SpacetimeDB

Rust Client SDK Quick Start

In this guide we'll show you how to get up and running with a simple SpacetimDB app with a client written in Rust.

We'll implement a command-line client for the module created in our Rust or C# Module Quickstart guides. Make sure you follow one of these guides before you start on this one.

Project structure

Enter the directory quickstart-chat you created in the Rust Module Quickstart or C# Module Quickstart guides:

cd quickstart-chat

Within it, create a client crate, our client application, which users run locally:

cargo new client

Depend on `spacetimedb-sdk` and `hex`

client/Cargo.toml should be initialized without any dependencies. We'll need two:

spacetimedb-sdk, which defines client-side interfaces for interacting with a remote SpacetimeDB module.
hex, which we'll use to print unnamed users' identities as hexadecimal strings.

Below the [dependencies] line in client/Cargo.toml, add:

spacetimedb-sdk = "1.0"
hex = "0.4"

Make sure you depend on the same version of spacetimedb-sdk as is reported by the SpacetimeDB CLI tool's spacetime version!

Clear `client/src/main.rs`

client/src/main.rs should be initialized with a trivial "Hello world" program. Clear it out so we can write our chat client.

In your quickstart-chat directory, run:

rm client/src/main.rs
touch client/src/main.rs

Generate your module types

The spacetime CLI's generate command will generate client-side interfaces for the tables, reducers and types referenced by tables or reducers defined in your server module.

In your quickstart-chat directory, run:

mkdir -p client/src/module_bindings
spacetime generate --lang rust --out-dir client/src/module_bindings --project-path server

Take a look inside client/src/module_bindings. The CLI should have generated a few files:

module_bindings/
├── identity_connected_reducer.rs
├── identity_disconnected_reducer.rs
├── message_table.rs
├── message_type.rs
├── mod.rs
├── send_message_reducer.rs
├── set_name_reducer.rs
├── user_table.rs
└── user_type.rs

To use these, we'll declare the module in our client crate and import its definitions.

To client/src/main.rs, add:

mod module_bindings;
use module_bindings::*;

Add more imports

We'll need additional imports from spacetimedb_sdk for interacting with the database, handling credentials, and managing events.

To client/src/main.rs, add:

use spacetimedb_sdk::{credentials, DbContext, Error, Event, Identity, Status, Table, TableWithPrimaryKey};

Define the main function

Our main function will do the following:

Connect to the database.
Register a number of callbacks to run in response to various database events.
Subscribe to a set of SQL queries, whose results will be replicated and automatically updated in our client.
Spawn a background thread where our connection will process messages and invoke callbacks.
Enter a loop to handle user input from the command line.

We'll see the implementation of these functions a bit later, but for now add to client/src/main.rs:

fn main() {
    // Connect to the database
    let ctx = connect_to_db();

    // Register callbacks to run in response to database events.
    register_callbacks(&ctx);

    // Subscribe to SQL queries in order to construct a local partial replica of the database.
    subscribe_to_tables(&ctx);

    // Spawn a thread, where the connection will process messages and invoke callbacks.
    ctx.run_threaded();

    // Handle CLI input
    user_input_loop(&ctx);
}

Connect to the database

A connection to a SpacetimeDB database is represented by a DbConnection. We configure DbConnections using the builder pattern, by calling DbConnection::builder(), chaining method calls to set various connection parameters and register callbacks, then we cap it off with a call to .build() to begin the connection.

In our case, we'll supply the following options:

An on_connect callback, to run when the remote database acknowledges and accepts our connection.
An on_connect_error callback, to run if the remote database is unreachable or it rejects our connection.
An on_disconnect callback, to run when our connection ends.
A with_token call, to supply a token to authenticate with.
A with_module_name call, to specify the name or Identity of our database. Make sure to pass the same name here as you supplied to spacetime publish.
A with_uri call, to specify the URI of the SpacetimeDB host where our module is running.

To client/src/main.rs, add:

/// The URI of the SpacetimeDB instance hosting our chat module.
const HOST: &str = "http://localhost:3000";

/// The database name we chose when we published our module.
const DB_NAME: &str = "quickstart-chat";

/// Load credentials from a file and connect to the database.
fn connect_to_db() -> DbConnection {
    DbConnection::builder()
        // Register our `on_connect` callback, which will save our auth token.
        .on_connect(on_connected)
        // Register our `on_connect_error` callback, which will print a message, then exit the process.
        .on_connect_error(on_connect_error)
        // Our `on_disconnect` callback, which will print a message, then exit the process.
        .on_disconnect(on_disconnected)
        // If the user has previously connected, we'll have saved a token in the `on_connect` callback.
        // In that case, we'll load it and pass it to `with_token`,
        // so we can re-authenticate as the same `Identity`.
        .with_token(creds_store().load().expect("Error loading credentials"))
        // Set the database name we chose when we called `spacetime publish`.
        .with_module_name(DB_NAME)
        // Set the URI of the SpacetimeDB host that's running our database.
        .with_uri(HOST)
        // Finalize configuration and connect!
        .build()
        .expect("Failed to connect")
}

Save credentials

SpacetimeDB will accept any OpenID Connect compliant JSON Web Token and use it to compute an Identity for the user. More complex applications will generally authenticate their user somehow, generate or retrieve a token, and attach it to their connection via with_token. In our case, though, we'll connect anonymously the first time, let SpacetimeDB generate a fresh Identity and corresponding JWT for us, and save that token locally to re-use the next time we connect.

The Rust SDK provides a pair of functions in File, save and load, for saving and storing these credentials in a file. By default the save and load will look for credentials in the $HOME/.spacetimedb_client_credentials/ directory, which should be unintrusive. If saving our credentials fails, we'll print a message to standard error, but otherwise continue; even though the user won't be able to reconnect with the same identity, they can still chat normally.

To client/src/main.rs, add:

fn creds_store() -> credentials::File {
    credentials::File::new("quickstart-chat")
}

/// Our `on_connect` callback: save our credentials to a file.
fn on_connected(_ctx: &DbConnection, _identity: Identity, token: &str) {
    if let Err(e) = creds_store().save(token) {
        eprintln!("Failed to save credentials: {:?}", e);
    }
}

Handle errors and disconnections

We need to handle connection errors and disconnections by printing appropriate messages and exiting the program. These callbacks take an ErrorContext, a DbConnection that's been augmented with information about the error that occured.

To client/src/main.rs, add:

/// Our `on_connect_error` callback: print the error, then exit the process.
fn on_connect_error(_ctx: &ErrorContext, err: Error) {
    eprintln!("Connection error: {:?}", err);
    std::process::exit(1);
}

/// Our `on_disconnect` callback: print a note, then exit the process.
fn on_disconnected(_ctx: &ErrorContext, err: Option<Error>) {
    if let Some(err) = err {
        eprintln!("Disconnected: {}", err);
        std::process::exit(1);
    } else {
        println!("Disconnected.");
        std::process::exit(0);
    }
}

Register callbacks

We need to handle several sorts of events:

When a new user joins, we'll print a message introducing them.
When a user is updated, we'll print their new name, or declare their new online status.
When we receive a new message, we'll print it.
If the server rejects our attempt to set our name, we'll print an error.
If the server rejects a message we send, we'll print an error.

To client/src/main.rs, add:

/// Register all the callbacks our app will use to respond to database events.
fn register_callbacks(ctx: &DbConnection) {
    // When a new user joins, print a notification.
    ctx.db.user().on_insert(on_user_inserted);

    // When a user's status changes, print a notification.
    ctx.db.user().on_update(on_user_updated);

    // When a new message is received, print it.
    ctx.db.message().on_insert(on_message_inserted);

    // When we fail to set our name, print a warning.
    ctx.reducers.on_set_name(on_name_set);

    // When we fail to send a message, print a warning.
    ctx.reducers.on_send_message(on_message_sent);
}

Notify about new users

For each table, we can register on-insert and on-delete callbacks to be run whenever a subscribed row is inserted or deleted. We register these callbacks using the on_insert and on_delete, which is automatically implemented for each table by spacetime generate.

These callbacks can fire in several contexts, of which we care about two:

After a reducer runs, when the client's cache is updated about changes to subscribed rows.
After calling subscribe, when the client's cache is initialized with all existing matching rows.

This second case means that, even though the module only ever inserts online users, the client's conn.db.user().on_insert(..) callbacks may be invoked with users who are offline. We'll only notify about online users.

on_insert and on_delete callbacks take two arguments: an &EventContext and the modified row. Like the ErrorContext above, EventContext is a DbConnection that's been augmented with information about the event that caused the row to be modified. You can determine whether the insert/delete operation was caused by a reducer, a newly-applied subscription, or some other event by pattern-matching on ctx.event.

Whenever we want to print a user, if they have set a name, we'll use that. If they haven't set a name, we'll instead print the first 8 bytes of their identity, encoded as hexadecimal. We'll define functions user_name_or_identity and identity_leading_hex to handle this.

To client/src/main.rs, add:

/// Our `User::on_insert` callback:
/// if the user is online, print a notification.
fn on_user_inserted(_ctx: &EventContext, user: &User) {
    if user.online {
        println!("User {} connected.", user_name_or_identity(user));
    }
}

fn user_name_or_identity(user: &User) -> String {
    user.name
        .clone()
        .unwrap_or_else(|| user.identity.to_hex().to_string())
}

Notify about updated users

Because we declared a #[primary_key] column in our User table, we can also register on-update callbacks. These run whenever a row is replaced by a row with the same primary key, like our module's ctx.db.user().identity().update(..) calls. We register these callbacks using the on_update method of the trait TableWithPrimaryKey, which is automatically implemented by spacetime generate for any table with a #[primary_key] column.

on_update callbacks take three arguments: the &EventContext, the old row, and the new row.

In our module, users can be updated for three reasons:

They've set their name using the set_name reducer.
They're an existing user re-connecting, so their online has been set to true.
They've disconnected, so their online has been set to false.

We'll print an appropriate message in each of these cases.

To client/src/main.rs, add:

/// Our `User::on_update` callback:
/// print a notification about name and status changes.
fn on_user_updated(_ctx: &EventContext, old: &User, new: &User) {
    if old.name != new.name {
        println!(
            "User {} renamed to {}.",
            user_name_or_identity(old),
            user_name_or_identity(new)
        );
    }
    if old.online && !new.online {
        println!("User {} disconnected.", user_name_or_identity(new));
    }
    if !old.online && new.online {
        println!("User {} connected.", user_name_or_identity(new));
    }
}

Print messages

When we receive a new message, we'll print it to standard output, along with the name of the user who sent it. Keep in mind that we only want to do this for new messages, i.e. those inserted by a send_message reducer invocation. We have to handle the backlog we receive when our subscription is initialized separately, to ensure they're printed in the correct order. To that effect, our on_message_inserted callback will check if the ctx.event type is an Event::Reducer, and only print in that case.

To find the User based on the message's sender identity, we'll use ctx.db.user().identity().find(..), which behaves like the same function on the server.

We'll print the user's name or identity in the same way as we did when notifying about User table events, but here we have to handle the case where we don't find a matching User row. This can happen when the module owner sends a message using the CLI's spacetime call. In this case, we'll print unknown.

Notice that our print_message function takes an &impl RemoteDbContext as an argument. This is a trait, defined in our module_bindings by spacetime generate, which is implemented by DbConnection, EventContext, ErrorContext and a few other similar types. (RemoteDbContext is actually a shorthand for DbContext, which applies to connections to any module, with its associated types locked to module-specific ones.) Later on, we're going to call print_message with a ReducerEventContext, so we need to be more generic than just accepting EventContext.

To client/src/main.rs, add:

/// Our `Message::on_insert` callback: print new messages.
fn on_message_inserted(ctx: &EventContext, message: &Message) {
    if let Event::Reducer(_) = ctx.event {
        print_message(ctx, message)
    }
}

fn print_message(ctx: &impl RemoteDbContext, message: &Message) {
    let sender = ctx
        .db()
        .user()
        .identity()
        .find(&message.sender.clone())
        .map(|u| user_name_or_identity(&u))
        .unwrap_or_else(|| "unknown".to_string());
    println!("{}: {}", sender, message.text);
}

Handle reducer failures

We can also register callbacks to run each time a reducer is invoked. We register these callbacks using the on_reducer method of the Reducer trait, which is automatically implemented for each reducer by spacetime generate.

Each reducer callback first takes a &ReducerEventContext which contains metadata about the reducer call, including the identity of the caller and whether or not the reducer call suceeded.

These callbacks will be invoked in one of two cases:

If the reducer was successful and altered any of our subscribed rows.
If we requested an invocation which failed.

Note that a status of Failed or OutOfEnergy implies that the caller identity is our own identity.

We already handle successful set_name invocations using our ctx.db.user().on_update(..) callback, but if the module rejects a user's chosen name, we'd like that user's client to let them know. We define a function on_set_name as a conn.reducers.on_set_name(..) callback which checks if the reducer failed, and if it did, prints a message including the rejected name and the error.

To client/src/main.rs, add:

/// Our `on_set_name` callback: print a warning if the reducer failed.
fn on_name_set(ctx: &ReducerEventContext, name: &String) {
    if let Status::Failed(err) = &ctx.event.status {
        eprintln!("Failed to change name to {:?}: {}", name, err);
    }
}

/// Our `on_send_message` callback: print a warning if the reducer failed.
fn on_message_sent(ctx: &ReducerEventContext, text: &String) {
    if let Status::Failed(err) = &ctx.event.status {
        eprintln!("Failed to send message {:?}: {}", text, err);
    }
}

SpacetimeDB is set up so that each client subscribes via SQL queries to some subset of the database, and is notified about changes only to that subset. For complex apps with large databases, judicious subscriptions can save each client significant network bandwidth, memory and computation. For example, in BitCraft, each player's client subscribes only to the entities in the "chunk" of the world where that player currently resides, rather than the entire game world. Our app is much simpler than BitCraft, so we'll just subscribe to the whole database.

When we specify our subscriptions, we can supply an on_applied callback. This will run when the subscription is applied and the matching rows become available in our client cache. We'll use this opportunity to print the message backlog in proper order.

We'll also provide an on_error callback. This will run if the subscription fails, usually due to an invalid or malformed SQL queries. We can't handle this case, so we'll just print out the error and exit the process.

To client/src/main.rs, add:

/// Register subscriptions for all rows of both tables.
fn subscribe_to_tables(ctx: &DbConnection) {
    ctx.subscription_builder()
        .on_applied(on_sub_applied)
        .on_error(on_sub_error)
        .subscribe(["SELECT * FROM user", "SELECT * FROM message"]);
}

Print past messages in order

Messages we receive live will come in order, but when we connect, we'll receive all the past messages at once. We can't just print these in the order we receive them; the logs would be all shuffled around, and would make no sense. Instead, when we receive the log of past messages, we'll sort them by their sent timestamps and print them in order.

We'll handle this in our function print_messages_in_order, which we registered as an on_applied callback. print_messages_in_order iterates over all the Messages we've received, sorts them, and then prints them. ctx.db.message().iter() is defined on the trait Table, and returns an iterator over all the messages in the client cache. Rust iterators can't be sorted in-place, so we'll collect it to a Vec, then use the sort_by_key method to sort by timestamp.

To client/src/main.rs, add:

/// Our `on_subscription_applied` callback:
/// sort all past messages and print them in timestamp order.
fn on_sub_applied(ctx: &SubscriptionEventContext) {
    let mut messages = ctx.db.message().iter().collect::<Vec<_>>();
    messages.sort_by_key(|m| m.sent);
    for message in messages {
        print_message(ctx, &message);
    }
    println!("Fully connected and all subscriptions applied.");
    println!("Use /name to set your name, or type a message!");
}

Notify about failed subscriptions

It's possible for SpacetimeDB to reject subscriptions. This happens most often because of a typo in the SQL queries, but can be due to use of SQL features that SpacetimeDB doesn't support. See SQL Support: Subscriptions for more information about what subscription queries SpacetimeDB supports.

In our case, we're pretty confident that our queries are valid, but if SpacetimeDB rejects them, we want to know about it. Our callback will print the error, then exit the process.

/// Or `on_error` callback:
/// print the error, then exit the process.
fn on_sub_error(_ctx: &ErrorContext, err: Error) {
    eprintln!("Subscription failed: {}", err);
    std::process::exit(1);
}

Handle user input

Our app should allow the user to interact by typing lines into their terminal. If the line starts with /name , we'll change the user's name. Any other line will send a message.

For each reducer defined by our module, ctx.reducers has a method to request an invocation. In our case, we pass set_name and send_message a String, which gets sent to the server to execute the corresponding reducer.

To client/src/main.rs, add:

/// Read each line of standard input, and either set our name or send a message as appropriate.
fn user_input_loop(ctx: &DbConnection) {
    for line in std::io::stdin().lines() {
        let Ok(line) = line else {
            panic!("Failed to read from stdin.");
        };
        if let Some(name) = line.strip_prefix("/name ") {
            ctx.reducers.set_name(name.to_string()).unwrap();
        } else {
            ctx.reducers.send_message(line).unwrap();
        }
    }
}

Run it

After setting everything up, change your directory to the client app, then compile and run it. From the quickstart-chat directory, run:

cd client
cargo run

You should see something like:

User d9e25c51996dea2f connected.

Now try sending a message by typing Hello, world! and pressing enter. You should see:

d9e25c51996dea2f: Hello, world!

Next, set your name by typing /name <my-name>, replacing <my-name> with your desired username. You should see:

User d9e25c51996dea2f renamed to <my-name>.

Then, send another message:

<my-name>: Hello after naming myself.

Now, close the app by hitting Ctrl+C, and start it again with cargo run. You'll see yourself connecting, and your past messages will load in order:

User <my-name> connected.
<my-name>: Hello, world!
<my-name>: Hello after naming myself.

What's next?

You can find the full code for this client in the Rust client SDK's examples.

Check out the Rust client SDK Reference for a more comprehensive view of the SpacetimeDB Rust client SDK.

Our basic terminal interface has some limitations. Incoming messages can appear while the user is typing, which is less than ideal. Additionally, the user's input gets mixed with the program's output, making messages the user sends appear twice. You might want to try improving the interface by using Rustyline, Cursive, or even creating a full-fledged GUI.

Once your chat server runs for a while, you might want to limit the messages your client loads by refining your Message subscription query, only subscribing to messages sent within the last half-hour.

You could also add features like:

Styling messages by interpreting HTML tags and printing appropriate ANSI escapes.
Adding a moderator flag to the User table, allowing moderators to manage users (e.g., time-out, ban).
Adding rooms or channels that users can join or leave.
Supporting direct messages or displaying user statuses next to their usernames.

Community

Rust Client SDK Quick Start

Project structure

Depend on `spacetimedb-sdk` and `hex`

Clear `client/src/main.rs`

Generate your module types

Add more imports

Define the main function

Connect to the database

Save credentials

Handle errors and disconnections

Register callbacks

Notify about new users

Notify about updated users

Print messages

Handle reducer failures

Subscribe to queries

Print past messages in order

Notify about failed subscriptions

Handle user input

Run it

What's next?