dev1.config
===> Writing tanodb/config/vars_dev2.config
===> Writing tanodb/config/vars_dev3.config
===> Writing tanodb/config/vm.args
===> Writing tanodb/config/dev1_vm.args
===> Writing tanodb/config/dev2_vm.args
===> Writing tanodb/config/dev3_vm.args

Building and Running

Before explaining what the files mean so you get an idea what just happened let’s run it!

cd tanodb
rebar3 release
rebar3 run

rebar3 release asks rebar3 to build a release of our project, for that it uses a tool called relx.

The initial build may take a while since it has to fetch all the dependencies and build them.

After the release is built (you can check the result by inspecting the folder _build/default/rel/tanodb/) we can run it, for this we use a rebar3 plugin called rebar3_run

When we run rebar3 run we get some noisy output that should end with something like this:

Eshell V7.0  (abort with ^G)
(tanodb@127.0.0.1)1>

This is the Erlang shell, something like a REPL connected to our system, we now can test our system by calling tanodb:ping() on it.

(tanodb@127.0.0.1)1> tanodb:ping().
{pong,1347321821914426127719021955160323408745312813056}

The response is the atom pong and a huge number that we will explain later, but to make it short, it’s the id of the process that replied to us.

Exploring the Template Files

The template created a lot of files and you are like me, you don’t like things that make magic and don’t explain what’s going on, that’s why we will get a brief overview of the files created here.

First this files are created:

apps/tanodb/src/tanodb.app.src
apps/tanodb/src/tanodb.erl
apps/tanodb/src/tanodb_app.erl
apps/tanodb/src/tanodb_sup.erl
apps/tanodb/src/tanodb_console.erl
apps/tanodb/src/tanodb_vnode.erl

Those are the meat of this project, the source code we start with, if you know a little of erlang you will recognice many of them, let’s explain them briefly, if you think you need more information I recommend you this awesome book which you can read online: Learn You Some Erlang for great good!

tanodb.app.src
This file is “The Application Resource File”, you can read it, it’s quite self descriptive. You can read more about it in the Building OTP Applications Section of Learn You Some Erlang or in the man page for app in the Erlang documentation.
tanodb.erl
This file is the main API of our application, here we expose all the things you can ask our application to do, for now it can only handle the ping() command but we will add some more in the future.
tanodb_app.erl
This file implements the application behavior it’s a set of callbacks that the Erlang runtime calls to start and stop our application.
tanodb_sup.erl
This file implements the supervisor behavior it’s a set of callbacks that the Erlang runtime calls to build the supervisor hierarchy.
tanodb_console.erl
This file is specific to riak_core, it’s a set of callbacks that will be called by the tanodb-admin command.
tanodb_vnode.erl
This file is specific to riak_core, it implements the riak_code_vnode behavior, which is a set of callbacks that riak_core will call to accomplish different tasks, it’s the main file we will edit to add new features.

Those were the source code files, but the template also created other files, let’s review them

rebar.config
This is the file that rebar3 reads to get information about our project like dependencies and build configuration, you can read more about it on the rebar3 documentation
.editorconfig
This file describes the coding style for this project, if your text editor understands editorconfig files then it will change it’s setting for this project to the ones described in this file, read more about editor config on the editorconfig website
.gitignore
A file to tell git which files to ignore from the repository.
README.rst
The README of the project
Makefile
A make file with some targets that will make it easier to achieve some complex tasks without copying and pasting too much.
config/nodetool
An escript that makes it easier to interact with an erlang node from the command line, it will be used by the tanodb and tanodb-admin commands.
config/extended_bin
A template for the tanodb command with some changes to support cuttlefish which is the library we use to load and validate our configuration
config/admin_bin
A template for the tanodb-admin command.
config/config.schema
The cuttlefish schema file that describes what configuration our application supports, it starts with some example configuration fields that we will use as the application grows.
config/advanced.config
This file is where we configure some advanced things of our application that don’t go on our tanodb.config file, here we configure riak_core and our logging library
config/sys.config
This is a standard Erlang application file, you can read more about it in the Erlang documentation for sys.config
config/vars.config
This file contains variables used by relx to build a release, you can read more about it in the rebar3 release documentation

The following files are like vars.config but with slight differences to allow running more than one node on the same machine:

config/vars_dev1.config
config/vars_dev2.config
config/vars_dev3.config

Normally when you have a cluster for your application one operating system instance runs one instance of your application and you have many operating system instances, but to test the clustering features of riak_core we will build 3 releases of our application using offsets for ports and changing the application name to avoid collisions.

config/vm.args
A file used to pass options to the Erlang VM when starting our application.

The following files are like vars_dev*.config but for vm.args:

config/dev1_vm.args
config/dev2_vm.args
config/dev3_vm.args

Those are all the files, follow the links to know more about them.

Playing with Clustering

Before starting to add features, let’s first play with clustering so we understand all those config files above work.

Build 3 releases that can run on the same machine:

make devrel

This will build 3 releases of the application using different parameters (the dev1, dev2 and dev3 files we saw earlier) and will place them under:

_build/dev1
_build/dev2
_build/dev3

This is achived by using the profiles feature from rebar3.

Now open 3 consoles and run the following commands one on each console:

make dev1-console
make dev2-console
make dev3-console

This will start the 3 nodes but they won’t know about eachother, for them to know about eachother we need to “join” them, that is to tell one of them about the other two, this is achieved using the tanodb-admin command, here is how you should run it manually (don’t run them):

_build/dev2/rel/tanodb/bin/tanodb-admin cluster join tanodb1@127.0.0.1
_build/dev3/rel/tanodb/bin/tanodb-admin cluster join tanodb1@127.0.0.1

We tell dev2 and dev3 to join tanodb1 (dev1), to make this easier and less error prone run the following command:

make devrel-join

Now let’s check the status of the cluster:

make devrel-status

You can read the Makefile to get an idea of what those commands do, in this case devrel-status does the following:

_build/dev1/rel/tanodb/bin/tanodb-admin member-status

You should see something like this:

================================= Membership ===============
Status     Ring    Pending    Node
------------------------------------------------------------
joining     0.0%      --      'tanodb2@127.0.0.1'
joining     0.0%      --      'tanodb3@127.0.0.1'
valid     100.0%      --      'tanodb1@127.0.0.1'
------------------------------------------------------------
Valid:1 / Leaving:0 / Exiting:0 / Joining:2 / Down:0

It should say that 3 nodes are joining, now check the cluster plan:

make devrel-cluster-plan

The output should be something like this:

=============================== Staged Changes ==============
Action         Details(s)
-------------------------------------------------------------
join           'tanodb2@127.0.0.1'
join           'tanodb3@127.0.0.1'
-------------------------------------------------------------


NOTE: Applying these changes will result in 1 cluster transition

#############################################################
                         After cluster transition 1/1
#############################################################

================================= Membership ================
Status     Ring    Pending    Node
-------------------------------------------------------------
valid     100.0%     34.4%    'tanodb1@127.0.0.1'
valid       0.0%     32.8%    'tanodb2@127.0.0.1'
valid       0.0%     32.8%    'tanodb3@127.0.0.1'
-------------------------------------------------------------
Valid:3 / Leaving:0 / Exiting:0 / Joining:0 / Down:0

WARNING: Not all replicas will be on distinct nodes

Transfers resulting from cluster changes: 42
  21 transfers from 'tanodb1@127.0.0.1' to 'tanodb3@127.0.0.1'
  21 transfers from 'tanodb1@127.0.0.1' to 'tanodb2@127.0.0.1'

Now we can commit the plan:

make devrel-cluster-commit

Which should say something like:

Cluster changes committed

Now riak_core started an internal process to join the nodes to the cluster, this involve some complex processes that we will explore in the following chapters.

You should see on the consoles where the nodes are running that some logging is happening describing the process.

Check the status of the cluster again:

make devrel-status

You can see the vnodes transfering, this means the content of some virtual nodes on one tanodb node are being transferred to another tanodb node:

================================= Membership =============
Status     Ring    Pending    Node
----------------------------------------------------------
valid      75.0%     34.4%    'tanodb1@127.0.0.1'
valid       9.4%     32.8%    'tanodb2@127.0.0.1'
valid       7.8%     32.8%    'tanodb3@127.0.0.1'
----------------------------------------------------------
Valid:3 / Leaving:0 / Exiting:0 / Joining:0 / Down:0

At some point you should see something like this, which means that the nodes are joined and balanced:

================================= Membership ==============
Status     Ring    Pending    Node
-----------------------------------------------------------
valid      34.4%      --      'tanodb1@127.0.0.1'
valid      32.8%      --      'tanodb2@127.0.0.1'
valid      32.8%      --      'tanodb3@127.0.0.1'
-----------------------------------------------------------
Valid:3 / Leaving:0 / Exiting:0 / Joining:0 / Down:0

When you are bored you can stop them:

make devrel-stop

Building a Production Release

Even when our application doesn’t have the features to merit a production release we are going to learn how to do it here since you can later do it at any step and get a full release of the app:

rebar3 as prod release

In that command we as rebar3 to run the release task using the prod profile, which has some configuration differences with the dev profiles we use so that it builds something we can unpack and run on another operating system without installing anything.

In my case I’m developing this on ubuntu, to show you that it works I will copy the release to a clean ubuntu 15.04 Virtualbox and run it there:

mkdir vm-ubuntu-1504
cd vm-ubuntu-1504

Inside I will create a file called Vagrantfile with the following content:

Vagrant.configure(2) do |config|
  config.vm.box = "ubuntu/vivid64"
  config.vm.provider "virtualbox" do |vb|
    vb.memory = "1024"
  end
end

And then run:

vagrant up

To start the virtual machine.

Now let’s package our release and copy it to a place where the VM can see it:

cd _build/prod/rel
tar -czf tanodb.tgz tanodb
cd -
mv _build/prod/rel/tanodb.tgz vm-ubuntu-1504

Let’s ssh into the virtual machine:

export TERM=xterm
vagrant ssh

Inside the virtual machine run:

cp /vagrant/tanodb.tgz .
tar -xzf tanodb.tgz
./tanodb/bin/tanodb console

And it runs!

Note

You should build the production release on the same operating system version you are intending to run it to avoid version problems, the main source of headaches are C extensions disagreeing on libc versions and similar.

So, even when you could build it on a version that is close and test it it’s better to build releases on the same version to avoid problems. More so if you are packaging the Erlang runtime with the release as we are doing here.

Wrapping Up

Now you know how to create a riak_core app from a template, how to build a release and run it, how to build releases for a development cluster, run the nodes, join them and inspect the cluster status and how to build a production release and run it on a fresh server.

Quite a lot for the first chapter I would say...