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Terraform

Terraform: Merge Maps Within a List - Complete Guide

By Filip on 11/13/2024

Learn how to efficiently merge maps within a list using Terraform to simplify your infrastructure as code and improve resource management.

Terraform: Merge Maps Within a List - Complete Guide

Table of Contents

Introduction

In Terraform, you can combine multiple maps into a single map using the merge function. This is particularly useful when you have a list of maps that you need to consolidate.

Step-by-Step Guide

You can merge a list of maps into a single map using the merge function in Terraform.

Example:

locals {
  list_of_maps = [
    {
      "key1" = "value1"
    },
    {
      "key2" = "value2"
    },
  ]

  merged_map = merge(local.list_of_maps...)
}

Explanation:

  1. local.list_of_maps: This defines a list containing two maps.
  2. merge(local.list_of_maps...): The merge function takes multiple maps as arguments and merges them into a single map. The ... after local.list_of_maps expands the list into individual map arguments for the merge function.

This code will output a single map:

{
  "key1" = "value1"
  "key2" = "value2"
}

Code Example

This Terraform code defines a list of maps and then uses the merge function to combine them into a single map. The resulting merged map is then outputted.

locals {
  list_of_maps = [
    {
      "key1" = "value1"
    },
    {
      "key2" = "value2"
    },
  ]

  merged_map = merge(local.list_of_maps...)
}

output {
  merged_map = local.merged_map
}

Explanation:

  1. local.list_of_maps: This defines a list containing two maps, each with a single key-value pair.
  2. merge(local.list_of_maps...): The merge function takes the list of maps and merges them into a single map. The ... operator expands the list local.list_of_maps into its individual map elements, which are then passed as arguments to the merge function.
  3. output { merged_map = local.merged_map }: This defines an output variable named "merged_map" that will display the contents of the merged map.

When you run Terraform with this code, the output will show the following:

merged_map = {
  "key1" = "value1"
  "key2" = "value2"
}

This demonstrates how the merge function combines the key-value pairs from multiple maps into a single map.

Additional Notes

  • Key Collisions: When merging maps, if the same key exists in multiple input maps, the value from the last map provided as an argument to merge will take precedence.
  • Use Cases: Merging maps is helpful for:
    • Combining configurations from different sources (e.g., default settings and environment-specific overrides).
    • Building up a complex data structure incrementally.
    • Flattening a list of maps into a single map for easier access.
  • Alternatives: While merge is convenient, consider these alternatives for specific scenarios:
    • Nested maps: For structured data, nesting maps might be more organized than merging everything into a single level.
    • lookup function: If you need to retrieve a specific value based on a key from a list of maps, lookup can be more efficient than merging.
  • Error Handling: Be mindful of potential errors when using merge:
    • Passing non-map arguments to merge will result in an error.
    • Unexpected key collisions might lead to unintended behavior. Carefully review the merged output.
  • Dynamic Merging: You can use merge within loops and dynamic blocks to merge maps dynamically based on your Terraform configuration.

Remember to consult the official Terraform documentation for the most up-to-date information on the merge function and its usage.

Summary

Feature Description
Merging Maps Terraform's merge function combines multiple maps into a single map.
Syntax merge(map1, map2, ...)
Example terraform<br>locals {<br> list_of_maps = [<br> { "key1" = "value1" },<br> { "key2" = "value2" }<br> ]<br> merged_map = merge(local.list_of_maps...)<br>}
Key Points - The ... operator expands a list into individual arguments.
- The resulting map contains all key-value pairs from the input maps.

Conclusion

The merge function in Terraform provides a simple and powerful way to combine multiple maps into a single map, facilitating the consolidation of configurations and data structures. By understanding its behavior with key collisions and exploring alternative approaches like nested maps and the lookup function, Terraform users can effectively leverage merge for a wide range of use cases, from managing configurations to building complex data structures. However, it's crucial to be aware of potential errors related to argument types and unexpected key collisions. Always review the merged output carefully and consult the official Terraform documentation for the most up-to-date information on using the merge function effectively.

References

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