12:46 a.m.
Am 30.03.11 02:32, schrieb Elias Ross:
I think it'd be BEST if you could support both models. I would
add:
interface Cache {
/**
* Returns a new or existing LargeObject object for the following key.
* @throws ClassCastException if the key exists and is not a LargeObject.
*/
LargeObject largeObject(K key);
}
OK, I'll keep that on my todo list, yet for the time being I'v opted to
start with implementing void writeToKey(K key, InputStream largeObject).
> This is certainly doable but leaves me wondering where that
proposed
> ChunkingInterceptor might come into play.
I would think ideally you don't need to create any new commands. Less
protocol messages is better.
It is my understanding that PutKeyValueCommand will
*always* attempt to
read the current value stored under the given key first. I'm not sure if
we want this in our situation where that current value may be several GB
in size. Anyway, it should be easy to refactor if reusing
PutKeyValueCommand should prove viable.
> 3. The design suggests to use a fresh UUID as the key for each
new
> chunk. While this in all likelihood gives us a unique new key for each
> chunk I currently fail to see how that guarantees that this key maps to
> a node that is different from all the nodes already used to store chunks
> of the same Large Object. But then again I know next to nothing about
> Infinispan's constant hashing algorithm.
I wouldn't use UUID. I'd just store (K, #) where # is the chunk.
Since this is important and might reveal a fundamental misunderstanding
on my part, I need to sort this out before moving on. These are my
assumptions, please point out any errors:
1. We want to partition a large object into chunks since, by definition,
a large object is too big to be stored in a single node in the cluster.
It follows that it is paramount that no two chunks be stored in the same
node, correct?
2. Constant hashing guarantees that any given key maps to *some* node in
the cluster. There is no way, however, such a key's creator could know
to what node exactly its key maps. In other words, there is no inverse
to the hash function, correct?
3. The current design mandates that for storing each chunk the existing
put(key, value) be reused, correct?
It follows that we have no way whatsoever of generating a set of keys
that guarantees that no two keys are mapped to the same node. In the
pathological case, *all* keys map to the same node, correct?
> I would think a use case for this API would be streaming audio
or
> video, maybe something like access logs even?
>
> In which case, you would want to read while you're writing. So,
> locking shouldn't be imposed. I would say, rely on the transaction
> manager to keep a consistent view. If transactions aren't being used,
> then the user might see some unexpected behavior. The API could
> compensate for that.
>
If I understand you correctly you propose two alternatives:
1. Use transactions, thus delegating all consistency requirements to the
transaction manager.
2. Don't use transactions and change the API so that readers may be told
that a large object they are interested in is currently being written.
Further, to support streaming use cases you propose that it should be
possible to read a large object while it is being written.
Is that correct?
Hmm, I need to think about this. If I understand Manik's comment and the
tx subsystem correctly each transaction holds its *entire* associated
state in memory. Thus, if we are to write all chunks of a given large
object within the scope of a single transaction we will blow up the
originator node's heap. Correct?
So many questions ...
Cheers,
Olaf