Django multiple databases and "cannot execute in a read-only transaction" error

Question

I used the Django docs to set up multiple databases in a master-slave configuration. My routers are set up correctly, and I can see that reads and write go to the correct respective database, but I am running into an error with some of my API calls:

cannot execute [UPDATE or DELETE] in a read-only transaction

In the API handlers where this happens, I am updating or deleting some other model as part of the same call, and Django seems to want to use a slave database for those transactions instead of master.

A workaround I found is to chain .using('default') before update() or delete(), which basically forces Django to use the master database for the update or delete. But this feels hacky and bandaid-ish, so I want to make sure there isn't a better way to do this.

Any help is much appreciated.

Have same issue. This django master-slave configuration is not for production use. In real life this configuration gots many issues. — Alexandr Shutko
– Alexandr Shutko, Commented Jul 27, 2021 at 3:36
It looks like this is a related manager bug - it allways select read database for all queries — Alexandr Shutko
– Alexandr Shutko, Commented Jul 27, 2021 at 4:24

Medhat Gayed · Accepted Answer · 2014-03-06 19:45:04Z

2

Have you tried the following? This was copied out of the Django docs page:

import random

class MasterSlaveRouter(object):
    def db_for_read(self, model, **hints):
        """
        Reads go to a randomly-chosen slave.
        """
        return random.choice(['slave1', 'slave2'])

    def db_for_write(self, model, **hints):
        """
        Writes always go to master.
        """
        return 'master'

    def allow_relation(self, obj1, obj2, **hints):
        """
        Relations between objects are allowed if both objects are
        in the master/slave pool.
        """
        db_list = ('master', 'slave1', 'slave2')
        if obj1._state.db in db_list and obj2._state.db in db_list:
            return True
        return None

    def allow_migrate(self, db, model):
        """
        All non-auth models end up in this pool.
        """
        return True

Finally, in the settings file, we add the following (substituting path.to. with the actual python path to the module(s) where the routers are defined):

DATABASE_ROUTERS = ['path.to.AuthRouter', 'path.to.MasterSlaveRouter']

answered Mar 6, 2014 at 19:45

Medhat Gayed

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2 Comments

Gady Over a year ago

Yes, I have set up the routers correctly. I'll update my question to say that.

Alexandr Shutko Over a year ago

Ok. I transferred a large project to cluster with write and read db replicas (amazon aurora rds). I added routers with db_for_read and db_for_write. I fixed MANY bugs related "object created in master but select in next view return doesn't exist" because read replicas has lags. But. Now I got error "cannot execute DELETE in a read-only transaction" on instance.binds.all().delete() query. Django routers doesn't work as expected...

Gady · Accepted Answer · 2014-04-15 00:02:30Z

0

It looks like there is no way to do this without sprinkling the .using('default') around in code.

answered Apr 15, 2014 at 0:02

Gady

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Alexandr Shutko · Accepted Answer · 2021-07-27 23:51:10Z

0

Got this too and posted a ticket for django developers. Here it is: https://code.djangoproject.com/ticket/32965

There was an answer. There is django documentation saying .delete() and .update() use same database (allways readonly!) that was used to retrieve the object. So this is not a bug, but future because they say it documented.

So there is two ways to solve this problem:

Add using('xxxx') before every .delete() and .update()
Replace .delete() by .clear() and .update() by .set()

answered Jul 27, 2021 at 23:51

Alexandr Shutko

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