Better constructor failure handling in ray serve #16922

jiaodong · 2021-07-07T03:11:53Z

Why are these changes needed?

Currently when exception is thrown in deployment constructor, which is expected to happen in case of transient failures or import error, we will retry forever in the controller. It's not handled nicely and repeatedly lead to user reported errors.

In addition, it's blocking #15820 that makes Http proxy a regular serve deployment.

Changes in this PR

Common flow of replica status tracking

Register actor defs, target state, return goal id as usual
async kick off constructor in RayServeWrappedReplica, will throw exception with logged error
- ActorReplicaWrapper's check_ready() will pick it up by ray.get and return ReplicaState.FAILED_TO_START
  - BackendReplica's check_started() will increase deployment retry counter by 1 (if we're still in valid tracking state) and return ReplicaStartingStatus back to update()

A sample deploy() flow, in case of transient failure that eventually passed:
- replica will go through steps above but eventually able to adjust itself to ReplicaState.RUNNING state and proceed as usual. Sample test output: https://gist.github.com/jiaodong/bde8fa2432cb02d8c66934fbf23614a8

Common flow of failed to start replica handling

In each update() loops's _check_completed_goals(), we factor in # of failed to start replicas and added a few more termination state of a deploy() goal:
- (1) target_replica_count = running_at_target_version_replica, clear success, complete goal
- (2) failed_to_start_replica > 0 and running_at_target_version_replica > 0, partial success, complete goal, also clear all tracking replicas in ReplicaState.FAILED_TO_START state as they failed and have nothing to do with subsequent deploy() goals anymore
  - NOTE: It might be worth changing this to something more strict like failed_to_start_replica + running_at_target_version_replica = target_replica_count in case we're still in transition to spawn all replicas
- (3) failed_to_start_replica >= retry threshold, target_replica_count > 0: clear failure, complete goal with exception.
- (4) Both success or failure completed goals will clear deployment retry counter as -1 and prevent controller from increasing it further.

A sample deploy() flow, in case of consistent failure on one replica, but eventually at least had one replica running at target version:
- Falls into (2) bucket above, blocking await on deploy() goal returns with success. Sample test output: https://gist.github.com/jiaodong/de24cf667ddd9e033ae7906a09453d53
A sample deploy() flow, in case of consistent failure on ALL replicas with no replica successfully started:
- Falls into (3) bucket above, blocking await on deploy() goal returns with exception object set by controller update() loop. Sample test output: https://gist.github.com/jiaodong/04635cd58fdea8fe8a98e6cfdde89305
  - NOTE: It might be worth adding better exception here or even surface original exception, I think probably a better solution here is to have a controller event loop level global exception handler like: https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.set_exception_handler so we can capture failure from all worker replica instances rather than creating our own hardcoded one. I don't want this to block this diff tho, so probably worth making it a followup for a clean solution focusing on global exception handling.

=== New in latest commit ===

If deploy() failed with exception, we track previous backend info and will call deploy_backend() if we have previous state, or delete_backend() if current call is the first one made, thus each deploy() is all or nothing with automatic rollback.

Related issue number

Closes #16597
Closes #16114

Checks

Full log

https://gist.github.com/jiaodong/3fd59057bca340a33e774f674479ec6b

I've run scripts/format.sh to lint the changes in this PR.
I've included any doc changes needed for https://docs.ray.io/en/master/.
I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
Testing Strategy
- Unit tests
- Release tests
- This PR is not tested :(

ijrsvt · 2021-07-07T15:27:43Z

python/ray/serve/async_goal_manager.py

Suggested change

Class to help ServeController to manage desirable states of replica.

Class to help ServeController to manage desired states of replica.

This comment is a leaky abstraction, the goal manager has nothing to do with managing replica states

ijrsvt · 2021-07-07T15:32:57Z

python/ray/serve/async_goal_manager.py

It be safer to do self._pending_goals.get(goal_id, asyncio.Future()).set_exception(e) to avoid some situation where this Except block is called when the future is removed early?

I see what you're suggesting .. but I wonder if that adds value to create a new Future instance out of event loop's API since no one should be waiting for its return value. Following pattern from current implementation we can get the actual future obj before try catch, thus we're always awaiting and setting exception on the same obj regardless if it's already poped from pending goals.

I think what we want to do is get a reference to the future outside the try-except then use it instead of doing another lookup in the dictionary:

fut = self._pending_goals[goal_id] try: await fut except Exception as e: fut.set_exception(e)

i don't think this is going to work? because await self._pendings[goal_id] means the future is already completed and has a result. you can't set_exception on a completed future.
this block should just be await self._pending_goals

I think the actual behavior for this file is not surfaced yet due to this commit directly called delete_deployment, but i totally agree with comments here. Let me change this a bit with prints as test plan on this PR so we can see a complete flow of what happened.

Just to confirm, i'm proposing the ideal state for async goals in case of constructor failure should be:

We throw exception from backend_worker's async function, which goes through backend_state_manager first;

But then we should let the exception surface to controller's async goal manager eventually, thus able to set the future tied up to original goal_id, therefore be able to surface it back to user script that called deploy() since the goal_id the represents blocking await for deploying returned with exception

Does this make sense as our deploy() API error handling ?

ijrsvt · 2021-07-07T15:34:58Z

python/ray/serve/backend_state_manager.py

ijrsvt · 2021-07-07T15:35:12Z

python/ray/serve/backend_state_manager.py

ijrsvt · 2021-07-07T15:35:18Z

python/ray/serve/backend_state_manager.py

ijrsvt · 2021-07-07T15:40:03Z

python/ray/serve/backend_state_manager.py

Maybe do:

replica.set_should_stop(0) replicas.add(ReplicaState.SHOULD_STOP, replica)

You may need to add some sort of 'if already dead logic' to BackendReplica::stop

I think deleting just the replica is a good idea because it is definitely possible for a constructor to fail at first, but succeed at a later time.

Yep I tried this for an earlier PR as well but it didn't play nicely without leveraging asyncio.Future to set exceptions. Let me give it another try, it's definitely preferable to only take action on replica level rather than shutting down entire deployment.

So there needs more changes in addition to marking its state as SHOULD_STOP since we still thinks target_replica should be 1, thus still causes it to loop forever. Let me send a followup commit to further change target_replica to 0, and let update() shut it down through controller's event loop.

ijrsvt · 2021-07-07T15:43:04Z

python/ray/serve/backend_worker.py

Can you move this break to directly after await sync_to_async(_callable.__init__)(*init_args)

ijrsvt

This overall looks pretty good! Would it be easy to test this with an actor that fails at first, but succeeds later? It could be something as simple as using the presence of a file to determine if this is the first or second call:

def __init__(self):
   if os.path.exists("file.name"):
      return True
    open("file.name", "w") as f:
       f.write("ONE")
    raise RuntimeError("First attempt is sad")

edoakes · 2021-07-07T19:09:34Z

python/ray/serve/async_goal_manager.py

This comment is a leaky abstraction, the goal manager has nothing to do with managing replica states

python/ray/serve/async_goal_manager.py

edoakes · 2021-07-07T19:15:21Z

python/ray/serve/async_goal_manager.py

I think what we want to do is get a reference to the future outside the try-except then use it instead of doing another lookup in the dictionary:

fut = self._pending_goals[goal_id] try: await fut except Exception as e: fut.set_exception(e)

edoakes · 2021-07-07T19:17:10Z

python/ray/serve/backend_state_manager.py

I'd prefer to return a boolean indiciating constructor failure rather than using the exception as the signaling mechanism here. That more closely matches the and would avoid errors where there is an exception in this code that trigger us to treat it as a constructor failure.

That indeed matches the current exception as signaling mechanism that I saw from current codebase, and i would like to keep it consistent :)

I wonder if there're better ways to throw initial exception to deploy() call's wait_for_goal() however, I assume ideally we want to be able to surface and set the original exception from constructor to the value of goal_id (which is an asyncio.Future now) ? But update() in backend_state handles it initially and we will lose it before reaching user code. Maybe we can have a mix of controller level global exception handling for cases like this that we want to set exception to the goal_id we're waiting for ?

edoakes · 2021-07-07T19:19:43Z

python/ray/serve/backend_state_manager.py

Hmm, don't we also need to somehow set the exception in the goal manager here? Otherwise how will this get propagated to the future?

simon-mo · 2021-07-07T19:22:29Z

python/ray/serve/async_goal_manager.py

i don't think this is going to work? because await self._pendings[goal_id] means the future is already completed and has a result. you can't set_exception on a completed future.
this block should just be await self._pending_goals

simon-mo · 2021-07-07T19:22:39Z

python/ray/serve/async_goal_manager.py

and move set_exception here

+1, I think it's better to have a single place to complete a goal. Maybe we should also let the caller pass in the right exception object instead of using a bare Exception?

edoakes · 2021-07-07T19:34:00Z

Please add unit tests for the logic to test_backend_state and integration tests to test_deploy.
We discussed rolling back to a previous version if an upgrade to a deployment fails, do you plan to handle that in a separate PR?
Right now we're treating a single constructor failure as a failed deployment and deleting the deployment if it fails. In some cases this might be an undesirable behavior. For example, if the deployment is scaled up and a single constructor fails, we'd delete the entire deployment (seems very dangerous). At a minimum we should only do this deletion/rollback behavior when the deployment is being created for the first time or upgraded to a new version.

edoakes · 2021-07-29T16:16:09Z

python/ray/serve/backend_state_manager.py

Hmm what's the difference between this and RUNNING?

Running means the deployment is available with at least 1 replica running with desired model version (technically we can also return deploy() so it doesn't block user code, and let controller to scale to target # eventually); while RUNNING_HEALTHY suggests it reached final desirable state such that no further action needed.

They can have better names and it's mostly draft for now

They don't necessarily need to exposed to user, but rather an enum to keep track of total possible internal states of a running deployment, which we already sort of do in the logic of update()

edoakes · 2021-07-29T16:17:20Z

python/ray/serve/backend_state_manager.py

Hmm it's kind of weird that these states are hierarchical/composable (e.g., what does it mean that it's both RUNNING and UPGRADING?). Can we make it so that a deployment is only in a single state at any given moment?

Ah sorry for the confusion, i meant it should be only one at a time. Line 52 primarily means all subsequent states requires the deployment to at least reach RUNNING state first so deployment states have its own transition DAG and precedence.

I see, makes sense. I think adding the DeploymentState here is fine but I'd prefer to avoid adding states that are not yet being used at all (in general I'm unilaterally opposed to checking in "dead code").

edoakes · 2021-07-29T16:19:05Z

python/ray/serve/backend_worker.py

It seems like we should probably do this retrying in the controller level instead of inside the replica given that we'll likely need retries there anyways and it'd be best to only have it in one place. Thoughts?

That's probably a better idea with current setup now to introduce deployment level states and per-replica status reporting. We will need to keep track of each replica's retry count and make decision to abort if all replicas failed to start, while they're probably newly spawned replicas retrying, ex:

Target replica 10, we had total of 10 unique replicas failed 3 times without ever reaching to running state, that's when we can abort rather than spawning replica #11; But if we had 1 running replica, even we accumulated 10 total replicas failed to start, it shouldn't prevent us from spawning #11 since it has reached "RUNNING" state.

edoakes · 2021-07-29T16:19:46Z

python/ray/serve/async_goal_manager.py

Please add details about the failure handling logic here. What is the behavior and what are the guarantees if a goal is errored? How should the client handle this?

edoakes · 2021-07-29T16:20:31Z

@jiaodong I left a few high-level comments after a quick scan. A full review will take me some more time. Is this in a working state btw?

jiaodong · 2021-07-29T18:46:57Z

@jiaodong I left a few high-level comments after a quick scan. A full review will take me some more time. Is this in a working state btw?

thanks for the comments, it's in a working state as i updated console outputs in test plan in a gist, but need more polishing and probably better to move replica retry on backend state / controller level

edoakes

See comments, mostly nits but one concern about the behavior: we're marking the deployment as successful when any replica succeeds to start. We should only be returning success when it failed or is fully successful.

edoakes · 2021-08-24T20:49:14Z

python/ray/serve/backend_state.py


        Returns:
-            (ready, past_slow_startup_threshold)
+            state (ReplicaState): Most recent state of replica by


Suggested change

state (ReplicaState): Most recent state of replica by

state (ReplicaStartingStatus): Most recent state of replica by

edoakes · 2021-08-24T20:49:24Z

python/ray/serve/backend_state.py

    STOPPING = 5


+class ReplicaStartingStatus(Enum):


Suggested change

class ReplicaStartingStatus(Enum):

class ReplicaStartupStatus(Enum):

nit

edoakes · 2021-08-24T20:49:46Z

python/ray/serve/backend_state.py

            f"*not* {self._state}.")

-        if self._actor.check_ready():
+        start_status = self._actor.check_ready()


Suggested change

start_status = self._actor.check_ready()

status = self._actor.check_ready()

edoakes · 2021-08-24T20:50:38Z

python/ray/serve/backend_state.py

+        self._replica_failed_to_start_counter: Dict[BackendTag,
+                                                    int] = defaultdict(int)
+        # Keep track of backend info in case of failed deploy() rollback
+        self._cur_backend_info: Dict[BackendTag, BackendInfo] = dict()


isn't this a duplicate of self._backend_metadata?

edoakes · 2021-08-24T20:50:52Z

python/ray/serve/backend_state.py

        if backend_tag not in self._replicas:
            self._replicas[backend_tag] = ReplicaStateContainer()

+        # Set deploy() retry counter and previous


didn't finish this sentence?

edoakes · 2021-08-24T20:56:49Z

python/ray/serve/backend_state.py

+        (complete_goal_ids, failed_goal_ids) = self._check_completed_goals()
+
+        for backend_tag, goal_id in complete_goal_ids:
+            self._replica_failed_to_start_counter[backend_tag] = -1


Shouldn't we be setting this counter to -1 as soon as any replica successfully starts up? Not sure why we're doing it here

that's one way of doing it too. It's doing it here in this revision because we makes a call when retry cap is reached and consider deployment as successful and this is where we handle goal ids that reached terminal state.

I think technically this counter can be set in different places to be considered correct, purely depends on how & where we make the call.

python/ray/serve/backend_state.py

edoakes · 2021-08-24T20:58:02Z

python/ray/serve/backend_state.py

+                logger.error("Reverting backend {backend_tag} to previous "
+                             f"backend info. {self._prev_backend_info}")


Logging the backend info object will not be useful to users. How about we log the version number instead?

also "backend" should be "deployment" in user-facing logs (confusing to keep track, I know)

I think mostly because we don't have repr for BackendInfo as i was expecting it will somehow print high-level description of the deployment setup .. but version number makes more sense for sure.

edoakes · 2021-08-24T20:58:15Z

python/ray/serve/backend_state.py

+                logger.error("Reverting backend to previous backend "
+                             f"info by deleting backend {backend_tag}")


confusing error message. just say "initial deployment failed, deleting the deployment"

edoakes · 2021-08-24T20:58:57Z

python/ray/serve/tests/test_backend_state.py

        return self.healthy


+class MockAsyncGoalManager:


I find it strange that we need to mock this class, I'd prefer if we could avoid needing to do this (by not using asyncio.Future unless necessary)

… running as expected

q

rebase

edoakes

Nice :) Just a few last nits and looks good to merge!

edoakes · 2021-08-26T16:39:16Z

python/ray/serve/backend_state.py

+        # import ipdb
+        # ipdb.set_trace()


edoakes · 2021-08-26T16:40:08Z

python/ray/serve/async_goal_manager.py

+        await async_goal.event.wait()
        logger.debug(
            f"Waiting for goal {goal_id} took {time.time() - start} seconds")
+
+        if async_goal.exception is not None:
+            return async_goal.exception


nit: consider adding a method here to avoid directly accessing the goal and exception: return await async_goal.wait()

that's my initial attempt too but i ran into quite a few pickling issues if we directly send AysncGoal object over .remote() calls so i only kept it minimal with exception object. But i can change the event within exposed wait() so that we don't need to directly touch it.

oh I was thinking that .wait() would just return a single object -- None if success or Exception if it errored

edoakes · 2021-08-26T16:42:08Z

python/ray/serve/backend_state.py

+                self._goal_manager.complete_goal(
+                    goal_id,
+                    RuntimeError(
+                        "Deployment failed, reverting to previous version "


add deployment name to the log

Deployment {backend_tag} failed due to constructor failure, reverting to previous version {version} asynchronously.

edoakes · 2021-08-26T16:42:18Z

python/ray/serve/backend_state.py

+                    RuntimeError(f"Deployment failed, deleting {backend_tag} "
+                                 "asynchronously."))


add deployment name to the log

i thought backend_tag is the same as deployment name ?

the log msg is a bit confusing, how about: Deployment {backend_tag} failed due to constructor failure, deleting it asynchronously.

edoakes · 2021-08-26T16:43:19Z

Oops, @ijrsvt can you rubber stamp to remove your change request?

edoakes · 2021-08-26T16:43:51Z

@jiaodong lint is failing BTW:
https://buildkite.com/ray-project/ray-builders-pr/builds/12286#e98195a3-d707-42cf-843f-f7d0e1fcd089/391-438

edoakes · 2021-08-26T16:44:27Z

Looks like there might have been an actual test failure in test_deploy also:
https://buildkite.com/ray-project/ray-builders-pr/builds/12257#d0e7ba05-7f57-4e2d-a421-f48351773bab/431-4618

ijrsvt

LGTM !

… lint noqa

edoakes · 2021-08-27T18:19:21Z

@jiaodong unfortunately the test_constructor_failure test didn't fully run on the PR. You need to add a if __name__ == "__main__" clause to actually run the tests due to a quirk in our test setup (see any of the other tests for an example).

See here, it ran in 0.7s: https://buildkite.com/ray-project/ray-builders-pr/builds/12312#9dafed28-5c32-4b17-8102-12f5907f5112/333-3318

edoakes · 2021-08-27T18:22:23Z

@jiaodong I push a commit to make the change ^

jiaodong requested review from edoakes, ijrsvt and simon-mo July 7, 2021 03:11

jiaodong assigned edoakes Jul 7, 2021

jiaodong mentioned this pull request Jul 7, 2021

[WIP] Fix serve constructor failure #16823

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jiaodong changed the title ~~[Ready for review] Better constructor failure handling in ray serve~~ [WIP] Better constructor failure handling in ray serve Jul 7, 2021

jiaodong added @author-action-required The PR author is responsible for the next step. Remove tag to send back to the reviewer. serve Ray Serve Related Issue labels Jul 7, 2021

jiaodong force-pushed the serve_constructor_new branch from 18dbab3 to 1211860 Compare July 29, 2021 00:44

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jiaodong added 2 commits August 23, 2021 21:04

working with all backend tests passing

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nits and address existing comment

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jiaodong added 4 commits August 25, 2021 16:42

a lot of progress on reverting async goal manager to event and almost…

f6c93e7

… running as expected

all test_deploy tests passed with right exception and revert

272bdfa

all tests passed

b06d724

q

nits

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jiaodong added tests-ok The tagger certifies test failures are unrelated and assumes personal liability. and removed @author-action-required The PR author is responsible for the next step. Remove tag to send back to the reviewer. labels Aug 26, 2021

jiaodong requested a review from edoakes August 26, 2021 06:06

jiaodong added 2 commits August 26, 2021 08:46

Merge branch 'master' into serve_constructor_new

f5592a5

rebase

nits

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split constructor tests into separate file, hide async goal event and…

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add clause to run tests

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edoakes merged commit c7e38ce into ray-project:master Aug 27, 2021

architkulkarni linked an issue Aug 28, 2021 that may be closed by this pull request

[Serve] Bad import in deployment __init__ causes deployment to restart forever #17823

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	Class to help ServeController to manage desirable states of replica.
	Class to help ServeController to manage desired states of replica.

	state (ReplicaState): Most recent state of replica by
	state (ReplicaStartingStatus): Most recent state of replica by

	class ReplicaStartingStatus(Enum):
	class ReplicaStartupStatus(Enum):

	start_status = self._actor.check_ready()
	status = self._actor.check_ready()

		logger.error("Reverting backend {backend_tag} to previous "
		f"backend info. {self._prev_backend_info}")

		logger.error("Reverting backend to previous backend "
		f"info by deleting backend {backend_tag}")

		RuntimeError(f"Deployment failed, deleting {backend_tag} "
		"asynchronously."))

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Better constructor failure handling in ray serve #16922

Better constructor failure handling in ray serve #16922

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Why are these changes needed?

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edoakes commented Jul 7, 2021

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jiaodong commented Jul 7, 2021 •

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