Key Concepts in Hyperledger Composer
Hyperledger Composer is a programming model containing a modeling language, and a set of APIs to quickly define and deploy business networks and applications that allow participants to send transactions that exchange assets.
Hyperledger Composer Components
You can experience Hyperledger Composer with our browser-based UI called Hyperledger Composer Playground. Playground is available as a hosted version (no install necessary) or a local install (good for editing and testing sample business networks offline).
Developers who want to use Hyperledger Composer's full application development capabilities should install the Developer Tools.
Key Concepts in Hyperledger Composer
Blockchain State Storage
All transactions submitted through a business network are stored on the blockchain ledger, and the current state of assets and participants are stored in the blockchain state database. The blockchain distributes the ledger and the state database across a set of peers and ensures that updates to the ledger and state database are consistent across all peers using a consensus algorithm.
Hyperledger Composer uses Connection Profiles to define the system to connect to. A connection profile is a JSON document the is part of a business network card. These profiles are usually provided by the creator of the system they refer to and should be used to create business network cards in order to be able to connect to that system.
Assets are tangible or intangible goods, services, or property, and are stored in registries. Assets can represent almost anything in a business network, for example, a house for sale, the sale listing, the land registry certificate for that house, and the insurance documents for that house may all be assets in one or more business networks.
Assets must have a unique identifier, but other than that, they can contain whatever properties you define. Assets may be related to other assets or participants.
Participants are members of a business network. They may own assets and submit transactions. Participant types are modeled, and like assets, must have an identifier and can have any other properties as required. A participant can be mapped to one or multiple identities.
An identity is a digital certificate and private key. Identities are used to transact on a business network and must be mapped to a participant in the business network. A single identity is stored in a business network card and if that identity has been mapped to a participant, it allows the user of that business network card to transact on a business network as that participant.
Business Network cards
Business network cards are a combination of an identity, a connection profile, and metadata, the metadata optionally containing the name of the business network to connect to. Business network cards simplify the process of connecting to a business network, and extend the concept of an identity outside the business network to a 'wallet' of identities, each associated with a specific business network and connection profile.
Transactions are the mechanism by which participants interact with assets. This could be as simple as a participant placing a bid on a asset in an auction, or an auctioneer marking an auction closed, automatically transferring ownership of the asset to the highest bidder.
Queries are used to return data about the blockchain world-state. Queries are defined within a business network, and can include variable parameters for simple customization. By using queries, data can be easily extracted from your blockchain network. Queries are sent by using the Hyperledger Composer API.
Events are defined in the business network definition in the same way as assets or participants. Once events have been defined, they can be emitted by transaction processor functions to indicate to external systems that something of importance has happened to the ledger. Applications can subscribe to emitted events through the
Business networks may contain a set of access control rules. Access control rules allow fine-grained control over what participants have access to what assets in the business network and under what conditions. The access control language is rich enough to capture sophisticated conditions declaratively, such as "only the owner of a vehicle can transfer ownership of the vehicle". Externalizing access control from transaction processor function logic makes it easier to inspect, debug, develop and maintain.
The historian is a specialised registry which records successful transactions, including the participants and identities that submitted them. The historian stores transactions as
HistorianRecord assets, which are defined in the Hyperledger Composer system namespace.