[Opensim-dev] Proposal: Implement a de-duplicating core ROBUST asset service

[Justin Clark-Casey]

I already have a working de-duplicating ROBUST asset service using hashing.  It was not at all hard to do (largely 
because SRAS had already demonstrated how it could be done) so complexity on this end is not an issue.

I have read many articles on filesystem vs blob storage.  There are pros and cons either way.  From what I've read, the 
performance difference is actually quite small.

As this service is for light to medium use, in my opinion the simplicity of managing just a database wins out over the 
advantages of a filesystem approach.  Anybody wanting filesystem storage right now can use SRAS [1], which is a third 
party project developed externally from opensim-core that provides this and other extra features or you can roll your 
own, which would not be difficult for anybody moderately competent in PHP to do.

If you're running a large grid this is always going to entail extra work and co-ordination of components, just like 
running a large website.

[1] https://github.com/coyled/sras

On 09/03/12 04:06, Wade Schuette wrote:
> Justin,
>
> I have to respectfully agree with Cory.
>
> Wouldn't something like the following address your valid concerns about complexity and reducing total load as well as
> perceived system response time to both filing and retrieving assets?
>
> First, if you use event-driven processes, there's no reason to rescan the entire database, and by separating the
> processes into distinct streams, they are decoupled which is actually a good thing and simplifies both sides. There's no
> reason I can see they need to be coupled, and separating them allows them to be optimized and tested separately, which
> is a good thing.
>
> In fact, the entire deduplication process could run overnight at a low-load time, which is even better, or have multiple
> "worker" processes assisgned to it, if it's taking too long. Seems very flexible.
>
> I'm assuming that a hash-code isn't unique, but just specifies the bucket into which this item can be categorized.
>
> When a new asset arrives, if the hash-code already exists, put the unique-ID in a pipe and finish filing it and move on.
> If the hash-code doesn't already exist, just file it and move on.
>
> At the other end of the pipe, this wakes up a process that can, as time allows, check in the background to see if not
> only the hash-code is the same, but the entire item is the same, and if so, change the handle to point to the existing
> copy. ( For all I know, this can be done in one step if CRC codes are sufficiently unique, but computing such a code is
> cpu intensive unless you can do it in hardware.)
>
> Of course, now the question arises of what happens when the original person DELETES the shared item. If you have solid
> database integrity, you only need to know how many pointers to it exist, and if someone deletes "their copy", you
> decrease the count by one, and when the count gets to one, the next delete can actually delete the entry.
>
>
>
> Wade
>
>
>
>
> On 3/8/12 7:41 PM, Justin Clark-Casey wrote:
>> On 08/03/12 22:00, Rory Slegtenhorst wrote:
>>> @Justin
>>> Can't we do the data de-duplication on a database level? Eg find the duplicates and just get rid of them on a regular
>>> interval (cron)?
>>
>> This would be enormously intricate. Not only would you have to keep rescanning the entire asset db but it adds another
>> moving part to an already complex system.
>>
>
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-- 
Justin Clark-Casey (justincc)
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