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Re: Rename a database that has connections
I've been helping out several customers recently who all seem to be wrestling with the same issue: wanting to update/refresh non-production databases from the latest corresponding prod version. Typically they have (fairly complex) scripts that at some point attempt to restore a dump into new database and then rename the to-be-retired db out of the way and rename the newly restored one to take over. In many cases such scripts would be simplified if a database could be renamed without requiring its connections terminated. I've been asked several times if this could be added... so I've caved in a done a patch that allows this. The default behavior is unchanged - it is required to specify an additional trailing FORCE keyword to elicit the more brutal behavior. Note that existing connections to the renamed database are unaffected, but obviously SELECT current_database() returns the new name (in the next transaction). regards Mark
Attachment: rename-force.patch.1
Description: text/plain (7.2 KB)
Description: text/plain (7.2 KB)
Mark Kirkwood wrote: > I've been helping out several customers recently who all seem to be > wrestling with the same issue: wanting to update/refresh non-production > databases from the latest corresponding prod version. Typically they > have (fairly complex) scripts that at some point attempt to restore a > dump into new database and then rename the to-be-retired db out of the > way and rename the newly restored one to take over. > > In many cases such scripts would be simplified if a database could be > renamed without requiring its connections terminated. I've been asked > several times if this could be added... so I've caved in a done a patch > that allows this. > > The default behavior is unchanged - it is required to specify an > additional trailing FORCE keyword to elicit the more brutal behavior. > Note that existing connections to the renamed database are unaffected, > but obviously SELECT current_database() returns the new name (in the > next transaction). Uh, it isn't save to copy a database when someone else is connected. How does this address that issue? -- Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us EnterpriseDB http://enterprisedb.com + It's impossible for everything to be true. +
Mark Kirkwood <mark(dot)kirkwood(at)catalyst(dot)net(dot)nz> writes: > I've been helping out several customers recently who all seem to be > wrestling with the same issue: wanting to update/refresh non-production > databases from the latest corresponding prod version. Typically they > have (fairly complex) scripts that at some point attempt to restore a > dump into new database and then rename the to-be-retired db out of the > way and rename the newly restored one to take over. > In many cases such scripts would be simplified if a database could be > renamed without requiring its connections terminated. I've been asked > several times if this could be added... so I've caved in a done a patch > that allows this. > The default behavior is unchanged - it is required to specify an > additional trailing FORCE keyword to elicit the more brutal behavior. > Note that existing connections to the renamed database are unaffected, > but obviously SELECT current_database() returns the new name (in the > next transaction). This patch seems to me to be pretty thoroughly misguided. Either renaming a database with open connections is safe, or it isn't. If it is safe, we should just allow it. If it isn't, making people write an extra FORCE keyword does not make it safe. It's particularly silly to allow someone to rename the database out from under other sessions (which won't know what happened) but not rename it out from under his own session (which would or at least could know it). What you need to be doing is investigating whether the comments about this in RenameDatabase() are really valid concerns or not. regards, tom lane
On 22/11/11 16:38, Bruce Momjian wrote: > Mark Kirkwood wrote: >> I've been helping out several customers recently who all seem to be >> wrestling with the same issue: wanting to update/refresh non-production >> databases from the latest corresponding prod version. Typically they >> have (fairly complex) scripts that at some point attempt to restore a >> dump into new database and then rename the to-be-retired db out of the >> way and rename the newly restored one to take over. >> >> In many cases such scripts would be simplified if a database could be >> renamed without requiring its connections terminated. I've been asked >> several times if this could be added... so I've caved in a done a patch >> that allows this. >> >> The default behavior is unchanged - it is required to specify an >> additional trailing FORCE keyword to elicit the more brutal behavior. >> Note that existing connections to the renamed database are unaffected, >> but obviously SELECT current_database() returns the new name (in the >> next transaction). > Uh, it isn't save to copy a database when someone else is connected. > How does this address that issue? > Copying a database is quite a different matter (compare with copying an open unix file vs mv'ing it... the latter is quite safe as the inode does not change). regards Mark
Mark Kirkwood wrote: > On 22/11/11 16:38, Bruce Momjian wrote: > > Mark Kirkwood wrote: > >> I've been helping out several customers recently who all seem to be > >> wrestling with the same issue: wanting to update/refresh non-production > >> databases from the latest corresponding prod version. Typically they > >> have (fairly complex) scripts that at some point attempt to restore a > >> dump into new database and then rename the to-be-retired db out of the > >> way and rename the newly restored one to take over. > >> > >> In many cases such scripts would be simplified if a database could be > >> renamed without requiring its connections terminated. I've been asked > >> several times if this could be added... so I've caved in a done a patch > >> that allows this. > >> > >> The default behavior is unchanged - it is required to specify an > >> additional trailing FORCE keyword to elicit the more brutal behavior. > >> Note that existing connections to the renamed database are unaffected, > >> but obviously SELECT current_database() returns the new name (in the > >> next transaction). > > Uh, it isn't save to copy a database when someone else is connected. > > How does this address that issue? > > > > Copying a database is quite a different matter (compare with copying an > open unix file vs mv'ing it... the latter is quite safe as the inode > does not change). Oh, I see, you are just renaming. Well, Tom is right that either it is safe, or it isn't --- a 'force' flag makes no sense. -- Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us EnterpriseDB http://enterprisedb.com + It's impossible for everything to be true. +
On 22/11/11 16:41, Tom Lane wrote: > Mark Kirkwood<mark(dot)kirkwood(at)catalyst(dot)net(dot)nz> writes: >> I've been helping out several customers recently who all seem to be >> wrestling with the same issue: wanting to update/refresh non-production >> databases from the latest corresponding prod version. Typically they >> have (fairly complex) scripts that at some point attempt to restore a >> dump into new database and then rename the to-be-retired db out of the >> way and rename the newly restored one to take over. >> In many cases such scripts would be simplified if a database could be >> renamed without requiring its connections terminated. I've been asked >> several times if this could be added... so I've caved in a done a patch >> that allows this. >> The default behavior is unchanged - it is required to specify an >> additional trailing FORCE keyword to elicit the more brutal behavior. >> Note that existing connections to the renamed database are unaffected, >> but obviously SELECT current_database() returns the new name (in the >> next transaction). > This patch seems to me to be pretty thoroughly misguided. Either > renaming a database with open connections is safe, or it isn't. If it > is safe, we should just allow it. If it isn't, making people write an > extra FORCE keyword does not make it safe. It's particularly silly > to allow someone to rename the database out from under other sessions > (which won't know what happened) but not rename it out from under his > own session (which would or at least could know it). > > What you need to be doing is investigating whether the comments about > this in RenameDatabase() are really valid concerns or not. > The reason I added FORCE was to preserve backwards compatibility - for any people out there that like the way it behaves right now. I am certainly willing to be convinced that such a concern is unneeded. You are quite right about the patch being inconsistent with respect to the renaming the current database, it should allow that too (will change if this overall approach makes sense). With respect to the concerns in RenameDatabase(), that seems to boil down to applications stashing the current dbname somewhere and caring about it. This was not viewed as a issue by any of the folks who I talked to about this (they are all application developers/architects etc so they understand that issue). However there may well be application frameworks out there that do care... which seemed to me to be another reason for making the forced rename require an extra keyword. I have not been able to find any other problems caused by this... renaming a db (many times) with hundreds of pgbench connections does not give rise to any issues. regards Mark
On 22/11/11 17:24, Mark Kirkwood wrote: > > I have not been able to find any other problems caused by this... > renaming a db (many times) with hundreds of pgbench connections does > not give rise to any issues. > > One point I did miss - the ps listing still uses the old dbname. pg_stat_activity is correct however. regards Mark
On 22/11/11 17:24, Mark Kirkwood wrote: > On 22/11/11 16:41, Tom Lane wrote: >> Mark Kirkwood<mark(dot)kirkwood(at)catalyst(dot)net(dot)nz> writes: >>> I've been helping out several customers recently who all seem to be >>> wrestling with the same issue: wanting to update/refresh non-production >>> databases from the latest corresponding prod version. Typically they >>> have (fairly complex) scripts that at some point attempt to restore a >>> dump into new database and then rename the to-be-retired db out of the >>> way and rename the newly restored one to take over. >>> In many cases such scripts would be simplified if a database could be >>> renamed without requiring its connections terminated. I've been asked >>> several times if this could be added... so I've caved in a done a patch >>> that allows this. >>> The default behavior is unchanged - it is required to specify an >>> additional trailing FORCE keyword to elicit the more brutal behavior. >>> Note that existing connections to the renamed database are unaffected, >>> but obviously SELECT current_database() returns the new name (in the >>> next transaction). >> This patch seems to me to be pretty thoroughly misguided. Either >> renaming a database with open connections is safe, or it isn't. If it >> is safe, we should just allow it. If it isn't, making people write an >> extra FORCE keyword does not make it safe. It's particularly silly >> to allow someone to rename the database out from under other sessions >> (which won't know what happened) but not rename it out from under his >> own session (which would or at least could know it). >> >> What you need to be doing is investigating whether the comments about >> this in RenameDatabase() are really valid concerns or not. >> > > The reason I added FORCE was to preserve backwards compatibility - for > any people out there that like the way it behaves right now. I am > certainly willing to be convinced that such a concern is unneeded. > > You are quite right about the patch being inconsistent with respect to > the renaming the current database, it should allow that too (will > change if this overall approach makes sense). > > With respect to the concerns in RenameDatabase(), that seems to boil > down to applications stashing the current dbname somewhere and caring > about it. This was not viewed as a issue by any of the folks who I > talked to about this (they are all application developers/architects > etc so they understand that issue). However there may well be > application frameworks out there that do care... which seemed to me to > be another reason for making the forced rename require an extra keyword. > > I have not been able to find any other problems caused by this... > renaming a db (many times) with hundreds of pgbench connections does > not give rise to any issues. > Minor change to be allow current database to be renamed as well if FORCE is used, which makes more sense. Cheers Mark
Attachment: rename-force.patch.2
Description: text/plain (8.5 KB)
Description: text/plain (8.5 KB)