Is there any documentation to state how can we have redundant nodes configured in Quorum network so that even if a single node goes down, the data in blockchain is still retrievable by the app?
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Since Quorum is based on Ethereum, node recovery is very straight forward and would behave in a similar manner as regular Ethereum nodes. In Quorum, you need to make sure to back up all geth node keys (generally, nodekey and wallets from nodedata folder). If this node goes down, you create a new node with the same keys and allow it to connect to the blockchain. Once connected, it will fetch all the blocks and recover its state.
One additional caveat with Quorum is recovery of Tessera (private transaction manager). The idea is the same tho: you will need to back up key pairs (and you should back up config file) the manager is responsible for and when recovering, start it up in a recovery mode before launching your quorum node. In this mode, it will reach out to the other transaction managers on the network and request payloads that were addressed to it be resent.
There are other options for Tessera as well and you can see it in our documentation here: https://docs.goquorum.com/en/latest/Privacy/Tessera/Tessera%20Services/Transaction%20Manager/#data-recovery.