Stateful Functions is an Apache Flink library for distributed applications and services, based on, well, you guessed it: stateful functions.
The project aims to simplify the development of distributed stateful applications by solving some of the common challenges in those applications: scaling, consistent state management, reliable interaction between distributed services, and resource management.
Stateful Functions uses Apache Flink for distributed coordination, state, and communication.
This README is meant as a brief walkthrough on the core concepts and how to set things up to get yourself started with Stateful Functions.
For a fully detailed documentation, please visit the official docs.
For code examples, please take a look at the examples.
Table of Contents
Core Concepts
Abstraction
A Stateful Functions application consists of the following primitives: stateful functions, ingresses, routers, and egresses.
Stateful functions
Stateful functions are the building blocks and namesake of the Stateful Functions framework. A function is a small piece of logic (currently simple Java functions) that are invoked through a message.
Each stateful function exist as uniquely invokable virtual instances of a function type. Each instance is addressed by its type, as well as an unique id (a string) within its type.
Stateful functions may be invoked from ingresses or any other stateful function (including itself). The caller simply needs to know the address of the target function.
Function instances are virtual, because they are not all active in memory at the same time. At any point in time, only a small set of functions and their state exists as actual objects. When a virtual instance receives a message, one of the objects is configured and loaded with the state of that virtual instance and then processes the message. Similar to virtual memory, the state of many functions might be “swapped out” at any point in time.
Each virtual instance of a function has its own state, which can be accessed in local variables. That state is private and local to that instance.
If you know Apache Flink’s DataStream API, you can think of stateful functions a bit like a lightweight KeyedProcessFunction. The function type is process function transformation, while the ID is the key. The difference is that functions are not assembled in a directed acyclic graph that defines the flow of data (the streaming topology), but rather send events arbitrarily to all other functions using addresses.
Ingresses and Egresses
Ingresses are the way that events initially arrive in a Stateful Functions application. Ingresses can be message queues, logs, or HTTP servers - anything that produces an event to be handled by the application.
Routers are attached to ingresses to determine which function instance should handle an event initially.
Egresses are a way to send events out from the application in a standardized way. Egresses are optional; it is also possible that no events leave the application and functions sink events or directly make calls to other applications.
Modules and extensibility
A module is the entry point for adding to a Stateful Functions application the core building block primitives, i.e. ingresses, egresses, routers, and stateful functions.
A single application may be a combination of multiple modules, each contributing a part of the whole application. This allows different parts of the application to be contributed by different modules; for example, one module may provide ingresses and egresses, while other modules may individually contribute specific parts of the business logic as stateful functions. This facilitates working in independent teams, but still deploying into the same larger application.
This extensibility is achieved by leveraging the Java Service Loader. In this context, each module is essentially a service provider.
Getting Started
Follow the steps here to get started right away with Stateful Functions.
This guide will walk you through locally building the project, running an existing example, and setup to start developing and testing your own Stateful Functions application.
Building the project
Prerequisites:
- Docker
- Maven 3.5.x or above
- Java 8 or above
Currently, the project does not have any publicly available artifacts or Docker images for use, so you would have to first build the project yourself before trying it out.
mvn clean install
If you want to deploy your applications using Docker, you should also build the base Docker image:
./tools/docker/build-stateful-functions.sh
Running a full example
As a simple demonstration, we will be going through the steps to run the Greeter example.
Before anything else, make sure that you have locally built the project as well as the base Stateful Functions Docker image. Then, follow the next steps to run the example:
cd statefun-examples/statefun-greeter-example docker-compose build docker-compose up
This example contains a very basic stateful function with a Kafka ingress and a Kafka egress.
To see the example in action, send some messages to the topic names, and see what comes out out of the topic greetings:
docker-compose exec kafka-broker kafka-console-producer.sh \
--broker-list localhost:9092 \
--topic names
docker-compose exec kafka-broker kafka-console-consumer.sh \
--bootstrap-server localhost:9092 \
--topic greetings --from-beginning
Project setup
You can quickly get started building Stateful Functions applications using the provided quickstart Maven archetype:
mvn archetype:generate \ -DarchetypeGroupId=org.apache.flink \ -DarchetypeArtifactId=statefun-quickstart \ -DarchetypeVersion=2.0.0
This allows you to name your newly created project. It will interactively ask you for the groupId, artifactId, and package name. There will be a new directory with the same name as your artifact id.
We recommend you import this project into your IDE to develop and test it. IntelliJ IDEA supports Maven projects out of the box. If you use Eclipse, the m2e plugin allows to import Maven projects. Some Eclipse bundles include that plugin by default, others require you to install it manually.
Running from the IDE
To test out your application, you can directly run it in the IDE without any further packaging or deployments.
Please see the Harness example on how to do that.
Deploying Applications
Stateful Functions applications can be packaged as either standalone applications or Flink jobs that can be submitted to a Flink cluster.
Deploying with a Docker image
Below is an example Dockerfile for building an image for an application called statefun-example:
FROM flink-statefun[:version-tag] RUN mkdir -p /opt/statefun/modules/statefun-example COPY target/statefun-example*jar /opt/statefun/modules/statefun-example/
Deploying as a Flink job
If you prefer to package your Stateful Functions application as a Flink job to submit to an existing Flink cluster, simply include statefun-flink-distribution as a dependency to your application.
<dependency>
<groupId>org.apache.flink</groupId>
<artifactId>statefun-flink-distribution</artifactId>
<version>2.0.0</version>
</dependency>
It includes all the runtime dependencies and configures the application's main entry-point. You do not need to take any action beyond adding the dependency to your Maven pom.
Bundle the distribution with your application as a fat jar, and then submit it as you normally would with any other Flink job:
{$FLINK_DIR}/bin/flink run ./statefun-example.jar
Contributing
There are many possible ways to enhance the Stateful Functions API for different types of applications. The runtime and operations will also evolve with the developments in Apache Flink. If you find these ideas interesting or promising, try Stateful Functions out and get involved!
You can learn more about how to contribute in the Apache Flink website. For code contributions, please read carefully the Contributing Code section and check the Stateful Functions component in Jira for an overview of ongoing community work.
License
The code in this repository is licensed under the Apache Software License 2.