[infinispan-dev] Re: Locking

Hi Vladimir I haven't spent too much time thinking about eager locking yet - swamped with the finishing touches on 4.0.0.ALPHA2 and getting a public splash ready. :-) Also, infinispan-dev (now cc'd!) is pretty informal so don't worry about getting a perfect, distilled design. The purpose of the list is to distil the design. I've included my initial thoughts inline, below. On 28 Apr 2009, at 03:18, Vladimir Blagojevic wrote: All makes sense, except that we need to think who the lock 'owner' is going to be if we are not running in a tx. The way the cache works, locks are held by the thread (if not in a tx) or the associated GlobalTransaction object (if in a tx). Let's take these cases one at a time. Let's start with (what I think) is the most pertinent: A. Transactions + Eager locking --------------------------------- when you do the following: 1. tx.begin() 2. cache.put(K, V) 3. tx.commit() what happens is, at 1., a new GTX instance is associated with the transaction. At 2. locks are acquired on K on the *local* cache, the owner being GTX. At 3., a prepare() is broadcast and the locks on K are acquired on all remote participating caches, the owner being GTX again. The GTX is propagated with the PrepareCommand as it is a field in AbstractTransactionBoundaryCommand. So now, an explicit lock in the scope of a transaction would also need to propagate the GTX - just so we know who the lock owner should be. E.g., 1. tx.begin() 2. cache.lock(K) 3. // read K,V and calculate V2 which is a function of V. E.g., V2 = V + 1 4. cache.put(K, V2) 5. tx.commit() In the above scenario, step 2 broadcasts a LockControlCommand which is constructed with the GTX and the necessary key(s). Remote caches acquire the locks using the GTX as the owner, and responds positively. (this RPC call would *have* to be sync regardless of cache mode). The way I see it, cache.unlock() should be *optional*. If unlock() is not called, when the transaction completes all locks associated with the transaction are released anyway. If unlock() is used, it would need some special behaviour. Consider: 1. tx.begin() 2. cache.lock(K) 3. cache.put(K2, V2) 4. cache.unlock(K) 5. tx.commit() In the above case, unlock() would release locks at step 4 since the tx has not actually modified K. If, however, we have: 1. tx.begin() 2. cache.lock(K) 3. cache.put(K, V) 4. cache.unlock(K) 5. tx.commit() then the unlock() should be a no-op (maybe don't even bother with the RPC) since we know that the tx has modified K and we can only feasibly release the lock once the transaction completes in 5. B. Transactions + Non-eager locking --------------------------------- So I guess non-eager would be that the acquisition of remote locks are deferred to when the prepare() broadcasts, and any potential lock acquisition failures happen at the time of prepare(). This is what we have in place right now anyway, I need to think about whether lock() and unlock() makes sense in a non-eager context. C. No transaction + Eager locking --------------------------------- This gets a bit more tricky, because in this case the lock owner is the Thread. And this does not translate to anything meaningful cluster-wide. Assume: 1. cache.lock(K) 2. cache.get(K) 3. cache.put(K, V) 4. cache.unlock() Here, we notice that explicit use of unlock() is needed to release locks acquired by lock(), unlike the case where a committing (or rolling back) transaction would do this for you. The tricky thing here is, who holds lock ownership across a cluster? :-) If this is standalone mode, this is simple - the Thread- owner paradigm works fine. Perhaps we need to change the locking code so that the non-tx lock owner is a combination of thread-name + cache address? I can see this leading to tricky stale locks though if we have a join/rejoin midway during a code sequence like above. D. No transaction + Non-eager locking --------------------------------- Just as in B., I'm not sure if this is a valid use case. I.e., how is this useful and what is its purpose. (perhaps just acquire local locks on lock() and only acquire the remote locks when the actual put() is performed?) Anyway, to sum things up: the way I see it, B and D are cases that still need further thought (i.e., the non-eager cases). C needs a lot more in-depth thought as it has the potential to break a lot of stuff as it changes lock ownership for all non-transactional cases. So I am inclined to suggest that lock() and unlock() only works for case A. For other cases, we do not support the use of these methods. At least for now, until we think of C in greater detail. What do you think? Apart from that, the detailed changes you mentioned below all make sense (except that I would add lockOwner as a field on LockControlCommand, etc). Cheers Manik -- Manik Surtani manik(a)jboss.org Lead, Infinispan Lead, JBoss Cache http://www.infinispan.org http://www.jbosscache.org