feat(update-app-base): add support to update base for application charms

This PR adds support to update the base in application charms by requiring a replace in case of a
machine charm, and perform the upgrade in case of a k8s charm.

We add the model_type computed field on the application schema, and we
use it to make a decision in the planmodifier RequiresReplaceIf.

re juju#635

Author: SimoneDutto

docs/resources/application.md

@@ -74,6 +74,7 @@ Notes:
 ### Read-Only
 
 - `id` (String) The ID of this resource.
+- `model_type` (String) The type of the model where the application is to be deployed. It is a computed field and is needed to determine if the application should be replaced or updated in case of base updates.
 - `principal` (Boolean, Deprecated) Whether this is a Principal application
 
 <a id="nestedblock--charm"></a>
@@ -85,7 +86,7 @@ Required:
 
 Optional:
 
-- `base` (String) The operating system on which to deploy. E.g. ubuntu@22.04.
+- `base` (String) The operating system on which to deploy. E.g. ubuntu@22.04. Changing this value for machine charms will trigger a replace by terraform.
 - `channel` (String) The channel to use when deploying a charm. Specified as \<track>/\<risk>/\<branch>.
 - `revision` (Number) The revision of the charm to deploy. During the update phase, the charm revision should be update before config update, to avoid issues with config parameters parsing.
 - `series` (String, Deprecated) The series on which to deploy.

internal/juju/applications.go

@@ -271,7 +271,8 @@ type transformedCreateApplicationInput struct {
 }
 
 type CreateApplicationResponse struct {
-	AppName string
+	AppName   string
+	ModelType string
 }
 
 type ReadApplicationInput struct {
@@ -284,6 +285,7 @@ type ReadApplicationResponse struct {
 	Channel          string
 	Revision         int
 	Base             string
+	ModelType        string
 	Series           string
 	Units            int
 	Trust            bool
@@ -307,9 +309,9 @@ type UpdateApplicationInput struct {
 	Trust     *bool
 	Expose    map[string]interface{}
 	// Unexpose indicates what endpoints to unexpose
-	Unexpose []string
-	Config   map[string]string
-	//Series    string // Unsupported today
+	Unexpose           []string
+	Config             map[string]string
+	Base               string
 	Placement          map[string]interface{}
 	Constraints        *constraints.Value
 	EndpointBindings   map[string]string
@@ -368,9 +370,16 @@ func (c applicationsClient) CreateApplication(ctx context.Context, input *Create
 	// If we have managed to deploy something, now we have
 	// to check if we have to expose something
 	err = c.processExpose(applicationAPIClient, transformedInput.applicationName, transformedInput.expose)
-
+	if err != nil {
+		return nil, err
+	}
+	modelType, err := c.ModelType(input.ModelName)
+	if err != nil {
+		return nil, err
+	}
 	return &CreateApplicationResponse{
-		AppName: transformedInput.applicationName,
+		AppName:   transformedInput.applicationName,
+		ModelType: modelType.String(),
 	}, err
 }
 
@@ -1114,6 +1123,7 @@ func (c applicationsClient) ReadApplication(input *ReadApplicationInput) (*ReadA
 		Channel:          appInfo.Channel,
 		Revision:         charmURL.Revision,
 		Base:             fmt.Sprintf("%s@%s", appInfo.Base.Name, baseChannel.Track),
+		ModelType:        modelType.String(),
 		Series:           seriesString,
 		Units:            unitCount,
 		Trust:            trustValue,
@@ -1194,7 +1204,7 @@ func (c applicationsClient) UpdateApplication(input *UpdateApplicationInput) err
 	// before the operations with config. Because the config params
 	// can be changed from one revision to another. So "Revision-Config"
 	// ordering will help to prevent issues with the configuration parsing.
-	if input.Revision != nil || input.Channel != "" || len(input.Resources) != 0 {
+	if input.Revision != nil || input.Channel != "" || len(input.Resources) != 0 || input.Base != "" {
 		setCharmConfig, err := c.computeSetCharmConfig(input, applicationAPIClient, charmsAPIClient, resourcesAPIClient)
 		if err != nil {
 			return err
@@ -1379,6 +1389,13 @@ func (c applicationsClient) computeSetCharmConfig(
 			newOrigin.Branch = strPtr(parsedChannel.Branch)
 		}
 	}
+	if input.Base != "" {
+		base, err := corebase.ParseBaseFromString(input.Base)
+		if err != nil {
+			return nil, err
+		}
+		newOrigin.Base = base
+	}
 
 	resolvedURL, resolvedOrigin, supportedBases, err := resolveCharm(charmsAPIClient, newURL, newOrigin)
 	if err != nil {
@@ -1391,10 +1408,6 @@ func (c applicationsClient) computeSetCharmConfig(
 		msg := fmt.Sprintf("the new charm does not support the current architecture %q", oldOrigin.Architecture)
 		return nil, errors.New(msg)
 	}
-	if !basesContain(oldOrigin.Base, supportedBases) {
-		msg := fmt.Sprintf("the new charm does not support the current operating system %q", oldOrigin.Base.String())
-		return nil, errors.New(msg)
-	}
 
 	// Ensure the new revision or channel is contained
 	// in the origin to be saved by juju when AddCharm
@@ -1406,6 +1419,14 @@ func (c applicationsClient) computeSetCharmConfig(
 		oldOrigin.Risk = newOrigin.Risk
 		oldOrigin.Branch = newOrigin.Branch
 	}
+	if input.Base != "" {
+		oldOrigin.Base = newOrigin.Base
+	}
+
+	if !basesContain(oldOrigin.Base, supportedBases) {
+		msg := fmt.Sprintf("the new charm does not support the current operating system %q", oldOrigin.Base.String())
+		return nil, errors.New(msg)
+	}
 
 	resultOrigin, err := charmsAPIClient.AddCharm(resolvedURL, oldOrigin, false)
 	if err != nil {

internal/provider/modifer_applicationbaseupdate.go

@@ -0,0 +1,32 @@
+// Copyright 2025 Canonical Ltd.
+// Licensed under the AGPLv3, see LICENCE file for details.
+
+package provider
+
+import (
+	"context"
+
+	"github.com/hashicorp/terraform-plugin-framework/path"
+	"github.com/hashicorp/terraform-plugin-framework/resource/schema/planmodifier"
+	"github.com/hashicorp/terraform-plugin-framework/resource/schema/stringplanmodifier"
+	"github.com/hashicorp/terraform-plugin-framework/types"
+	"github.com/juju/juju/core/model"
+)
+
+// baseApplicationRequiresReplaceIf is a plan modifier that sets the RequiresReplace field if the
+// model type is IAAS. The reason is that with CAAS the application can be updated in place.
+// With IAAS the application needs to be replaced. To make this decision the model type is needed.
+// Since you can't access the juju client in the plan modifiers we've added a computed field `model_type`.
+// This is set in the state by means of the `stringplanmodifier.UseStateForUnknown()`, so when we update the base
+// is always guaranteed to be set.
+func baseApplicationRequiresReplaceIf(ctx context.Context, req planmodifier.StringRequest, resp *stringplanmodifier.RequiresReplaceIfFuncResponse) {
+	if req.State.Raw.IsKnown() {
+		var modelType types.String
+		diags := req.State.GetAttribute(ctx, path.Root("model_type"), &modelType)
+		if diags.HasError() {
+			resp.Diagnostics.Append(diags...)
+			return
+		}
+		resp.RequiresReplace = modelType.ValueString() == model.IAAS.String()
+	}
+}

internal/provider/resource_application.go

@@ -82,6 +82,7 @@ type applicationResourceModel struct {
 	Constraints       types.String `tfsdk:"constraints"`
 	Expose            types.List   `tfsdk:"expose"`
 	ModelName         types.String `tfsdk:"model"`
+	ModelType         types.String `tfsdk:"model_type"`
 	Placement         types.String `tfsdk:"placement"`
 	EndpointBindings  types.Set    `tfsdk:"endpoint_bindings"`
 	Resources         types.Map    `tfsdk:"resources"`
@@ -147,6 +148,14 @@ func (r *applicationResource) Schema(_ context.Context, _ resource.SchemaRequest
 					stringplanmodifier.RequiresReplaceIfConfigured(),
 				},
 			},
+			"model_type": schema.StringAttribute{
+				Description: "The type of the model where the application is to be deployed. It is a computed field and " +
+					"is needed to determine if the application should be replaced or updated in case of base updates.",
+				Computed: true,
+				PlanModifiers: []planmodifier.String{
+					stringplanmodifier.UseStateForUnknown(),
+				},
+			},
 			"units": schema.Int64Attribute{
 				Description: "The number of application units to deploy for the charm.",
 				Optional:    true,
@@ -316,11 +325,12 @@ func (r *applicationResource) Schema(_ context.Context, _ resource.SchemaRequest
 							DeprecationMessage: "Configure base instead. This attribute will be removed in the next major version of the provider.",
 						},
 						BaseKey: schema.StringAttribute{
-							Description: "The operating system on which to deploy. E.g. ubuntu@22.04.",
+							Description: "The operating system on which to deploy. E.g. ubuntu@22.04. Changing this value for machine charms will trigger a replace by terraform.",
 							Optional:    true,
 							Computed:    true,
 							PlanModifiers: []planmodifier.String{
 								stringplanmodifier.UseStateForUnknown(),
+								stringplanmodifier.RequiresReplaceIf(baseApplicationRequiresReplaceIf, "", ""),
 							},
 							Validators: []validator.String{
 								stringvalidator.ConflictsWith(path.Expressions{
@@ -581,7 +591,6 @@ func (r *applicationResource) Create(ctx context.Context, req resource.CreateReq
 		return
 	}
 	r.trace(fmt.Sprintf("read application resource %q", createResp.AppName))
-
 	// Save plan into Terraform state
 
 	// Constraints do not apply to subordinate applications. If the application
@@ -590,6 +599,7 @@ func (r *applicationResource) Create(ctx context.Context, req resource.CreateReq
 	plan.Placement = types.StringValue(readResp.Placement)
 	plan.Principal = types.BoolNull()
 	plan.ApplicationName = types.StringValue(createResp.AppName)
+	plan.ModelType = types.StringValue(readResp.ModelType)
 	planCharm.Revision = types.Int64Value(int64(readResp.Revision))
 	planCharm.Base = types.StringValue(readResp.Base)
 	planCharm.Series = types.StringValue(readResp.Series)
@@ -692,6 +702,12 @@ func (r *applicationResource) Read(ctx context.Context, req resource.ReadRequest
 	}
 	r.trace("read application", map[string]interface{}{"resource": appName, "response": response})
 
+	modelType, err := r.client.Applications.ModelType(modelName)
+	if err != nil {
+		resp.Diagnostics.Append(handleApplicationNotFoundError(ctx, err, &resp.State)...)
+		return
+	}
+
 	state.ApplicationName = types.StringValue(appName)
 	state.ModelName = types.StringValue(modelName)
 
@@ -700,6 +716,7 @@ func (r *applicationResource) Read(ctx context.Context, req resource.ReadRequest
 	state.Placement = types.StringValue(response.Placement)
 	state.Principal = types.BoolNull()
 	state.UnitCount = types.Int64Value(int64(response.Units))
+	state.ModelType = types.StringValue(modelType.String())
 	state.Trust = types.BoolValue(response.Trust)
 
 	// state requiring transformation
@@ -919,16 +936,18 @@ func (r *applicationResource) Update(ctx context.Context, req resource.UpdateReq
 		} else if !planCharm.Revision.Equal(stateCharm.Revision) {
 			updateApplicationInput.Revision = intPtr(planCharm.Revision)
 		}
-
-		if !planCharm.Series.Equal(stateCharm.Series) || !planCharm.Base.Equal(stateCharm.Base) {
+		if !planCharm.Base.Equal(stateCharm.Base) {
+			updateApplicationInput.Base = planCharm.Base.ValueString()
+		}
+		if !planCharm.Series.Equal(stateCharm.Series) {
 			// This violates Terraform's declarative model. We could implement
 			// `juju set-application-base`, usually used after `upgrade-machine`,
 			// which would change the operating system used for future units of
 			// the application provided the charm supported it, but not change
 			// the current. This provider does not implement an equivalent to
 			// `upgrade-machine`. There is also a question of how to handle a
 			// change to series, revision and channel at the same time.
-			resp.Diagnostics.AddWarning("Not Supported", "Changing an application's operating system after deploy.")
+			resp.Diagnostics.AddWarning("Not Supported", "Changing operating system's series after deploy.")
 		}
 	}
 
@@ -1051,7 +1070,8 @@ func (r *applicationResource) Update(ctx context.Context, req resource.UpdateReq
 	if updateApplicationInput.Channel != "" ||
 		updateApplicationInput.Revision != nil ||
 		updateApplicationInput.Placement != nil ||
-		updateApplicationInput.Units != nil {
+		updateApplicationInput.Units != nil ||
+		updateApplicationInput.Base != "" {
 		readResp, err := r.client.Applications.ReadApplicationWithRetryOnNotFound(ctx, &juju.ReadApplicationInput{
 			ModelName: updateApplicationInput.ModelName,
 			AppName:   updateApplicationInput.AppName,
@@ -1090,6 +1110,7 @@ func (r *applicationResource) Update(ctx context.Context, req resource.UpdateReq
 		}
 	}
 
+	plan.ModelType = state.ModelType
 	plan.ID = types.StringValue(newAppID(plan.ModelName.ValueString(), plan.ApplicationName.ValueString()))
 	plan.Principal = types.BoolNull()
 	r.trace("Updated", applicationResourceModelForLogging(ctx, &plan))

internal/provider/resource_application_test.go

@@ -276,6 +276,37 @@ func TestAcc_CharmUpdates(t *testing.T) {
 	})
 }
 
+func TestAcc_CharmUpdateBase(t *testing.T) {
+	modelName := acctest.RandomWithPrefix("tf-test-charmbaseupdates")
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:                 func() { testAccPreCheck(t) },
+		ProtoV6ProviderFactories: frameworkProviderFactories,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccApplicationUpdateBaseCharm(modelName, "ubuntu@22.04"),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr("juju_application.this", "charm.0.base", "ubuntu@22.04"),
+				),
+			},
+			{
+				// move to base ubuntu 20.04
+				Config: testAccApplicationUpdateBaseCharm(modelName, "ubuntu@20.04"),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr("juju_application.this", "charm.0.base", "ubuntu@20.04"),
+				),
+			},
+			{
+				// move back to ubuntu 22.04
+				Config: testAccApplicationUpdateBaseCharm(modelName, "ubuntu@22.04"),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr("juju_application.this", "charm.0.base", "ubuntu@22.04"),
+				),
+			},
+		},
+	})
+}
+
 func TestAcc_ResourceRevisionUpdatesLXD(t *testing.T) {
 	if testingCloud != LXDCloudTesting {
 		t.Skip(t.Name() + " only runs with LXD")
@@ -1021,6 +1052,41 @@ func testAccResourceApplicationUpdatesCharm(modelName string, channel string) st
 	}
 }
 
+func testAccApplicationUpdateBaseCharm(modelName string, base string) string {
+	if testingCloud == LXDCloudTesting {
+		return fmt.Sprintf(`
+		resource "juju_model" "this" {
+		  name = %q
+		}
+		
+		resource "juju_application" "this" {
+		  model = juju_model.this.name
+		  name = "test-app"
+		  charm {
+			name     = "ubuntu"
+			base = %q
+		  }
+		}
+		`, modelName, base)
+	} else {
+		return fmt.Sprintf(`
+		resource "juju_model" "this" {
+		  name = %q
+		}
+		
+		resource "juju_application" "this" {
+		  model = juju_model.this.name
+		  name = "test-app"
+		  charm {
+			name     = "coredns"
+			channel = "1.25/stable"
+			base = %q
+		  }
+		}
+		`, modelName, base)
+	}
+}
+
 // testAccResourceApplicationConstraints will return two set for constraint
 // applications. The version to be used in K8s sets the juju-external-hostname
 // because we set the expose parameter.