#event-sourcing #wasm #embedded-database

app thalo_cli

Cli tool for interacting with thalo through the quic protocol

1 unstable release

0.8.0 Nov 21, 2023

#2 in #thalo

28 downloads per month

Apache-2.0 OR MIT


Thalo — Event sourcing runtime for wasm

Crates.io Documentation MIT OR Apache-2.0 Stargazers Last commit Discord

Thalo is an event sourcing runtime that leverages the power of WebAssembly (wasm) through wasmtime, combined with sled as an embedded event store. It is designed to handle commands using compiled aggregate wasm components and to persist the resulting events, efficiently managing the rebuilding of aggregate states from previous events.

Key Features

  • Wasmtime Integration: High-performance, isolated aggregate computation using WebAssembly.
  • Sled Event Store: Fast, reliable embedded database for event storage.
  • Actor-Based Aggregates: Efficient, lock-free management supporting over 10,000 commands/sec.
  • Projections: Projections to handle interested events with an at-least-once gaurantee.
  • LRU Caching: Fast aggregate rebuilding with optimized memory usage.
  • Streamlined State Management: Automated reconstruction of aggregate states from events.
  • Multi-Language Support: Compile aggregates to wasm, enabling diverse language use (currently Rust supported).
  • Event Relaying: Events can be relayed to external services (redis streams, etc) with an at-least-once guarantee.


  1. Rust: Latest stable version of the Rust programming language.
  2. Thalo: Required for building aggregates to wasm.
    • Install with: cargo install thalo_cli
  3. Thalo Runtime: Required for running the Thalo event sourcing environment.
    • Install with: cargo install thalo_runtime

Thalo can be started by running thalo-runtime.

For a list of supported arguments, use thalo_runtime --help.

Writing an Aggregate

An aggregate is a regular rust crates which exposes the type implementing thalo::Aggregate with the thalo::export_aggregate! macro.

struct Counter { ... }
impl thalo::Aggregate for Counter { ... }

It can then be compiled with the thalo cli.

$ thalo build counter -o ./modules

The thalo build command is essentially a wrapper around cargo build, handling wasm specifics including converting the resulting binary to a wasm component.

Thalo will automatically load all modules within this the ./modules directory, and use them to handle commands.

You can restart the thalo runtime and you should see a message saying our counter was loaded from the modules/counter.wasm file.

$ ./target/debug/thalo-runtime
INFO: loaded module from file file="modules/counter.wasm"

We can now send commands to this module with the thalo execute command for testing.

$ thalo execute counter abc123 Increment '{"amount":10}'
Executed with 1 events:
    Incremented  {"amount":10}
Counter Example
use serde::{Deserialize, Serialize};
use thalo::{events, export_aggregate, Aggregate, Apply, Command, Event, Handle};


pub struct Counter {
    count: u64,

impl Aggregate for Counter {
    type Command = CounterCommand;
    type Event = CounterEvent;

    fn init(_id: String) -> Self {
        Counter { count: 0 }

#[derive(Command, Deserialize)]
pub enum CounterCommand {
    Increment { amount: u64 },

impl Handle<CounterCommand> for Counter {
    type Error = Infallible;

    fn handle(&self, cmd: CounterCommand) -> Result<Vec<CounterEvent>, Self::Error> {
        match cmd {
            CounterCommand::Increment { amount } => events![Incremented { amount }],

#[derive(Event, Serialize, Deserialize)]
pub enum CounterEvent {

#[derive(Serialize, Deserialize)]
pub struct Incremented {
    pub amount: u64,

impl Apply<Incremented> for Counter {
    fn apply(&mut self, event: Incremented) {
        self.count += event.amount;

Understanding Event Sourcing

Event sourcing is a design pattern in which changes to the application state are stored as a sequence of events. Instead of storing just the current state, event sourcing involves persisting the full series of actions taken on the data. This allows for:

  • Complete State History: Every change is recorded, enabling full traceability and audit trails.
  • Easy Reversion: The ability to revert to any previous state by replaying events.
  • Complex Event Reconstruction: States can be rebuilt or projections created by processing the event log.
  • Improved Scalability and Performance: Decouples data commands from updates, often leading to better system performance.

Event sourcing is particularly useful in systems where understanding the history of operations is crucial, such as financial systems, order management systems, and other domains where auditability and traceability are important.

Getting Help

As Thalo is in pre-release, the API is not stable yet. If you'd like to get started using Thalo, you can checkout the examples directory, or chat on the Discord server.


Examples can found in the examples directory.


🎈 Thanks for your help improving the project! As we don't currently have a contributing guide, you can ping us on the Discord server or open an issue.


This project is licensed under the MIT OR Apache-2.0 license.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in Thalo by you, shall be licensed as MIT, without any additional terms or conditions.


~1M SLoC