nicholasjackson/terraform-plugin-framework

Comprehensive documentation and best practices for building Terraform providers with terraform-plugin-framework (v1.17.0). Covers providers, resources, schemas, types, validators, testing, and common pitfalls.

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Plan Modifiers

Name: nicholasjackson/terraform-plugin-framework
Author: nicholasjackson

Control Terraform plan behavior with built-in and custom plan modifiers.

Overview

Plan modifiers alter the planned value for an attribute during the planning phase. They:

Run during terraform plan to modify planned values
Control whether changes trigger resource replacement
Prevent unnecessary "known after apply" values
Can use state values to inform plan values

Critical pattern: Use UseStateForUnknown() for computed attributes to prevent unnecessary resource replacements.

Built-in Plan Modifiers

String Plan Modifiers

import "github.com/hashicorp/terraform-plugin-framework/resource/schema/planmodifier"
import "github.com/hashicorp/terraform-plugin-framework/resource/schema/stringplanmodifier"

"id": schema.StringAttribute{
    Computed: true,
    PlanModifiers: []planmodifier.String{
        // Use state value if plan value is unknown
        stringplanmodifier.UseStateForUnknown(),
    },
},

"region": schema.StringAttribute{
    Required: true,
    PlanModifiers: []planmodifier.String{
        // Require resource replacement if value changes
        stringplanmodifier.RequiresReplace(),
    },
},

"name": schema.StringAttribute{
    Optional: true,
    Computed: true,
    PlanModifiers: []planmodifier.String{
        // Only if null, use default value
        stringplanmodifier.RequiresReplaceIfConfigured(),
    },
},

Int64 Plan Modifiers

import "github.com/hashicorp/terraform-plugin-framework/resource/schema/int64planmodifier"

"port": schema.Int64Attribute{
    Optional: true,
    Computed: true,
    PlanModifiers: []planmodifier.Int64{
        int64planmodifier.UseStateForUnknown(),
        int64planmodifier.RequiresReplace(),
    },
},

Bool Plan Modifiers

import "github.com/hashicorp/terraform-plugin-framework/resource/schema/boolplanmodifier"

"enabled": schema.BoolAttribute{
    Optional: true,
    Computed: true,
    PlanModifiers: []planmodifier.Bool{
        boolplanmodifier.UseStateForUnknown(),
    },
},

Float64 Plan Modifiers

import "github.com/hashicorp/terraform-plugin-framework/resource/schema/float64planmodifier"

"threshold": schema.Float64Attribute{
    Optional: true,
    Computed: true,
    PlanModifiers: []planmodifier.Float64{
        float64planmodifier.UseStateForUnknown(),
    },
},

List/Set/Map Plan Modifiers

import "github.com/hashicorp/terraform-plugin-framework/resource/schema/listplanmodifier"
import "github.com/hashicorp/terraform-plugin-framework/resource/schema/setplanmodifier"
import "github.com/hashicorp/terraform-plugin-framework/resource/schema/mapplanmodifier"

"tags": schema.ListAttribute{
    ElementType: types.StringType,
    Optional:    true,
    Computed:    true,
    PlanModifiers: []planmodifier.List{
        listplanmodifier.UseStateForUnknown(),
    },
},

"roles": schema.SetAttribute{
    ElementType: types.StringType,
    Optional:    true,
    Computed:    true,
    PlanModifiers: []planmodifier.Set{
        setplanmodifier.UseStateForUnknown(),
    },
},

"labels": schema.MapAttribute{
    ElementType: types.StringType,
    Optional:    true,
    Computed:    true,
    PlanModifiers: []planmodifier.Map{
        mapplanmodifier.UseStateForUnknown(),
    },
},

Object Plan Modifiers

import "github.com/hashicorp/terraform-plugin-framework/resource/schema/objectplanmodifier"

"config": schema.SingleNestedAttribute{
    Optional: true,
    Computed: true,
    Attributes: map[string]schema.Attribute{
        "host": schema.StringAttribute{Required: true},
        "port": schema.Int64Attribute{Required: true},
    },
    PlanModifiers: []planmodifier.Object{
        objectplanmodifier.UseStateForUnknown(),
    },
},

UseStateForUnknown Pattern

Most important plan modifier - prevents "known after apply" when value doesn't actually change.

Without UseStateForUnknown

"id": schema.StringAttribute{
    Computed: true,
    // NO plan modifier
},

Result:

# terraform plan
  ~ resource "example_pet" "fluffy" {
      ~ id      = "pet-123" -> (known after apply)  # Unnecessary!
        name    = "Fluffy"
    }

With UseStateForUnknown

"id": schema.StringAttribute{
    Computed: true,
    PlanModifiers: []planmodifier.String{
        stringplanmodifier.UseStateForUnknown(),
    },
},

Result:

# terraform plan
resource "example_pet" "fluffy" {
    id   = "pet-123"  # Stays as is
    name = "Fluffy"
}

When to Use

Use UseStateForUnknown() for:

IDs - Computed identifiers that don't change
Computed timestamps - Created/updated times that don't change on every apply
Computed attributes from API - Values computed by API that are stable

Don't use for:

Truly dynamic values - Values that change on every apply (e.g., current timestamp)
Values that depend on other changes - Computed values that recalculate when inputs change

RequiresReplace Pattern

Force resource replacement when attribute changes.

Basic Usage

"region": schema.StringAttribute{
    Required: true,
    PlanModifiers: []planmodifier.String{
        stringplanmodifier.RequiresReplace(),
    },
},

Result:

# terraform plan
  # example_server.web must be replaced
-/+ resource "example_server" "web" {
      ~ region = "us-west-1" -> "us-east-1" # forces replacement
    }

When to Use

Use RequiresReplace() for immutable attributes:

Region/Zone - Cannot be changed after creation
Image ID - Changing requires new instance
Disk type - Cannot be changed for existing disk
Network configuration - Immutable network settings

RequiresReplaceIf

Conditional replacement:

"size": schema.StringAttribute{
    Optional: true,
    PlanModifiers: []planmodifier.String{
        stringplanmodifier.RequiresReplaceIf(
            func(ctx context.Context, req planmodifier.StringRequest, resp *stringplanmodifier.RequiresReplaceIfFuncResponse) {
                // Only require replacement if downgrading
                if req.StateValue.IsNull() {
                    return
                }

                oldSize := req.StateValue.ValueString()
                newSize := req.PlanValue.ValueString()

                sizeOrder := map[string]int{
                    "small":  1,
                    "medium": 2,
                    "large":  3,
                }

                if sizeOrder[newSize] < sizeOrder[oldSize] {
                    resp.RequiresReplace = true
                }
            },
            "Downgrading size requires replacement",
            "Downgrading size requires replacement",
        ),
    },
},

RequiresReplaceIfConfigured

Only replace if user explicitly sets value:

"override": schema.StringAttribute{
    Optional: true,
    Computed: true,
    PlanModifiers: []planmodifier.String{
        stringplanmodifier.RequiresReplaceIfConfigured(),
    },
},

Custom Plan Modifiers

Create custom plan modifiers for domain-specific logic.

String Plan Modifier Example

package planmodifiers

import (
    "context"

    "github.com/hashicorp/terraform-plugin-framework/resource/schema/planmodifier"
)

var _ planmodifier.String = (*defaultValueModifier)(nil)

type defaultValueModifier struct {
    defaultValue string
}

func DefaultValue(defaultValue string) planmodifier.String {
    return &defaultValueModifier{
        defaultValue: defaultValue,
    }
}

func (m *defaultValueModifier) Description(ctx context.Context) string {
    return "Sets a default value if not configured"
}

func (m *defaultValueModifier) MarkdownDescription(ctx context.Context) string {
    return "Sets a default value if not configured"
}

func (m *defaultValueModifier) PlanModifyString(ctx context.Context, req planmodifier.StringRequest, resp *planmodifier.StringResponse) {
    // If value is null in config, set default
    if req.ConfigValue.IsNull() {
        resp.PlanValue = types.StringValue(m.defaultValue)
    }
}

// Use in schema
"environment": schema.StringAttribute{
    Optional: true,
    Computed: true,
    PlanModifiers: []planmodifier.String{
        planmodifiers.DefaultValue("production"),
    },
},

Timestamp Plan Modifier Example

var _ planmodifier.String = (*immutableTimestampModifier)(nil)

type immutableTimestampModifier struct{}

func ImmutableTimestamp() planmodifier.String {
    return &immutableTimestampModifier{}
}

func (m *immutableTimestampModifier) Description(ctx context.Context) string {
    return "Ensures timestamp is only set once and never changes"
}

func (m *immutableTimestampModifier) MarkdownDescription(ctx context.Context) string {
    return "Ensures timestamp is only set once and never changes"
}

func (m *immutableTimestampModifier) PlanModifyString(ctx context.Context, req planmodifier.StringRequest, resp *planmodifier.StringResponse) {
    // If state has a value, always use it
    if !req.StateValue.IsNull() {
        resp.PlanValue = req.StateValue
        return
    }

    // If no state value (creation), set current time
    if req.PlanValue.IsUnknown() {
        resp.PlanValue = types.StringValue(time.Now().UTC().Format(time.RFC3339))
    }
}

// Use in schema
"created_at": schema.StringAttribute{
    Computed: true,
    PlanModifiers: []planmodifier.String{
        planmodifiers.ImmutableTimestamp(),
    },
},

Conditional Replace Plan Modifier

var _ planmodifier.String = (*conditionalReplaceModifier)(nil)

type conditionalReplaceModifier struct {
    attributePath path.Path
}

func RequiresReplaceIfAttributeSet(attributePath path.Path) planmodifier.String {
    return &conditionalReplaceModifier{
        attributePath: attributePath,
    }
}

func (m *conditionalReplaceModifier) Description(ctx context.Context) string {
    return "Requires replacement if specified attribute is set"
}

func (m *conditionalReplaceModifier) MarkdownDescription(ctx context.Context) string {
    return "Requires replacement if specified attribute is set"
}

func (m *conditionalReplaceModifier) PlanModifyString(ctx context.Context, req planmodifier.StringRequest, resp *planmodifier.StringResponse) {
    // Check if values changed
    if req.StateValue.Equal(req.PlanValue) {
        return
    }

    // Get the conditional attribute
    var attrValue types.Bool
    resp.Diagnostics.Append(req.Plan.GetAttribute(ctx, m.attributePath, &attrValue)...)
    if resp.Diagnostics.HasError() {
        return
    }

    // Require replacement if attribute is true
    if !attrValue.IsNull() && attrValue.ValueBool() {
        resp.RequiresReplace = true
    }
}

Plan Modifier Patterns

Incrementing Version

var _ planmodifier.Int64 = (*incrementVersionModifier)(nil)

type incrementVersionModifier struct{}

func IncrementVersion() planmodifier.Int64 {
    return &incrementVersionModifier{}
}

func (m *incrementVersionModifier) PlanModifyInt64(ctx context.Context, req planmodifier.Int64Request, resp *planmodifier.Int64Response) {
    // If state exists, increment version
    if !req.StateValue.IsNull() {
        currentVersion := req.StateValue.ValueInt64()
        resp.PlanValue = types.Int64Value(currentVersion + 1)
    } else {
        // First version is 1
        resp.PlanValue = types.Int64Value(1)
    }
}

Normalize Case

func (m *lowercaseModifier) PlanModifyString(ctx context.Context, req planmodifier.StringRequest, resp *planmodifier.StringResponse) {
    if req.PlanValue.IsNull() || req.PlanValue.IsUnknown() {
        return
    }

    value := req.PlanValue.ValueString()
    resp.PlanValue = types.StringValue(strings.ToLower(value))
}

Generate Random ID

func (m *generateIDModifier) PlanModifyString(ctx context.Context, req planmodifier.StringRequest, resp *planmodifier.StringResponse) {
    // If state exists, keep it
    if !req.StateValue.IsNull() {
        resp.PlanValue = req.StateValue
        return
    }

    // Generate new ID on creation
    if req.PlanValue.IsUnknown() {
        id := fmt.Sprintf("id-%d", time.Now().UnixNano())
        resp.PlanValue = types.StringValue(id)
    }
}

Testing Plan Modifiers

func TestDefaultValueModifier(t *testing.T) {
    modifier := DefaultValue("default")

    req := planmodifier.StringRequest{
        ConfigValue: types.StringNull(),  // Not set in config
        PlanValue:   types.StringUnknown(),
    }
    resp := &planmodifier.StringResponse{
        PlanValue: req.PlanValue,
    }

    modifier.PlanModifyString(context.Background(), req, resp)

    require.Equal(t, "default", resp.PlanValue.ValueString())
}

func TestImmutableTimestampModifier(t *testing.T) {
    modifier := ImmutableTimestamp()

    // Test with existing state
    stateTime := "2025-01-01T00:00:00Z"
    req := planmodifier.StringRequest{
        StateValue: types.StringValue(stateTime),
        PlanValue:  types.StringUnknown(),
    }
    resp := &planmodifier.StringResponse{
        PlanValue: req.PlanValue,
    }

    modifier.PlanModifyString(context.Background(), req, resp)

    // Should preserve state value
    require.Equal(t, stateTime, resp.PlanValue.ValueString())
}

Common Patterns

Computed ID Pattern

"id": schema.StringAttribute{
    Computed: true,
    PlanModifiers: []planmodifier.String{
        stringplanmodifier.UseStateForUnknown(),
    },
},

Immutable Attribute Pattern

"immutable_field": schema.StringAttribute{
    Required: true,
    PlanModifiers: []planmodifier.String{
        stringplanmodifier.RequiresReplace(),
    },
},

Optional with Computed Default Pattern

"status": schema.StringAttribute{
    Optional: true,
    Computed: true,
    PlanModifiers: []planmodifier.String{
        planmodifiers.DefaultValue("active"),
    },
},

Timestamp Pattern

"created_at": schema.StringAttribute{
    Computed: true,
    PlanModifiers: []planmodifier.String{
        planmodifiers.ImmutableTimestamp(),
    },
},

"updated_at": schema.StringAttribute{
    Computed: true,
    // No modifier - always update
},

External References

Navigation

Previous: Validators - Built-in and custom validators
Next: Functions - Provider functions
Up: Index - Documentation home

Continue to Functions to learn about implementing provider-defined Terraform functions.

Install with Tessl CLI

npx tessl i nicholasjackson/terraform-plugin-framework@0.1.6

nicholasjackson/terraform-plugin-framework

plan-modifiers.md.css-3qkkll{font-size:var(--chakra-font-sizes-sm);font-weight:var(--chakra-font-weights-normal);color:var(--chakra-colors-gray-300);}docs/

Plan Modifiers

Overview

Built-in Plan Modifiers

String Plan Modifiers

Int64 Plan Modifiers

Bool Plan Modifiers

Float64 Plan Modifiers

List/Set/Map Plan Modifiers

Object Plan Modifiers

UseStateForUnknown Pattern

Without UseStateForUnknown

With UseStateForUnknown

When to Use

RequiresReplace Pattern

Basic Usage

When to Use

RequiresReplaceIf

RequiresReplaceIfConfigured

Custom Plan Modifiers

String Plan Modifier Example

Timestamp Plan Modifier Example

Conditional Replace Plan Modifier

Plan Modifier Patterns

Incrementing Version

Normalize Case

Generate Random ID

Testing Plan Modifiers

Common Patterns

Computed ID Pattern

Immutable Attribute Pattern

Optional with Computed Default Pattern

Timestamp Pattern

External References

Navigation

plan-modifiers.mddocs/