Querrel

Querrel makes it easy to query multiple databases in parallel (threads) using ActiveRecord, as if you were querying one database.

Add this line to your application's Gemfile:

gem 'querrel'

And then execute:

$ bundle

Or install it yourself as:

$ gem install querrel

You can use with Querrel directly via it's top level namespace, or by creating an instance:

all_brands = Querrel.query(Brand.all, on: ['db1', 'db2', 'db3'])
# is the same as
all_brands = Querrel.new(['db1', 'db2', 'db3']).query(Brand.all)

Either of the above will give you an array of all the Brand objects from all the given databases. The records will be marked as readonly.

query will yield a block with the passed in ActiveRecord::Relation, this allows you to do additional operations on the results before they are merged, for example you could use pluck:

all_brand_names = q.query(Brand.all) do |s|
  s.pluck(:name)
end

There is also a run method which instead of running a preprescribed scope and merging, will take a block which allows you do anything you want with the specified database connection, for instance:

require 'thread'
all_brands = []
b_s = Mutex.new
all_products = []
p_s = Mutex.new

Querrel.run(on: dbs) do |db|
  b_s.synchronize { all_brands += Brand.all.to_a }
  p_s.synchronize { all_products += Product.all.to_a }
end

There are three ways in which you can instruct Querrel which databases to use:

1. Pass in an array of environments, e.g. Querrel.new([:customer1, :customer2])

2. Pass in an array of database names, e.g. Querrel.new(['dbs/customer1.sqlite3', 'dbs/customer2.sqlite3'], db_names: true)

3. Pass in a hash of named connection configurations, e.g.:

   Querrel.new({
     one: {
       adapter: "sqlite3",
       database: "test/dbs/test_db_1.sqlite3"
     },
     two: {
       adapter: "sqlite3",
       database: "test/dbs/test_db_2.sqlite3"
     }
   })

By default Querrel will use a maximum of 20 threads, but you can adjust this using the :threads option:

q = Querrel.new(dbs, threads: 50)

1. Fork it ( https://github.com/meritec/querrel/fork )
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request