EventSourced

Event sourced entities in Rust.

Concepts

EventSourced is inspired to a large degree by the amazing Akka Persistence library. It provides a framework for implementing Event Sourcing and CQRS.

The [EventSourced] trait defines the event type and handling for event sourced entities. These are identifiable by a type name and ID and can be created with the EventSourcedExt::entity extension method. Commands can be defined via the [Command] trait which contains a command handler function to either reject a command or return an event. An event gets persisted to the event log and then applied to the event handler to return the new state of the entity.

                 ┌───────┐   ┌ ─ ─ ─ Entity─ ─ ─ ─
                 │Command│                        │
┌ ─ ─ ─ ─ ─ ─    └───────┘   │ ┌────────────────┐
    Client   │────────────────▶│ handle_command │─┼─────────┐
└ ─ ─ ─ ─ ─ ─                │ └────────────────┘           │
       ▲                           │    │         │         │ ┌─────┐
        ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─│─ ─ ─     │read               │ │Event│
                  ┌─────┐               ▼         │         ▼ └─────┘
                  │Reply│    │     ┌─────────┐       ┌ ─ ─ ─ ─ ─ ─
                  │  /  │          │  State  │    │     EventLog  │
                  │Error│    │     └─────────┘       └ ─ ─ ─ ─ ─ ─
                  └─────┘               ▲         │         │ ┌─────┐
                             │     write│                   │ │Event│
                                        │         │         │ └─────┘
                             │ ┌────────────────┐           │
                               │  handle_event  │◀┼─────────┘
                             │ └────────────────┘
                                                  │
                             └ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─

The [EventLog] and [SnapshotStore] traits define a pluggable event log and a pluggable snapshot store respectively. For NATS and Postgres these are implemented in the respective crates.

EventSourcedEntity::spawn puts the event sourced entity on the given event log and snapshot store, returning an [EntityRef] which can be cheaply cloned and used to pass commands to the entity. Conversion of events and snapshot state to and from bytes happens via the given [Binarize] implementation; for prost and serde_json these are already provided. Snapshots are taken after the configured number of processed events to speed up future spawning.

EntityRef::handle_command either returns Command::Error for a rejected command or Command::Reply for an accepted one, wrapped in another Result dealing with technical errors.

Events can be queried from the event log by ID or by entity type. These queries can be used to build read side projections. There is early support for projections in the eventsourced-projection crate.

Requirements for building the project and examples

Before building the project and examples, please make sure you have installed the protobuf dependency that is not only needed for the optional byte conversion with prost, but also for eventsourced-nats. The only way to get away without protobuf is to not use prost and not build eventsourced-nats.

On macOS protobuf can be installed via Homebrew:

brew install protobuf

Counter example (no pun intended)

The counter package in the example directory contains a simple example: a counter which handles Inc and Dec commands and emits/handles Increased and Decreased events.

#[derive(Debug)]
struct Increase(u64);

impl Command<Uuid, Event, Counter> for Increase {
    type Error = Overflow;

    type Reply = u64;

    fn handle(self, id: &Uuid, state: &Counter) -> Result<Event, Self::Error> {
        if u64::MAX - state.0 < self.0 {
            Err(Overflow)
        } else {
            Ok(Event::Increased(*id))
        }
    }

    fn reply(state: &Counter) -> Self::Reply {
        state.0
    }
}

#[derive(Debug)]
struct Overflow;

#[derive(Debug)]
struct Decrease(u64);

impl Command<Uuid, Event, Counter> for Decrease {
    type Error = Underflow;

    type Reply = Counter;

    fn handle(self, id: &Uuid, state: &Counter) -> Result<Event, Self::Error> {
        if state.0 < self.0 {
            Err(Underflow)
        } else {
            Ok(Event::Decreased(*id))
        }
    }

    fn reply(state: &Counter) -> Self::Reply {
        *state
    }
}

#[derive(Debug, PartialEq, Eq)]
struct Underflow;

#[derive(Debug, Serialize, Deserialize)]
enum Event {
    Increased(Uuid),
    Decreased(Uuid),
}

#[derive(Debug, Default, Clone, Copy, Serialize, Deserialize)]
struct Counter(u64);

impl Counter {
    fn handle_event(self, evt: Event) -> Self {
        match evt {
            Event::Increased(_) => Self(self.0 + 1),
            Event::Decreased(_) => Self(self.0 - 1),
        }
    }
}

There are also the two counter-nats and counter-postgres packages, with a binary crate each, using eventsourced-nats and eventsourced-postgres respectively for the event log.

...
let evt_log = evt_log.clone();
let snapshot_store = snapshot_store.clone();
let counter = Counter::default();
let counter = counter
    .spawn(
        id,
        unsafe { NonZeroUsize::new_unchecked(42) },
        evt_log,
        snapshot_store,
        convert::serde_json::binarizer(),
    )
    .await
    .context("cannot spawn entity")?;

tasks.spawn(async move {
    for n in 0..config.evt_count / 2 {
        if n > 0 && n % 2_500 == 0 {
            println!("{id}: {} events persisted", n * 2);
        }
        counter
            .handle_command(Command::Inc(n as u64))
            .await
            .context("cannot handle Inc command")
            .unwrap();
        ...
    }
});
...

Take a look at the examples directory for more details.

Running the counter-nats example

For the counter-nats example, nats-server needs to be installed. On macOS just use Homebrew:

brew install nats-server

Before running the example, start the nats-server with the jetstream feature enabled:

nats-server --jetstream

Then use the following command to run the example:

RUST_LOG=info \
    CONFIG_DIR=examples/counter-nats/config \
    cargo run \
    --release \
    --package counter-nats

Notice that you can change the configuration either by changing the defaul.toml file at examples/counter-nats/config or by overriding the configuration settings with environment variables, e.g. APP__COUNTER__EVT_COUNT=42:

RUST_LOG=info \
    APP__COUNTER__EVT_COUNT=42 \
    CONFIG_DIR=examples/counter-nats/config \
    cargo run \
    --release \
    --package counter-nats

Running the counter-postgres example

For the counter-postgres example, PostgreSQL needs to be installed. On macOS just use Homebrew:

brew install postgresql@14

Before running the example, start PostgreSQL:

brew services run postgresql@14

Make sure you know the following connection parameters:

• host
• port
• user
• password
• dbname

Change the configuration either by changing the default.toml file at examples/counter-postgres/config or by overriding the configuration settings with environment variables, e.g. APP__EVT_LOG__DBNAME=test or APP__COUNTER__EVT_COUNT=42:

Then use the following command to run the example:

RUST_LOG=info \
    APP__EVT_LOG__DBNAME=test \
    APP__COUNTER__EVT_COUNT=42 \
    CONFIG_DIR=examples/counter-postgres/config \
    cargo run \
    --release \
    --package counter-postgres

License

This code is open source software licensed under the Apache 2.0 License.