Small. Fast. Reliable.
Choose any three.
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  Changing SQLite to incorporate such custodian logic would defeat the object of having an embeddable database that doesn't need its own separate process. 
  
  Therefore, a solution would probably be to develop such shared memory custodian as as a separate addon, along with a protocol to format the request/response conversation between this custodian and the processes that need to access a common database with high concurrency.
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+ By the way, at a lower level you will find that the operating system already intelligently caches disk sectors that are being accessed regularly. Even though the database server itself would probably outperform the underlying OS in knowing how and what exactly to cache, the underlying caching already contributes substantially to improving SQLite's performance under high concurrency.
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