Cartesi - Platform for DApps Based on Layer-2 Scalability Solution
Introduction
Take a moment to reflect on the complexity of the software infrastructure that increasingly permeates all aspects of our lives. This involves operating systems, software libraries, tools, protocols, programming languages, compilers, and other technologies. This environment is the result of ongoing global efforts over the past 50 years and has spent billions of hours of work. This interdependent network of components enables software to become so pervasive. Without it, none of the services and applications that we rely on will be possible, or even possible.
Blockchains is a relatively new breakthrough. The main innovation of Bitcoin is the mechanism by which a network of agents can maintain a decentralized consensus on a shared situation. In this context, decentralization means that the state is managed by the participants themselves in a way that does not require mutual trust. This situation contains, among other data, payment systems. The bets each participant has in the resulting economy serves as their incentive to make the country widely available and to reject unauthorized changes to the country. This virtuous cycle creates a standalone system: The payment system is built on a decentralized consensus, which only functions because of incentives created by the payment system itself.
Ethereum significantly generalized this idea by allowing transactions to be expressed as arbitrary programs, or smart contracts. It soon became apparent that programmable transactions over a decentralized state could be used for much more than simply maintaining a payment system. Blockchains essentially became equitable, decentralized ‘’world computers’’ that cannot be turned off and cannot be controlled by individuals or institutions. A new kind of (decentralized) application emerged, called DApp, to run on these world computers.
When compared with traditional computer programs, DApps face two major limitations. The first is one is scalability. Because smart contracts must be executed by all participants in a blockchain, DApps are crippled by stringent limits on computation, slow transaction rates, tight storage space, and high price. This issue has been widely recognized as one of the key factors limiting the growth of blockchain adoption. As a result, several current projects focus on developing scalability solutions for blockchains (e.g. Plasma, sharding, side chains, state channels, etc).
The second limitation has received much less attention: primitive software infrastructure. Here, the culprit is the lack of an operating system. Operating systems are the basis upon which all of the past 50 years of software development is built. Cut off from all this previous work, DApp developers struggle to accomplish tasks that are trivial for conventional developers, like transferring files, compressing data, finding the color of a pixel in an image, or querying a database for a record. Even the concept of a file itself is missing!
There is a chasm between today’s immature blockchain software and the vast software infrastructure that is readily available and running on our laptops and smartphones.
DApps should be capable of performing computations like the ones “centralized” applications regularly do. DApp development should have access to the languages, libraries, and tools that are standard in modern operating systems like Linux. Going even further, we should strive for a higher-level infrastructure that is generic and scalable while abstracting away the nuts and bolts of individual blockchains and consensus layers. Developers should be able to focus on their specific DApp logic instead of dealing with the intricacies of state channels, side-chains, and the blockchain itself. DApps should be easily portable across different blockchains.
This kind of infrastructure would blur the lines between the skills required to develop centralized and decentralized applications. It would pave the way for a new generation of DApps that are as inconceivable today as the modern Internet was 50 years ago. We at Cartesi have been pursuing this inevitable future. To that end, we specified and implemented a decentralized Linux infrastructure for scalable blockchain applications.
Before getting into the weeds of our project, let us take a short detour into the basics of layer-2 systems for scalable DApps. Systems like Plasma, state channels, TrueBit, and Cartesi differ from each other in many respects, but they are based on the same fundamental concepts.
Layer-2 Solutions
Blockchains achieve consensus by redundancy, replicating computation and data across the entire network. Because of that, DApps cannot rely on them for high-frequency transactions, for complex processing, or for storing large amounts of data. This would be expensive, wasteful, and inefficient. The range of practical DApps is severely constrained by these limitations. In contrast, centralized applications running on servers on the cloud (or even on our personal devices) do not suffer from these limitations. What we need is to combine the performance and generality of conventional software systems with the consensus and security guarantees of blockchains.
Although global consensus is an expensive resource, most of the data and processing associated with a DApp (i.e., apart from financial aspects) are of no interest to the entire network. They only matter to the few parties involved in that specific DApp. Because of this, it is almost always sufficient for the involved parties to maintain their own local consensus. The strong guarantees offered by the global consensus of the entire blockchain network can be used sparingly to resolve eventual disputes between these parties.
The general idea is as follows. All parties involved with a given DApp are bound together by smart contracts with well defined crypto-economic incentives that reward honest behavior and punish dishonesty. A common design is to have smart contracts require all involved parties to place a deposit as collateral before engaging with the DApp. When all participants agree with a change to the local state of the DApp, they communicate their agreement and set of changes to the blockchain. The blockchain then reacts by modifying the relevant global state. When any involved party has proof of misbehavior, they can appeal to the blockchain before the changes become permanent.
The blockchain becomes a referee that arbitrates in favor of the honest party, rewarding him with the collateral seized from the bad actor.
This simple yet powerful idea minimizes blockchain resource consumption while preserving the strong security guarantees of the entire network. This arbitration mechanism is inspired by age-old interactions between humans. We don’t engage the supreme court before each human interaction. The vast majority of transactions, even those involving money, are settled locally. We don’t call a lawyer before we go out and buy fruits in a street market. That would be extremely inefficient. Instead, we have a system in place that is used only when necessary: The judicial system is only engaged in case of legal disputes.
Applying this metaphor to well-designed blockchain systems involving crypto economic incentives is the key to efficient, scalable DApps.
Cartesi Linux on the blockchain
We have just seen how a layer-2 system can decouple local interactions from the blockchain while preserving its security guarantees. We can now use this decoupling to significantly increase the expressiveness and power of the infrastructure. In particular, we create a paradigm shift in DApp development by transforming the platform from what feels like a toy spreadsheet into something that behaves like a real computer running Linux.
The Cartesi Core (see figure 1) is a set of basic technologies that constitute the first step towards the vision we have been discussing. It includes on-chain and off-chain components. The off-chain component is what we call the Cartesi Node. In a nutshell, each participant that wants to interact with a Cartesi DApp does so through a Cartesi Node. They can either run their own Cartesi Node or hire a third party that does to represent their interests.
The essential module in the Cartesi Node is the Cartesi Machine, a custom virtual machine that emulates a RISC-V microprocessor and runs an embedded Linux distribution. When developing a DApp with Cartesi, most of the decentralized logic will be coded to run under Linux off-chain, instead of coded as smart contracts and deployed on-chain.
Cartesi Machines
Cartesi Machines are special in many ways. First, they are completely self-contained. This means that there are no external sources of entropy that can alter the machine state in a non-deterministic way (such as a keyboard, or mouse, or a wall-time clock). Second, they are reproducible. In other words, when two Cartesi Machines start from the same initial state and run for the same number of processor cycles, they reach exactly the same final state. This is true regardless of the architecture and operating system hosting the Cartesi Node. Finally, Cartesi Machines offer full transparency. The state is specified and exposed in such a way that any external observer with access to the entire state at a given processor cycle can predict the entire state at the next processor cycle.
The self-containment and reproducibility properties ensure that any two parties that have access to the initial state of a Cartesi Machine will obtain the same results when they run the machine in their Cartesi Nodes. Naturally, this is vital for consensus. Unfortunately, it is not, in general, guaranteed by hardware architectures or virtual machines. The Cartesi Machine was designed specifically to satisfy these requirements. The transparency property means the entire state of a Cartesi Machine can be Merklized. Among other things, this enables the blockchain to represent a machine with a single hash. As we will see, these properties allow the blockchain to efficiently settle any disputes over the results of executing Cartesi Machines off-chain.
Settling Disputes
Consider two actors, Alice and Bob, both involved in a transaction controlled by a smart contract. Let us assume that the transaction involves some complex computation that is impractical to execute on-chain. Now assume further that the smart contract knows the initial root hash for a Cartesi Machine that, when booted, runs a program in drive 1 that performs the required computation using the input from drive 2 and writes the output to drive 3. Using efficient hash operations, the smart contract can replace drive 2 with its desired input. It can then demand from Bob the root hash that results from executing the corresponding Cartesi Machine off-chain.
There are now two possibilities. If Alice agrees with Bob, the smart contract can proceed by requesting from Bob details about drive 3, satisfied that both Alice and Bob will agree with the consequences. If Alice disagrees, she starts a dispute by depositing a collateral. If Bobs wants to defend against the dispute, he answers by depositing the same collateral. (Otherwise, after a timeout, he is considered to have forfeited.) What follows is an interactive protocol, involving Alice and Bob and mediated by the blockchain, that guarantees that whoever is being honest will win the dispute.
The verification game [Feige and Kilian 1997] is an algorithm that allows an arbiter with limited computational resources to mediate a game between two computationally unlimited players. In our case, the limited mediator is a set of smart contracts running on the blockchain. The unlimited players are the Cartesi Machines running on Alice’s and Bob’s Cartesi Nodes. This procedure is also used by TrueBit, and a detailed explanation can be found in this great article by Sina Habibian.
In a nutshell, using an n-ary search, a smart contract identifies the last processor cycle for which Alice and Bob agree on the root hash for the entire state of the Cartesi Machine. Then, the blockchain executes a single RISC-V instruction to obtain the root hash for the state that follows. Alice and Bob are known to disagree on this hash. By identifying who posted the correct results, the smart contract can arbitrate in favor of the honest party and reward him or her with both collaterals.
Running a single RISC-V instruction is painless and very cost-effective for the blockchain. Locating the instruction takes only logarithmic time and space. Moreover, this process is only engaged in case a dispute arises. Disputes are exceedingly rare, because cheating will always be caught, and cheating brings punishment.
The Future
Within the past 20 months, most of the Cartesi Core, as described in our white paper, has been implemented. In future articles, we will showcase its power and flexibility by building example DApps. Needless to say, these DApps will involve verifiable computations over large amounts of data, while leveraging the vast software infrastructure available to developers on Linux.
Cartesi’s Core is only the first step towards our vision. Cartesi’s mission is to close the gap between centralized and decentralized applications, both in terms of possibility and convenience.
We will continue to work towards eliminating all limitations from DApps powered by Cartesi. We will offer seamless state channels and a verifiable off-chain data-exchange protocol. We will create a new programming environment that integrates the development of all the components of a Cartesi DApp. Ultimately, we will make DApps easily portable across different blockchains.
Our hope is that, by empowering DApp developers, we will help them build products that are ever more compelling to users, and bring us all closer to a future in which decentralization becomes less burdensome.
Conclusion
This paper lays the foundation on which the Cartesi platform stands. Cartesi's mission is to help DApp developers build more products that appeal to their clients. As the paradigm changes, the blockchain presents opportunities for real innovation and the risk of "reinvention wheels". In the immediate application of the principle at least with amazement, Cartesi's core allows developers to increase their existing knowledge and tools to increase their productivity. The remaining components of the Cartesi platform, explained in future documents [Teixeira and Nehab 2019a], will help developers unleash their creativity when exploiting the uniqueness of the potential blockchain.
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