Obsidian Clients

Client programs, which interface with smart contracts that are installed on the blockchain, can also be written in Obsidian. Client programs must have a transaction called main that takes a remote reference to the smart contract as an argument. For example:

import "Auction.obs"

main contract AuctionClient {
    transaction main (remote Auction@Shared auction) {
        ...
    }
}

Important things to note about the above example:

• The client file imports the file that implements the smart contract that was installed on the blockchain (Auction.obs).
• The client has a transaction called main.
• The main transaction takes a remote reference to an Auction@Shared. The top-level smart contract is always Shared because many client programs can reference it. However, as a special exception to the usual rule that assets must have owners and there must be no shared references to assets, the blockchain itself is considered the owner of the top-level smart contract. Shared references are permitted because otherwise clients would not be able to use it.

Within the client, when transactions are invoked on remote objects, the invocations happen over the network, and become individual transactions on the blockchain. If any of these transactions fails, the whole client program will abort.

When a reference to a local (i.e. not remote) object is passed as an argument to a remote transaction, the local object is packed up and a copy is sent to the blockchain. The local reference will still point to the local version, even though the blockchain how has a separate copy. If you want to get a reference to the copy on the blockchain, you will need to do so via separate means, e.g. having the transaction return a reference to the new object you are interested in. This part of the design is a work in progress.