Capistrano with PostgreSQL, error: database is being accessed by other users

Question

I have a Rails app that uses PostgreSQL as a backend with a cert environment that tries to mimic production, except that it needs to have the database reset periodically for QA.

When I attempt to execute db:reset from a Capistrano task during deployment I get the error:

ERROR: database "database_name" is being accessed by other users

and the database cannot be dropped as part of the reset task resulting in deployment failing. Is there a way I can reset database connections from Capistrano so I can successfully drop the table? Piping the SQL to psql from a Capistrano task might work but I was wondering if there was a better way to go about this.

Answer 1

With PostgreSQL you can issue the following statement to return the backend pids of all open connections other than then this one:

SELECT pid FROM pg_stat_activity where pid <> pg_backend_pid(); 

Then you can issue a a termination request to each of those backends with

SELECT pg_terminate_backend($1); 

Binding the pids returned from the first statement to each pg_terminate_backend exec.

If the other connections are not using the same user as you, you will have to connect as a superuser to successfully issue the terminates.

• Admin signalling functions docs
• Monitoring stats functions
• pg_stat_activity view docs

UPDATE: Incorporating comments and expressing as Capistrano task:

desc "Force disconnect of open backends and drop database" task :force_close_and_drop_db do   dbname = 'your_database_name'   run "psql -U postgres",       :data => <<-"PSQL"          REVOKE CONNECT ON DATABASE #{dbname} FROM public;          ALTER DATABASE #{dbname} CONNECTION LIMIT 0;          SELECT pg_terminate_backend(pid)            FROM pg_stat_activity            WHERE pid <> pg_backend_pid()            AND datname='#{dbname}';          DROP DATABASE #{dbname};       PSQL end 

Answer 2

I have combined dbenhur's answer with this Capistrano task to achieve the result I needed works like a charm:

desc 'kill pgsql users so database can be dropped' task :kill_postgres_connections do   run 'echo "SELECT pg_terminate_backend(procpid) FROM pg_stat_activity WHERE datname=\'database_name\';" | psql -U postgres' end 

This assumes the auth_method for user postgres set to 'trust' in pg_hba.conf

Then you can just call it in your deploy task after update_code and before migrate

after 'deploy:update_code', 'kill_postgres_connections'