No downtime rolling upgrades with migration

[Sandip Bhattacharya]

New to django, so wanted to ask some questions that I didn’t find good resources for:

1. If you deploy django apps using docker, do you run migrations before launching gunicorn/uwsgi etc within the docker image?

2. If you do, how does rolling migrations work? Won’t the non-upgraded replicas start erroring if the new schema is not backward compatible?

3. How do you do non-backward compatible schema upgrades? Do you do it out of band from deployments? Do you shift traffic to a different cluster, and then zero traffic upgrade the whole cluster at once?

Thanks,
Sandip

[Damanjeet Singh]

Hello,

Please read my reply below.

Best of luck.

Regards

On Sun, 26 Mar 2023, 21:17 Sandip Bhattacharya, <san...@showmethesource.org> wrote:

New to django, so wanted to ask some questions that I didn’t find good resources for:

1. If you deploy django apps using docker, do you run migrations before launching gunicorn/uwsgi etc within the docker image?

Daman:This is all upto you. Better to do when creating container. As you can also upgrade from git. 

2. If you do, how does rolling migrations work? Won’t the non-upgraded replicas start erroring if the new schema is not backward compatible?

Daman:Create blue green deployment.

3. How do you do non-backward compatible schema upgrades? Do you do it out of band from deployments? Do you shift traffic to a different cluster, and then zero traffic upgrade the whole cluster at once?

Daman: Blue Green deployment can help. You can distribute traffic to old and new. When all stable then move everything. You can use kubernetes with helm.

Thanks,
  Sandip

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[Sandip Bhattacharya]

On Mar 26, 2023, at 4:32 PM, Damanjeet Singh <mailt...@gmail.com> wrote:

3. How do you do non-backward compatible schema upgrades? Do you do it out of band from deployments? Do you shift traffic to a different cluster, and then zero traffic upgrade the whole cluster at once?

Daman: Blue Green deployment can help. You can distribute traffic to old and new. When all stable then move everything. You can use kubernetes with helm.

Do you do blue-green deployments with two different databases? 

- Sandip

[Damanjeet Singh]

Depending on the requirements. Either two different databases or point to one database.

Or

Link one database to master (read and write)

Another database instance to read only.

And you can change Another database to main.

Cheers 

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[Sandip Bhattacharya]

On Mar 26, 2023, at 6:47 PM, Damanjeet Singh <mailt...@gmail.com> wrote:

Depending on the requirements. Either two different databases or point to one database.

Or

Link one database to master (read and write)

Another database instance to read only.

And you can change Another database to main.

Thanks for your patience in answering my doubts. But I still have some follow up questions.

If it is the same database, and the migration changes the schema, won’t the older application code instances have the same problem with backward incompatible schema changes?

And if it is two databases, how does it work? If you set the older database to read-only, then traffic on the write path of the older application will break. If you somehow connect the two in sync, won’t the DDL changes on the primary replicate to the other replica as well, again causing the older application to break?

- Sandip