GSN Provider is deprecated. We are no longer developing new features nor addressing issues. Read here for more info.

Testing GSN Applications

A GSN-capable smart contract or web application cannot be properly tested without interacting with relayers and the RelayHub contract. While these exist on all testnets, it is often desirable to be able to perform tests on a local blockchain.

This guide will teach you how to have a slimmed-down version of the GSN on your development machine.

Deploying the RelayHub Contract

The on-chain part of the GSN cannot be stubbed or faked: the contracts will only recognize the canonical addresses.

Doing this locally however is not a complex process: head to the OpenZeppelin GSN Helpers to learn how to deploy a RelayHub instance, the core contract of the GSN, both from the terminal and programatically.

Using the Development Provider

In addition to the GSNProvider, this package includes GSNDevProvider: a provider that also acts as a GSN relayer.

Any transactions sent through it will be signed by the sender, and relayed by another address. It will register itself in the relay hub as

The GSNDevProvider still needs RelayHub to be deployed on the network.

It requires two addresses with funds: one to act as the relayer, and one to act as its owner (who will register it on the RelayHub). If these are not set when constructing the provider, they will default to the first two accounts on the local node.

const { GSNDevProvider } = require("@openzeppelin/gsn-provider");
const gsnDevProvider = new GSNDevProvider("http://localhost:8545", {
  ownerAddress: accounts[0],
  relayerAddress: accounts[1]

The `GSNDevProvider’s basic usage is the same as the regular `GSNProvider’s: transactions sent using it will go over the GSN by default.

With this setup in place, you will be able to test interactions that rely on the acceptRelayedCall, preRelayedCall, and postRelayedCall methods of your contracts. However, unlike regular relayers GSNDevProvider will never fail: this part of the testing process needs to be carried out on a real network.