Performance: remove unnecessary workflow execution update

* This fix drastically improves performance since it eliminates
  an unnecessary update of the workflow execution object when
  processing "on_action_complete" operation. W/o this fix
  all such transactions would have to compete for the workflow
  executions table that causes lots of DB deadlocks (on MySQL)
  and transaction retries. In some cases the number of retries
  even exceeds the limit (currently hardcoded 50) and such tasks
  can be fixed only with the integrity checker over time.
  See the code comment in the "dispatcher.py" module that explains
  why this specific change eliminates the update of the workflow
  execution object.
* Style changes in api.py and aciton_execution_checker.py

Closes-Bug: #1790079

Change-Id: I08cb561e252d31e35fcfb61984d87a7bfc387a4d

mistral/db/v2/sqlalchemy/api.py

@@ -1266,6 +1266,7 @@ def get_expired_executions(expiration_time, limit=None, columns=(),
 @b.session_aware()
 def get_running_expired_sync_actions(expiration_time, session=None):
     query = b.model_query(models.ActionExecution)
+
     query = query.filter(
         models.ActionExecution.last_heartbeat < expiration_time
     )

mistral/engine/dispatcher.py

@@ -98,11 +98,20 @@ def _save_command_to_backlog(wf_ex, cmd):
 
 
 def _poll_commands_from_backlog(wf_ex):
-    backlog_cmds = wf_ex.runtime_context.pop(BACKLOG_KEY, [])
-
-    if not backlog_cmds:
+    # NOTE: We need to always use a guard condition that checks
+    # if a persistent structure is empty and, as in this case,
+    # return immediately w/o doing any further manipulations.
+    # Otherwise, if we do pop() operation with a default value
+    # then the ORM framework will consider it a modification of
+    # the persistent object and generate a corresponding SQL
+    # UPDATE operation. In this particular case it will increase
+    # contention for workflow executions table drastically and
+    # decrease performance.
+    if not wf_ex.runtime_context.get(BACKLOG_KEY):
         return []
 
+    backlog_cmds = wf_ex.runtime_context.pop(BACKLOG_KEY)
+
     return [
         commands.restore_command_from_dict(wf_ex, cmd_dict)
         for cmd_dict in backlog_cmds

mistral/services/action_execution_checker.py

@@ -34,25 +34,32 @@ SCHEDULER_KEY = 'handle_expired_actions_key'
 def handle_expired_actions():
     LOG.debug("Running heartbeat checker...")
 
-    try:
-        interval = CONF.action_heartbeat.check_interval
-        max_missed = CONF.action_heartbeat.max_missed_heartbeats
-        exp_date = utils.utc_now_sec() - datetime.timedelta(
-            seconds=max_missed * interval
-        )
+    interval = CONF.action_heartbeat.check_interval
+    max_missed = CONF.action_heartbeat.max_missed_heartbeats
 
+    exp_date = utils.utc_now_sec() - datetime.timedelta(
+        seconds=max_missed * interval
+    )
+
+    try:
         with db_api.transaction():
             action_exs = db_api.get_running_expired_sync_actions(exp_date)
+
             LOG.debug("Found {} running and expired actions.".format(
                 len(action_exs))
             )
+
             if action_exs:
-                LOG.info("Actions executions to transit to error, because "
-                         "heartbeat wasn't received: {}".format(action_exs))
+                LOG.info(
+                    "Actions executions to transit to error, because "
+                    "heartbeat wasn't received: {}".format(action_exs)
+                )
+
                 for action_ex in action_exs:
                     result = mistral_lib.Result(
                         error="Heartbeat wasn't received."
                     )
+
                     action_handler.on_action_complete(action_ex, result)
     finally:
         schedule(interval)
@@ -62,14 +69,19 @@ def setup():
     interval = CONF.action_heartbeat.check_interval
     max_missed = CONF.action_heartbeat.max_missed_heartbeats
     enabled = interval and max_missed
+
     if not enabled:
         LOG.info("Action heartbeat reporting disabled.")
+
         return
 
     wait_time = interval * max_missed
-    LOG.debug("First run of action execution checker, wait before "
-              "checking to make sure executors have time to send "
-              "heartbeats. ({} seconds)".format(wait_time))
+
+    LOG.debug(
+        "First run of action execution checker, wait before "
+        "checking to make sure executors have time to send "
+        "heartbeats. ({} seconds)".format(wait_time)
+    )
 
     schedule(wait_time)
 

releasenotes/notes/remove_unnecessary_workflow_execution_update-bdc9526bd39539c4.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+    Eliminated an unnecessary update of the workflow execution object
+    when processing "on_action_complete" operation. W/o this fix all
+    such transactions would have to compete for the workflow executions
+    table that causes lots of DB deadlocks (on MySQL) and transaction
+    retries. In some cases the number of retries even exceeds the limit
+    (currently hardcoded 50) and such tasks can be fixed only with the
+    integrity checker over time.