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AWS OpsWorks
User Guide (API Version 2013-02-18)

Example 8: Managing Services

Packages such as application servers typically have an associated service that must be started, stopped, restarted, and so on. For example, you need to start the Tomcat service after installing the package or after the instance finishes booting, and restart the service each time you modify the configuration file. This topic discusses the basics of how to manage a service on a Linux instance, using a Tomcat application server as an example. The service resource works much the same way on Windows instances, although there are some differences in detail. For more information, see service.

Note

The example does a very minimal Tomcat installation, just enough to demonstrate the basics of how to use a service resource. For an example of how to implement recipes for a more functional Tomcat server, see Creating a Custom Tomcat Server Layer.

Defining and Starting a Service

This section shows the basics of how to define and start a service.

To get started

  1. In the opsworks_cookbooks directory, create a directory named tomcat and navigate to it.

  2. Add a metadata.rb file to tomcat with the following content.

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    name "tomcat" version "0.1.0"
  3. Initialize and configure Test Kitchen, as described in Example 1: Installing Packages, and remove CentOS from the platforms list.

  4. Add a recipes subdirectory to tomcat.

You use a service resource to manage a service. The following default recipe installs Tomcat and starts the service.

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execute "install_updates" do command "apt-get update" end package "tomcat7" do action :install end include_recipe 'tomcat::service' service 'tomcat' do action :start end

The recipe does the following:

  • The execute resource runs apt-get update to install the current system updates.

    For the Ubuntu 12.04 LTS instance used in this example, you must install the updates before installing Tomcat. Other systems might have different requirements.

  • The package resource installs Tomcat 7.

  • The includedtomcat::service recipe defines the service and is discussed later.

  • The service resource starts the Tomcat service.

    You can also use this resource to issue other commands, such as stopping and restarting the service.

The following example shows the tomcat::service recipe.

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service 'tomcat' do service_name "tomcat7" supports :restart => true, :reload => false, :status => true action :nothing end

This recipe creates the Tomcat service definition as follows:

  • The resource name, tomcat, is used by other recipes to reference the service.

    For example, default.rb references tomcat to start the service.

  • The service_name resource specifies the service name.

    When you list the services on the instance, the Tomcat service will be named tomcat7.

  • supports specifies how Chef manages the service's restart, reload, and status commands.

    • true indicates that Chef can use the init script or other service provider to run the command.

    • false indicates that Chef must attempt to run the command by other means.

Notice that action is set to :nothing, which directs the resource to take no action. The service resource does support actions such as start and restart. However, this cookbook follows a standard practice of using a service definition that takes no action and starting or restarting the service elsewhere. Each recipe that starts or restarts a service must first define it, so the simplest approach is to put the service definition in a separate recipe and include it in other recipes as needed.

Note

For simplicity, the default recipe for this example uses a service resource to start the service after running the service definition. A production implementation typically starts or restarts a service by using notifies, as discussed later.

To run the recipe

  1. Create a default.rb file that contains the default recipe example and save it to recipes.

  2. Create a service.rb file that contains the service definition example and save it to recipes.

  3. Run kitchen converge, then log in to the instance and run the following command to verify that the service is running.

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    sudo service tomcat7 status

Note

If you were running service.rb separately from default.rb, you would have to edit .kitchen.yml to add tomcat::service to the run list. However, when you include a recipe, its code is incorporated into the parent recipe before the recipe is executed. service.rb is therefore basically a part of default.rb and doesn't require a separate run list entry.

Using notifies to Start or Restart a Service

Production implementations typically do not use service to start or restart a service. Instead, they add notifies to any of several resources. For example, if you want to restart the service after modifying a configuration file, you include notifies in the associated template resource. Using notifies has the following advantages over using a service resource to explicitly restart the service.

  • The notifies element restarts the service only if the associated configuration file has changed, so there's no risk of causing an unnecessary service restart.

  • Chef restarts the service at most once at the end of each run, regardless of how many notifies the run contains.

    For example, Chef run might include multiple template resources, each of which modifies a different configuration file and requires a service restart if the file has changed. However, you typically want to restart the service only once, at the end of the Chef run. Otherwise, you might attempt to restart a service that is not yet fully operational from an earlier restart, which can lead to errors.

This example modifies tomcat::default to include a template resource that uses notifies to restart the service. A realistic example would use a template resource that creates a customized version of one of the Tomcat configuration files, but those are rather long and complex. For simplicity, the example just uses the template resource from Creating a File from a Template. It doesn't have anything to do with Tomcat, but it provides a simple way to show how to use notifies. For an example of how to use templates to create Tomcat configuration files, see Setup Recipes.

To set up the cookbook

  1. Add a templates subdirectory to tomcat and a default subdirectory to templates.

  2. Copy the example_data.json.erb template from the createfile cookbook to the templates/default directory.

  3. Add an attributes subdirectory to tomcat.

  4. Copy the default.rb attribute file from the createfile cookbook to the attributes directory.

The following recipe uses notifies to restart the Tomcat service.

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execute "install_updates" do command "apt-get update" end package "tomcat7" do action :install end include_recipe 'tomcat::service' service 'tomcat' do action :enable end directory "/srv/www/shared" do mode 0755 owner 'root' group 'root' recursive true action :create end template "/srv/www/shared/example_data.json" do source "example_data.json.erb" mode 0644 variables( :a_boolean_var => true, :a_string_var => "some string" ) only_if {node['createfile']['install_file']} notifies :restart, resources(:service => 'tomcat') end

The example merges the recipe from Creating a File from a Template into the recipe from the preceding section, with two significant changes:

  • The service resource is still there, but it now serves a somewhat different purpose.

    The :enable action enables the Tomcat service at boot.

  • The template resource now includes notifies, which restarts the Tomcat service if example_data.json has changed.

    This ensures that the service is started when Tomcat is first installed and restarted after every configuration change.

To run the recipe

  1. Run kitchen destroy to start with a clean instance.

  2. Replace the code in default.rb with the preceding example.

  3. Run kitchen converge, then log in to the instance and verify that the service is running.

Note

If you want to restart a service but the recipe doesn't include a resource such as template that supports notifies, you can instead use a dummy execute resource. For example

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execute 'trigger tomcat service restart' do command 'bin/true' notifies :restart, resources(:service => 'tomcat') end

The execute resource must have a command attribute, even if you are using the resource only as a way to run notifies. This example gets around that requirement by running /bin/true, which is a shell command that simply returns a success code.