File I/O in Rust Golem Agents

Overview

Golem Rust agents compile to wasm32-wasip1 which provides WASI filesystem access. Use the standard std::fs module for all filesystem operations — it works out of the box with WASI.

To provision files into an agent’s filesystem, See the golem-add-initial-files guide.

Reading Files

Text Files


use std::fs;
 
let content = fs::read_to_string("/data/config.json")
    .expect("Failed to read file");

Binary Files


use std::fs;
 
let bytes = fs::read("/data/image.png")
    .expect("Failed to read file");

Writing Files

Only files provisioned with read-write permission (or files in non-provisioned paths) can be written to.


use std::fs;
 
fs::write("/tmp/output.txt", "Hello, world!")
    .expect("Failed to write file");

Appending to Files


use std::fs::OpenOptions;
use std::io::Write;
 
let mut file = OpenOptions::new()
    .append(true)
    .create(true)
    .open("/tmp/agent.log")
    .expect("Failed to open file");
 
writeln!(file, "Log message").expect("Failed to write");

Checking File Existence


use std::path::Path;
 
if Path::new("/data/config.json").exists() {
    let content = std::fs::read_to_string("/data/config.json").unwrap();
}

Listing Directories


use std::fs;
 
let entries = fs::read_dir("/data").expect("Failed to read directory");
for entry in entries {
    let entry = entry.expect("Failed to read entry");
    println!("{}", entry.file_name().to_string_lossy());
}

Complete Agent Example


use golem_rust::{agent_definition, agent_implementation};
use std::fs;
use std::io::Write;
 
#[agent_definition]
pub trait FileReaderAgent {
    fn new(name: String) -> Self;
    fn read_greeting(&self) -> String;
    fn write_log(&mut self, message: String);
}
 
struct FileReaderAgentImpl {
    name: String,
}
 
#[agent_implementation]
impl FileReaderAgent for FileReaderAgentImpl {
    fn new(name: String) -> Self {
        Self { name }
    }
 
    fn read_greeting(&self) -> String {
        fs::read_to_string("/data/greeting.txt")
            .expect("Failed to read greeting")
            .trim()
            .to_string()
    }
 
    fn write_log(&mut self, message: String) {
        let mut file = std::fs::OpenOptions::new()
            .append(true)
            .create(true)
            .open("/tmp/agent.log")
            .expect("Failed to open log");
        writeln!(file, "{}", message).expect("Failed to write log");
    }
}

Key Constraints

Use standard std::fs — it works with WASI out of the box
Files provisioned via golem-add-initial-files with read-only permission cannot be written to
The filesystem is per-agent-instance — each agent has its own isolated filesystem
File changes within an agent are persistent across invocations (durable state)
Third-party crates that use std::fs (e.g., serde_json’s file reading) also work