# Relayers

Since `shielder` is just a smart contract, each transaction in shielder must be submitted by some `AccountId`. However, if this was done just by the user itself, then it would lead to privacy loss. For this reason we introduce relayers: parties that post shielder transactions to chain, on behalf of normal users.

To support relayers we enrich `Operation`s for which it makes sense with two new fields: `relayer_address` and `relayer_fee`. Consider the example of the `WithdrawETH` operation

```rust
struct WithdrawETH {
    relayer_address: AccountId,
    relayer_fee: u128,
    withdraw_address: AccountId,
    amount_eth: u128,
}
```

Now the corresponding methods would be implemented as follows:

```rust
fn update(acc: Account, op: WithdrawETH) -> Account {
    decrease balance of AZERO in acc by op.relayer_fee
    decrease balance of ETH in acc by op.amount_eth
    return acc;
}
```

The corresponding implementation of `public_exec` is

```rust
fn public_exec(op: WithdrawETH) {
    transfer op.relayer_fee AZERO from shielder to op.relayer_address
    transfer op.amount_eth ETH from shielder to op.withdraw_address
}
```

The typical flow of sending a transaction by the user would be then:

* Contact a relayer and negotiate fee.
* Create a transaction placing the relayer's address in `relayer_address` field and the negotiated fee in `relayer_fee` (in AZERO). Generate the corresponding snark proof. Note that `withdraw_address` should be a fresh account with no history, for max privacy.
* Pass all the data to the relayer.
* The relayer validates that the `relayer_address` and `relayer_fee` is correct, and simulates the transaction to make sure it passes (the proof is correct etc.).
* The relayer sends the transaction, bears it's gas fee, but gains `relayer_fee` which should be larger than gas cost.


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