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From Benjamin Bannier <benjamin.bann...@mesosphere.io>
Subject Re: Discussion: Scheduler API for Operation Reconciliation
Date Wed, 16 Jan 2019 21:15:26 GMT
Hi,

have we reached a conclusion here?

From the Mesos side of things I would be strongly in favor of proposal (III). This is not
only consistent with what we do with task status updates, but also would allow us to provide
improved operation status (e.g., `OPERATION_UNREACHABLE` instead of just `OPERATION_UNKNOWN`
to better distinguish non-terminal from terminal operation states. To accomplish that we wouldn’t
need to introduce extra information leakage (e.g., explicitly keeping master up to date on
local resource provider state and associated internal consistency complications).

This approach should also simplify framework development as a framework would only need to
watch a single channel to see operation status updates (no need to reconcile different information
sources). The benefits of better status updates and simpler implementation IMO outweigh any
benefits of the current approach (disclaimer: I filed the slightly inflammatory MESOS-9448).

What is keeping us from moving forward with (III) at this point?


Cheers,

Benjamin

> On Jan 3, 2019, at 11:30 PM, Benno Evers <bevers@mesosphere.com> wrote:
> 
> Hi Chun-Hung,
> 
> > imagine that there are 1k nodes and 10 active + 10 gone LRPs per node, then the
master need to maintain 20k entries for LRPs.
> 
> How big would the required additional storage be in this scenario? Even if it's 1KiB
per LRP, using 20 MiB of extra memory doesn't sound too bad for such a big custer.
> 
> In general, it seems hard to discuss the trade-offs between your proposals without looking
at the users of that API - do you know if there are ayn frameworks out there that already
use
>  operation reconciliation, and if so what do they do based on the reconciliation response?
> 
> As far as I know, we don't have any formal guarantees on which operations status changes
the framework will receive without reconciliation. So putting on my framework-implementer
hat it seems like I'd have no choice but to implement a continously polling background loop
anyways if I care about knowing the latest operation statuses. If this is indeed the case,
having a synchronous `RECONCILE_OPERATIONS` would seem to have little additional benefit.
> 
> Best regards,
> Benno
> 
> On Wed, Dec 12, 2018 at 4:07 AM Chun-Hung Hsiao <chhsiao@apache.org> wrote:
> Hi folks,
> 
> Recently I've being discussing the problems of the current design of the
> experimental
> `RECONCILE_OPERATIONS` scheduler API with a couple people. The discussion
> was started
> from MESOS-9318 <https://issues.apache.org/jira/browse/MESOS-9318>: when a
> framework receives an `OPERATION_UNKNOWN`, it doesn't know
> if it should retry the operation or not (further details described below).
> As the discussion
> evolves, we realize there are more issues to consider, design-wise and
> implementation-wise, so
> I'd like to reach out to the community to get valuable opinions from you
> guys.
> 
> Before I jump right into the issues I'd like to discuss, let me fill you
> guys in with some
> background of operation reconciliation. Since the design of this feature
> was informed by the
> pre-existing implementation of task reconciliation, I'll begin there.
> 
> *Task Reconciliation: Design*
> 
> The scheduler API has a `RECONCILE` call for a framework to query the
> current statuses
> of its tasks. This call supports the following modes:
> 
>    - *Explicit reconciliation*: The framework specifies the list of tasks
>    it wants to know
>    about, and expects status updates for these tasks.
> 
>    - *Implicit reconciliation*: The framework does not specify a list of
>    tasks, and simply
>    expects status updates for all tasks the master knows about.
> 
> In both cases, the master looks into its in-memory task bookkeeping and
> sends
> *one or more`UPDATE` events* to respond to the reconciliation request.
> 
> *Task Reconciliation: Problems*
> 
> This API design of task reconciliation has the following shortcomings:
> 
>    - (1) There is no clear boundary of when the "reconciliation response"
>    ends, and thus
>    there is
> *no 1-1 correspondence between the reconciliation request and the response*.
>    For explicit reconciliation, the framework might wait for an extended period
>    of time before it receives all status updates; for implicit
>    reconciliation, there is no way for
>    a framework to tell if it has learned about all of its tasks, which
>    could be inconvenient if
>    the framework has lost its task bookkeeping.
> 
>    - (2) The "reconciliation response" may be outdated. If an agent
>    reregisters after a task
>    reconciliation has been responded,
> *the framework wouldn't learn about the tasks **from this recovered agent*.
>    Mesos relies on the framework to call the `RECONCILE` call
>    *periodically* to get up-to-date task statuses.
> 
> 
> 
> *Operation Reconciliation: Design & Problems*
> 
> When designing operation reconciliation, we made the `RECONCILE_OPERATIONS`
> call
> *asynchronous request-response style call* that returns a 200 OK with a
> list of operation status
> to avoid (1). However, this design does not resolve (2), and also
> introduces new problems:
> 
>    - (3) *The synchronous response could race with the event stream* and
>    the framework
>    does not know which contains the latest operation status.
> 
>    - (4) To ensure scalability, the master does not manage local resource
>    providers (LRPs);
>    the agents do. So the master cannot tell if an LRP is temporarily
>    unreachable/recovering
>    or permanently gone. As a result, if the framework explicitly reconciles
>    an LRP operation
>    that the master does not know about, it can only reply
>    `OPERATION_UNKNOWN`, but
>    then *the framework would not know if the operation would come back in
>    the future*,
>    and thus cannot decide if it should reissue another operation, which
>    leads to MESOS-9318 <https://issues.apache.org/jira/browse/MESOS-9318>.
> 
>    Note that this is less of a problem for explicit task reconciliation,
>    because in most cases
>    the master can infer task statuses from agent statuses, and in the rare
>    cases that it
>    replies `TASK_UNKNOWN`, it is generally safe for the framework to
>    relaunch another
>    task.
> 
> 
> *The Open Question*
> 
> Now, the big question here is:
> *are the benefits of a synchronous request-responsestyle
> `RECONCILE_OPERATIONS` call worth the complexity it introduces* in order to
> address (3) and (4) in the code? To explain what the complexity would be,
> let me lay out a
> couple proposals we've been discussing:
> 
> I. Keep `RECONCILE_OPERATIONS` synchronous
> 
> To address (3), we could add a *timestamp* to every operation status as
> well as the
> reconciliation response, so the framework can infer which one is the latest
> status, and if
> it receives a stale operation status update after the reconciliation
> response, it can just
> ack the status update without updating its bookkeeping. But, the framework
> needs to
> deal with a corner case:
> 
> *when it receives a reconciliation response containing aterminal operation
> status, it may or may not receive one or more status updatesfor that
> operation later *because of the race.
> 
> 
> To address (4), we could either: (a) surface the unreachable/gone LRPs to
> the master, or
> (b) forward the explicit reconciliation request to the corresponding agent.
> The complexity
> of (a) is that
> *it might not be scalable for the master to maintain the list ofunreachable
> and gone LRPs*: imagine that there are 1k nodes and 10 active + 10 gone
> LRPs per node, then the master need to maintain 20k entries for LRPs. The
> complexity
> of (b) is that the response wouldn't be computed based on the master's
> state; instead,
> *the master needs to wait for the agent's reply to respond to the framework*.
> Note
> that it's probably not scalable to forward implicit reconciliation requests
> to all agents, so
> implicit reconciliation might have to still be responded based on the
> master's state.
> 
> 
> II. Make `RECONCILE_OPERATIONS` "semi-synchronous"
> 
> Instead of returning a 200 OK, the master could return a 202 Accepted with
> an empty
> body, and then
> *reply a single event containing the operation status of all
> requestedoperations in the event stream asynchronously*. Although the
> framework loses the
> 1-1 correspondence between the request and the response, there's still a
> clear boundary
> for a reconciliation response. The advantage of this approach compared to
> proposal I is
> that we don't have a race between the reconciliation response and the event
> stream, so
> no timestamp is required. Still, we have to address (4) through either (a)
> or (b) described
> above, thus the complexity remains. That said, this approach fits with (b)
> better since no
> synchronous response is needed.
> 
> 
> III. Make `RECONCILE_OPERATIONS` an asynchronous trigger
> 
> This would be similar to what we have for task reconciliation. The master
> would return a
> 202 Accepted, and then send
> *one or more `UPDATE_OPERATION_STATUS` events*based on its state for an
> implicit reconciliation, or
> *forward the request to some agent*for an explicit reconciliation. In other
> words, this call plays the role of a trigger of the
> operation status updates. This approach is the simplest in terms of the
> implementation,
> but the trade-off is that the framework needs to live with (1).
> 
> 
> So far we haven't discussed much about (2) for operation reconciliation, so
> let's also briefly talk
> about it. Potentially (2) can be addressed by making the agent *actively
> push *
> *operation statusupdates to the framework when an LRP is resubscribed*, so
> the framework won't need to do
> periodic operation reconciliation. If we do this in the future, it would
> also be more aligned with
> proposal II or III.
> 
> So the question again: is it worth the complexity to keep
> `RECONCILE_OPERATIONS`
> synchronous? I'd like to hear the opinions from the community so we can
> drive towards a better
> API design!
> 
> Best,
> Chun-Hung
> 
> 
> -- 
> Benno Evers
> Software Engineer, Mesosphere


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