Support cloud fleet in-place update

src/dstack/_internal/cli/services/configurators/fleet.py

@@ -34,9 +34,10 @@
     InstanceGroupPlacement,
 )
 from dstack._internal.core.models.instances import InstanceStatus, SSHKey
-from dstack._internal.core.services.diff import diff_models
+from dstack._internal.core.services.diff import copy_model, diff_models
 from dstack._internal.utils.common import local_time
 from dstack._internal.utils.logging import get_logger
+from dstack._internal.utils.nested_list import NestedList, NestedListItem
 from dstack._internal.utils.ssh import convert_ssh_key_to_pem, generate_public_key, pkey_from_str
 from dstack.api.utils import load_profile
 
@@ -85,14 +86,10 @@ def _apply_plan(self, plan: FleetPlan, command_args: argparse.Namespace):
                 )
             confirm_message += "Create the fleet?"
         else:
+            effective_spec = plan.get_effective_spec()
+            diff = _render_fleet_spec_diff(plan.current_resource.spec, effective_spec)
             action_message += f"Found fleet [code]{plan.spec.configuration.name}[/]."
-            if plan.action == ApplyAction.CREATE:
-                delete_fleet_name = plan.current_resource.name
-                action_message += (
-                    " Configuration changes detected. Cannot update the fleet in-place"
-                )
-                confirm_message += "Re-create the fleet?"
-            elif plan.current_resource.spec == plan.effective_spec:
+            if plan.current_resource.spec == effective_spec:
                 if command_args.yes and not command_args.force:
                     # --force is required only with --yes,
                     # otherwise we may ask for force apply interactively.
@@ -103,8 +100,26 @@ def _apply_plan(self, plan: FleetPlan, command_args: argparse.Namespace):
                 delete_fleet_name = plan.current_resource.name
                 action_message += " No configuration changes detected."
                 confirm_message += "Re-create the fleet?"
+            elif plan.action == ApplyAction.CREATE:
+                delete_fleet_name = plan.current_resource.name
+                if diff is not None:
+                    # TODO: Highlight only the fields that block in-place update instead of
+                    # showing the full detected diff here.
+                    action_message += (
+                        f" Detected changes that [error]cannot[/] be updated in-place:\n{diff}"
+                    )
+                else:
+                    action_message += (
+                        " Configuration changes detected. Cannot update the fleet in-place."
+                    )
+                confirm_message += "Re-create the fleet?"
             else:
-                action_message += " Configuration changes detected."
+                if diff is not None:
+                    action_message += (
+                        f" Detected changes that [code]can[/] be updated in-place:\n{diff}"
+                    )
+                else:
+                    action_message += " Configuration changes detected."
                 confirm_message += "Update the fleet in-place?"
 
         console.print(action_message)
@@ -357,6 +372,44 @@ def _resolve_ssh_key(ssh_key_path: Optional[str]) -> Optional[SSHKey]:
         exit()
 
 
+def _render_fleet_spec_diff(old_spec: FleetSpec, new_spec: FleetSpec) -> Optional[str]:
+    old_spec = copy_model(old_spec)
+    new_spec = copy_model(new_spec)
+    changed_spec_fields = list(diff_models(old_spec, new_spec))
+    if not changed_spec_fields:
+        return None
+
+    nested_list = NestedList()
+    for spec_field in changed_spec_fields:
+        if spec_field == "merged_profile":
+            continue
+        if spec_field == "configuration":
+            item = NestedListItem(
+                "Configuration properties:",
+                children=[
+                    NestedListItem(field)
+                    for field in diff_models(old_spec.configuration, new_spec.configuration)
+                ],
+            )
+        elif spec_field == "profile":
+            item = NestedListItem(
+                "Profile properties:",
+                children=[
+                    NestedListItem(field)
+                    for field in diff_models(old_spec.profile, new_spec.profile)
+                ],
+            )
+        elif spec_field == "configuration_path":
+            item = NestedListItem("Configuration path")
+        else:
+            item = NestedListItem(spec_field.replace("_", " ").capitalize())
+        nested_list.children.append(item)
+
+    if not nested_list.children:
+        return None
+    return nested_list.render()
+
+
 def _print_plan_header(plan: FleetPlan):
     def th(s: str) -> str:
         return f"[bold]{s}[/bold]"

src/dstack/_internal/cli/services/configurators/run.py

@@ -152,6 +152,8 @@ def apply_configuration(
                 confirm_message = "Stop and override the run?"
             elif not run_plan.current_resource.status.is_finished():
                 stop_run_name = run_plan.current_resource.run_spec.run_name
+                # TODO: Highlight only the fields that block in-place update instead of
+                # showing the full detected diff here.
                 console.print(
                     f"Active run [code]{conf.name}[/] already exists."
                     f" Detected changes that [error]cannot[/] be updated in-place:\n{diff}"

src/dstack/_internal/server/background/pipeline_tasks/fleets.py

@@ -63,7 +63,7 @@ def __init__(
         workers_num: int = 10,
         queue_lower_limit_factor: float = 0.5,
         queue_upper_limit_factor: float = 2.0,
-        min_processing_interval: timedelta = timedelta(seconds=30),
+        min_processing_interval: timedelta = timedelta(seconds=15),
         lock_timeout: timedelta = timedelta(seconds=20),
         heartbeat_trigger: timedelta = timedelta(seconds=10),
         *,

src/dstack/_internal/server/services/fleets.py

@@ -1219,26 +1219,36 @@ def _check_can_update_inner(current: M, new: M, updatable_fields: tuple[str, ...
     return diff
 
 
-@_check_can_update("configuration", "configuration_path")
+@_check_can_update("configuration", "configuration_path", "merged_profile")
 def _check_can_update_fleet_spec(current: FleetSpec, new: FleetSpec, diff: ModelDiff):
+    # Allow `merged_profile` only to absorb derived changes from supported configuration updates
+    # such as `configuration.reservation` and `configuration.tags`.
+    # Direct `profile` updates are still not in-place updatable.
     if "configuration" in diff:
         _check_can_update_fleet_configuration(current.configuration, new.configuration)
 
 
-@_check_can_update("ssh_config")
-def _check_can_update_fleet_configuration(
-    current: FleetConfiguration, new: FleetConfiguration, diff: ModelDiff
-):
+def _check_can_update_fleet_configuration(current: FleetConfiguration, new: FleetConfiguration):
+    diff = diff_models(current, new)
+    current_ssh_config = current.ssh_config
+    new_ssh_config = new.ssh_config
+    if current_ssh_config is None:
+        if new_ssh_config is not None:
+            raise ServerClientError("Fleet type changed from Cloud to SSH, cannot update")
+        # TODO: Support best-effort `nodes.target` apply semantics:
+        # create missing instances and terminate extra idle instances.
+        # Current in-place update only persists `target`; FleetPipeline reconciles `min`/`max`.
+        #
+        # For `reservation` and `tags`, update affects only future provisioning.
+        _check_can_update_inner(current, new, ("nodes", "reservation", "tags"))
+        return
+
+    if new_ssh_config is None:
+        raise ServerClientError("Fleet type changed from SSH to Cloud, cannot update")
+
+    _check_can_update_inner(current, new, ("ssh_config",))
     if "ssh_config" in diff:
-        current_ssh_config = current.ssh_config
-        new_ssh_config = new.ssh_config
-        if current_ssh_config is None:
-            if new_ssh_config is not None:
-                raise ServerClientError("Fleet type changed from Cloud to SSH, cannot update")
-        elif new_ssh_config is None:
-            raise ServerClientError("Fleet type changed from SSH to Cloud, cannot update")
-        else:
-            _check_can_update_ssh_config(current_ssh_config, new_ssh_config)
+        _check_can_update_ssh_config(current_ssh_config, new_ssh_config)
 
 
 @_check_can_update("hosts")