Would it be possible to have multiple database connection pools in rails to switch between?

A little background

I have been using the Apartment gem for running a multi-tenancy app for years. Now recently the need to scale the database out into separate hosts has arrived, the db server simply can't keep up any more (both reads and writes are getting too too much) - and yes, I scaled the hardware to the max (dedicated hardware, 64 cores, 12 Nvm-e drives in raid 10, 384Gb ram etc.).

I was considering doing this per-tenant (1 tenant = 1 database connection config / pool) as that would be a "simple" and efficient way to get up to number-of-tenants-times more capacity without doing loads of application code changes.

Now, I am running rails 4.2 atm., soon upgrading to 5.2. I can see that rails 6 adds support for a per-model connection definitions, however that is not really what I need, as I have a completely mirrored database schema for each of my 20 tenants. Typically I switch "database" per request (in middleware) or per background job (sidekiq middleware), however this is currently trivial and handled ny the Apartment gem, as it just sets the search_path in Postgresql and does not really change the actual connection. When switching to a per-tenant hosting strategy I will need to switch the entire connection per request.

Questions:

1. I understand that I could do an ActiveRecord::Base.establish_connection(config) per request / background job - however, as I also understand, that triggers an entirely new database connection handshake to be made and a new db pool to spawn in rails - right? I guess that would be a performance suicide to make that kind of overhead on every single request to my application.
2. I am therefore wondering if anyone can see the option with rails of e.g. pre-establishing multiple (total of 20) database connections/pools from the beginning (e.g. on boot of the application), and then just switch between those pools per request? So that he db connections are already made and ready to be used.
3. Is all this just a poor poor idea, and should I instead look for a different approach? E.g. 1 app instance = one specific connection to one specific tenant. Or something else.

As I understand, there are 4 pattern for multi-tenancy app:

1. Dedicated model/Multiple Production Environments

Each instance or database instance entirely host different tenant application and nothing is shared among tenants.

This is 1 instance app and 1 database for 1 tenant. The development would be easy as if you serve 1 tenant only. But will be nightmare for devops if you have, say, 100 tenants.

2. Physical Segregation of Tenants

1 instance app for all tenant but 1 database for 1 tenant. This is what you are searching for. You can use ActiveRecord::Base.establish_connection(config), or using gems, or update to Rails 6 as other suggests. See the answer for (2) below.

3. Isolated schema model/Logical Segregations

In an Isolated Schema, the tenant tables or database components are group under a logical schema or name-space and separated from other tenant schemas, however the schema are hosted in the same database instance.

1 instance app and 1 database for all tenant, like you do with apartment gem.

4. Partially Isolated Component

In this model, components that have common functionalities are shared among tenants while components with unique or unrelated functions are isolated. At the data layer, common data such as data that identify tenants are grouped or kept in single table while tenant specific data are isolated at table or instance layer.

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As for (1), ActiveRecord::Base.establish_connection(config) not handshaking to db per request if you use it correctly. You can check here and read all the comment here.

As for (2), If you don't want to use establish_connection, you can use gem multiverse (it works for rails 4.2), or other gems. Or, as other suggest, you can update to Rails 6.

Edit: Multiverse gem is using establish_connection. It will append the database.yml, and create a base class so that each subclass shares the same connection/pool. Basically, it reducing our effort to use establish_connection.

As for (3), the answer:

If you don't have so many tenants, and your application is pretty complex, I suggest you use Dedicated Model pattern. So you go for 1 app instance = one specific connection to one specific tenant. You don't have to make your apps more complex by adding multiple database connections.

But if you have many tenants, I suggest you use Physical Segregation of Tenants or Partially Isolated Component depends on your business process.

Either way, you have to update/rewrite your application to comply with the new architecture.