# Pulsar Functions overview

Pulsar Functions are lightweight compute processes that

• consume messages from one or more Pulsar topics,
• apply a user-supplied processing logic to each message,
• publish the results of the computation to another topic.

## Goals​

With Pulsar Functions, you can create complex processing logic without deploying a separate neighboring system (such as Apache Storm, Apache Heron, Apache Flink). Pulsar Functions are computing infrastructure of Pulsar messaging system. The core goal is tied to a series of other goals:

• Developer productivity (language-native vs Pulsar Functions SDK functions)
• Easy troubleshooting
• Operational simplicity (no need for an external processing system)

## Inspirations​

Pulsar Functions are inspired by (and take cues from) several systems and paradigms:

Pulsar Functions can be described as

• Lambda-style functions that are
• specifically designed to use Pulsar as a message bus.

## Programming model​

Pulsar Functions provide a wide range of functionality, and the core programming model is simple. Functions receive messages from one or more input topics. Each time a message is received, the function will complete the following tasks.

• Apply some processing logic to the input and write output to:
• Write logs to a log topic (potentially for debugging purposes)
• Increment a counter

You can use Pulsar Functions to set up the following processing chain:

• A Python function listens for the raw-sentences topic and "sanitizes" incoming strings (removing extraneous whitespace and converting all characters to lowercase) and then publishes the results to a sanitized-sentences topic.
• A Java function listens for the sanitized-sentences topic, counts the number of times each word appears within a specified time window, and publishes the results to a results topic
• Finally, a Python function listens for the results topic and writes the results to a MySQL table.

### Word count example​

If you implement the classic word count example using Pulsar Functions, it looks something like this:

To write the function in Java with Pulsar Functions SDK for Java, you can write the function as follows.

package org.example.functions;import org.apache.pulsar.functions.api.Context;import org.apache.pulsar.functions.api.Function;import java.util.Arrays;public class WordCountFunction implements Function<String, Void> {    // This function is invoked every time a message is published to the input topic    @Override    public Void process(String input, Context context) throws Exception {        Arrays.asList(input.split(" ")).forEach(word -> {            String counterKey = word.toLowerCase();            context.incrCounter(counterKey, 1);        });        return null;    }}

Bundle and build the JAR file to be deployed, and then deploy it in your Pulsar cluster using the command line as follows.

### Functions, messages and message types​

Pulsar Functions take byte arrays as inputs and spit out byte arrays as output. However in languages that support typed interfaces(Java), you can write typed Functions, and bind messages to types in the following ways.

## Fully Qualified Function Name (FQFN)​

Each Pulsar Function has a Fully Qualified Function Name (FQFN) that consists of three elements: the function tenant, namespace, and function name. FQFN looks like this:

tenant/namespace/name

FQFNs enable you to create multiple functions with the same name provided that they are in different namespaces.

## Supported languages​

Currently, you can write Pulsar Functions in Java, Python, and Go. For details, refer to Develop Pulsar Functions.

## Processing guarantees​

Pulsar Functions provide three different messaging semantics that you can apply to any function.

Delivery semanticsDescription
At-most-once deliveryEach message sent to the function is likely to be processed, or not to be processed (hence "at most").
At-least-once deliveryEach message sent to the function can be processed more than once (hence the "at least").
Effectively-once deliveryEach message sent to the function will have one output associated with it.

### Apply processing guarantees to a function​

You can set the processing guarantees for a Pulsar Function when you create the Function. The following pulsar-function create command creates a function with effectively-once guarantees applied.