Arweave is a decentralized, durable data storage solution. To store data in Arweave, one would need to send it as a transaction. Once the transaction is accepted and processed by Arweave, one can query that data by the transaction ID, which is a hash of the transaction signature. As such, from a usability perspective, the biggest difference between Arweave and a traditional KV store is that one cannot pick his own key, and consequently an index data structure is needed to map a key to the transactions that may contain it.
Since it's not economic to send one transaction per key-value pair, we will batch store multiple key-value pairs in one transaction. The key-to-transaction index will therefore be a key-range-to-transaction index. To make the index size more compact, we will require the key range to be truncated (or padded) as fix-sized bytes, which means it's possible to have multiple transactions for the same (truncated) key range, so all of such transactions need to be read at query time to find where the exact key resides. Each version will have its own index, and each index version is stored as individual transaction on Arweave. The transaction IDs for each index version are stored on a local leveldb; which is the only data required to be stored locally.