Understanding Nested Variables in Jinja2/Ansible

Описание: Learn how to effectively use nested variables in Jinja2/Ansible with this comprehensive guide, including practical examples and solutions.
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This video is based on the question https://stackoverflow.com/q/71252128/ asked by the user 'programpee' ( https://stackoverflow.com/u/9437958/ ) and on the answer https://stackoverflow.com/a/71252462/ provided by the user 'Zeitounator' ( https://stackoverflow.com/u/9401096/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Nested Variables in Jinja2/Ansible: A Step-by-Step Guide

Managing configurations in Ansible can sometimes be tricky, especially when you're dealing with nested variables in Jinja2. Many users struggle with how to effectively handle these variables, particularly when it comes to looking up specific values within complex data structures. In this guide, we will explore how to use nested variables within an Ansible context, breaking down the steps to achieve the desired output.

The Problem

Imagine you have a YAML file containing configurations of dual-homed workloads. Your goal is to retrieve VLAN tags and their descriptions for a specific switch, based on the value provided by inventory_hostname. Here’s the YAML structure we’re working with:

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Your aim is to loop through the VLANs for a specific switch and display the required information. If you are querying switch1, you want your output to look like:

[[See Video to Reveal this Text or Code Snippet]]

The Solution

To achieve this, you'll need to write a Jinja2 template that effectively retrieves the necessary data based on inventory_hostname. Below, we provide the groundwork for how to filter the VLANs for the specific switch.

Looping through VLANs

Here’s how to create a loop that targets the VLANs for the switch that matches the current inventory_hostname:

[[See Video to Reveal this Text or Code Snippet]]

Filtering by LAGs

Now, let's dig into a more specific requirement: you might need to filter VLANs based on whether they contain a specific value in their lags list. For instance, if you only wanted to keep VLANs that have a lag of 1, you can modify your template as follows:

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More Advanced Filtering

If you want to be able to filter by multiple lags, for example, it could get a bit more complex. Below are two variations depending on your needs:

Keep VLANs that are in either lags 1 or 2:

[[See Video to Reveal this Text or Code Snippet]]

Only keep VLANs which have both lags 1 and 2:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Handling nested variables in Jinja2/Ansible can be a challenge, but with a clear understanding of the syntax and structure, it becomes much easier. By following the examples provided, you can effectively filter and retrieve the necessary VLAN information based on specific conditions.

Remember, the flexibility of Jinja2 allows you to expand upon this template to suit your particular needs. Happy coding!