Momento Functions

Momento Functions are how you can extend Momento.

A work in progress, you can learn more about Functions by reaching out to [email protected]

Functions run on Momento's service hosts, and offer a powerful scripting capability.

• Use the Momento host interfaces to interact with Momento features within your cache
• Use the AWS host interfaces to use a managed, hot channel to talk to AWS resources
• Use the HTTP host interfaces to reach out to anything you want

This repository holds crates for Momento Functions guest code.

To see some of what you can do with Functions, you can look at the examples.

Getting started

One-time setup

• Install Rust: https://rustup.rs
• Add the Momento Functions compile target: rustup target add wasm32-wasip2

Make a project

cargo init --lib hello

Set up build configuration

Add a file .cargo/config.toml that sets the build target, for convenience.

[build]
target = "wasm32-wasip2"

Set up Cargo.toml

Add this to build the right kind of artifact:

[lib]
crate-type = ["cdylib"]

Pull in only the crates you actually need. For a basic web function, the guest macro and the off-guest buffer crate are enough:

[dependencies]
momento-functions-bytes     = { version = "0" }
momento-functions-guest-web = { version = "0" }

For a Spawn function, swap guest-web for guest-spawn. Other capabilities live in their own focused crates — add them as you go: momento-functions-cache, momento-functions-http, momento-functions-token, momento-functions-topic, momento-functions-valkey, momento-functions-aws-s3, momento-functions-aws-secrets-manager, momento-functions-aws-auth, momento-functions-host-log.

Write a Function

The simplest function is a pong response web function. You can put this in lib.rs.

use momento_functions_bytes::Data;
use momento_functions_guest_web::invoke;

invoke!(ping);
fn ping(_payload: Data) -> &'static str {
    "pong"
}

Data is a buffer that can stay on the host instead of being copied into your function's memory — useful when you're just passing bodies through. Call Data::into_bytes() when you actually need the bytes.

For typed JSON in/out, swap the payload type for momento_functions_bytes::encoding::Json<T>:

use momento_functions_bytes::encoding::Json;
use momento_functions_guest_web::invoke;

#[derive(serde::Deserialize)]
struct Request { name: String }

#[derive(serde::Serialize)]
struct Response { message: String }

invoke!(greet);
fn greet(Json(request): Json<Request>) -> Json<Response> {
    Json(Response { message: format!("Hello, {}!", request.name) })
}

For a portfolio of more substantial v2 Functions — cache, Valkey, HTTP integrations (Turbopuffer, OpenAI), AWS S3 / Secrets Manager, disposable token vending, structured logging, Spawn — see examples/. Each is its own minimal crate so you can copy a Cargo.toml as a starter.

Build and deploy

Build: cargo build --release

Deploy

First, base64 encode the function, then upload. Note that the path here includes "manage". The output from using curl -v should include an HTTP status code of 204.

MOMENTO_CACHE_NAME=your_cache

base64_data=$(cat target/wasm32-wasip2/release/hello.wasm | base64)

curl -v \
  https://api.cache.$MOMENTO_CELL_HOSTNAME/functions/manage/$MOMENTO_CACHE_NAME/ping \
  -XPUT \
  -H "authorization: $MOMENTO_API_KEY" \
  -H "Content-Type: application/json" \
  --data "{\"inline_wasm\":\"$base64_data\"}"

Alternatively, you can use the Momento CLI, which will handle the encoding for you:

momento preview function put-function \
   --cache-name "$MOMENTO_CACHE_NAME" \
   --name ping \
   --wasm-file target/wasm32-wasip2/release/hello.wasm

Invoke

Invoke the function by sending a request directly to the function name.

MOMENTO_CACHE_NAME=your_cache

curl \
  https://api.cache.$MOMENTO_CELL_HOSTNAME/functions/$MOMENTO_CACHE_NAME/ping \
  -H "authorization: $MOMENTO_API_KEY" \
  -d 'ping'

Alternatively, you can use the CLI:

momento preview function invoke-function \
  --cache-name "$MOMENTO_CACHE_NAME" \
  --name ping
  --data 'ping'

Going further

From here, you should look at the examples. Momento Functions are a limited environment, but the supported feature set is growing.

Developing a Function

Wasi support and standard library

Using wasm32-wasip2, you have access to std::time. Most other std wasip2 interfaces will panic at runtime.

Other wasi interfaces are not defined and will result in a linking error when you upload your Function.

Environment details

You are running under a wasmtime host. Unless otherwise specified, the host you're running on is undefined. You are effectively running as a stateless web server.

As the ecosystem matures, new limits may be created and execution location semantics may change.

If you hit an error you don't think you should - e.g., you updated Rust locally and now your Functions don't link - please reach out to [email protected]