summary: Cardstack is at the core of a four-sided market between users, developers, providers, and businesses. 
content: Cardstack is at the core of a four-sided market between users, developers, providers, and businesses. Cardstack provides everything the users need at a good price, with good services, and a good selection. Similarly to other open-sourced software like Wordpress, users can choose any hosting provider to manage and run their Cardstack-based app for them. A large number of hosting providers in the Cardstack ecosystem will ensure that users from around the world get tailored services that fit their budget and requirements. Overview Card SDK (Software Development Kit) is a composable UI system, allowing developers to easily build card templates, which end users then use to make their own cards. The Card SDK is deployed to the Cardstack Hub, which is the server. But that server is unique: In most systems, the developers run the server, so the users’ files are stored in the developer’s data center, being controlled by the developer. At Cardstacks, developers deploy their cards to the hub, which is run by service providers of their choice. This way, all users have their own hub, their own server, their own little personal cloud, where they have all their code — whether it’s for note-taking or music-making or image-editing. It is run by a service provider, but it belongs to them. Finally, the intersections between the users, the developers, the providers, and the businesses are important pieces as we create this full ecosystem, where all the participants and the value network are connected in a seamless way. The CARD Protocol The CARD Protocol integrates the Card SDK and the Cardstack Hub together in a decentralized way. If you only have one service and one SDK (for example, Instagram with an Instagram SDK), developers can only deploy their software to one company. But in our open-network approach, inspired by blockchain and implemented using open-source technology, we can essentially build a multi-currency payment and billing network The CARD Protocol allows a user to choose any service provider who runs a hub and say, “I pick you to help me store my image files, or manage my wallet, or count my sales, or process my credit card payments or cryptocurrency transactions.” Users are not limited to one provider or one hub. They can have different hubs and different service providers serving different cards, because one provider may be good at dealing with data, while another one may be better at dealing with a content management system. But it’s all built using the same Card SDK, thus offers one cohesive experience. And the Cardstack Hub is the server where it all comes together. Users are customers who deposit money to pay the service providers. Their funds get accumulated in a reward pool. Based on the rendered service and billable usage, the service providers receive a portion of that reward pool, which they can withdraw. The hub reports the relevant data to the protocol. The CARD Protocol coordinates between these six aspects — deposits, market rates, billable usage, withdrawals, purchases, and spendable balance — to ensure that this is a fair network. When it comes to payments, you get what your underlying asset is worth; so customers can be billed in a fair and accurate way. 