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.

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Intermediate resource-management exception-safety blocks transactions rollback
1module FileResource
2 # Opens a file, yields it to the caller, and guarantees the
3 # handle is closed even if the block raises an exception.
4 def self.open(path, mode = "r")
5 handle = File.open(path, mode)
6 begin
7 yield handle
8 ensure
9 handle.close unless handle.closed?
10 end
11 end
12end
13 
14class Transaction
15 def initialize
16 @committed = false
17 @actions = []
18 end
19 
20 def perform(&action)
21 @actions << action
22 begin
23 action.call
24 yield if block_given?
25 @committed = true
26 ensure
27 rollback unless @committed
28 end
29 end
30 
31 private
32 
33 def rollback
34 @actions.reverse_each do |a|
35 a.respond_to?(:undo) ? a.undo : nil
36 end
37 @actions.clear
38 end
39end
01 / 01
STEP 01

Walkthrough

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

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