Linking to EVM packages

ZeppelinOS allows us to link packages that have been already deployed to the blockchain, instead of wasting resources deploying them again every time we need them in a project. And of course, developers can upgrade their packages and we can update our links to point to the newest versions, so we are finally making an efficient use of all the developments in the community.

To use EVM packages we need first to initialize a ZeppelinOS project. Luckily, we already have one after following the guide about Deploying your first project.

To continue with this exploration, let's write a new contract called MyLinkedContract.sol and place it in the contracts folder. Then, let's make it import a very common contract from the OpenZeppelin EVM package:

pragma solidity ^0.5.0;

import "openzeppelin-eth/contracts/token/ERC721/ERC721Mintable.sol";

contract MyLinkedContract {
  ERC721Mintable private _token;

  function setToken(ERC721Mintable token) external {
    require(address(token) != address(0));
    _token = token;
  }
}

One thing to notice is that instead of importing openzeppelin-solidity we are importing openzeppelin-eth. This is the name of the OpenZeppelin EVM package, the one we have to use if we want to reuse the package already deployed. For more information, see this article which explains the difference between openzeppelin-solidity and openzeppelin-eth.

Now, let's link our project to the openzeppelin-eth package by running:

npx zos link openzeppelin-eth

This command will install the openzeppelin-eth contracts locally, which is needed to compile the contracts that import it; but it will also update the zos.json file with a reference to the linked package to make sure that when you deploy the project it will use the EVM package that already exists on the blockchain.

The following commands will be familiar to you. Just as we did before, we have to add the contract to the project:

npx zos add MyLinkedContract

If any of the following commands fail with an A network name must be provided to execute the requested action error, it means our session has expired. In that case, renew it by running the command npx zos session --network local --from 0x1df62f291b2e969fb0849d99d9ce41e2f137006e --expires 3600 again.

Now, let's push our changes to the blockchain:

npx zos push --deploy-dependencies

There is one caveat here with the --deploy-dependencies flag. We mentioned that this feature is about reusing packages that were already deployed, and we do have already deployed versions of openzeppelin-eth and many other packages to mainnet, ropsten, rinkeby and kovan. However, on this guide we are not using any of those networks, instead we started a local development network with Truffle that is empty. So, --deploy-dependencies tells ZeppelinOS to deploy to the network the EVM packages we depend on. This has to be done only once, so if you are using one of the real networks mentioned above, or you are running the command again on the local network, this flag won't be needed.

Repeating ourselves from before, let's make an upgradeable instance of the contract:

npx zos create MyLinkedContract

We also need an instance of the ERC721 token from the EVM package:

npx zos create openzeppelin-eth/StandaloneERC721 --init initialize --args MyToken,TKN,[<address>],[<address>]

<address> will be the minter and pauser of the token. For local development you can use one of the 10 addresses that ganache-cli created by default.

Finally, we can start a new Truffle console to interact with our contract by running:

npx truffle console --network local

Now, let's jump to that Truffle console and connect our two deployed upgradeable contracts:

Make sure you replace <my-linked-contract-address> and <my-erc721-address> with the addresses of the upgradeable instances your created of MyLinkedContract and StandaloneERC721 respectively. Both addresses were returned by the create commands we ran above.

truffle(local)> myLinkedContract = await MyLinkedContract.at('<my-linked-contract-address>')
truffle(local)> myLinkedContract.setToken('<my-erc721-address>')

Remember that the addresses of both your contract and the token, can also be found in the zos.dev-<network_id>.json configuration file.

This is just the beginning of a better blockchain ecosystem, where developers share their knowledge and their cool ideas in EVM packages, and we all contribute by using and improving those packages. This soon will be a swarm of packages that implement crazy new ways for a society to work, and they will all be available for you to just link into your project and build on top of them.

But so far we have only seen the side of the users of EVM packages. I'm sure you are now asking yourself how can you publish all the packages that you are developing. That's what we'll explore next.