2023-04-09
When a server process like MDriven Turnkey or MDrivenServer service holds several ecospaces in the same process, we now have a mechanism called SharedBigValue.
What this does is:
- If a loaded attribute value is a byte[] or string
- Larger than 8192 bytes
- Is the maxint in version (latest version)
- Shares the same object ID, and attribute ID
- Shares typesystem checksum
...if the above is true, the cache will 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 key Cache - 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:
- 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.
- 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, and 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;