On Tuesday, October 27, 2015 at 11:54:40 AM UTC-7, Janko Marohnić wrote:
>
> Hey Jeremy,
>
> I'm trying to do a column rename in production, and I want to have zero
> downtime, so I want to somehow write to the original column, and then when
> I run the migration I want to start writing to the renamed column. However,
> if I run the migration, the schema won't be updated. Is it somehow possible
> to refresh the schema inside the method? Or just `Sequel::Model.column`
> would also be great. I thought it would be good to do something like this:
>
> class Company
> def similar_domains
> refresh_schema
> column = columns.grep(/similar_domains/)
> send(column)
> end
>
> def similar_domains=(value)
> refresh_schema
> column = columns.grep(/similar_domains/)
> send("#{column}=", value)
> end
> end
>
> I'm renaming `similar_domains_array` to `similar_domains` (the rest of the
> production code is currently referencing `smilar_domains`, which I'm
> currently delegating to `similar_domain_array`).
>
You should do zero downtime migrations in the following manner:
1) Modify the code so that it will work both with the old schema and the
new schema
2) Modify the schema (run the migration)
3) Remove support in the code for running with the old schema.
This should not require refreshing the schema inside a running application.
I think that's probably impossible to do without a race condition.
For renaming of a column, on PostgreSQL, one way to do a zero downtime
rename is:
1) Add a new similar_domains column via a migration
2) Have your application write changes to both the similar_domains and
similar_domains_array column
3) Run a migration that syncs similar_domains_array to similar_domains in
the cases where they differ
4) Modify your code to only write to similar_domains
5) Remove similar domains_array
Alternatively, you may be able to use a view instead of a table:
1) Add a view for the table that aliases similar_domains array to
similar_domains.
2) Have your application use the view instead of the table
3) Run a migration that renames the similar_domains_array to
similar_domains in the table, and updates the view to match.
I'm not sure if you can add a rule that will allow for a zero downtime
migration; I don't have much experience with rules.
Thanks,
Jeremy
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