A Social Media Example Application

This example illustrates how to use channels for cross-chain messages.

For simplicity, each microchain represents one user—its owner. They can subscribe to other users and make text posts that get sent to their subscribers.

How it Works

The application's state on every microchain contains a set of posts created by this chain owner, and a set of posts received from other chains that it has subscribed to. The received posts are indexed by timestamp, sender and index.

There are Subscribe and Unsubscribe operations: If a chain owner includes these in a new block, they can subscribe to or unsubscribe from another chain.

There is also a Post operation: It creates a new post and sends it to a channel, so that it reaches all subscribers.

There are corresponding Subscribe, Unsubscribe and Posts cross-chain message variants that are created when these operations are handled. The first two are sent directly to the chain we want to subscribe to or unsubscribe from. The latter goes to the channel.

Usage

To try it out, build Linera and add it to your path:

cargo build
PATH=$PWD/target/debug:$PATH

Then, using the helper function defined by linera net helper, set up a local network with two wallets and define variables holding their wallet paths ($LINERA_WALLET_0, $LINERA_WALLET_1) and storage paths ($LINERA_STORAGE_0, $LINERA_STORAGE_1).

eval "$(linera net helper)"
linera_spawn_and_read_wallet_variables \
    linera net up \
        --extra-wallets 1

Compile the social example and create an application with it:

linera --with-wallet 0 project publish-and-create examples/social

This will output the new application ID, e.g.:

e476187f6ddfeb9d588c7b45d3df334d5501d6499b3f9ad5595cae86cce16a65010000000000000001000000e476187f6ddfeb9d588c7b45d3df334d5501d6499b3f9ad5595cae86cce16a65030000000000000000000000

With the wallet show command you can find the ID of the application creator's chain:

linera --with-wallet 0 wallet show

e476187f6ddfeb9d588c7b45d3df334d5501d6499b3f9ad5595cae86cce16a65

Now start a node service for each wallet, using two different ports:

linera --with-wallet 0 service --port 8080 &

# Wait for it to complete
sleep 2

linera --with-wallet 1 service --port 8081 &

# Wait for it to complete
sleep 2

Point your browser to http://localhost:8081. This is the wallet that didn't create the application, so we have to request it from the creator chain. As the chain ID specify the one of the chain where it isn't registered yet:

mutation {
    requestApplication(
        chainId: "1db1936dad0717597a7743a8353c9c0191c14c3a129b258e9743aec2b4f05d03",
        applicationId: "e476187f6ddfeb9d588c7b45d3df334d5501d6499b3f9ad5595cae86cce16a65010000000000000001000000e476187f6ddfeb9d588c7b45d3df334d5501d6499b3f9ad5595cae86cce16a65030000000000000000000000"
    )
}

Now in both http://localhost:8080 and http://localhost:8081, this should list the application and provide a link to its GraphQL API. Remember to use each wallet's chain ID:

query {
    applications(
        chainId: "1db1936dad0717597a7743a8353c9c0191c14c3a129b258e9743aec2b4f05d03"
    ) {
        id
        link
    }
}

Open both URLs under the entry link. Now you can use the application on each chain. E.g. in the 8081 tab subscribe to the other chain:

mutation {
    subscribe(
        chainId: "e476187f6ddfeb9d588c7b45d3df334d5501d6499b3f9ad5595cae86cce16a65"
    )
}

Now make a post in the 8080 tab:

mutation {
    post(
        text: "Linera Social is the new Mastodon!"
    )
}

Since 8081 is a subscriber. Let's see if it received any posts:

query { receivedPosts { keys { timestamp author index } } }

This should now list one entry, with timestamp, author and an index. If we view that entry, we can see the posted text:

query {
    receivedPosts {
        entry(
            key: {
                timestamp: 1705504131018960,
                author: "e476187f6ddfeb9d588c7b45d3df334d5501d6499b3f9ad5595cae86cce16a65",
                index: 0
            }
        ) { value }
    }
}

{
    "data": {
        "receivedPosts": {
            "entry": {
                "value": "Linera Social is the new Mastodon!"
            }
        }
    }
}