Zero-fee transactions

Question

Need so help to understand the below transaction :

[
    {
        "type": "transaction",
        "id": 279,
        "level": 26,
        "timestamp": "2018-06-30T18:07:27Z",
        "hash": "opDckVrXacor6B25F3SD2epxC13iCoN6xjwsvvJDYo5aa9AMbqz",
        "counter": 5,
        "originalSender": {
            "address": "tz1NKVAxzJusWgKewn4LEViPSQVRE5Kg6XFV"
        },
        "sender": {
            "alias": "Vested funds 5",
            "address": "KT1Um7ieBEytZtumecLqGeL56iY6BuWoBgio"
        },
        "nonce": 0,
        "gasLimit": 0,
        "gasUsed": 107,
        "storageLimit": 0,
        "storageUsed": 0,
        "bakerFee": 0,
        "storageFee": 0,
        "allocationFee": 0,
        "target": {
            "alias": "Foundation baker 8",
            "address": "tz3NExpXn9aPNZPorRE4SdjJ2RGrfbJgMAaV"
        },
        "amount": 199041301565,
        "parameters": "{\"prim\":\"Unit\"}",
        "status": "applied",
        "hasInternals": false
    }

Please help me validate my understanding

Address - KT1Um7ieBEytZtumecLqGeL56iY6BuWoBgio has "X" tez.

with the above transaction KT1Um7ieBEytZtumecLqGeL56iY6BuWoBgio has transferred 199041301565 tezos to target tz3NExpXn9aPNZPorRE4SdjJ2RGrfbJgMAaV.

1. Amount left with KT1Um7ieBEytZtumecLqGeL56iY6BuWoBgio = ("X" - ( 199041301565 + bakerFee)) - Is my assumption correct in this case?

2. Why is the baker fee zero in this case ?

Answer 1

1. No, it's not correct.

There are two types of operations in Tezos: ordinary and internal (aka a result of smart contracts execution). If there is a nonce field or originalSender field, then it's an internal operation.

In case of internal operation, original sender pays for all the fees, so here is the correct code:

if (tx.originalSender != null)
{
    tx.sender.Balance -= tx.amount;
    tx.originalSender.Balance -= tx.storageFee + tx.allocationFee;
    //bakerFee in internal operations is always 0
}
else
{
    tx.sender.Balance -= tx.amount + tx.bakerFee + tx.storageFee + tx.allocationFee;
}

2. In this particular case bakerFee is 0, because it's an internal operation.

But it may be 0 even if it's not an internal operation (sender can set any fee value). Also, on the early blocks many operations have zero fee...

Answer 2

I've edited my answer to take into account the answer of Groxan.

1. Yes, the contract KT1... that is the emitter of the internal transaction only pays the amount of the transaction. However, as detailed below in the answer of Groxan, fees are payed by the original sender.

Now follows a discussion on other cases where the fees of the outer transaction may have zero fees:

There is no fixed baker fee. Instead, when someone injects a transaction onto the network, they propose a fee. A baker is free to accept, or not, to bake that transactions for the given fee:

Another interesting thing we learn from the receipt is that there are more costs being added on top of the transfer and the burn: fees. In order to encourage a baker to include our operation, and in general to pay for the cost of running the blockchain, each operation usually includes a fee that goes to the baker. Fees are variable over time and depend on many factors but the tezos client selects a default for us.

Lets try this out in a sandbox. When submitting a transaction using tezos-client, we can set the fee using the --fee flag (search "--fee" in the CLI reference):

$ tezos-client transfer 100 from bootstrap1 to bootstrap2 --fee 0
Node is bootstrapped, ready for injecting operations.
Estimated gas: 10207 units (will add 100 for safety)
Estimated storage: no bytes added
Fatal error:
  The proposed fee (ꜩ0) are lower than the fee that baker expect by default (ꜩ0.001281).
   Use `--force-low-fee` to emit this operation anyway.

The client complains about the low fee, but we can force it with --force-low-fee:

tezos-client transfer 100 from bootstrap1 to bootstrap2 --fee 0 --force-low-fee

The transactions is now injected, but the command will wait until it has been validated by a baker. In a separate terminal, let's bake it:

$ tezos-client bake for bootstrap3
Feb 17 14:34:27 - alpha.baking.forge: found 0 valid operations (1 refused) for timestamp 2020-02-17T13:34:27-00:00 (fitness 01::0000000000000001)

The baker refuses transactions because of the low fee. But we can force it by setting minimum fees requires:

$ tezos-client bake for bootstrap3 --minimal-fees 0 --minimal-nanotez-per-gas-unit 0 --minimal-nanotez-per-byte 0
Feb 17 14:37:41 - alpha.baking.forge: found 1 valid operations (0 refused) for timestamp 2020-02-17T13:37:41-00:00 (fitness 01::0000000000000004)

We get the following receipt:

Operation found in block: BMRSeaNGPB5AfqzqEVkdnEBzJfNCoP9WyXK2zZTY18H9hvrcx1H (pass: 3, offset: 0)
This sequence of operations was run:
  Manager signed operations:
    From: tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx
    Fee to the baker: ꜩ0
    Expected counter: 1
    Gas limit: 10307
    Storage limit: 0 bytes
    Transaction:
      Amount: ꜩ100
      From: tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx
      To: tz1gjaF81ZRRvdzjobyfVNsAeSC6PScjfQwN
      This transaction was successfully applied
      Consumed gas: 10207
      Balance updates:
        tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx ... -ꜩ100
        tz1gjaF81ZRRvdzjobyfVNsAeSC6PScjfQwN ... +ꜩ100

We can find the transaction in the RPC and compare it to what you've got:

$ tezos-client rpc get /chains/main/blocks/head/operations/3
[ { "protocol": "ProtoALphaALphaALphaALphaALphaALphaALphaALphaDdp3zK",
    "chain_id": "NetXdQprcVkpaWU",
    "hash": "opYqCQPfhDEMGeCrTa55XrHTFrQweThEFNSTQuWpUVVgKzGDHt3",
    "branch": "BLfZzHax3gG7GyQzee6AqS6UremB9hJpidqRpmo56FoLRiRaj6Z",
    "contents":
      [ { "kind": "transaction",
          "source": "tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx", "fee": "0",
          "counter": "1", "gas_limit": "10307", "storage_limit": "0",
          "amount": "100000000",
          "destination": "tz1gjaF81ZRRvdzjobyfVNsAeSC6PScjfQwN",
          "metadata":
            { "balance_updates": [],
              "operation_result":
                { "status": "applied",
                  "balance_updates":
                    [ { "kind": "contract",
                        "contract": "tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx",
                        "change": "-100000000" },
                      { "kind": "contract",
                        "contract": "tz1gjaF81ZRRvdzjobyfVNsAeSC6PScjfQwN",
                        "change": "100000000" } ], "consumed_gas": "10207" } } } ],
    "signature":
      "sigvyNn4gV5N4hC9VyGMq5rCZ62mNp1pn6GYPH57WM2PLh8UV6WNJDQ7sNKzD8dsc1ndfTodqYpCtMSrH7ptsP8cgL4jWo1m" } ]

As you can see, the fee is 0.

However in the case of your question, the reason is that the transaction is internal, as detailed below in the answer of Groxan.