Quickstart Chat App

In this tutorial, we'll implement a simple chat server as a SpacetimeDB module.

A SpacetimeDB module is code that gets compiled to a WebAssembly binary and is uploaded to SpacetimeDB. This code becomes server-side logic that interfaces directly with the SpacetimeDB relational database.

Each SpacetimeDB module defines a set of tables and a set of reducers.

Each table is defined as a Rust struct annotated with #[table(name = table_name)]. An instance of the struct represents a row, and each field represents a column.

By default, tables are private. This means that they are only readable by the table owner, and by server module code. The #[table(name = table_name, public)] macro makes a table public. Public tables are readable by all users but can still only be modified by your server module code.

A reducer is a function that traverses and updates the database. Each reducer call runs in its own transaction, and its updates to the database are only committed if the reducer returns successfully. In Rust, reducers are defined as functions annotated with #[reducer], and may return a Result<()>, with an Err return aborting the transaction.

Install SpacetimeDB

If you haven't already, start by installing SpacetimeDB. This will install the spacetime command line interface (CLI), which provides all the functionality needed to interact with SpacetimeDB.

Install the SpacetimeDB CLI tool

Install Rust

Next we need to install Rust so that we can create our database module.

On macOS and Linux run this command to install the Rust compiler:

curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

If you're on Windows, go here.

Project structure

Let's start by running spacetime init to initialize our project's directory structure:

spacetime init --lang rust quickstart-chat

spacetime init will ask you for a project path in which to put your project. By default this will be ./quickstart-chat. This basic project will have a few helper files like Cursor rules for SpacetimeDB and a spacetimedb directory which is where your SpacetimeDB module code will go.

How to Compile

[!IMPORTANT] While it is possible to use the traditional cargo build to build SpacetimeDB server modules, spacetime build makes this process easier. Keep this in mind when using an IDE that assumes using cargo for building.

cd spacetimedb 
spacetime build

Declare imports

spacetime init should have pre-populated spacetimedb/src/lib.rs with a trivial module. Clear it out so we can write a new, simple module: a bare-bones chat server.

To the top of spacetimedb/src/lib.rs, add some imports we'll be using:

use spacetimedb::{table, reducer, Table, ReducerContext, Identity, Timestamp};

From spacetimedb, we import:

table, a macro used to define SpacetimeDB tables.
reducer, a macro used to define SpacetimeDB reducers.
Table, a rust trait which allows us to interact with tables.
ReducerContext, a special argument passed to each reducer.
Identity, a unique identifier for each user.
Timestamp, a point in time. Specifically, an unsigned 64-bit count of milliseconds since the UNIX epoch.

Define tables

To get our chat server running, we'll need to store two kinds of data: information about each user, and records of all the messages that have been sent.

For each User, we'll store their Identity, an optional name they can set to identify themselves to other users, and whether they're online or not. We'll designate the Identity as our primary key, which enforces that it must be unique, indexes it for faster lookup, and allows clients to track updates.

To spacetimedb/src/lib.rs, add the definition of the table User:

#[table(name = user, public)]
pub struct User {
    #[primary_key]
    identity: Identity,
    name: Option<String>,
    online: bool,
}

For each Message, we'll store the Identity of the user who sent it, the Timestamp when it was sent, and the text of the message.

To spacetimedb/src/lib.rs, add the definition of the table Message:

#[table(name = message, public)]
pub struct Message {
    sender: Identity,
    sent: Timestamp,
    text: String,
}

Set users' names

We want to allow users to set their names, because Identity is not a terribly user-friendly identifier. To that effect, we define a reducer set_name which clients can invoke to set their User.name. It will validate the caller's chosen name, using a function validate_name which we'll define next, then look up the User record for the caller and update it to store the validated name. If the name fails the validation, the reducer will fail.

Each reducer must accept as its first argument a ReducerContext, which includes the Identity and ConnectionId of the client that called the reducer, and the Timestamp when it was invoked. It also allows us access to the db, which is used to read and manipulate rows in our tables. For now, we only need the db, Identity, and ctx.sender.

It's also possible to call set_name via the SpacetimeDB CLI's spacetime call command without a connection, in which case no User record will exist for the caller. We'll return an error in this case, but you could alter the reducer to insert a User row for the module owner. You'll have to decide whether the module owner is always online or always offline, though.

To spacetimedb/src/lib.rs, add:

#[reducer]
/// Clients invoke this reducer to set their user names.
pub fn set_name(ctx: &ReducerContext, name: String) -> Result<(), String> {
    let name = validate_name(name)?;
    if let Some(user) = ctx.db.user().identity().find(ctx.sender) {
        ctx.db.user().identity().update(User { name: Some(name), ..user });
        Ok(())
    } else {
        Err("Cannot set name for unknown user".to_string())
    }
}

For now, we'll just do a bare minimum of validation, rejecting the empty name. You could extend this in various ways, like:

Comparing against a blacklist for moderation purposes.
Unicode-normalizing names.
Rejecting names that contain non-printable characters, or removing characters or replacing them with a placeholder.
Rejecting or truncating long names.
Rejecting duplicate names.

To spacetimedb/src/lib.rs, add:

/// Takes a name and checks if it's acceptable as a user's name.
fn validate_name(name: String) -> Result<String, String> {
    if name.is_empty() {
        Err("Names must not be empty".to_string())
    } else {
        Ok(name)
    }
}

Send messages

We define a reducer send_message, which clients will call to send messages. It will validate the message's text, then insert a new Message record using ctx.db.message().insert(..), with the sender identity and sent timestamp taken from the ReducerContext. Because the Message table does not have any columns with a unique constraint, ctx.db.message().insert() is infallible and does not return a Result.

To spacetimedb/src/lib.rs, add:

#[reducer]
/// Clients invoke this reducer to send messages.
pub fn send_message(ctx: &ReducerContext, text: String) -> Result<(), String> {
    let text = validate_message(text)?;
    log::info!("{}", text);
    ctx.db.message().insert(Message {
        sender: ctx.sender,
        text,
        sent: ctx.timestamp,
    });
    Ok(())
}

We'll want to validate messages' texts in much the same way we validate users' chosen names. As above, we'll do the bare minimum, rejecting only empty messages.

To spacetimedb/src/lib.rs, add:

/// Takes a message's text and checks if it's acceptable to send.
fn validate_message(text: String) -> Result<String, String> {
    if text.is_empty() {
        Err("Messages must not be empty".to_string())
    } else {
        Ok(text)
    }
}

You could extend the validation in validate_message in similar ways to validate_name, or add additional checks to send_message, like:

Rejecting messages from senders who haven't set their names.
Rate-limiting users so they can't send new messages too quickly.

Set users' online status

Whenever a client connects, the database will run a special reducer, annotated with #[reducer(client_connected)], if it's defined. By convention, it's named client_connected. We'll use it to create a User record for the client if it doesn't yet exist, and to set its online status.

We'll use ctx.db.user().identity().find(ctx.sender) to look up a User row for ctx.sender, if one exists. If we find one, we'll use ctx.db.user().identity().update(..) to overwrite it with a row that has online: true. If not, we'll use ctx.db.user().insert(..) to insert a new row for our new user. All three of these methods are generated by the #[table(..)] macro, with rows and behavior based on the row attributes. ctx.db.user().find(..) returns an Option<User>, because of the unique constraint from the #[primary_key] attribute. This means there will be either zero or one matching rows. If we used try_insert here it would return a Result<(), UniqueConstraintViolation> because of the same unique constraint. However, because we're already checking if there is a user with the given sender identity we know that inserting into this table will not fail. Therefore, we use insert, which automatically unwraps the result, simplifying the code. If we want to overwrite a User row, we need to do so explicitly using ctx.db.user().identity().update(..).

To spacetimedb/src/lib.rs, add the definition of the connect reducer:

#[reducer(client_connected)]
// Called when a client connects to a SpacetimeDB database 
pub fn client_connected(ctx: &ReducerContext) {
    if let Some(user) = ctx.db.user().identity().find(ctx.sender) {
        // If this is a returning user, i.e. we already have a `User` with this `Identity`,
        // set `online: true`, but leave `name` and `identity` unchanged.
        ctx.db.user().identity().update(User { online: true, ..user });
    } else {
        // If this is a new user, create a `User` row for the `Identity`,
        // which is online, but hasn't set a name.
        ctx.db.user().insert(User {
            name: None,
            identity: ctx.sender,
            online: true,
        });
    }
}

Similarly, whenever a client disconnects, the database will run the #[reducer(client_disconnected)] reducer if it's defined. By convention, it's named client_disconnected. We'll use it to un-set the online status of the User for the disconnected client.

#[reducer(client_disconnected)]
// Called when a client disconnects from SpacetimeDB database 
pub fn identity_disconnected(ctx: &ReducerContext) {
    if let Some(user) = ctx.db.user().identity().find(ctx.sender) {
        ctx.db.user().identity().update(User { online: false, ..user });
    } else {
        // This branch should be unreachable,
        // as it doesn't make sense for a client to disconnect without connecting first.
        log::warn!("Disconnect event for unknown user with identity {:?}", ctx.sender);
    }
}

Start the Server

If you haven't already started the SpacetimeDB server, run the spacetime start command in a separate terminal and leave it running while you continue following along.

Publish the module

And that's all of our module code! We'll run spacetime publish to compile our module and publish it on SpacetimeDB. spacetime publish takes an optional name which will map to the database's unique Identity. Clients can connect either by name or by Identity, but names are much more user-friendly. If you'd like, come up with a unique name that contains only URL-safe characters (letters, numbers, hyphens and underscores), and fill it in where we've written quickstart-chat.

From the quickstart-chat directory, run in another tab:

spacetime publish --server local --project-path spacetimedb quickstart-chat

Call Reducers

You can use the CLI (command line interface) to run reducers. The arguments to the reducer are passed in JSON format.

spacetime call --server local quickstart-chat send_message "Hello, World!"

Once we've called our send_message reducer, we can check to make sure it ran by running the logs command.

spacetime logs --server local quickstart-chat

You should now see the output that your module printed in the database.

<timestamp>  INFO: spacetimedb: Creating table `message`
<timestamp>  INFO: spacetimedb: Creating table `user`
<timestamp>  INFO: spacetimedb: Database initialized
<timestamp>  INFO: src/lib.rs:43: Hello, world!

SQL Queries

SpacetimeDB supports a subset of the SQL syntax so that you can easily query the data of your database. We can run a query using the sql command.

spacetime sql --server local quickstart-chat "SELECT * FROM message"

 sender                                                             | sent                             | text
--------------------------------------------------------------------+----------------------------------+-----------------
 0x93dda09db9a56d8fa6c024d843e805d8262191db3b4ba84c5efcd1ad451fed4e | 2025-04-08T15:47:46.935402+00:00 | "Hello, world!"

You've just set up your first Rust module in SpacetimeDB! You can find the full code for this module in the SpacetimeDB module examples.

Creating the client

Next, we'll show you how to get up and running with a simple SpacetimeDB 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 database.
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 database is running.

To client/src/main.rs, add:

/// The URI of the SpacetimeDB instance hosting our chat database and 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.

Install SpacetimeDB​

Install the SpacetimeDB CLI tool

Install Rust​

Project structure​

How to Compile​

Declare imports​

Define tables​

Set users' names​

Send messages​

Set users' online status​

Start the Server​

Publish the module​

Call Reducers​

SQL Queries​

Creating the client

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?​

Install SpacetimeDB

Install Rust

Project structure

How to Compile

Declare imports

Define tables

Set users' names

Send messages

Set users' online status

Start the Server

Publish the module

Call Reducers

SQL Queries

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?