EVM package deployment with ZeppelinOS - Part I Introduction and Creating a LinkedList Contract

Introduction to ZeppelinOS and Package Managers

If you're familiar with Node.js, then you will be familiar with NPM (Node Package Manager). You will also know that the ability to npm install existing code in your project makes your life as a programmer easier and, frequently, more secure. Being able to import existing code is a hallmark of a mature developer ecosystem and one of the fundamental tools that allow a programming language to reach an ecosystem of scale. The immense success of Node.js is due in no small part to the NPM ecosystem. In the last month alone, NPM saw 33,839,343,034 downloads (at the time of writing)!

In comparison, Ethereum, and the primary language for smart contracts, Solidity, is in its infancy. Yet the Solidity ecosystem is growing rapidly, and as it grows, so does the complexity of tasks programmers are trying to solve. Starting from scratch on every project can be a major impediment to progress, especially when code can potentially store (and lose!) millions of dollars in real money. A frugal blockchain developer will try to write as little "new code" as possible, instead importing existing audited code libraries so as to reduce introducing new and unforeseen bugs as much as possible.

To this end, Zeppelin created OpenZeppelin, an open source framework of battle-tested, reusable smart contracts for Ethereum (as well as other EVM and eWASM blockchains). OpenZeppelin smart contracts allow you to get up and running with standard blockchain tasks, with the confidence that the code you're using has been through rigorous testing by auditors and the open source community.

ZeppelinOS takes this idea even further by offering a development platform designed specifically for smart contract projects. It allows for seamless upgrades and provides economic incentives for creating a healthy ecosystem of secure applications. Most importantly, it allows you to build projects that leverage the power of code that is already deployed on-chain. Not only does this dramatically increase the transparency of your project to your potential users, but it also saves you money, as you don't need to pay the gas cost to deploy entirely new copies of the existing code to the network. You just reuse the logic from contracts that someone else has already deployed!

With ZeppelinOS, not only can you now create upgradable smart contracts (where the upgrading process can be controlled by any number of governance techniques), you can also create EVM packages to allow others to reuse, remix, and develop on an open ecosystem.

What you will learn

In this series, we're going to go over the complete process for creating, deploying, and linking on-chain bytecode using NPM and the ZeppelinOS system.


Who this tutorial is meant for

This tutorial aims to be as detailed as possible to cater to Solidity developers at all levels of experience. For beginners, it's best to follow the entire document, as I will build on steps from previous sections. For more advanced Solidity developers, feel free to skip ahead.

What are EVM packages?

EVM packages are collections of deployed on-chain code that you can incorporate and reuse in your own project via package managers such as NPM. This makes it easy to build upon open source libraries created and verified by others in the ecosystem. ZeppelinOS makes the process of creating and using EVM packages simple.

What are we going to build?

For this tutorial, we're going to create our own EVM package for a Solidity Linked List implementation: custom code that tackles a real use case. Our linked list will give your Solidity projects an easy-to-use data structure that we will deploy on-chain and that you can reuse over and over for any project you might like. No Hello World here!

What do I need to get started?

If you are an existing Solidity developer familiar with tools such as Truffle, Ganache, Node.js, and Remix, you're probably already set up. If not, don't worry, we'll cover the steps necessary to get you started.


While this guide will be oriented toward Unix-based operating systems such as Ubuntu or macOS, as long as you can get the core requirements running, you should be able to follow along with this guide.


Ganache is a development blockchain that runs locally on your computer and is used for testing. Previously called TestRPC, Ganache runs a full ephemeral Ethereum blockchain on our machine that behaves and acts like the real thing (with customizable differences). We'll use it for testing our contracts.

npm install -g ganache-cli

ZeppelinOS is the development platform that will allow us to not only create and deploy our EVM package but also to upgrade smart contracts when we want to add new features or fix bugs. Additionally, ZeppelinOS introduces economic incentives to support a healthy ecosystem of secure applications. Learn more at ZeppelinOS.

npm install --global zos

To see a list of command line options once they're installed:

zos --help

To learn more, check out the API reference docs.


Go ahead and get Truffle. Truffle is one of several development frameworks for Ethereum that can make developing dApps (distributed applications) easier. Recently, Truffle upgraded to v5, which introduces a number of breaking changes. If you have Truffle v4, be sure to upgrade.

npm install [email protected]

Project Setup

First, create a new directory and navigate into it.

mkdir LinkedList
cd LinkedList

Feel free to name your directory and project whatever suits you best.

We'll now use NPM to create your package.json file.

npm init

Take care to answer the prompts, as this information will be needed when you publish to NPM.

Now we're ready to initialize our ZeppelinOS project.

zos init LinkedList>>Successfully written zos.json

Like npm init , zos init will create its own JSON file, zos.json, to track the details of your ZeppelinOS project. The command will also initialize Truffle, which will create two directories, contracts and migrations, as well as a truffle-config.js file.

Your directory structure should look like this:


Both contracts and migrations are empty folders.

The truffle-config.js file exports an object that defines the networks Truffle can use and the settings for deployment. For now, you should only see a "local" network. This network will be our development network.

"use strict";
module.exports = {
  networks: {
    local: {
      host: "localhost",
      port: 9545,
      gas: 5000000,
      gasPrice: 5e9,
      network_id: "*"

The zos.json file will fill up as soon as you start adding and deploying contracts. The information about our contracts will be stored here.

  "zosversion": "2",
  "name": "LinkedList",
  "version": "0.1.0",
  "contracts": {}

The files and folders so far are package.json (created by npm init), two empty directories named contracts and migrations (created by zos for Truffle), and a zos.json file (created by zos for ZeppelinOS).

Creating the LinkedList contract

The goal of the LinkedList.sol contract is to create a data structure that will allow us to add and remove nodes from the head of a list. In essence, we're going to create a stack. Custom data structures are useful, because Solidity has limited built-in array methods, and array manipulation can quickly become an expensive (or even impossible!) operation to complete on-chain because of gas costs.

Need a linked list refresher? Here are some references I used for this tutorial:

The Little Guide of Linked List in JavaScript A Linked List is a list of nodes that are represented by a head that is the first node in the list and the tail that is the last one. Read more at hackernoon.

JS Data Structures: Linked ListLearn what a Linked List is and how to write one in JavaScript. Read more at codeburst.ioOpen your code editor of choice and navigate to your project. Create a Solidity contract called LinkedList.sol and save it in the contracts folder.

If you're interested in an excellent tutorial about building a linked list in Solidity, I highly recommend Austin Thomas Griffith's "Linked Lists inSolidity", which I used as a starting point for the following code:

pragma solidity >=0.4.24 <0.6.0;

import "zos-lib/contracts/Initializable.sol";

contract LinkedList is Initializable{

    event EntryAdded(bytes32 head, string data, bytes32 next);

    //Struct will be our Node
    struct Node {
        bytes32 next;
        string data;

    //Mappping will hold nodes
    mapping (bytes32 => Node) public nodes;

    //Length of LinkedList (initialize with constructor/initalizer)
    uint public length;

    //Head of list;
    bytes32 public head;

    //Name of LinkedList (the purpose for the list)
    string public listName;

    function initialize(string memory _listName) initializer public {
        require(bytes(_listName).length >= 0);
        length = 0;
        listName = _listName;

    function addNode(string memory _data) public returns (bool){
        Node memory node = Node(head, _data);
        bytes32 id = keccak256(abi.encodePacked(node.data, length, now));
        nodes[id] = node;
        head = id;
        length = length+1;

        emit EntryAdded(head, node.data, node.next);

    function popHead() public returns (bool) {
        require(length > 0, "error...head is empty");
        //hold this to delete it
        bytes32 newHead = nodes[head].next;
        //delete it
        delete nodes[head];
        head = newHead;
        length = length-1;

    //Contract interface
    function getNodeExternal(bytes32 _node) external view returns (bytes32, string memory){
        return (nodes[_node].next, nodes[_node].data);


On line 3, we're importing:

import "zos-lib/contracts/Initializable.sol";

Initializable.sol is not installed by default with ZeppelinOS, but you will need it in order to compile the contract. To install it:

npm install zos-lib

Here we're importing from the zos-lib a contract called Initializable.sol, and we're  telling the compiler on line 5 that our contract LinkedList is initializable. Then on line 26, we create a function called initialize(), which uses the library modifier initializer which can be found in the zos-lib/contract/Initializable.sol contract.

It's not entirely critical to understand the purpose of the initializer pattern for this tutorial, but you should know that when building upgradable smart contracts using the ZeppelinOS system, you must use initializer functions instead of constructor functions in your contracts. Constructor functions are incompatible with the way ZeppelinOS proxy contracts manage upgradable smart contracts. Learn more from "Deploying your first contract."

Strictly speaking, in this example it's not really necessary to use the initializer function, as the LinkedList.sol contract only uses the initialize() function to set the value of length to zero. In Solidity, undeclared values are by default zero, making this initializer redundant. I am including it as a reminder that in the ZeppelinOS system, we do not use constructor functions.

Deploying to our development network

To get started with testing the contract (to be sure it works!), first deploy it to Ganache, our development blockchain environment.

Open a new terminal window and type the following:

ganache-cli --port 9545 --deterministic

You will want to run this in a new window, because you'll need to keep it running during development. If you close it, you will lose your development blockchain state, including all of your locally deployed contracts, and be forced to start again. After starting ganache-cli, you should see an output like this:

Ganache CLI v6.1.8 (ganache-core: 2.2.1)
 Available Accounts
 (0) 0x90f8bf6a479f320ead074411a4b0e7944ea8c9c1 (~100 ETH)
 (1) …
 (9) 0x1df62f291b2e969fb0849d99d9ce41e2f137006e (~100 ETH)
 Private Keys
 (0) 0xca9aca918d1c47237a858be560fa444k4qoid23s4e46d63ffc5edeeab2918f
 (1) …
 HD Wallet
 Mnemonic: <>;
 Base HD Path: m/44'/60'/0'/0/{account_index}
 Gas Price
 Gas Limit
 Listening on

This is a list of pre-generated and pre-funded accounts that you can use on your development blockchain, along with a generated mnemonic, details about gas price, gas limit, and base HD (hierarchical deterministic) wallet path.


Note that as arguments for the command ganache-cli, we have both --port and --deterministic.

The --port argument has the same value (9545) as we saw in our truffle-config.js file.

The --deterministic argument means that every time we run ganache-cli, we're going to use the same mnemonic so that we can ensure we get the same test wallet addresses and corresponding private keys. Without this flag, we would have a list of random addresses each time, which can be confusing when testing projects.

I like to try and keep a single development mnemonic for all my dev work, so I recommend copying down this mnemonic and saving it. If you're doing development work across multiple teams or platforms, it can be useful for everyone to work with the same set of addresses. To use your mnemonic, run ganache-cli with the following additional argument:

–mnemonic "your twelve word mnemonic here"

Remember that this mnemonic is for DEVELOPMENT ONLY. For a complete list of command line options, see GitHub for ganache-cli.

Return to your original terminal window and create a new session for ZeppelinOS in the top-level folder of your project.

zos session --network local --from 0x1df62f291b2e969fb0849d99d9ce41e2f137006e --expires 3600

The command line argument --network local instructs zos to use the local network as specified in our truffle-config.js file, while the --from argument specifies the address from which ZeppelinOS will be deploying contracts and managing them. In this case, the --from address is the tenth address that ganache-cli generated.\ Notice that we're using the tenth address that Ganache provides as the address for our ZeppelinOS session. Essentially, the proxy contract that is deployed when creating upgradable contracts responds differently depending on whether or not the address calling the contract is the same as the address that manages it. Only the address that deploys the proxy can interact directly with the proxy, while all other addresses that call the proxy will be proxied transparently onward toward the contract that the proxy points to.

Fortunately, ZeppelinOS takes care of proxies for us, and as a developer, it is only important to remember that  --from needs to be an address that will never be used to interact with the contract except for upgrading it. So in this case we use the last address Ganache offers: the tenth. But you can use any address as long as you remember that it controls the proxy but won't be proxied itself. To read more, see ZeppelinOS upgrades pattern section.

Finally, --expires 3600 sets the duration for which we want to keep the ZeppelinOS session open. f you time out, the command will need to be re-entered. You will know your session has timed out if you receive this message when you try to execute other zos commands:

A network name must be provided to execute the requested action.

Not to worry, re-enter the following and you'll have a new session to work with.

zos session --network local --from 0x1df62f291b2e969fb0849d99d9ce41e2f137006e --expires 3600

Once you have a ZeppelinOS session running, you can always check the file .zos.session in your main project. Here you will see the information regarding your current session; the network you're using, the from address, and when the session expires.

Adding our contract

Now that you have set up your ZeppelinOS session, created your contract, and have your development blockchain running, you need to add your contract to the ZeppelinOS project. In the same terminal window, in the top-level folder of your project, run the following:

zos add LinkedList

If you look at the zos.json file, you will see that your JSON object now contains our contract "LinkedList" under the "contracts" key value.

Similarly, you now have a top-level folder in your project named build that contains a subfolder called contracts, which is where the JSON objects representing the contract's ABI (application binary interface) are stored.

Now that you have added your LinkedList.sol contract to the ZeppelinOS project, we can deploy the project to the development blockchain.

zos push

When it comes time to deploy to a different network (such as a public test-net or mainnet), you will run the zos session command again with a new network, as defined in your truffle-config.js file. You can also add the --network flag followed by your network name if you want to push without starting a session.

Note that if you take a look at your second terminal where Ganache is running, you'll see that a transaction has been created.

Return to your first terminal window where you issued the zos push command. The final line of output should be something like this:

Created zos.dev-{some number here}.json

If you look at your project folder structure, you will see a new JSON file has been created in your project with the filename zos.dev-{ome number here}.json. The "{some number here}" will be the network-id of your development blockchain. This will most likely be a pseudo-random number when working with a development chain and is used just to identify the network. There's an unofficial list of public networks and their identification numbers.

The zos.dev-{some number here}.json file holds all the information regarding the current deployment of our contracts on this particular network. If you open it up, you'll see the address the contracts are deployed at along with information about them.

Here's more information regarding networks.

Creating an instance of our contract

Now that we have deployed our project, we need to make an instance of our contract to interact with it.

zos create LinkedList --init initialize --args "NameOfYourLinkedListHere"

This command creates an instance of our LinkedList.sol and calls the initialize function in place of what would normally have been a constructor. It will return an address where the instance was created (these addresses will be unique to your project):

Using session with network local, sender address 0x1df62f291b2e969fb0849d99d9ce41e2f137006e
Creating proxy to logic contract 0x2adf8b30d4dd24a05ccd9afbdc06a5b49c9c758d and initializing by calling initialize with:

Instance created at 0x25f96b23947f3e57b29d15760fd8af926694fa81

The zos create command creates an instance of the contract on the blockchain that we can directly interact with. While zos push deploys the project, our project isn't intended to be directly manipulated. Rather, ZeppelinOS gives us the ability to create proxies of contracts that point to the original contracts we wrote. This gives us the ability to upgrade our contracts by telling ZeppelinOS we want to point our instance of a contract to a newer version of our original contract.

If you take a look again at the zos.dev-{some number here}.json file and scroll to the bottom, you will now see a top-level value named "proxies" near the bottom of the file. This is where ZeppelinOS keeps track of your contract deployments on this particular network. The address refers to the location of the proxy of your contract, version is the version of our instance, and implementation points to the logic contract our contract calls will be proxied to.

The address entry will be the address that you will use when you want to connect to your contract (via the proxy) for something like testing or via web3 in a dApp.

Thanks to Santiago Palladino for reading early drafts and providing feedback.