# How does a contract interact with the GSN?

**URL:** <https://forum.openzeppelin.com/t/how-does-a-contract-interact-with-the-gsn/2310>\
**Category:** SDK\
**Created:** [February 19, 2020, 11:41am UTC](https://forum.openzeppelin.com/t/how-does-a-contract-interact-with-the-gsn/2310 "2020-02-19T11:41:36Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![vasa-develop](https://sea2.discourse-cdn.com/flex022/user_avatar/forum.openzeppelin.com/vasa-develop/32/1744_2.png) [@vasa-develop](https://forum.openzeppelin.com/u/vasa-develop)\
**Post date:** [February 19, 2020, 11:41am UTC](https://forum.openzeppelin.com/t/how-does-a-contract-interact-with-the-gsn/2310/1 "2020-02-19T11:41:36Z")

</div>

I have been checking out the [Sending Gasless Transactions](https://docs.openzeppelin.com/learn/sending-gasless-transactions) tutorial.

As discussed in the tutorial, we have a `Counter` contract that implements the `GSNRecipient`.

I’m trying to see how the react-app that we made in the tutorial, is interacting with the Relayer that we deployed locally.

The `App.js` file contains an `increase()` function:

```auto
  const increase = async () => {
    console.log("test");
    await counterInstance.methods.increase().send({ from: accounts[0] });
    getCount();
  };

```

which sends a transaction to the deployed `Counter` contract.

Here is the `Counter` contract.

```auto
// contracts/Counter.sol
pragma solidity ^0.5.0;

import "@openzeppelin/contracts-ethereum-package/contracts/GSN/GSNRecipient.sol";

contract Counter is GSNRecipient {
    uint256 public value;

    function increase() public {
        value += 1;
    }

    function acceptRelayedCall(
        address relay,
        address from,
        bytes calldata encodedFunction,
        uint256 transactionFee,
        uint256 gasPrice,
        uint256 gasLimit,
        uint256 nonce,
        bytes calldata approvalData,
        uint256 maxPossibleCharge
    ) external view returns (uint256, bytes memory) {
        return _approveRelayedCall();
    }

    // We won't do any pre or post processing, so leave _preRelayedCall and _postRelayedCall empty
    function _preRelayedCall(bytes memory context) internal returns (bytes32) {}

    function _postRelayedCall(
        bytes memory context,
        bool,
        uint256 actualCharge,
        bytes32
    ) internal {}
}

```

**Question** : I can’t figure out, how calling the `increase()` function triggers the Relayer? I mean I can’t find anything interacting with the `HTTP` interface of the Relayer.

---

<div class="post-metadata">

**Author:** ![abcoathup](https://sea2.discourse-cdn.com/flex022/user_avatar/forum.openzeppelin.com/abcoathup/32/415_2.png) [@abcoathup](https://forum.openzeppelin.com/u/abcoathup)\
**Post date:** [February 20, 2020, 3:32am UTC](https://forum.openzeppelin.com/t/how-does-a-contract-interact-with-the-gsn/2310/2 "2020-02-20T03:32:00Z")

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Hi @vasa-develop,

> [@vasa-develop](#):
>
> **Question** : I can’t figure out, how calling the `increase()` function triggers the Relayer? I mean I can’t find anything interacting with the `HTTP` interface of the Relayer.

In our `App.js` we use [OpenZeppelin Network.js](https://docs.openzeppelin.com/network-js) and specify that we want to use the GSN.

```javascript
  const context = useWeb3Network(PROVIDER_URL, {
    gsn: { dev: true }
  });

```

[OpenZeppelin Network.js](https://docs.openzeppelin.com/network-js) uses [OpenZeppelin GSN Provider](https://docs.openzeppelin.com/gsn-provider/) where transactions sent to contracts will then be automatically routed through the GSN.

---

<div class="post-metadata">

**Author:** ![vasa-develop](https://sea2.discourse-cdn.com/flex022/user_avatar/forum.openzeppelin.com/vasa-develop/32/1744_2.png) [@vasa-develop](https://forum.openzeppelin.com/u/vasa-develop)\
**Post date:** [February 20, 2020, 6:12pm UTC](https://forum.openzeppelin.com/t/how-does-a-contract-interact-with-the-gsn/2310/4 "2020-02-20T18:12:23Z")

</div>

Hey @abcoathup!

Just wanted to check if I understood the flow correctly:

- There is a `RelayHub` Contract.
- There are a number of `Relays`(or `Relayers`) with owners, all registered on the `RelayHub` Contract.
- We have a GSN compatible `Recipient Contract` that has a function named `increase()` that we want to execute. This `Recipient Contract` is funded and is charged by the `Relay` for fueling the transactions.
- We have a react app(to interact with the dapp) that uses OpenZeppelin Network.js which uses [OpenZeppelin GSN Provider](https://docs.openzeppelin.com/gsn-provider/).
- The [OpenZeppelin GSN Provider](https://docs.openzeppelin.com/gsn-provider/) selects the best `Relay` from the available relays.
- It contacts the winner relayer via the relay’s HTTP interface to send the required data.
- The relayer gets the required data via the HTTP interface and signs the transaction.
- The Relay submits the transaction to the `RelayHub` contract.
- The `RelayHub` publishes the transaction to the Network.
- The `Recipient Contract` is charged a fee according to the Relayer’s fee structure.

Is this right?

P.S. I have omitted the details like `stakes` etc. and assumed that all the conditions of the Relay to filter an eligible transaction are met, and nothing messes up.

---

<div class="post-metadata">

**Author:** ![abcoathup](https://sea2.discourse-cdn.com/flex022/user_avatar/forum.openzeppelin.com/abcoathup/32/415_2.png) [@abcoathup](https://forum.openzeppelin.com/u/abcoathup)\
**Post date:** [February 21, 2020, 5:59am UTC](https://forum.openzeppelin.com/t/how-does-a-contract-interact-with-the-gsn/2310/5 "2020-02-21T05:59:14Z")

</div>

Hi @vasa-develop

That flow is correct. Though it is the Relayer (not the RelayHub) that publishes the transaction to the Network.

In addition, a recipient contract will generally use a [GSN Strategy](https://docs.openzeppelin.com/contracts/2.x/gsn-strategies) to decide whether or not to accept a requested relayed call.

We can see the available relays using the following website with injected web3 (e.g. MetaMask) and setting the desired network in injected web3. [https://gsn.openzeppelin.com/relay-hubs/0xd216153c06e857cd7f72665e0af1d7d82172f494?listRelays=true](https://gsn.openzeppelin.com/relay-hubs/0xd216153c06e857cd7f72665e0af1d7d82172f494?listRelays=true)

Feel free to ask all the questions that you need.