--- a/libervia/server/tasks/manager.py	Sun May 03 21:07:22 2020 +0200
+++ b/libervia/server/tasks/manager.py	Sun May 03 21:07:23 2020 +0200
@@ -18,6 +18,7 @@
 import os
 import os.path
 from pathlib import Path
+from typing import Dict
 import importlib.util
 from twisted.internet import defer
 from sat.core.log import getLogger
@@ -26,6 +27,7 @@
 from sat.tools import utils
 from libervia.server.constants import Const as C
 from . import implicit
+from .task import Task
 
 log = getLogger(__name__)
 
@@ -81,9 +83,61 @@
                     raise ValueError(_("Unexpected value for {var}: {value!r}").format(
                         var=var, value=value))
 
-    async def parseTasksDir(self, dir_path):
+    async def importTask(
+        self,
+        task_name: str,
+        task_path: Path,
+        to_import: Dict[str, Path]
+    ) -> None:
+        if task_name in self.tasks:
+            log.debug(f"skipping task {task_name} which is already imported")
+            return
+        module_name = f"{self.site_name or C.SITE_NAME_DEFAULT}.task.{task_name}"
+
+        spec = importlib.util.spec_from_file_location(module_name, task_path)
+        task_module = importlib.util.module_from_spec(spec)
+        spec.loader.exec_module(task_module)
+        task = task_module.Task(self, task_name)
+        if task.AFTER is not None:
+            for pre_task_name in task.AFTER:
+                log.debug(
+                    f"task {task_name!r} must be run after {pre_task_name!r}")
+                try:
+                    pre_task_path = to_import[pre_task_name]
+                except KeyError:
+                    raise ValueError(
+                        f"task {task_name!r} must be run after {pre_task_name!r}, "
+                        f"however there is no task with such name")
+                await self.importTask(pre_task_name, pre_task_path, to_import)
+
+        # we launch prepare, which is a method used to prepare
+        # data at runtime (e.g. set WATCH_DIRS using config)
+        try:
+            prepare = task.prepare
+        except AttributeError:
+            pass
+        else:
+            log.info(_('== preparing task "{task_name}" for {site_name} =='.format(
+                task_name=task_name, site_name=self.site_name or DEFAULT_SITE_LABEL)))
+            try:
+                await utils.asDeferred(prepare)
+            except exceptions.CancelError as e:
+                log.debug(f"Skipping {task_name} which cancelled itself: {e}")
+                return
+
+        self.tasks[task_name] = task
+        self.validateData(task)
+        if self.host.options['dev_mode']:
+            dirs = task.WATCH_DIRS or []
+            for dir_ in dirs:
+                self.host.files_watcher.watchDir(
+                    dir_, auto_add=True, recursive=True,
+                    callback=self._autorunTask, task_name=task_name)
+
+    async def parseTasksDir(self, dir_path: Path) -> None:
         log.debug(f"parsing tasks in {dir_path}")
         tasks_paths = sorted(dir_path.glob('task_*.py'))
+        to_import = {}
         for task_path in tasks_paths:
             if not task_path.is_file():
                 continue
@@ -95,34 +149,10 @@
                     "A task with the name [{name}] already exists".format(
                         name=task_name))
             log.debug(f"task {task_name} found")
-            module_name = f"{self.site_name or C.SITE_NAME_DEFAULT}.task.{task_name}"
-
-            spec = importlib.util.spec_from_file_location(module_name, task_path)
-            task_module = importlib.util.module_from_spec(spec)
-            spec.loader.exec_module(task_module)
-            task = task_module.Task(self)
+            to_import[task_name] = task_path
 
-            # we launch prepare, which is a method used to prepare
-            # data at runtime (e.g. set WATCH_DIRS using config)
-            try:
-                prepare = task.prepare
-            except AttributeError:
-                pass
-            else:
-                try:
-                    await utils.asDeferred(prepare)
-                except exceptions.CancelError as e:
-                    log.debug(f"Skipping {task_name} which cancelled itself: {e}")
-                    continue
-
-            self.tasks[task_name] = task
-            self.validateData(task)
-            if self.host.options['dev_mode']:
-                dirs = task.WATCH_DIRS or []
-                for dir_ in dirs:
-                    self.host.files_watcher.watchDir(
-                        dir_, auto_add=True, recursive=True,
-                        callback=self._autorunTask, task_name=task_name)
+        for task_name, task_path in to_import.items():
+            await self.importTask(task_name, task_path, to_import)
 
     async def parseTasks(self):
         # implicit tasks are always run