Thread: How to solve the problem of one backend process crashing and causing other processes to restart?

yuansong <yyuansong@126.com> writes:
> In PostgreSQL, when a backend process crashes, it can cause other backend processes to also require a restart,
primarilyto ensure data consistency. I understand that the correct approach is to analyze and identify the cause of the
crashand resolve it. However, it is also important to be able to handle a backend process crash without affecting the
operationof other processes, thus minimizing the scope of negative impact and improving availability. To achieve this
goal,could we mimic the Oracle process by introducing a "pmon" process dedicated to rolling back crashed process
transactionsand performing resource cleanup? I wonder if anyone has attempted such a strategy or if there have been
previousdiscussions on this topic. 

The reason we force a database-wide restart is that there's no way to
be certain that the crashed process didn't corrupt anything in shared
memory.  (Even with the forced restart, there's a window where bad
data could reach disk before we kill off the other processes that
might write it.  But at least it's a short window.)  "Corruption"
here doesn't just involve bad data placed into disk buffers; more
often it's things like unreleased locks, which would block other
processes indefinitely.

I seriously doubt that anything like what you're describing
could be made reliable enough to be acceptable.  "Oracle does
it like this" isn't a counter-argument: they have a much different
(and non-extensible) architecture, and they also have an army of
programmers to deal with minutiae like undoing resource acquisition.
Even with that, you'd have to wonder about the number of bugs
existing in such necessarily-poorly-tested code paths.

            regards, tom lane

On Sun, 2023-11-12 at 21:55 -0500, Tom Lane wrote:
> yuansong <yyuansong@126.com> writes:
> > In PostgreSQL, when a backend process crashes, it can cause other backend
> > processes to also require a restart, primarily to ensure data consistency.
> > I understand that the correct approach is to analyze and identify the
> > cause of the crash and resolve it. However, it is also important to be
> > able to handle a backend process crash without affecting the operation of
> > other processes, thus minimizing the scope of negative impact and
> > improving availability. To achieve this goal, could we mimic the Oracle
> > process by introducing a "pmon" process dedicated to rolling back crashed
> > process transactions and performing resource cleanup? I wonder if anyone
> > has attempted such a strategy or if there have been previous discussions
> > on this topic.
>
> The reason we force a database-wide restart is that there's no way to
> be certain that the crashed process didn't corrupt anything in shared
> memory.  (Even with the forced restart, there's a window where bad
> data could reach disk before we kill off the other processes that
> might write it.  But at least it's a short window.)  "Corruption"
> here doesn't just involve bad data placed into disk buffers; more
> often it's things like unreleased locks, which would block other
> processes indefinitely.
>
> I seriously doubt that anything like what you're describing
> could be made reliable enough to be acceptable.  "Oracle does
> it like this" isn't a counter-argument: they have a much different
> (and non-extensible) architecture, and they also have an army of
> programmers to deal with minutiae like undoing resource acquisition.
> Even with that, you'd have to wonder about the number of bugs
> existing in such necessarily-poorly-tested code paths.

Yes.
I think that PostgreSQL's approach is superior: rather than investing in
code to mitigate the impact of data corruption caused by a crash, invest
in quality code that doesn't crash in the first place.

Euphemistically naming a crash "ORA-600 error" seems to be part of
their strategy.

Yours,
Laurenz Albe

On Mon, Nov 13, 2023 at 5:14 PM yuansong <yyuansong@126.com> wrote:
>
> Enhancing the overall fault tolerance of the entire system for this feature is quite important. No one can avoid
bugs,and I don't believe that this approach is a more advanced one. It might be worth considering adding it to the
roadmapso that interested parties can conduct relevant research. 
>
> The current major issue is that when one process crashes, resetting all connections has a significant impact on other
connections.Is it possible to only disconnect the crashed connection and have the other connections simply roll back
thecurrent transaction without reconnecting? Perhaps this problem can be mitigated through the use of a connection
pool.

It's not about the other connections, it's that the crashed connection
has no way to rollback.

>
> If we want to implement this feature, would it be sufficient to clean up or restore the shared memory and disk
changescaused by the crashed backend? Besides clearing various known locks, what else needs to be changed? Does anyone
haveany insights? Please help me. Thank you. 
>
>
>
>
>
>
>
> At 2023-11-13 13:53:29, "Laurenz Albe" <laurenz.albe@cybertec.at> wrote:
> >On Sun, 2023-11-12 at 21:55 -0500, Tom Lane wrote:
> >> yuansong <yyuansong@126.com> writes:
> >> > In PostgreSQL, when a backend process crashes, it can cause other backend
> >> > processes to also require a restart, primarily to ensure data consistency.
> >> > I understand that the correct approach is to analyze and identify the
> >> > cause of the crash and resolve it. However, it is also important to be
> >> > able to handle a backend process crash without affecting the operation of
> >> > other processes, thus minimizing the scope of negative impact and
> >> > improving availability. To achieve this goal, could we mimic the Oracle
> >> > process by introducing a "pmon" process dedicated to rolling back crashed
> >> > process transactions and performing resource cleanup? I wonder if anyone
> >> > has attempted such a strategy or if there have been previous discussions
> >> > on this topic.
> >>
> >> The reason we force a database-wide restart is that there's no way to
> >> be certain that the crashed process didn't corrupt anything in shared
> >> memory.  (Even with the forced restart, there's a window where bad
> >> data could reach disk before we kill off the other processes that
> >> might write it.  But at least it's a short window.)  "Corruption"
> >> here doesn't just involve bad data placed into disk buffers; more
> >> often it's things like unreleased locks, which would block other
> >> processes indefinitely.
> >>
> >> I seriously doubt that anything like what you're describing
> >> could be made reliable enough to be acceptable.  "Oracle does
> >> it like this" isn't a counter-argument: they have a much different
> >> (and non-extensible) architecture, and they also have an army of
> >> programmers to deal with minutiae like undoing resource acquisition.
> >> Even with that, you'd have to wonder about the number of bugs
> >> existing in such necessarily-poorly-tested code paths.
> >
> >Yes.
> >I think that PostgreSQL's approach is superior: rather than investing in
> >code to mitigate the impact of data corruption caused by a crash, invest
> >in quality code that doesn't crash in the first place.
> >
> >Euphemistically naming a crash "ORA-600 error" seems to be part of
> >their strategy.
> >
> >Yours,
> >Laurenz Albe
> >



--
Regards
Junwang Zhao

On 11/13/23 00:53, Laurenz Albe wrote:
> On Sun, 2023-11-12 at 21:55 -0500, Tom Lane wrote:
>> yuansong <yyuansong@126.com> writes:
>> > In PostgreSQL, when a backend process crashes, it can cause other backend
>> > processes to also require a restart, primarily to ensure data consistency.
>> > I understand that the correct approach is to analyze and identify the
>> > cause of the crash and resolve it. However, it is also important to be
>> > able to handle a backend process crash without affecting the operation of
>> > other processes, thus minimizing the scope of negative impact and
>> > improving availability. To achieve this goal, could we mimic the Oracle
>> > process by introducing a "pmon" process dedicated to rolling back crashed
>> > process transactions and performing resource cleanup? I wonder if anyone
>> > has attempted such a strategy or if there have been previous discussions
>> > on this topic.
>> 
>> The reason we force a database-wide restart is that there's no way to
>> be certain that the crashed process didn't corrupt anything in shared
>> memory.  (Even with the forced restart, there's a window where bad
>> data could reach disk before we kill off the other processes that
>> might write it.  But at least it's a short window.)  "Corruption"
>> here doesn't just involve bad data placed into disk buffers; more
>> often it's things like unreleased locks, which would block other
>> processes indefinitely.
>> 
>> I seriously doubt that anything like what you're describing
>> could be made reliable enough to be acceptable.  "Oracle does
>> it like this" isn't a counter-argument: they have a much different
>> (and non-extensible) architecture, and they also have an army of
>> programmers to deal with minutiae like undoing resource acquisition.
>> Even with that, you'd have to wonder about the number of bugs
>> existing in such necessarily-poorly-tested code paths.
> 
> Yes.
> I think that PostgreSQL's approach is superior: rather than investing in
> code to mitigate the impact of data corruption caused by a crash, invest
> in quality code that doesn't crash in the first place.


While true, this does nothing to prevent OOM kills, which are becoming 
more prevalent as, for example, running Postgres in a container (or 
otherwise) with a cgroup memory limit becomes more popular.

And in any case, there are enterprise use cases that necessarily avoid 
Postgres due to this behavior, which is a shame.

-- 
Joe Conway
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com