SharedBigValue

When a server process like MDriven Turnkey or MDrivenServer service holds several ecospaces in the same process, we now (from 2023-04-09) have a mechanism called SharedBigValue.

What this does is:

1. If a loaded attribute value is a byte[] or string
2. Larger than 8192 bytes
3. Is the maxint in version (latest version)
4. Shares the same object id, and attribute id
5. Shares typesystem checksum

...if the above is true, the cache will really hold a SharedBigValue.

• All public access methods to get to a cache value will screen for a SharedBigValue - and if found - resolve to a real value and return this.

Only when objects are loaded from PS and hit the ApplyDataBlock method do we consider creating or looking up SharedBigValue.

• We do this by keeping a static dictionary on the Cache that is key, SharedBigValue.
• If the key already exists, we return the existing SharedBigValue - otherwise, we create a SharedBigValue and return it (and store it in the dictionary).

Reading is protected by a ReadLock that can be upgraded to WriteLock if we need to create.

Limitations I consider okay until reality proves otherwise:

1. It is only DB loaded (old value) that is the target for SharedBigValue - thus write/update of large blocks are handled as before - and we do not try to share this.
2. We do not actively destroy SharedBigValue's if a new model is uploaded - changing the checksum - and forcing all existing ecospaces to be recreated - this is considered to be an uncommon production scenario.

Ways to test: Model with Image and Text, run Turnkey with two different users, or two different browsers, update large text and image in one - make sure it updates in the other.

Expected positive effect: Only one instance of large things is held in memory even if 1000 users look at this same thing.

Expected negative effect: additional overhead for large texts and byte arrays but kept low by checks above - I do not expect it to be noticeable.

Currently, this feature is always on, you can stop it from having an effect by setting:

FetchedBlockHandler.kBigValueThreshold=int.MaxValue;