ConnectionHandler is a collection of ConnectionPool objects. It is used for keeping separate connection pools for Active Record models that connect to different databases.
For example, suppose that you have 5 models, with the following hierarchy:
| +-- Book | | | +-- ScaryBook | +-- GoodBook +-- Author +-- BankAccount
Suppose that Book is to connect to a separate database (i.e. one other than the default database). Then Book, ScaryBook and GoodBook will all use the same connection pool. Likewise, Author and BankAccount will use the same connection pool. However, the connection pool used by Author/BankAccount is not the same as the one used by Book/ScaryBook/GoodBook.
Normally there is only a single ConnectionHandler instance, accessible via ActiveRecord::Core.connection_handler. Active Record models use this to determine the connection pool that they should use.