In certain scenarios, you may want to control the number of hosts on which your playbook tasks should be executed. This can be useful when you have a large number of hosts in your inventory, but you want to limit the execution to a specific subset of hosts at a time.

To achieve this, you can follow these steps:

1. Create a dynamic inventory script: Ansible supports dynamic inventory scripts that provide a dynamic view of your infrastructure. These scripts can retrieve host information from various sources like files, APIs, databases, or external services. In this case, you would create a custom dynamic inventory script that also retrieves the host count.

2. Implement logic to fetch the host count: Within the dynamic inventory script, you need to implement the logic to fetch the host count dynamically. The source of the host count can be any data store or calculation based on your infrastructure. For example, you may retrieve the host count from a file, query an API, or perform a database query. This logic should return the host count as part of the inventory structure.

3. Specify the maximum number of hosts for playbook execution: Once you have the dynamic inventory script in place and it includes the host count field, you can run your playbook with the --limit option. The --limit option allows you to restrict the execution to a subset of hosts based on a pattern or expression. In this case, you would use a pattern that limits the execution to a certain number of hosts, using the host count retrieved from the inventory.

4. Adjust playbook configuration if needed: Depending on your requirements, you may need to modify your playbook to handle the desired number of hosts. For example, you can use the serial keyword in your playbook to control the number of hosts on which tasks are executed simultaneously. This allows you to limit the concurrency and ensure that tasks are executed on a subset of hosts at a time.

By combining these steps, you can effectively limit the execution of an Ansible playbook based on the number of hosts in your inventory. This gives you flexibility and control over task execution when dealing with a large number of hosts.

Limit the execution of an Ansible playbook in code snippet:

To limit the execution of an Ansible playbook based on the number of hosts in the inventory, you can make use of Ansible's --limit option along with a dynamic inventory script that provides the host count.

Step 1. Create a dynamic inventory script: Ansible supports various dynamic inventory scripts, such as the one for AWS EC2, OpenStack, etc. In this case, we can create a custom script to retrieve the host count. The script should output a JSON representation of the inventory, including a host count field.

Here's an example code of a dynamic inventory script (inventory.py) that retrieves the host count from a source (e.g., a file, API, or database):

    # Retrieve host count from the source and store it in a variable

    host_count = 42  # Replace with your own logic to get the host count

    # Generate the inventory JSON structure

        "host_count": host_count  # Add the host count field

Make sure to replace the logic in get_inventory() to fetch the host count from your source.

Step 2. Make the script executable: Run the following command to make the script executable:

Step 3. Run the playbook with the --limit option: Now, when executing your playbook, you can use the --limit option to limit the execution based on the host count. Specify the maximum number of hosts you want to target.

For example, to limit the playbook execution to a maximum of 5 hosts:

ansible-playbook -i inventory.py playbook.yml --limit 'all:&lt;5'

The all:&lt;5 filter in --limit will match all hosts and limit the execution to a maximum of 5.

Adjust the 5 to your desired host count threshold.

Step 4. Updating dynamic inventory script: Update the dynamic inventory script to generate the inventory based on the desired logic or source. In the provided example, the host count is hardcoded as 42. Replace this logic with your own implementation to retrieve the host count dynamically.

The inventory structure generated by the script should include a "host_count" field, as shown in the example. This field will be used to limit the playbook execution.

Step 5. Designing your Ansible playbook: Make sure your playbook is designed to handle the desired number of hosts. For example, if you want to execute tasks only on a specific number of hosts, you can use the serial keyword in your playbook. The serial keyword controls how many hosts Ansible will run the tasks on simultaneously.

Here's an example playbook (playbook.yml) that demonstrates the usage of serial to limit the execution to a certain number of hosts:

  serial: 5  # Limit execution to 5 hosts at a time

        msg: "This task will be executed on a maximum of 5 hosts at a time."

In this playbook, the serial parameter is set to 5, which means Ansible will execute the tasks on a maximum of 5 hosts concurrently. Adjust the 5 to your desired host count threshold.

Step 6. Execute the playbook: Execute the playbook with the --limit option as explained in step 3. Ansible will use the dynamic inventory script to retrieve the host count and limit the execution based on the specified number.

By combining the dynamic inventory script, the host count field, and the --limit option with the serial parameter in your playbook, you can restrict the execution of Ansible tasks based on the number of hosts in your inventory.

In conclusion, by utilizing a dynamic inventory script, retrieving the host count, and using the --limit option in Ansible playbooks, you can effectively limit the execution of tasks based on the number of hosts in your inventory. This approach provides flexibility and control over task execution, especially when dealing with a large number of hosts.

Limit Ansible playbook execution based on host count using dynamic inventory script and --limit option with desired number.

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