We use multiple MongoDB clusters for our application. (We are intentionally not using a sharded cluster, as we want greater control/isolation.)

Current Behavior

When we perform a session on one cluster, it becomes impossible to execute
queries on any other cluster. Such queries fail with "The configuration of the client used to create this session does not match that of the client owning this operation..."

Desired Behavior

We would like to request the following changes:

1. When performing a session/transaction on Cluster A, queries on Cluster B should still execute outside that session/transaction.
2. Moreover, it should be possible to execute a session/transaction specific to Cluster B within the Ruby block for the session on Cluster A.
3. Within a session/transaction Ruby block, we should be able to "escape" out of that Session, i.e. explicitly execute commands outside of the transaction (see Workaround patch below.)

Workaround

We've made the following monkey patch which allows us to "escape" out of the session for the purpose of executing commands outside a transaction. It relies on the coincidental behavior that Ruby threads escape out of the session. However, it would be be much nicer if we could do this without spawning a thread.

module Mongo
  class Session
    def self.escape(&)
      # Note: #value joins the thread
      Thread.new(&).value
    end
  end
end

Example Use Case

We have Customer object and Audit object. Audit is a changelog that is created each time a Customer is saved. It therefore looks something like:

class Customer
  include Mongoid::Document
  store_in client: :default
  after_save do |customer|
    Audit.new(customer.changes).save!
  end
end
class Audit
  include Mongoid::Document
  store_in client: :other_database
end

Note that Audits are stored in a separate cluster. Executing a session/transaction with the Customer model will fail when it's callback attempts to save the corresponding Audit model.