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from the cumulus tutorial:
... You cannot, at this time, connect a parachain with any previous state to a relay chain. All parachains must start from block 0 on the relay chain...
If that is the case, how does it work when a parachain's lease ends? How does the parachain re-connect to the relay without losing it previous state?
The tutorial is technically wrong because it is possible to connect a parachain with previous state to a realychain.
What is not possible (yet), is convert a solo chain to parachain without hardfork (and that will be solved in future soonish).
Back to the question, basically a parachain/parathread have its header/state and wasm stored in relay chain and that will be used to verify new block produced by collator.
When a parachain is downgraded into parathread, the header & wasm is kept. So when it is upgraded back to parachain, the previous header & wasm will be used. i.e. the chain just continue to run as nothing happened (just a very long block time between two blocks).
That also means it is possible to register a parachain on a different relaychain, which is useful for testing purpose when the relaychain died due to the epoch issue. Then you can restart the relaychain, register the current head & wasm of the parachain, update parachain to use new relaychain,and the parachain will continue to run with a new relaychain without loosing state.
When a parachain lease ends, it becomes a parathread, and its existing state root and Wasm stays on the relay chain.
When the parathread protocol is complete, a chain in this state would still be able to produce new blocks, but it may need to pay per block, rather than being guaranteed to get a new block for free like a parachain.
As such, a parathread can also simply win a new auction and be re-upgraded to a parachain, and in this case, everything moves forward like normal.
So your statement:
How does the parachain re-connect to the relay without losing it previous state?
Assumes that some state is lost, but this is not true.
If a parathread was fully offboard, and all state deleted, it would be possible to re-onboard with the old state root and Wasm, and things would appear to work just like before. The only potential issue here would be that they may have a different ParaId, and so other chains that interacted with them would need to migrate their past storage, or we could use governance to try and let parachains re-register their old ID.
