feat: use exponential backoff algorithm when polling actions (#524)

##### SUMMARY

Replace the constant poll interval of 1 second, with a truncated
exponential back off algorithm with jitter.

Below is a suite of poll interval (in seconds) generated by the new
algorithm:
```
1.49
2.14
5.46
6.51
6.57
5.57
5.98
7.13
6.59
7.10
5.54
5.03
6.56
5.96
6.72
7.21
7.05
5.31
5.60
6.33
6.82
5.42
6.08
6.60
TOTAL: 140.77
```

changelogs/fragments/exponential-actions-polling-interval.yml

@@ -0,0 +1,2 @@
+minor_changes:
+  - Use a truncated exponential backoff algorithm when polling actions from the API.

plugins/module_utils/client.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 from contextlib import contextmanager
+from random import random
 
 from ansible.module_utils.basic import missing_required_lib
 
@@ -106,3 +107,22 @@ def cached_session(self):
             yield
         finally:
             self._requests_session = requests.Session()
+
+
+def exponential_backoff_poll_interval(*, base: float, multiplier: int, cap: float, jitter: float):
+    """
+    Return a poll interval function, implementing a truncated exponential backoff with jitter.
+
+    :param base: Base for the exponential backoff algorithm.
+    :param multiplier: Multiplier for the exponential backoff algorithm.
+    :param cap: Value at which the interval is truncated.
+    :param jitter: Proportion of the interval to add as random jitter.
+    """
+
+    def func(retries: int) -> float:
+        interval = base * multiplier**retries  # Exponential backoff
+        interval = min(cap, interval)  # Cap backoff
+        interval += random() * interval * jitter  # Add jitter
+        return interval
+
+    return func

plugins/module_utils/hcloud.py

@@ -15,7 +15,12 @@
     check_required_one_of,
 )
 
-from .client import ClientException, client_check_required_lib, client_get_by_name_or_id
+from .client import (
+    ClientException,
+    client_check_required_lib,
+    client_get_by_name_or_id,
+    exponential_backoff_poll_interval,
+)
 from .vendor.hcloud import APIException, Client, HCloudException
 from .vendor.hcloud.actions import ActionException
 from .version import version
@@ -81,6 +86,9 @@ def _build_client(self) -> None:
             api_endpoint=self.module.params["api_endpoint"],
             application_name="ansible-module",
             application_version=version,
+            # Total waiting time before timeout is > 117.0
+            poll_interval=exponential_backoff_poll_interval(base=1.0, multiplier=2, cap=5.0, jitter=0.5),
+            poll_max_retries=25,
         )
 
     def _client_get_by_name_or_id(self, resource: str, param: str | int):

plugins/modules/certificate.py

@@ -204,7 +204,7 @@ def _create_certificate(self):
                     resp = self.client.certificates.create_managed(**params)
                     # Action should take 60 to 90 seconds on average, wait for 5m to
                     # allow DNS or Let's Encrypt slowdowns.
-                    resp.action.wait_until_finished(max_retries=300)
+                    resp.action.wait_until_finished(max_retries=62)  # 62 retries >= 302 seconds
                 except HCloudException as exception:
                     self.fail_json_hcloud(exception)
 

plugins/modules/server.py

@@ -471,7 +471,7 @@ def _create_server(self):
                 self.result["root_password"] = resp.root_password
                 # Action should take 60 to 90 seconds on average, but can be >10m when creating a
                 # server from a custom images
-                resp.action.wait_until_finished(max_retries=1800)
+                resp.action.wait_until_finished(max_retries=362)  # 362 retries >= 1802 seconds
                 for action in resp.next_actions:
                     action.wait_until_finished()
 
@@ -671,17 +671,18 @@ def _update_server_server_type(self) -> None:
 
         self.stop_server_if_forced()
 
-        upgrade_disk = self.module.params.get("upgrade_disk")
-        # Upgrading a server takes 160 seconds on average, upgrading the disk should
-        # take more time
-        upgrade_timeout = 600 if upgrade_disk else 180
-
         if not self.module.check_mode:
+            upgrade_disk = self.module.params.get("upgrade_disk")
+
             action = self.hcloud_server.change_type(
                 server_type=self._get_server_type(),
                 upgrade_disk=upgrade_disk,
             )
-            action.wait_until_finished(max_retries=upgrade_timeout)
+            # Upgrading a server takes 160 seconds on average, upgrading the disk should
+            # take more time
+            # 122 retries >= 602 seconds
+            # 38 retries >= 182 seconds
+            action.wait_until_finished(max_retries=122 if upgrade_disk else 38)
         self._mark_as_changed()
 
     def _update_server_ip(self, kind: Literal["ipv4", "ipv6"]) -> None:
@@ -867,9 +868,9 @@ def rebuild_server(self):
         try:
             if not self.module.check_mode:
                 image = self._get_image(self.hcloud_server.server_type)
-                # When we rebuild the server progress takes some more time.
                 resp = self.client.servers.rebuild(self.hcloud_server, image)
-                resp.action.wait_until_finished(1000)
+                # When we rebuild the server progress takes some more time.
+                resp.action.wait_until_finished(max_retries=202)  # 202 retries >= 1002 seconds
             self._mark_as_changed()
 
             self._get_server()

tests/unit/module_utils/test_client.py

@@ -0,0 +1,14 @@
+from __future__ import annotations
+
+from ansible_collections.hetzner.hcloud.plugins.module_utils.client import (
+    exponential_backoff_poll_interval,
+)
+
+
+def test_exponential_backoff_poll_interval():
+    poll_interval = exponential_backoff_poll_interval(base=1.0, multiplier=2, cap=5.0, jitter=0.0)
+    poll_max_retries = 25
+
+    results = [poll_interval(i) for i in range(poll_max_retries)]
+    assert sum(results) == 117.0
+    assert results[:6] == [1.0, 2.0, 4.0, 5.0, 5.0, 5.0]