Domain modeling with Lagom

I’m currently trying to build an application that handles personal finances. I’m struggling with Lagom ways of doing because I can’t find any example of “real” application built with Lagom. I have to guess what are best practises and I’m constantly afraid of falling into pitfalls.

My case is the following: I have Users, Accounts and Transactions. Accounts belong to users but can be “shared” between them (with some sort of authorization system, one user is admin and other can read or edit the account). Transactions have an optional “debit” account, an optional “credit” account and an amount which is always positive.

The scenarios I was considering are the followings:

  1. I consider that transactions belong to accounts and are parts of the account entity as a list of entries. In that scenario, a transfert transaction must have a “sister” entry in the other account. This seems easy to implement but I’m concerned by :
    • the potential size of the entity (and the snapshots). What happen if I have accounts that contain thousands of ten of thousands of transactions?
    • the duplication of the transaction in several accounts.
  2. I consider that transactions have their own service. I that case I can use Kafka to publish events when transactions are recorded so the Account entity can “update” it’s balance. In that case does it make sense to have a “balance” property in the entity or a read-side event listener for transaction events that update the read-database?
  3. I can have two Persistent Entities in the same service but in that case I’m struggling with the read-side. Let say I have a transaction, I want to insert into the “transactions” table and update the “accounts” table. Should I have multiple read-side processors that listen to different events but write in the same db?

What do you think?