In postgres you can make a particular database or all databases of a postgres cluster.

default_transaction_read_only parameter controls whether database is in read only or read write .

default_transaction_read_only -> on ( for read only )
default_transaction_read_only -> off ( for read write )

For making single db read only.( DB_NAME – > edbstore)

–connect to database other than the edbstore ( here i connected to default postgres db)

[enterprisedb@localhost ~]$ psql -d postgres
psql (12.4.5)
Type "help" for help.

postgres=# \conninfo
You are connected to database "postgres" as user "enterprisedb" via socket in "/tmp" at port "5444".


postgres=# alter database edbstore set default_transaction_read_only=on;
ALTER DATABASE

— Now restart the postgres cluster(either using pg_ctl or service_name)

[root@localhost ~]# systemctl stop edb-as-12
[root@localhost ~]# systemctl start edb-as-12

— Now login to database and check:

edbstore=# \conninfo
You are connected to database "edbstore" as user "enterprisedb" via socket in "/tmp" at port "5444".
edbstore=# create table test as select  * from pg_settings;
ERROR:  cannot execute CREATE TABLE AS in a read-only transaction

We can see it is not allowing write operations.

Now if you wish to remove the database from read only mode, then

postgres=# \conninfo
You are connected to database "postgres" as user "enterprisedb" via socket in "/tmp" at port "5444".


postgres=# alter database edbstore set default_transaction_read_only=off;
ALTER DATABASE

[root@localhost ~]# systemctl stop edb-as-12
[root@localhost ~]# systemctl start edb-as-12

For making all the databases of the postgres cluster read only:

postgres=# show default_transaction_read_only;
 default_transaction_read_only 
-------------------------------
 off
(1 row)

postgres=# alter system set default_transaction_read_only=on;
ALTER SYSTEM

-- Restart the pg cluster:
[root@localhost ~]# systemctl stop edb-as-12
[root@localhost ~]# systemctl start edb-as-12