Re: Re: How to solve the problem of one backend process crashing and causing other processes to restart?
On Mon, Nov 13, 2023 at 3:14 AM yuansong 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 roadmap so 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 the current transaction without reconnecting? Perhaps this > problem can be mitigated through the use of a connection pool. > > If we want to implement this feature, would it be sufficient to clean up > or restore the shared memory and disk changes caused by the crashed > backend? Besides clearing various known locks, what else needs to be > changed? Does anyone have any insights? Please help me. Thank you. > One thing that's really key to understand about postgres is that there are a different set of rules regarding what is the database's job to solve vs supporting libraries and frameworks. It isn't that hard to wait and retry a query in most applications, and it is up to you to do that.There are also various connection poolers that might implement retry logic, and not having to work through those concerns keeps the code lean and has other benefits. While postgres might implement things like a built in connection pooler, 'o_direct' type memory management, and things like that, there are long term costs to doing them. There's another side to this. Suppose I had to choose between a hypothetical postgres that had some kind of process local crash recovery and the current implementation. I might still choose the current implementation because, in general, crashes are good, and the full reset has a much better chance of clearing the underlying issue that caused the problem vs managing the symptoms of it. merlin
Re: How to solve the problem of one backend process crashing and causing other processes to restart?
On 11/13/23 00:53, Laurenz Albe wrote: On Sun, 2023-11-12 at 21:55 -0500, Tom Lane wrote: yuansong 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
Re: Re: How to solve the problem of one backend process crashing and causing other processes to restart?
On Mon, Nov 13, 2023 at 5:14 PM yuansong 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 roadmap so 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 the current 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 changes caused by the crashed backend? > Besides clearing various known locks, what else needs to be changed? Does > anyone have any insights? Please help me. Thank you. > > > > > > > > At 2023-11-13 13:53:29, "Laurenz Albe" wrote: > >On Sun, 2023-11-12 at 21:55 -0500, Tom Lane wrote: > >> yuansong 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
Re: How to solve the problem of one backend process crashing and causing other processes to restart?
On Sun, 2023-11-12 at 21:55 -0500, Tom Lane wrote: > yuansong 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
Re: How to solve the problem of one backend process crashing and causing other processes to restart?
yuansong 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. regards, tom lane
