ruby 39 lines · 8 steps

Guaranteeing cleanup with begin/ensure in Ruby

Two small patterns that use ensure to run cleanup code whether the work succeeds or blows up.

Explained by highlit

Intermediate resource-management exception-safety blocks transactions rollback

1module FileResource
# Opens a file, yields it to the caller, and guarantees the
# handle is closed even if the block raises an exception.
def self.open(path, mode = "r")
  handle = File.open(path, mode)
  begin
    yield handle
  ensure
    handle.close unless handle.closed?
  end
end
12end
13 
14class Transaction
def initialize
  @committed = false
  @actions = []
end
19 
def perform(&action)
  @actions << action
  begin
    action.call
    yield if block_given?
    @committed = true
  ensure
    rollback unless @committed
  end
end
30 
private
32 
def rollback
  @actions.reverse_each do |a|
    a.respond_to?(:undo) ? a.undo : nil
  end
  @actions.clear
end
39end

01 / 01

STEP 01

Walkthrough

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Three takeaways

1An ensure clause runs on both normal exit and exception, making it the right home for cleanup.
2Yielding a resource to a caller's block lets the method own acquisition and release while the caller owns usage.
3Tracking a success flag lets ensure distinguish a clean finish from a failure that needs undoing.

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