Re: Stopping writes in master - Mailing list pgsql-admin

Rui DeSousa

Thursday, June 14, 2018 11:42 AM

I did think about it after I hit sent; but it really doing to depend on if they setup all permissions using roles; which is good practice to do.

Did you just mean to send this to me? You should suggest it to thread as an option.

MichaelDBA

Thursday, June 14, 2018 11:38 AM

I wonder why nobody suggested access control changes to accomplish the read only.  That is change permissions on some or all users that can log into the database and adjust them accordingly.

Regards,
Michael Vitale

Rui DeSousa

Thursday, June 14, 2018 11:20 AM

I can think of two other ways but the latter is more a pause than read-only. Neither are ideal solutions but hacks that may work depending on your needs. 

1. If you’re restarting the server; then just make it a replia of nobody.

Con: 

1. Requires database restart which could take a while depending on database activity

2. Truly is read only… i.e. no temp tables, etc.

 i.e. Create a recovery.conf and restart the server… the database is truly read only.

recovery.conf:

standby_mode = 'on'

primary_conninfo = ‘host=nowhere'

2. Without restarting server; enable synchronous replication to nowhere and reload config.

Cons: 

1. More of a pause than read-only

2. Clients can still write changes but will hang on commit (causing applications to hang).

3. Ctrl-c on a hanged session will actually commit the record on the master (oops)

Pro or Con depending on your view: 

1. When releasing the synchronous setup to nowhere; hung sessions will carry on with their commit.

postgres.conf:

synchronous_standby_names = ‘1 (nowhere)’

If you trust your clients; then setting the default transaction state and killing the sessions forcing a reconnect might be the best solution like has already been suggested; however, if you dealing with end users they can change their session transaction level back.

Debraj Manna

Thursday, June 14, 2018 9:46 AM

Thanks Shreeyansh.  

Can this be handled from the server side may be setting some config parameters and then restarting the master so that existing connections gets dropped and the new ones not able to write?

Shreeyansh Dba

Thursday, June 14, 2018 8:06 AM

Hi Debraj Manna,

Yes,You can make  the database read only as well  just by setting the below parameter is equal to true at the session level.But you need reconnect your existing connections.

ALTER DATABASE YOUR_DATABASE_NAME  SET default_transaction_read_only = true;

Hope this helps..