Each 'postgres' instance starts a background worker 'raftable' and creates a

shared memory segment for storing the 'state'. All 'raftable' workers are

communicating Raft protocol over UDP.

When 'frontend' issues a read command, the 'backend' returns the data from the

local replica of the 'state' which is in the shared memory of current instance.

When 'frontend' issues a write command, the 'backend' connects to the current

'raftable' leader directly through TCP. Raftable leader returns the state

version number V after the update gets applied (acked by the majority of

'raftable' workers). Then the backend waits until the local state version

number becomes at least V and returns 'ok' to the frontend.

The backend can also issue commands to itself through C API.

fork ┏━━━━━━━━━┓ sql ┏━━━━━━━━━━┓

┌─────────────┨ backend ┠───────┨ frontend ┃

│ ┗┯━━━━━━━┯┛ ┗━━━━━━━━━━┛

│ │shm │

┏━━━━┷━━━━━┓ ┏━━━┷━━━┓ │

┃ postgres ┠────┨ state ┃ │

┗━━━━┯━━━━━┛ ┗━━━┯━━━┛ │ tcp

│ │shm │

│ fork ┏━━━━┷━━━━━┓ │

└─────────┨ raftable ┃ │

┗━┯━┯━┯━┯━┯┛ │

│ raft│ │ │

┏━┷━┷━┷━┷━┷┓ │

┏┻━━━━━━━━━┓┠─┘

┏┻━━━━━━━━━┓┣┛

┃ raftable ┣┛

┗━━━━━━━━━━┛

* Adds/updates value by key. Returns when the value gets replicated on

* current machine. Storing NULL will delete the item from the table.