mongo-ruby-driver/docs/TAILABLE_CURSORS.md

Tailable cursors in Ruby

Tailable cursors are cursors that remain open even after they've returned a final result. This way, if more documents are added to a collection (i.e., to the cursor's result set), then you can continue to call Cursor#next to retrieve those results. Here's a complete test case that demonstrates the use of tailable cursors.

Note that tailable cursors are for capped collections only.

  require 'mongo'
  require 'test/unit'

  class TestTailable < Test::Unit::TestCase
    include Mongo

    def test_tailable

      # Create a connection and capped collection.
      @con = Connection.new
      @db  = @con['test']
      @db.drop_collection('log')
      @capped = @db.create_collection('log', :capped => true, :size => 1024)

      # Insert 10 documents.
      10.times do |n|
        @capped.insert({:n => n})
      end

      # Create a tailable cursor that iterates the collection in natural order
      @tail = Cursor.new(@capped, :tailable => true, :order => [['$natural', 1]])

      # Call Cursor#next 10 times. Each call returns a document.
      10.times do
        assert @tail.next
      end

      # But the 11th time, the cursor returns nothing.
      assert_nil @tail.next

      # Add a document to the capped collection.
      @capped.insert({:n => 100})

      # Now call Cursor#next again. This will return the just-inserted result.
      assert @tail.next

      # Close the cursor.
      @tail.close
    end

  end