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    <p>i'm pretty unclear on what is meant by this. i think the simplest implementation is simply</p> <pre><code>Hash </code></pre> <p>that is to say the built-in ruby hash <em>is</em> threadsafe if by threadsafe you mean will not blow up if > 1 threads tries to access it. this code will run safely forever</p> <pre><code>n = 4242 hash = {} loop do a = Thread.new do n.times do hash[:key] = :val end end b = Thread.new do n.times do hash.delete(:key) end end c = Thread.new do n.times do val = hash[:key] raise val.inspect unless [nil, :val].include?(val) end end a.join b.join c.join p :THREADSAFE end </code></pre> <p>i suspect by thread safe you really mean ACID - for instance a write like hash[:key]=:val followed by a read if has[:key] would return :val. but no amount of trickery with locking can provide that - the last in would always win. for example, say you have 42 thread all updating a threadsafe hash - which value should be read by the 43'rd?? surely by threasafe you don't mean some sort of total ordering on writes - therefore if 42 threads were actively writing the 'correct' value is <em>any</em> right? but ruby's built-in Hash works in just this way...</p> <p>perhaps you mean something like</p> <pre><code>hash.each do ... </code></pre> <p>in one thread and</p> <pre><code>hash.delete(key) </code></pre> <p>would not interfere with one another? i can imagine wanting that to be threadsafe, but that's not even safe in a <em>single</em> thread with the MRI ruby (obviously you cannot modify a hash while iterating over it)</p> <p>so can you be more specific about what you mean by 'threadsafe' ??</p> <p>the only way to give ACID semantics would be a gross lock (sure this could be a method that took a block - but still an external lock).</p> <p>ruby's thread scheduler isn't just going to schedule a thread smack in the middle of some arbitrary c function (like the built-in hash aref aset methods) so those are effectively threadsafe.</p>
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    1. USara t howard
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    1. COAt the very least, I require that if two threads attempt to set a key at the same time, both are set, and that if multiple threads do a write + read in sequence to the same key, the only two possible read values would be the ones that are set (i.e. no nil). I'm not content to observe MRI behavior, because I'd like the solution to work in cases of true simultaneous reads and/or writes, such as in JRuby.
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      1. USYehuda Katz
    2. COi must say it seems like a bug in jruby that hash writes and reads are not atomic - but that is indeed the case. i will point out that ruby's Hash does give you this for free in 1.8 and 1.9, so really you want a factory that simply returns Hash unless you're in jruby.
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    3. COI'm not convinced that "threads don't operate simultaneously" can be considered a feature of Ruby. I consider the thread scheduler in MRI (with it's 10ms timeslices) to be an implementation detail -- and I *want* both MRI and other implementations to be free to improve threading semantics without such improvements being considered a bug.
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