Using deploy hooks

Overview

Deploy hooks are YAML-formatted configuration sets that that allow you to automate actions (such as running commands) at various points during the build and deployment process for your applications. If you’ve never used hooks before, please follow our tutorial to get a grasp of the basics.

Deploy hooks are a powerful tool for customising the components on your servers, and automating the setup process whenever you add new servers. However they require careful configuration to work exactly as expected. This guide will explain all the necessary detail for you to write your own custom hooks. For am exhaustive list of all the options available, please consult our Reference guide.

Creating or editing deploy hooks

All deploy hooks for Rails applications use a file called deploy_hooks.yml. This file is found within a folder named .cloud66, which is in turn located in the root of your source code.

/.cloud66/deploy_hooks.yml

This file should be YAML formatted, and you can use a service like YAMLlint to validate it.

Every deploy hook needs, at a minimum:

a hook point - where in the deployment process the hook must be invoked
a hook type - either a command or one of the two script types
a target - which type(s) of servers will use this hook
a hook field - the command or script being run (or invoked)

Specifiying app environment

Unless you explicitly specify an environment - e.g. production - as the top node of your yaml, your hook will be applied to all of your application environments. For example:

  production: #Environment
    first_thing: # Hook point
      - command: apt-get install nmap -y # Hook type
        target: any # Hook fields
        execute: true

Understanding hook points and ordering

Hook points are used to define the point in your deployment process at which a hook should be invoked. This is obviously critical when there are tight dependencies between the components of your application (i.e. one component relies on another component being installed first), but it is also important in terms of what actions and commands are possible. For example, running tasks against a database before the database server is installed will not work!

It’s important to understand the order in which hook points will occur in the flow of deployment. The simplified deployment process below shows where each deploy hook is triggered. Hooks are marked in bold italic. Some hooks have several possible values (x, y, z). Click on the hook name to see a list of available options:

Deployment process (simplified)

Server is fired up
Operating system and standard system components installed → first_thing
before_agent → Cloud 66 Agent is installed → after_agent
before_x → Database is installed → after_x
before_y → Database replication is configured → after_y
before_data_mount → Data is mounted → after_data_mount (GlusterFS specific)
custom_server (runs on custom servers only)
before_nginx → NGINX is installed → after_nginx
before_docker → Docker is installed after_docker
last_thing

Notes

x, y and z represent database & storage engine installation , replication configuration and application framework installation respectively.
The last_thing hook runs only when ALL servers reach that point

For more details on hook points, please read our Reference guide.

Targets

The target of a hook is the server or set of servers on which it must be executed.

You can use any to run the hook across your entire application, but you can also choose to run it on a specific type of server.

Calling environment variables in deploy hooks

You can pull environment variables from your application configuration into your hooks using the following format:

username: <%= ENV['DB_USER'] %>

These environment variables need to be available in your app before the deploy hook will work correctly. Please follow our tutorial if you’re unsure of how to do this.

Hook types

There are different types of deploy hooks, and the fields available (and required) vary by type:

Snippets: use “off the shelf” scripts to install common packages. These snippets are open source, and are created by Cloud 66 or other third parties.
Commands: run standard Linux shell commands.
Inline Scripts: use your own inline shell scripts for more complex procedures
Existing Scripts: use your own existing scripts for more comprehensive procedures

Using command hooks

The simplest type of hook is the command. This simply executes a command in the operating system as part of the deployment process.

You may recognise this example from our tutorial:

production: # Environment
    first_thing: # Hook point
      - command: apt-get install nmap -y # Hook type
        target: any # Hook fields
        execute: true

This hook will install the nmap package on our server during the deployment process. You can find all the avaible options for command hooks in our reference guide.

Using snippet hooks

Snippets are pre-defined hook scripts hosted in a Cloud 66 repository. These are commonly used scripts that we provide for ease of use. Like scripts (see below) these snippets are written in bash.

For example the Cloud 66 ImageMagick snippet installs ImageMagick, the popular image processing application. You can use the example below to test it in your demo app:

production: # Environment
    first_thing: # Hook point
      - snippet: cloud66/imagemagick # Hook type
        target: any # Hook fields
        execute: true

You can test this in the same way as you did with the command hook in our tutorial.

Using inline script hooks

Script hooks allow you to add your own logic to a deploy hook, either by calling a separate file using source or by using inline. Both of methods use bash scripting (as do snippets above).

The hook below will create an arbitrary log file in /tmp using a simple inline script:

first_thing: # Hook point
 - inline: |

     #!/usr/bin/env bash
     echo "script called!" >> /tmp/inline_script.log
   target: any
   execute: true
   apply_during: all
   owner: root:root

Advanced examples of Deploy Hooks

We’ve put together some examples of the most common use cases, but there an almost infinite number of possibilities. Essentially if you can install it on a Linux server or code it into a shell script, you can run it from a deploy hook.

Querying server metadata

You can query a server’s metadata (including deployment details) using the following command:

curl -s -H "Accept: application/json" https://$CLOUD66_ACCOUNT_API_KEY:X@app.cloud66.com/api/tooling/metadata/$CLOUD66_APPLICATION_API_KEY

You can use this command in a deploy hook, for example:

first_thing:
- target: any
  execute: true
  apply_during: all 
  sudo: false
  inline: |
    #!/usr/bin/env bash
    current_deployer=$(curl -s -H "Accept: application/json" https://$CLOUD66_ACCOUNT_API_KEY:X@app.cloud66.com/api/tooling/metadata/$CLOUD66_APPLICATION_API_KEY/deployment/triggered_by | jq -r '.response')
    echo "Deployed by: $current_deployer"

For more details please read our full Querying server metadata doc.

Install packages on each server deployed

If your application relies on a package or software library that is not installed by default by Ubuntu, or by Cloud 66, you may ask, how do I install a package or library on each server I deploy?

You can use a deploy hook to fetch and install the package whenever you deploy a server.

For this example we’re going to use glances - a popular FOSS process monitor - but you can install literally anything that has an Ubuntu repository using this method.

We need to set up our deploy hook to install the package from the Ubuntu. Every deploy hook needs:

a hook point - in this case we are using the first_thing hook point to ensure this is installed as early as possible in case any other processes depend on it
A hook type - we are using the command hook type
A target - in this case we are using any because we want the package on all our servers
A hook field - this is where we put the commands we’re going to run.

Our hook code will look like this:

first_thing:
 - command: sudo apt-get install glances
   target: any
   execute: true

Now that we have our hook code ready, we need to add it to our application. We do this by creating a file in the /.cloud66 directory of our repo called deploy_hooks.yml and dropping in the YAML code we just wrote above. If you’re not well versed in YAML, it’s often helpful to use an online linter to test that your code is valid and conformant.

Once you have added the deploy_hooks.yml file to your repo you can deploy (or redeploy) your application and glances will be installed during the process. To test that it is installed correctly you can SSH into your server and run glances - it should bring up the monitoring interface.

Installing non-standard versions of packages

If your application depends on a specific version of a Linux package or software library, the best way to ensure the correct version is installed is by using a deploy hook.

To illustrate how this is done we’re going to use ImageMagick - the beloved and ubiquitous image processing software installed on millions of servers around the world. Suppose your application relies on an older version of ImageMagick - in this case Version 6.9.9-51.

We need to set up our deploy hook to download the source code for this version and then build and compile it. Every deploy hook needs:

a hook point - in this case we are using the first_thing hook point to ensure this is installed as early as possible in case any other processes depend on it
A hook type - we are using the inline hook type - we’ll go into the details of this later
A target - in this case we are using rails but any would also work
A hook field - this is where we put the commands we’re going to run. We’re going to use these instructions as the basis for our commands.

Hooks are written in YAML and ours will look like this:

first_thing:
- target: rails
  execute: true
  apply_during: build_only 
  sudo: true
  inline: |
    #!/usr/bin/env bash
    set -e
    version="6.9.9-51"
    echo "Installing ImageMagick v$version"

    echo "1. Fetching Tarball"
    curl -sL https://codeload.github.com/ImageMagick/ImageMagick6/tar.gz/"$version" --output /tmp/image-magick-"$version".tar.gz 

    echo "2. Extracting Tarball"
    cd /tmp
    mkdir -p /opt/image-magick/v"$version"
    tar -xvzf image-magick-"$version".tar.gz -C /opt/image-magick/v"$version" --strip 1

    echo "3. Building ImageMagick"
    cd /opt/image-magick/v"$version"
    ./configure
    make
    make install 
    ldconfig /usr/local/lib

    echo "4. Tidying up"
    rm -f /tmp/image-magick-"$version".tar.gz

    echo "ImageMagick install complete!"

Two things to take note of here:

In order to execute properly, the script needs a place on the disk to run - we are using the /tmp/ directory
We have added another option - apply_during - this allows us to set our hook to only run when a server is first built, and not on every deploy. You can find out more about these options in our reference doc.

Once you have added the deploy_hooks.yml file to your repo you can deploy (or redeploy) your application and version 6.9.9-51 of ImageMagick will be installed during the process. To test that it is installed correctly you can SSH into your server and run identify -version.

Switching config files during build

This hook simply overwrites a standard config file with the config file of your choice at the start of the deployment process. The example is for a database.yml but you could use it for any config file.

after_checkout:
command: mv $STACK_PATH/config/database.yml.cloud66 $STACK_PATH/config/database.yml
target: rails
run_on: all_servers
execute: true

For more info on doing this for external databases, read our separate guide to database management.

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