In Terraform, how do I apply DiffSuppressFunc on a TypeList with each element in the list being an object following a schema?

Question

Brief Context

I am working on coding a Terraform Provider (using the Terraform SDK), which has a resource. The schema of the resource has a couple of attributes, of which one is

            "sample_attribute": {
                Type:        schema.TypeList,
                Optional:    true,
                Description: "A sample attribute whose details have been redacted.",
                Elem:        sampleAttributeSchemaExpanded()
            },

where sampleAttributeSchemaExpanded() returns a list of attributes (*schema.Resource) of which some are integers, some are boolean, some are strings, etc.

What I would like to achieve here, is to apply a DiffSuppressFunc on sample_attribute.

The Problem I'm Trying To Solve

When this resource is created (i.e. running terraform apply) and the resource is read (i.e. upon terraform plan after the first terraform apply to check for updates), API calls are sent from the respective methods in the resource to create the resource, and read the resource respectively.

The problem is that the order of elements (in sample_attribute) in the create API call's request and response and the order of elements in the read API call are dissimilar. Owing to this, after the first terraform apply, even if no changes are made to the Terraform Configuration, upon running terraform plan, a drift (non-null list of changes) is shown, suggesting a change to be made to elements belonging to sample_attribute. Restructuring the order of elements in sample_attribute in the Terraform Configuration eliminates the drift, but this should trivially not be the solution.

To fix this, I would like to use DiffSuppressFunc in such a way that differences are suppressed if the difference between the current configuration(new) and the configuration in the state (old) pertaining to sample_attribute is only the order of elements. If real changes are made to an element in sample_attribute, the differences are to be shown as usual.

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Unfortunately, I haven't been able to find a good example on using DiffSuppressFunc with TypeList with each element as an object (following a schema), which is why I am blocked. Any insights or a sample on this (or even suggestions, if you think other methods would work) would really be appreciated :)

P.S. One suggestion I've taken from my circles is to sort the elements of sample_attribute obtained in the read response to align them with the order of elements in sample_attribute in the Terraform Configuration, but I'd not want to do that (unless it is impossible to do this by any other means such as DiffSuppressFunc).

Answer 1

The signature of DiffSuppressFunc is:

func(k, old, new string, d *schema.ResourceData) bool

Unfortunately this signature means that it's only able to work with "string-like" attributes, because other attribute types cannot be provided as a single string in old and new. This is a limitation of the old Terraform plugin SDK that was originally designed for much older versions of Terraform that didn't yet have full support for lists and maps.

For requirements like this it's typically better to use the modern Plugin Framework instead of the legacy SDK. The new framework was designed for modern Terraform and so it supports all of the same types and data structures that Terraform itself supports, including lists of objects.

The framework does not have a feature that's directly analogous to DiffSuppressFunc, but you can use the more general plan modification features to achieve the same effect. The SDK uses the DiffSupressFunc result to reset an attribute back to its value from the prior state, which you can implement yourself in a plan modifier by comparing req.StateValue with req.PlanValue and then setting req.PlanValue to exactly req.StateValue to "suppress the diff".

Alternatively, you can define a custom type with a semantic equality rule to tell the framework how to compare two values of your custom type and decide if they are equivalent. If your type indicates that the two values are equivalent then the framework will automatically retain the prior value, in a similar way as DiffSuppressFunc is treated in the old SDK.

If you cannot migrate to the modern Plugin Framework then an an alternative with the old SDK is the resource-level CustomizeDiff function, which is similar in scope to the plugin framework's "plan modification" concept. In a CustomizeDiff function you can implement a rule which compares the old and new values for an attribute and uses d.Set to override the new value back to the old value when appropriate.