Custom Snapshots in Rust

Golem agents can implement custom save_snapshot and load_snapshot functions to support manual (snapshot-based) updates and snapshot-based recovery.

When to Use Snapshotting

Snapshotting solves two distinct problems:

1. Manual / snapshot-based component updates — required when updating agents between incompatible component versions.
2. Fast recovery and oplog compaction — for long-running agents whose oplog grows over time (heartbeats, polling loops, recurring tasks, agents with frequent state changes). Without snapshotting, every recovery replays the full oplog from the beginning, which becomes increasingly expensive. With periodic snapshotting (every(N) or periodic(...)), recovery starts from the latest snapshot and replays only the entries after it.

You cannot opt out of oplog writes for a durable agent. If you are worried about oplog volume or replay cost, do not try to skip persistence — enable snapshot-based recovery here instead.

Enabling Snapshotting

Snapshotting must be enabled via the snapshotting attribute on #[agent_definition]. Without it, no snapshot exports are generated:

#[agent_definition(mount = "/counters/{name}", snapshotting = "every(1)")]
pub trait CounterAgent {
    fn new(name: String) -> Self;

    #[endpoint(post = "/increment")]
    fn increment(&mut self) -> u32;
}

Snapshotting Modes

The snapshotting attribute accepts these values:

Mode	Example	Description
"disabled"	(default when omitted)	No snapshotting
"enabled"	snapshotting = "enabled"	Enable snapshot support with the server's default policy. The server default is disabled, so this may have no effect. Use "every(N)" or "periodic(…)" to guarantee snapshotting is active.
"every(N)"	snapshotting = "every(1)"	Snapshot every N successful function calls (use "every(1)" for every invocation)
"periodic(duration)"	snapshotting = "periodic(30s)"	Snapshot at most once per time interval (uses humantime durations)

#[agent_definition(mount = "/periodic/{name}", snapshotting = "periodic(30s)")]
pub trait PeriodicAgent { ... }

#[agent_definition(mount = "/batch/{name}", snapshotting = "every(10)")]
pub trait BatchAgent { ... }

Automatic Snapshotting (Default)

If the agent's struct implements serde::Serialize and serde::de::DeserializeOwned, the SDK automatically provides JSON-based snapshotting — no custom code needed. The #[agent_implementation] macro detects Serialize/DeserializeOwned on the agent type and auto-generates snapshot handlers.

use serde::{Serialize, Deserialize};
use golem_rust::{agent_definition, agent_implementation, endpoint};

#[agent_definition(mount = "/counters/{name}", snapshotting = "every(1)")]
pub trait CounterAgent {
    fn new(name: String) -> Self;

    #[endpoint(post = "/increment")]
    fn increment(&mut self) -> u32;
}

#[derive(Serialize, Deserialize)]  // This enables automatic snapshotting
struct CounterImpl {
    name: String,
    count: u32,
}

#[agent_implementation(mount = "/counters/{name}")]
impl CounterAgent for CounterImpl {
    fn new(name: String) -> Self {
        Self { name, count: 0 }
    }

    #[endpoint(post = "/increment")]
    fn increment(&mut self) -> u32 {
        self.count += 1;
        self.count
    }
    // No save_snapshot/load_snapshot needed — serde handles it automatically
}

Custom Snapshotting

For custom binary formats, compatibility with non-Rust components, or migration between different state schemas, implement both save_snapshot and load_snapshot on the agent implementation:

use golem_rust::{agent_definition, agent_implementation, endpoint};

#[agent_definition(mount = "/snapshot-counters/{name}", snapshotting = "every(1)")]
pub trait CounterWithSnapshotAgent {
    fn new(name: String) -> Self;

    #[endpoint(post = "/increment")]
    fn increment(&mut self) -> u32;
}

struct CounterImpl {
    _name: String,
    count: u32,
}

#[agent_implementation(mount = "/snapshot-counters/{name}")]
impl CounterWithSnapshotAgent for CounterImpl {
    fn new(name: String) -> Self {
        Self {
            _name: name,
            count: 0,
        }
    }

    #[endpoint(post = "/increment")]
    fn increment(&mut self) -> u32 {
        self.count += 1;
        log::info!("The new value is {}", self.count);
        self.count
    }

    async fn load_snapshot(&mut self, bytes: Vec<u8>) -> Result<(), String> {
        let arr: [u8; 4] = bytes
            .try_into()
            .map_err(|_| "Expected a 4-byte long snapshot")?;
        self.count = u32::from_be_bytes(arr);
        Ok(())
    }

    async fn save_snapshot(&self) -> Result<Vec<u8>, String> {
        Ok(self.count.to_be_bytes().to_vec())
    }
}

Rules

• Both save_snapshot and load_snapshot must be implemented together, or neither. The macro enforces this at compile time.
• When custom implementations are present, automatic serde-based snapshotting is bypassed.
• save_snapshot returns Result<Vec<u8>, String> — the bytes are the snapshot payload.
• load_snapshot receives Vec<u8> and returns Result<(), String> — it must restore the agent's state from the bytes.
• Both methods are async — they can perform asynchronous operations during serialization/deserialization.
• Returning Err from load_snapshot causes the update to fail and the agent reverts to the old version.

Method Signatures

// Save: serialize the agent's current state to bytes
async fn save_snapshot(&self) -> Result<Vec<u8>, String>

// Load: restore the agent's state from previously saved bytes
async fn load_snapshot(&mut self, bytes: Vec<u8>) -> Result<(), String>

Best Practices

1. Prefer automatic (serde) snapshotting unless you need a compact binary format or cross-version migration logic.
2. Keep snapshots small — large snapshots impact recovery and update time.
3. Version your snapshot format — include a version byte or tag so load_snapshot can handle snapshots from older versions.
4. Test round-trips — verify that save_snapshot → load_snapshot produces equivalent state.
5. Handle migration — when the state schema changes between versions, load_snapshot in the new version should be able to parse snapshots from the old version.

Project Template

A ready-made project with snapshotting can be created using:

golem new --yes --language rust --template snapshotting my-project