Starting¶
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 and that you have Erlang activated.
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/admin_bin
===> Writing tanodb/priv/01-tanodb.schema
===> Writing tanodb/config/advanced.config
===> Writing tanodb/config/vars.config
===> Writing tanodb/config/vars_dev1.config
===> Writing tanodb/config/vars_dev2.config
===> Writing tanodb/config/vars_dev3.config
Note
The content of this chapter is in the 01-template branch.
Building and Running¶
Before explaining what the files mean so you get an idea what just happened let’s run it!
cd tanodb
make
make console
make runs rebar3 commands 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.
When we run make console we get some noisy output that should end with something like this:
Eshell V9.2 (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 if 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/admin_bin
- A template for the tanodb-admin command.
- priv/01-tanodb.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/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.
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 each other, for them to know about each other 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:
make prod-release
In that command rebar3 runs 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.
Let’s package our release:
cd _build/prod/rel
tar -czf tanodb.tgz tanodb tanodb_config
cd -
mv _build/prod/rel/tanodb.tgz .
You can copy it to a clean OS and run:
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.