Hi @pauldaoust I think what you describe is more on the security and how shard ascertain the validity of incoming transactions from another shard which is another topic (and also a major issue in sharded blockchains) altogether.
Another link you can take a look and have a feel on issues i am taking about is: https://slideslive.com/38919733/devcon5-hall-a2-day2 Skip to straight to 5:16:55
It talks about complication arising when there are asynchronous communications between different smart contracts with different shards.
It talks about if there are various steps needed to happen in sequence or in parallel between different smart contracts in a “single atomic transaction” in a “single blockchain” - it is easier verify things that are suppose to happen, happen and if any steps did not happen, everything can be seamlessly unwound as the transaction reverse.
This can’t happen with smart contracts across shards. If any issue happens in say, step 2 of a cross shard communication/transaction, i understand that the process can’t be unwound automatically. There will be manual intervention needed and this causes friction and inconvenience (i think).
The issues get more complicated when it involved more smart contracts interactions with each smart contract living in different shards. Say 5 smart contracts interactions across 5 shards.
In a sharded blockchain, there are issues in both scenarios whether a app live in a single shard or an app lives in multiple shards.
I agree with you holochain apps totally different world/design architecture out there compared to sharded blockchain. Every app will always be their own independent DHT network and i think they will communicate with each other across the network via bridges and api calls.
What i am more concern is the composability and ability of holochain apps to interact/build on top of each other seamlessly without the complication that apps/developers will have to face in sharded blockchains as mentioned in all the links i posted so far.
I do hope that holochain doesnt face the same issue while retaining high composability as i believe it will help greatly with network effects and “compounded innovation”. Await your thoughts.