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SmartModule FilterMaps are used to both transform and potentially filter records from a stream at the same time. This can be useful for a number of scenarios including working with data with nullable fields, or working with subsets of event data. In these cases, FilterMap allows us discard irrelevant data - such as records with null fields or event types that we don’t care about - while also performing meaningful work with relevant data - such as reformatting fields we’ve extracted or events we’ve gathered.

FilterMap functions work by returning an Option of a new record. To discard a record from the stream, return None. Otherwise, transform the record according to your needs and return it as Some(record).

SmartModule FilterMap

Let’s dive in and see how to use this in practice. You can find the full code for this doc’s example in the fluvio-smartmodule-examples repository.


Example project

For this example, we’ll write a simplified version of the online grocery-notifications example we blogged about. In this simplified example, we’ll consider a stream that has two event types: Account Created and Order Ready. We’ll use FilterMap to discard account_created events, and to transform order_ready events into text messages that should be sent to the user.

Let’s take a look at some sample input for this example:


We’d like to discard the account_created events, keep the order_ready events, and transform the order_ready events to a nice message that should be sent to the user. The output records in this stream should look like this:

{"number":"1-800-234-5678","message":"Hello Bill, your groceries have been collected and are ready to pick up!"}
{"number":"1-222-222-2222","message":"Hello Mary, your groceries have been collected and are ready to pick up!"}

Create a new Project

We can use the cargo-generate tool to create a new SmartModules project that is ready to go. If you don’t already have it, you can install cargo-generate using this command:

$ cargo install cargo-generate

Then, use the following command to create a new SmartModules FilterMap project.

$ cargo generate --git="https://github.com/infinyon/fluvio-smartmodule-template"
⚠️   Unable to load config file: ~/.cargo/cargo-generate.toml
🤷   Project Name : filter-map
🔧   Generating template ...
✔ 🤷   Which type of SmartModule would you like? · array-map
[1/7]   Done: .cargo/config.toml
[2/7]   Done: .cargo
[3/7]   Done: .gitignore
[4/7]   Done: Cargo.toml
[5/7]   Done: README.md
[6/7]   Done: src/lib.rs
[7/7]   Done: src
🔧   Moving generated files into: `filter-map`...
✨   Done! New project created filter-map

We’ll want to cd into the project directory for the rest of the commands to work:

$ cd filter-map

Now, let’s jump right into the code. Copy and paste the following block into the src/lib.rs file in the new SmartModule project:

use fluvio_smartmodule::{smartmodule, Record, RecordData, Result};
use serde::{Deserialize, Serialize};

/// Events that may take place in an online grocery service
#[derive(Debug, Serialize, Deserialize)]
#[serde(tag = "type", rename_all = "snake_case")]
enum GroceryEvent {
    AccountCreated {
        account_id: String,
        username: String,
        preferred_name: String,
        phone_number: String,
    OrderReady {
        account_id: String,
        sms_number: String,
        sms_name: String,

fn filter_map(record: &Record) -> Result<Option<(Option<RecordData>, RecordData)>> {
    let event: GroceryEvent = match serde_json::from_slice(record.value.as_ref()) {
        Ok(event) => event,
        Err(_) => return Ok(None), // Skip if we fail to parse JSON

    let sms_message = match event {
        GroceryEvent::OrderReady {
        } => serde_json::json!({
            "number": sms_number,
            "message": format!(
                "Hello {}, your groceries have been collected and are ready to pick up!",
        _ => return Ok(None),

    let message_json = serde_json::to_string(&sms_message)?;
    Ok(Some((record.key.clone(), message_json.into())))

Let’s break down what’s happening here. First, we have a GroceryEvent enum which represents the different types of input record we are expecting. We’re using serde to automatically serialize and deserialize our JSON into this enum type.

Next, we check what type of event we received. If it’s an order_ready event, we transform it, picking the phone number and account preferred name and creating a record that contains a text-friendly message that should be sent to the user. If it is any other event type, we filter it out by returning Ok(None).

Let’s take a look at how we can run this SmartModule with our sample data!


Running the FilterMap

Before getting started, make sure you have downloaded the Fluvio CLI and followed the getting started guide to get up and running with a Fluvio cluster. Then, if you haven’t done so already, you’ll need to install the wasm32-unknown-unknown target for Rust using the following command:

$ rustup target add wasm32-unknown-unknown

Now we’ll be able to compile the FilterMap SmartModule. Let’s use release mode so we get the smallest WASM binary possible:

$ cargo build --release

Next, we’ll need to create a new Fluvio topic to produce and consume our data using this command:

$ fluvio topic create filter-map
topic "array-map" created

Now, let’s put our sample data into a file and produce it to our topic. Create groceries.txt with the following contents:


Now we can produce the sample data to our topic.

$ fluvio produce filter-map -f ./groceries.txt

Finally, let’s consume our data using our FilterMap SmartModule and see that our output records match the format we expect, with the phone number and text message to send:

$ fluvio consume filter-map -B --filter-map=target/wasm32-unknown-unknown/release/filter_map.wasm
Consuming records from the beginning of topic 'filter-map'
{"message":"Hello Bill, your groceries have been collected and are ready to pick up!","number":"1-800-234-5678"}
{"message":"Hello Mary, your groceries have been collected and are ready to pick up!","number":"1-222-222-2222"}

Congratulations, you just completed your first FilterMap example! You can find the full source code for this example on GitHub, along with the full sources for many other SmartModules examples.


Register the SmartModule with Fluvio

After building a SmartModule as a WASM binary, it may be registered with Fluvio using the fluvio smart-module command:

$ fluvio smart-module create filter-groceries --wasm-file target/wasm32-unknown-unknown/release/filter_map.wasm

Use the fluvio smart-module list command to see all available SmartModules:

$ fluvio smart-module list
 NAME             STATUS             SIZE
filter-groceries  SmartModuleStatus  178931 

Once the SmartModule is created, it can be used by other areas of the system (consumers, producers, connectors, etc):

$ fluvio consume filter-map -B --filter-map=filter-groceries

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