Revert transaction

[altitude]

Summary

We should be able to revert a transaction without having to build the reverse transaction

Proposal

POST /:ledger/transactions/:txid/revert

Expected behaviors

Given the transaction:

{
  "txid": 0,
  "postings": [
    {
      "source": "world",
      "destination": "users:001",
      "amount": 100,
      "asset": "COIN"
    },
    {
      "source": "users:001",
      "destination": "payments:001",
      "amount": 100,
      "asset": "COIN"
    }
  ]
}

The revert endpoint should attempt to commit the exact reverse transaction:

{
  "txid": 1,
  "postings": [
    {
      "source": "payments:001",
      "destination": "users:001",
      "amount": 100,
      "asset": "COIN"
    },
    {
      "source": "users:001",
      "destination": "world",
      "amount": 100,
      "asset": "COIN"
    }
  ]
}

[henry-jackson]

@altitude What do you think about adding a "Reverse" method to the Transaction struct in core/transaction.go. The method would reverse the order of the postings in the Transaction, and switch the source and destination strings on each posting.

Then when the endpoint is hit, we will query for transaction using the ID in the URL, reverse the transaction using that method, and then post the new transaction to the ledger.

[henry-jackson]

Another consideration is that we should mark that a Transaction has been reverted - do you think this is best achieved with the metadata table?

[altitude]

@henry-jackson sounds great to me!

1/ Sharing that vision for core/transaction.go, agreed it's a great place to host the reverse-transaction-building-logic

2/ Awesome idea. Would go along well with the scheme concept we're working on for materialized transactions, where we introduce specific keys and values to use on the metadata table. I think I'd go for two meta fields addition on the reverted transaction upon successful commit of the reverting transaction:

scheme/state with a reverted value
scheme/state/reverted-by with a value of the txid of the reverting transaction