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Terraform

Terraform S3 Etag Changes On Every Apply: Fix

By Ondřej Dolanský on 01/06/2025

Learn why your Terraform S3 object etags are changing on every apply and how to fix this common issue.

Terraform S3 Etag Changes On Every Apply: Fix

Table of Contents

Introduction

When working with Terraform's aws_s3_bucket_object resource, it's crucial to understand how the etag attribute influences object updates. This attribute, representing the MD5 hash of your object's content, can trigger unintended updates even when the file itself remains unchanged.

Step-by-Step Guide

The etag attribute in the aws_s3_bucket_object resource represents the MD5 hash of the object content. This means that any change to the file content, even without modifying the file itself, will result in a different etag.

resource "aws_s3_bucket_object" "example" {
  bucket = "your-bucket-name"
  key    = "your-file.zip"
  source = "path/to/your-file.zip"
}

Terraform uses this etag to determine if the object needs to be updated. If the etag changes, Terraform considers the object to be modified and will update it in S3.

This behavior can be problematic when the file content doesn't change, but the etag does. For example, if you re-download or copy the file, the timestamp might change, leading to a different etag even though the content remains the same.

To avoid unnecessary updates, you can:

  • Use source_code_hash for Lambda functions: If you're deploying Lambda functions, use the source_code_hash argument in the aws_lambda_function resource instead of relying on the etag of the zip file. This argument calculates a hash of the function's code, ensuring that the function is only updated when the code itself changes.
resource "aws_lambda_function" "example" {
  # ... other configurations ...

  source_code_hash = filebase64sha256("path/to/your-function.zip")
}
  • Trigger updates manually: If you need to update the object even when the content hasn't changed, you can:
    • Manually delete the object in S3 and run terraform apply to recreate it.
    • Use the lifecycle block with the create_before_destroy meta-argument to force the creation of a new object before deleting the old one.
resource "aws_s3_bucket_object" "example" {
  # ... other configurations ...

  lifecycle {
    create_before_destroy = true
  }
}
  • Consider alternative solutions: For managing large files or complex deployments, explore other tools and services like AWS CodeDeploy, AWS CodePipeline, or third-party solutions that integrate with Terraform.

Remember that while using etag can be helpful for detecting changes, it's essential to understand its limitations and employ appropriate strategies to manage your S3 objects effectively.

Code Example

This code demonstrates how Terraform uses etag to manage changes in AWS S3 bucket objects and Lambda functions. It provides examples of how to upload a file to an S3 bucket, define a Lambda function with source_code_hash to track code changes, and force an update using create_before_destroy. The code emphasizes the importance of adapting solutions to specific needs, considering alternative deployment tools, and thorough testing.

This example showcases the etag behavior with aws_s3_bucket_object and provides solutions for managing unnecessary updates.

1. Basic Example with etag:

resource "aws_s3_bucket" "example" {
  bucket = "your-bucket-name"
}

resource "aws_s3_bucket_object" "example" {
  bucket = aws_s3_bucket.example.id
  key    = "your-file.zip"
  source = "path/to/your-file.zip"
}

This code defines an S3 bucket and uploads a file. Terraform uses the etag to track changes.

2. Lambda Function with source_code_hash:

resource "aws_lambda_function" "example" {
  function_name = "your-function-name"
  # ... other configurations ...

  s3_bucket = aws_s3_bucket.example.id
  s3_key    = "your-function.zip"
  source_code_hash = filebase64sha256("path/to/your-function.zip")
}

This code defines a Lambda function and uses source_code_hash to track changes in the function code, avoiding reliance on the zip file's etag.

3. Manual Update with create_before_destroy:

resource "aws_s3_bucket_object" "example" {
  bucket = aws_s3_bucket.example.id
  key    = "your-file.zip"
  source = "path/to/your-file.zip"

  lifecycle {
    create_before_destroy = true
  }
}

This code uses the create_before_destroy meta-argument to force the creation of a new object before deleting the old one, ensuring an update even if the content hasn't changed.

Remember:

  • This is a simplified example. You might need to adapt it based on your specific needs and infrastructure.
  • Consider using alternative solutions like AWS CodeDeploy or CodePipeline for managing complex deployments.
  • Always test your Terraform code thoroughly before applying changes to your production environment.

Additional Notes

Here are some additional points to consider regarding the behavior of etag with the aws_s3_bucket_object resource in Terraform:

Understanding etag Limitations:

  • Platform-Specific Calculations: The way etag is calculated can vary depending on how you upload the file to S3. Uploads from different operating systems or S3 clients might result in different etag values for the same file content.
  • Multipart Uploads: For large files uploaded using multipart uploads, the etag represents a hash of all the parts and their order, making it even more sensitive to changes in the upload process.
  • Not a Content Integrity Check: While etag can help detect changes, it shouldn't be solely relied upon for content integrity verification. Use checksums like SHA-256 for that purpose.

Best Practices:

  • Minimize Unnecessary Changes: Avoid actions that modify file timestamps or metadata if the content remains the same. Use tools that preserve these attributes during file operations.
  • Versioning for Sensitive Data: Enable S3 versioning for critical objects to maintain a history of changes and enable rollbacks if needed.
  • Automate with Caution: When using create_before_destroy or manually deleting objects to trigger updates, ensure your infrastructure and applications can handle potential downtime or inconsistencies during the process.

Alternatives to Consider:

  • Terraform External Data Sources: For situations where you need to fetch and compare file hashes outside of Terraform's core functionality, consider using external data sources to retrieve and compare checksums.
  • Configuration Management Tools: For more complex deployments involving software installations and configurations, explore dedicated configuration management tools like Ansible, Chef, or Puppet in conjunction with Terraform.

Troubleshooting etag Issues:

  • Taint the Resource: If you've made changes to the file outside of Terraform and need to force an update, use the terraform taint command to mark the aws_s3_bucket_object resource as requiring recreation.
  • Inspect State File: Examine the Terraform state file to understand the current etag value and compare it with the actual etag of the object in S3 to identify discrepancies.

By understanding the nuances of etag and employing these best practices, you can effectively manage your S3 objects in Terraform while minimizing unexpected updates and ensuring the desired state of your infrastructure.

Summary

Feature Description Potential Issue Solution
etag Attribute Represents the MD5 hash of the S3 object content. Any change to the file, even without modifying content (e.g., re-download), results in a different etag, triggering unnecessary updates by Terraform. See solutions below.
source_code_hash for Lambda Use this argument in aws_lambda_function instead of relying on etag when deploying Lambda functions. N/A Calculates a hash of the function's code, ensuring updates only occur when the code changes.
Manual Updates Force object updates even if the content hasn't changed. N/A 1. Manually delete the object in S3 and run terraform apply.
2. Use the lifecycle block with create_before_destroy = true to force new object creation before deleting the old one.
Alternative Solutions Consider for managing large files or complex deployments. N/A Explore tools like AWS CodeDeploy, AWS CodePipeline, or third-party solutions integrated with Terraform.

Key Takeaway: While etag helps detect changes, understand its limitations and use appropriate strategies for efficient S3 object management in Terraform.

Conclusion

By understanding the nuances of etag and employing best practices, you can effectively manage your S3 objects in Terraform. Leverage source_code_hash for Lambda deployments, utilize manual updates when necessary, and explore alternative tools for complex scenarios. Minimizing unnecessary changes, versioning sensitive data, and automating with caution are crucial for a robust and efficient infrastructure. Troubleshooting etag issues can be achieved by tainting resources or inspecting the state file. By mastering these concepts, you can confidently manage your S3 objects in Terraform while ensuring the desired state of your infrastructure.

References

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