refactor: deprecate plan field in cluster resource

- Add IgnoreChangesString PlanModifier to suppress all diffs for deprecated fields
- Add DeprecationMessage to plan field in schema
- Modify Read() to preserve state value instead of overwriting from API

This ensures existing resources won't trigger unexpected diffs when the
plan field value changes, and users will see deprecation warnings.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: yellow-shine

docs/resources/cluster.md

@@ -79,7 +79,7 @@ resource "zillizcloud_cluster" "enterprise_plan_cluster" {
 - `desired_status` (String) The desired status of the cluster. Possible values are RUNNING and SUSPENDED. Defaults to RUNNING.
 - `labels` (Map of String) A map of labels to assign to the cluster. Labels are key-value pairs that can be used to organize and categorize clusters.
 - `load_balancer_security_groups` (Set of String, Deprecated) A set of security group IDs to associate with the load balancer of the cluster.
-- `plan` (String) The plan tier of the Zilliz Cloud service. Available options are Serverless, Standard and Enterprise.
+- `plan` (String, Deprecated) The plan tier of the Zilliz Cloud service. Available options are Serverless, Standard and Enterprise.
 - `region_id` (String) The ID of the region where the cluster exists.
 - `replica` (Number) The number of replicas for the cluster. Defaults to 1.
 - `replica_settings` (Attributes) Query compute unit configuration for the cluster. The cu_settings and cu_size cannot be set simultaneously. (see [below for nested schema](#nestedatt--replica_settings))

internal/cluster/cluster_resource.go

@@ -27,6 +27,7 @@ import (
 	"github.com/hashicorp/terraform-plugin-log/tflog"
 	zilliz "github.com/zilliztech/terraform-provider-zillizcloud/client"
 	util "github.com/zilliztech/terraform-provider-zillizcloud/client/retry"
+	customplanmodifier "github.com/zilliztech/terraform-provider-zillizcloud/internal/planmodifier"
 	customvalidator "github.com/zilliztech/terraform-provider-zillizcloud/internal/validator"
 )
 
@@ -100,6 +101,10 @@ func (r *ClusterResource) Schema(ctx context.Context, req resource.SchemaRequest
 				MarkdownDescription: "The plan tier of the Zilliz Cloud service. Available options are Serverless, Standard and Enterprise.",
 				Optional:            true,
 				Computed:            true,
+				DeprecationMessage:  "The plan field is deprecated and will be removed in a future major version. The plan is determined automatically based on cu_size and cu_type.",
+				PlanModifiers: []planmodifier.String{
+					customplanmodifier.IgnoreChangesString(),
+				},
 				Validators: []validator.String{
 					stringvalidator.OneOf(FreePlan, ServerlessPlan, StandardPlan, EnterprisePlan, BusinessCriticalPlan),
 				},
@@ -534,7 +539,10 @@ func (r *ClusterResource) Read(ctx context.Context, req resource.ReadRequest, re
 	state.ConnectAddress = cluster.ConnectAddress
 	state.PrivateLinkAddress = cluster.PrivateLinkAddress
 	state.CreateTime = cluster.CreateTime
-	state.Plan = cluster.Plan
+	// plan field is deprecated - only set from API if state is empty (first read after import)
+	if state.Plan.IsNull() || state.Plan.IsUnknown() {
+		state.Plan = cluster.Plan
+	}
 	state.Replica = cluster.Replica
 	state.CuSize = cluster.CuSize
 	state.CuType = cluster.CuType

internal/planmodifier/ignore_changes_string.go

@@ -0,0 +1,38 @@
+package planmodifier
+
+import (
+	"context"
+
+	"github.com/hashicorp/terraform-plugin-framework/resource/schema/planmodifier"
+)
+
+// ignoreChangesString implements a PlanModifier that suppresses all diffs
+// for deprecated fields by keeping the prior state value.
+type ignoreChangesString struct{}
+
+func (m ignoreChangesString) Description(ctx context.Context) string {
+	return "Ignore any configuration changes by keeping the prior state value."
+}
+
+func (m ignoreChangesString) MarkdownDescription(ctx context.Context) string {
+	return m.Description(ctx)
+}
+
+func (m ignoreChangesString) PlanModifyString(ctx context.Context, req planmodifier.StringRequest, resp *planmodifier.StringResponse) {
+	// If the resource already exists (state is not null/unknown), keep the state value
+	// This completely suppresses any diff for this field
+	if !req.StateValue.IsNull() && !req.StateValue.IsUnknown() {
+		resp.PlanValue = req.StateValue
+		return
+	}
+
+	// For new resources: use the config value (or null if not set)
+	// This allows the first apply to write a value to state, which is then preserved
+	resp.PlanValue = req.ConfigValue
+}
+
+// IgnoreChangesString returns a PlanModifier that suppresses all diffs
+// by keeping the prior state value. Use this for deprecated fields.
+func IgnoreChangesString() planmodifier.String {
+	return ignoreChangesString{}
+}