Estimated reading time: 7 minutes
Last updated on November 8th, 2024 at 03:43 pm
Terraform is a powerful open-source tool to manage infrastructure as a code (IaC) that allows you to define and provision the entire infrastructure with the configuration code.
Terraform provides a way to manage the structure code with the merge lists. Terraform merge lists combine multiple values and lists into one with the aggregate list.
Table of Contents
Understanding Terraform Merge Lists
A list is an ordered collection of the same data type values. In Terraform, lists are used extensively to manage the configuration and resources. Terrafrom list can be used to define server IPs, resource tags, or other configuration items.
How to use Terraform Merge Function: Tips, Examples, and Best Practices
Why Merge Lists in Terraform?
Terraform Merge Lists simplify complex configurations and provide modular and reusable code structure. Combining the multiple lists into a single list you can reduce the code complexity and make your configuration more readable and maintainable.
Top 5 Terraform Module Versioning Best Practices
Common Use Case for Merging Lists
Terraform merge lists are useful in various scenarios:
- Combine the multiple modules’ resource tags.
- Aggregate IP addresses from different sources.
- Merge the different configurations into one.
Methods to Merge Lists in Terraform
1. Use the “merge” Function
The merge
function in Terraform is used to combine multiple maps into a single map. It takes n number of maps as an argument and returns a single map that contains all the elements. if the key is present in multiple inputs, the value from the last map with the key is used.
variable "map1" {
type = map(string)
default = {
key1 = "value1"
key2 = "value2"
}
}
variable "map2" {
type = map(string)
default = {
key2 = "override_value2"
key3 = "value3"
}
}
output "merged_map" {
value = merge(var.map1, var.map2)
}
var.map1
and var.map2
are merged into a single map, resulting in:
{
key1 = "value1"
key2 = "override_value2"
key3 = "value3"
}
To merge the list you can use the Terraform concat function.
2. Use the “concat” Function
The concat
function is designed to merge lists. The concate
function takes multiple lists as an argument and returns a list containing all the values in a single list.
variable "list1" {
type = list(string)
default = ["apple", "banana"]
}
variable "list2" {
type = list(string)
default = ["cherry", "date"]
}
output "merged_list" {
value = concat(var.list1, var.list2)
}
var.list1
and var.list2
are merged into a single list using the concat
function, resulting in:
["apple", "banana", "cherry", "date"]
💡 You must read this:
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Top 5 Terraform Module Versioning Best Practices
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Terraform Merge Lists: Advanced
1. Terraform Merge Lists Maps
We define the map of objects with {}
The merge
function combines multiple maps into a single map. This is useful for aggregating the configuration from various sources.
locals {
team1 = {
alice = "developer"
bob = "designer"
carol = "manager"
}
team2 = {
dave = "developer"
eve = "analyst"
frank = "tester"
}
}
output "merged_team" {
value = merge(local.team1, local.team2)
}
In this example, local.team1
and local.team2
are merged into a single map using the merge
function. The resulting map is:
{
alice = "developer"
bob = "designer"
carol = "manager"
dave = "developer"
eve = "analyst"
frank = "tester"
}
2. Terraform Merge Lists of Objects
Terraform does not have a built-in function for merging a list of objects, but you can use the concat
function
locals {
fruits1 = [
{ name = "apple", color = "red" },
{ name = "banana", color = "yellow" }
]
fruits2 = [
{ name = "grape", color = "purple" },
{ name = "lemon", color = "yellow" }
]
combined_fruits = [for fruit in concat(local.fruits1, local.fruits2) : fruit]
}
output "combined_fruits" {
value = local.combined_fruits
}
In this example, local.fruits1
and local.fruits2
are concatenated and then iterated over using a for
expression to produce a single merged list:
[
{ name = "apple", color = "red" },
{ name = "banana", color = "yellow" },
{ name = "grape", color = "purple" },
{ name = "lemon", color = "yellow" }
]
3. Terraform Merge Lists Using “flatten”
Another way to merge lists of objects is by using the flatten
function.
locals {
nested_lists = [
[
{ name = "tomato", type = "vegetable" },
{ name = "carrot", type = "vegetable" }
],
[
{ name = "strawberry", type = "fruit" },
{ name = "blueberry", type = "fruit" }
]
]
flat_list = flatten(local.nested_lists)
}
output "flat_list" {
value = local.flat_list
}
In this example, local.nested_lists
is flattened into a single list using the flatten
function:
[
{ name = "tomato", type = "vegetable" },
{ name = "carrot", type = "vegetable" },
{ name = "strawberry", type = "fruit" },
{ name = "blueberry", type = "fruit" }
]
4. Terraform Merge Tags
One common use case for using the merge
the function is to “merge lists” of tags applied to cloud resources.
locals {
base_tags = {
Application = "MyApp"
Owner = "DevTeam"
}
additional_tags = {
Environment = "Production"
Region = "us-west-2"
}
merged_tags = merge(local.base_tags, local.additional_tags)
}
output "merged_tags" {
value = local.merged_tags
}
This example merges local.base_tags
and local.additional_tags
into a single map of tags:
{
Application = "MyApp"
Owner = "DevTeam"
Environment = "Production"
Region = "us-west-2"
}
5. Merging Maps with Conflicting Keys
When merging maps with conflicting keys, the value from the last map with that key will be used.
variable "map1" {
type = map(string)
default = {
key = "value1"
}
}
variable "map2" {
type = map(string)
default = {
key = "value2"
}
}
output "merged_map" {
value = merge(var.map1, var.map2)
}
In this example, var.map1
and var.map2
both have the same key key
. The value from var.map2
(“value2”) overrides the value from var.map1
(“value1”). The resulting map is:
{
key = "value2"
}
6. Merging Maps with Nested Structures
You can merge maps with nested structures. The merge is shallow, meaning it does not recursively merge nested maps.
variable "map1" {
type = map(any)
default = {
config = {
param1 = "value1"
}
}
}
variable "map2" {
type = map(any)
default = {
config = {
param2 = "value2"
}
}
}
output "merged_map" {
value = merge(var.map1, var.map2)
}
Here, var.map1
and var.map2
both have a nested map under the key config
. The resulting map will have config
from var.map2
entirely, without merging the nested config
maps:
{
config = {
param2 = "value2"
}
}
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Terraform Merge Lists: Best Practices
Keep Code Maintainable
- Use descriptive and readable variable names.
- Use Terraform Modules to keep your configuration organized and manageable.
Error Handling and Debugging
- Use the
terraform validate
command to check syntax errors for configuration. - Use the Terraform built-in function to handle the edge cases and debugging.
Performance Considerations
- Merging large lists can cause a performance bottleneck, avoid merging large lists if possible.
- Optimize Terrafrom’s performance with the state management for large-scale configuration.
Conclusion
In this guide, we learned how to use Terraform Merge Lists with various examples and the best practices to keep Terraform configuration maintainable.
Further Reading and Resources
Terraform Where to Store Secrets: Best Practices and Solutions
How to use Terraform Merge Function: Tips, Examples, and Best Practices