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From Chun-Hung Hsiao <chhs...@mesosphere.io>
Subject Re: Follow up to discussion regarding use : in paths on Windows (MESOS-9109)
Date Thu, 23 Aug 2018 22:34:28 GMT
I'm a bit concerned about the recovery logic and backward compatibility:
The changes we're making shouldn't affect existing users,
and we should try hard to avoid any future backward compatibility problem.

Say if there is already some custom framework using task ID 'Hello%3AWorld',
then if we blindly decode the task path during recovery, we will get the
wrong ID 'Hello:World'.
On the other hand, if we don't decode the task path during recovery,
then later on during checkpointing for the same task,
we shouldn't blindly encode the task ID, because it might create a
different path,
and we'll need to introduce some migration code to avoid such duplication.

Fortunately, we do checkpoint the executor IDs and task IDs in the info
files under the meta dir.
So I'm considering the following design to minimize the backward
compatibility issue we might have:
During recovery, we don't decode the recovered task path,
but get the executor/task ID from the info file instead of relying on
parsing the executor/task path.
When checkpointing, we only encode executor/task IDs if they contain
reserved characters.
The set of reserved characters could be defined as a platform-dependent
variable,
similar to what we have done for `PATH_SEPARATOR`.

The above design would look a bit more complicated then just blindly
applying percent encoding
to when constructing checkpoint paths, but it doesn't require extra
checkpoint migration logic,
and would keep the exact same behavior we have now for "normal"
executor/task IDs.

What did you guys think? Please feel free to raise any concern :)
And we don't need to implement the whole thing for now.
For example, we could start with just dealing with colons,
and extend the implementation later on,
as long as the partial solution we're going to have right now doesn't
create future tech debts!

Best,
Chun-Hung

On Thu, Aug 23, 2018 at 1:42 PM Greg Mann <greg@mesosphere.io> wrote:

> Thanks Andy! Responses inlined below.
>
>
>
>> No: As the only character we've run into a problem with is `:`
>> (MESOS-9109), it might not be worth it to generalize this to solve a bunch
>> of problems that we haven't encountered.
>>
>>
> It's true that I'm not aware of other scenarios where
> filesystem-disallowed characters in task/executor IDs have caused issues
> for users, and this issue has existed for a long time. However, when
> feasible I would like to fix issues that we're aware of before they cause
> problems for users, rather than after. I would suggest that since we have
> one compelling case that we need to address now, it's worth formulating an
> approach for the general case, so that we can be sure any current work
> doesn't get in our way later on.
>
>
>> I'm somewhat comfortable doing so only for Windows, as we don't really
>> need to worry about the recovery scenario; but very uncomfortable about
>> doing so for Linux etc., for precisely that reason.
>>
>> So expanding this is definitely up for debate; but we must fix the bug
>> with `:`.
>>
>>
> Indeed, addressing the general case may prove to be much more complex - I
> can certainly identify with this situation, where a fix for a smaller issue
> turns into a big project :)
> It may turn out to be possible to implement a scoped-down solution for the
> colon case now, and extend it later on. I think it would be good if we
> could at least get an idea of how we want to handle the general case now,
> so that any short-term solutions can be a constructive step toward the
> long-term.
>
> Cheers,
> G
>

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