Sandbox classic and Sandbox next handle empty transactions differently

Consider the following DAML Model:

template T
  with
    p : Party
  where
    signatory p
    nonconsuming choice C : ()
      controller p
      do pure ()

together with this REPL session (it doesn’t matter if you use the REPL or anything else)

daml> p <- allocatePartyWithHint "p" (PartyIdHint "p")
daml> cid <- submit p (createCmd (T p))
daml> submit p (exerciseCmd cid C)

If you now look at the transaction stream, you will see two transactions for Sandbox classic:

{
  "transactions": [
    {
      "transactionId": "3",
      "commandId": "2890353f-9788-4b4b-984e-5ace58e30ff2",
      "effectiveAt": "2020-08-03T19:38:37.989539Z",
      "events": [
        {
          "created": {
            "eventId": "#3:0",
            "contractId": "#3:0",
            "templateId": {
              "packageId": "ed52b012e14eba3719e0edd43e0db33e5140ce4fa6c084de844b90001c2d387c",
              "moduleName": "Main",
              "entityName": "T"
            },
            "createArguments": {
              "fields": [
                {
                  "value": {
                    "party": "p"
                  }
                }
              ]
            },
            "witnessParties": [
              "p"
            ],
            "agreementText": "",
            "signatories": [
              "p"
            ]
          }
        }
      ],
      "offset": "0000000000000003"
    }
  ]
}
{
  "transactions": [
    {
      "transactionId": "4",
      "commandId": "b94d4442-09c3-4116-9e91-e8b3c18556c3",
      "effectiveAt": "2020-08-03T19:38:52.187258Z",
      "offset": "0000000000000004"
    }
  ]
}

But for sandbox next, you get only one

{
  "transactions": [
    {
      "transactionId": "0A2432366565653138662D396637352D336663312D393334612D323766336539346637396164",
      "commandId": "8bf38d6e-ce70-4354-826b-8bee12e84162",
      "effectiveAt": "2020-08-03T19:35:28.286117Z",
      "events": [
        {
          "created": {
            "eventId": "#0A2432366565653138662D396637352D336663312D393334612D323766336539346637396164:0",
            "contractId": "00aa688090465067ffe871ed9e439befd56ad303e7059b10cc2f59f733d783d722",
            "templateId": {
              "packageId": "ed52b012e14eba3719e0edd43e0db33e5140ce4fa6c084de844b90001c2d387c",
              "moduleName": "Main",
              "entityName": "T"
            },
            "createArguments": {
              "fields": [
                {
                  "value": {
                    "party": "p"
                  }
                }
              ]
            },
            "witnessParties": [
              "p"
            ],
            "agreementText": "",
            "signatories": [
              "p"
            ]
          }
        }
      ],
      "offset": "00000000000000040000000000000000"
    }
  ]
}

The completion stream contains both commands in both cases and it also contains a transactionId in both cases.

I would expect to get the empty transaction like I do in Sandbox classic.

[nmarton-da]

Current implementation for flat transactions:

• if TX killed by filter, then not returning anything (no empty TX neither).
• if TX only having transient pairs, and the union of all parties in TransactionFilter has intersection with the TX’s submitters then returning an empty transaction.

Suggestion by @mziolekda @cocreature @nmarton-da

• keeping the current behavior for persistence queries
• fixing IMFO so it behaves the same as persistence
• NOT changing Ledger API, so this feature is not explicit
• Fixing the original issue with Triggers: this will wait for the unified stream project to have a proper implementation

[cocreature]

If you’re confident that the performance will be reasonable, I think this can work. It feels a bit odd to have to resort to only the completion stream + pointwise queries instead of subscribing to the completion & transaction stream but I don’t feel super strongly about this.