All but the simplest of Ethereum applications are composed of several smart contracts. This is because of a hard limit of in any deployed contract, and because your sanity will slip away as the complexity of a smart contract grows. 24KB How to make your smart contracts cooperate safely Once you break down your code into manageable contracts you will certainly find that one contract has a function that should only be called by another contract. For example, in , only the contract factory is supposed to initialize a Uniswap Pair. Uniswap v2 The Uniswap team solved their problem with a simple check, but wherever I look I find more examples of orchestration solutions being coded from scratch for each project. In the process of understanding this problem and developing a pattern, we understood better how to build applications out of several smart contracts, which made more robust and secure. Yield In this article, I’m going to look in depth at smart contract orchestration with examples from well-known projects. By the time you are done reading it, you will be able to look at the orchestration requirements of your own project, and decide which one of the existing approaches suits you. I made this to get you going, when you are ready. example repository Let’s dive in. Background I advanced already that you will need to break down your project into a collection of smart contracts because two limits, one technical, one mental. The technical limit was imposed in November 2016. At that time the was implemented in the Ethereum mainnet, including . This changelimited the size of deployed smart contracts to a maximum of 24576 bytes. Spurious Dragon hard fork EIP-170 Bitten by the Dragon Without this limit, an attacker could deploy smart contracts taking infinite amounts of computation during deployment. It wouldn’t impact any of the data stored in the blockchain, but could be used as a kind of Denial-Of-Service attack on Ethereum nodes. At the time the block gas limits didn’t allow for smart contracts of that size anyway, so the change : was considered non-breaking “The solution is to put a hard cap on the size of an object that can be saved to the blockchain, and do so non-disruptively by setting the cap at a value slightly higher than what is feasible with current gas limits (an pathological worst-case contract can be created with ~23200 bytes using 4.7 million gas, and a normally created contract can go up to ~18 kb).” That was well before the DeFi explosion. For we coded 2000 lines of smart contract code, which deployed would add up to close to 100 KB. Our auditors didn’t even consider our project to be very complex. Yield Yet we still had to break down our solution into multiple contracts. Complexity and Object-Oriented Programming The second reason to break down a blockchain application into several smart contracts has nothing to do with technology limits, but with human limits. We can only keep that much information in our brains at a given time. We perform better if we can work in smaller problems that interact in limited ways, than in one single large problem where everything can interact with everything. It is probably fair to say that Object-Oriented Programming allowed software to reach a higher level of complexity. By defining “objects”  that represented some concept, and defining variables and functions as properties of an object, developers could make better mental images of the problem they were solving. Solidity uses Object-Oriented Programming, but at the contract level. You can think of a contract as an object, with its variables and functions. A complex blockchain application will be easier to picture in your mind as a collection of contracts, each representing one single entity. For example, in MakerDAO you have separate contracts for each one of the cryptocurrencies, another contract for recording debt, separate contracts to represent gateways between the debt vault and the external world, among many others. Trying to code all this in a single contract might be impossible. If possible at all, it would be really hard. Breaking big problems into smaller ones that interact in limited ways really helps. Implementation In the next section, we will look at the orchestration implementations of Uniswap, MakerDAO and . It will be fun. Yield The simple one — Uniswap and Ownable.sol I like Uniswap v2 because it is so simple. They managed to build a hugely successful decentralized exchange in 410 lines of smart contract code. They only have two types of deployed contracts: One factory, and an unlimited number of pair exchange contracts. Because of the way their factory contract is designed, the deployment of a new pair-trading contract is a two step process. First the contract is deployed, and then it is initialized with the two tokens it will trade. I don’t know the vulnerability they were protecting themselves against, but they needed to ensure that only the factory that created the pair-trading contract would initialize that contract. To solve this problem they reimplemented the pattern. Ownable It worked out for them, and it will work out for you if your case is as simple as theirs. If you know that your contract only needs to give privileged access to only one other contract, then you can use . You don’t even need to use a factory like Uniswap. You can have one user deploy two contracts ( and , with inheriting from ), and then execute . Ownable.sol Boss Minion Minion Ownable.sol minion.transferOwnership(address(boss)) The complete one — Yield For we didn’t manage to code a solution as simple as Uniswap v2. Our core is five contracts, and privileged access relationships are not one-to-one. Some contracts have restricted functionality that we need to make available to several contracts in the core group. Yield So we just extended to have two access tiers, one of them with multiple members: Ownable.sol The contract owner can add any addresses to a privileged list ( ). Inheriting contracts can include the modifier which will restrict access to authorized addresses. authorized onlyOrchestrated As an additional security check, each address is registered along with a , narrowing down the access to a single function in the Orchestrated contract. Check the for details on this. function signature example repository There isn’t a function to revoke access because we would the contracts during deployment, and then would renounce to its own privileged access by calling on all contracts. orchestrate owner transferOwnership(address(0)) Our own platform token, , would inherit from and limit to specific contracts set up before renounces to its privileges: yDai Orchestrated mint owner This pattern is relatively easy to implement and debug, and allows us to implement functions that should be used only by contracts we control. The obfuscated — MakerDAO MakerDAO is infamous for using nonsensical terminology, which makes it extremely hard to understand. It was only well after I solved orchestration for that I realized they use almost exactly the same implementation. Yield The contract deployer is the original member of the . wards can on other people ( ) so that they also become . wards rely usr wards Functions can be restricted ( ) so that only can execute them. auth wards As an example, the function in MakerDAO’s contract is used to update an interest rate accumulator, and should only be called by another of the contracts in their collection ( contract, function). If you look at the function you will see the modifier, which is what they use for orchestration: fold Vat.sol Jug.sol drip auth In a way, and the other orchestration implementations are an extension of the and functions concept, only for access control between contracts. auth private internal The MakerDAO implementation is very similar to the one we came up with ourselves. The contract deployer is the original member of the . In it would be . wards Yield owner can on other people ( ) so that they also become wards. In , only the can other addresses as . wards rely usr Yield owner orchestrate authorized Functions can be restricted ( ) so that only wards can execute them. In we say that addresses can call the marked functions. We further restrict access to a single function. auth Yield onlyOrchestrated Except that in we used two access tiers ( and ) and individual function restrictions, the implementation is the same. Contract orchestration is a common pattern that could be implemented once and reused often. Yield owner authorized To make auditors and users even happier, we also developed a script that and paints a picture of ownership and orchestration for our contracts. This script will be available from our website at go-live and proves that no one has or ever will have privileged access to them, except the contracts set up at deployment. trawls the blockchain events After all, that’s the whole point of contract orchestration. Conclusion Orchestration of smart contracts is a problem that repeats itself in most projects, and which most projects implement from scratch. Often the solutions implemented are nearly identical to each other. While a canon emerges that we all can use to implement orchestration safely and efficiently, please use the examples in this article to understand and implement a solution for your requirements. Use the code in the if it suits you. example repository Finally, thanks for reading this, and if you have any questions or feedback, please don’t hesitate to contact me. Thanks to , and for their excellent feedback while coding the smart contracts. Allan Niemerg Dan Robinson Georgios Konstantopoulos Yield

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