Terraform's lifecycle can sometimes lead to unexpected changes in attributes managed by providers, particularly when external factors modify them. This blog post explores strategies to prevent Terraform from reverting these intentional external modifications. It focuses on using ignore_changes within a resource's lifecycle block to specify the attributes to disregard during the plan and apply phases. The post demonstrates this with an AWS security group example, where an external tool might add ingress rules that Terraform shouldn't overwrite. It emphasizes the importance of carefully choosing which attributes to ignore, as it can mask legitimate changes and potentially introduce drift. The author recommends using ignore_changes sparingly and considering alternative solutions like null_resource or data sources to manage externally controlled resources when possible.
This blog post by Matt Titmus addresses the common Terraform challenge of managing unwanted changes to resource attributes introduced by providers, particularly during updates. The author meticulously explores different strategies for mitigating this issue, highlighting the nuances and potential pitfalls of each approach.
The core problem arises when a Terraform provider, which acts as an interface to various infrastructure platforms, modifies the default values or behavior of certain resource attributes. This can lead to unexpected changes during terraform apply, even when the user hasn't explicitly altered their Terraform configuration. Such changes can disrupt existing infrastructure and necessitate manual intervention.
The post begins by discussing the simplest approach: accepting the changes proposed by the provider. While straightforward, this method isn't always desirable, especially in production environments. It then delves into more sophisticated techniques.
The primary focus lies on leveraging the lifecycle block within a resource definition. Specifically, the ignore_changes argument allows users to specify a list of attributes that Terraform should disregard during state comparisons. This effectively tells Terraform to overlook modifications to these attributes made by the provider, preventing unintended updates. The author provides clear examples demonstrating how to utilize ignore_changes for various scenarios, including ignoring changes to specific attributes and using wildcards for broader matching.
However, the post also cautions against the overuse of ignore_changes. Ignoring changes can mask legitimate updates and lead to configuration drift, where the actual infrastructure state deviates from the desired state defined in Terraform. Therefore, the author stresses the importance of understanding the implications of ignoring changes and using this feature judiciously.
Furthermore, the blog post explores alternative strategies for managing provider-driven changes. It discusses setting default values explicitly in the Terraform configuration, thereby overriding the provider defaults and preventing unexpected modifications. It also touches upon the concept of pinning provider versions to avoid updates that introduce undesirable behavior.
Finally, the post acknowledges scenarios where ignoring changes isn't feasible or advisable. In such cases, the author recommends manually updating the Terraform state to align with the provider changes. While this requires more effort, it ensures consistency and prevents further discrepancies.
In summary, the blog post provides a comprehensive guide to handling unwanted attribute changes introduced by Terraform providers. It emphasizes the use of lifecycle and ignore_changes while also highlighting alternative approaches and the potential drawbacks of ignoring changes. The author encourages careful consideration and understanding of the implications before implementing any of these strategies.
Summary of Comments ( 11 )
https://news.ycombinator.com/item?id=43454642
The Hacker News comments discuss practical approaches to the problem of Terraform providers sometimes changing attributes unexpectedly. Several users suggest using
ignore_changeslifecycle arguments within Terraform configurations, emphasizing its utility but also cautioning about potential risks if misused. Others propose leveraging thenullprovider or generating local values to manage these situations, offering specific code examples. The discussion touches on the complexities of state management and the potential for drift, with recommendations for robust testing and careful planning. Some commenters highlight the importance of understanding why the provider is making changes, advocating for addressing the root cause rather than simply ignoring the symptoms. The thread also features a brief exchange on the benefits and drawbacks of the presentedignore_changessolution versus simply overriding the changed value every time, with arguments made for both sides.The Hacker News post "Ignoring unwanted Terraform attribute changes" discussing the linked blog post has generated several comments. Many revolve around the complexities and frustrations of managing Terraform state, particularly when dealing with external forces modifying resources managed by Terraform.
One commenter highlights the common scenario where a provider might default a value that the user didn't explicitly set, leading to Terraform wanting to revert it on the next apply. They suggest this is especially problematic when combined with
for_eachresources. They appreciate the blog post's solution usingignore_changeslifecycle meta-argument but express a desire for Terraform to handle this more elegantly by default. This sentiment of wishing for better default behavior from Terraform echoes through other comments as well.Another user mentions the struggles of managing resources where underlying providers or external systems might alter values outside of Terraform's purview. They describe their current strategy of manually editing the state file which, while functional, is clearly not ideal. They see the
ignore_changesapproach as a much cleaner and more maintainable way to handle these situations.The discussion then delves into the nuances of when to utilize
ignore_changes. One participant cautions against overusing it as a catch-all solution. They emphasize the importance of understanding why a value is drifting and whether ignoring the change is truly the appropriate course of action. They suggest investigating if a provider's default behavior can be configured or if the external system modifying the resource can be adjusted. Ignoring changes should be a conscious decision made with full awareness of the potential implications.Another commenter reiterates this caution, pointing out that blindly using
ignore_changescould mask legitimate problems and create unexpected side effects down the line. They suggest treating it as a temporary fix while a more robust solution is investigated.Some users suggest alternative approaches like using
nullvalues for certain attributes to avoid conflicts or leveraging theprevent_destroylifecycle argument to prevent accidental deletion of resources. These suggestions highlight the various tools available in Terraform for managing state drift, but also reinforce the complexity of choosing the right approach for a given scenario.Finally, a commenter touches upon the broader issue of state management in Infrastructure-as-Code and expresses hope for future improvements in Terraform that could simplify these kinds of challenges.