[FLINK-21865] Add a Docker Compose greeter example

This closes #4.

java/greeter/Dockerfile

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+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements.  See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.
+# The ASF licenses this file to You under the Apache License, Version 2.0
+# (the "License"); you may not use this file except in compliance with
+# the License.  You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Build the functions code ...
+FROM maven:3.6.3-jdk-11 AS builder
+COPY src /usr/src/app/src
+COPY pom.xml /usr/src/app
+# TODO remove these commented lines and the jar; this is needed now only because we don't have the latest Java SDK published to Maven central yet
+# COPY statefun-sdk-java-2.3-SNAPSHOT.jar /usr/src/app
+# RUN mvn install:install-file \
+#     -Dfile=/usr/src/app/statefun-sdk-java-2.3-SNAPSHOT.jar \
+#     -DgroupId=org.apache.flink \
+#     -DartifactId=statefun-sdk-java \
+#     -Dversion=2.3-SNAPSHOT \
+#     -Dpackaging=jar \
+#     -DgeneratePom=true
+RUN mvn -f /usr/src/app/pom.xml clean package
+
+# ... and run the web server!
+FROM openjdk:8
+WORKDIR /
+COPY --from=builder /usr/src/app/target/greeter-functions-app*jar-with-dependencies.jar greeter-functions-app.jar
+EXPOSE 1108
+CMD java -jar greeter-functions-app.jar

java/greeter/README.md

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+# Greeter Example with Docker Compose
+
+This example is intended as a follow-up after completion of the [Java SDK Showcase Tutorial](../showcase). If you're
+already familiar with the Java SDK fundamentals and would like to get a better understanding of how a realistic StateFun
+application looks like, then you're in the right place! Otherwise, we highly suggest taking a look at the Showcase
+tutorial first.
+
+This example works with Docker Compose, and runs a few services that build up an end-to-end StateFun application:
+- Functions service that runs your functions and expose them through an HTTP endpoint.
+- StateFun runtime processes (a manager plus workers) that will handle ingress, egress, and inter-function messages as
+  well as function state storage in a consistent and fault-tolerant manner.
+- Apache Kafka broker for the application ingress and egress. StateFun currently natively supports AWS Kinesis as well,
+  and you can also extend to connect with other systems.
+
+To motivate this example, we'll implement a simple user greeter application, which has two functions - a `UserFn` that
+expects `UserLogin` JSON events from an ingress and keeps in state storage information about users, and a `GreetingsFn`
+that accepts user information to generate personalized greeting messages that are sent to users via an egress.
+
+## Directory structure
+
+- `src/`, `pom.xml` and `Dockerfile`: These files and directories are the contents of a Java Maven project which builds
+  our functions service, hosting the `UserFn` and `UserLogin` behind a HTTP endpoint. Check out the source code under
+  `src/main/java`. The `Dockerfile` is used to build a Docker image for our functions service.
+- `user-logins.txt`: A file with multiple JSON objects per line; this is used as test events produced to our application ingress.
+- `module.yaml`: The [Module Specification]() file to be mounted to the StateFun runtime process containers. This
+  configures a few things for a StateFun application, such as the service endpoints of the application's functions, as
+  well as definitions of [Ingresses and Egresses]() which the application will use.
+- `docker-compose.yml`: Docker Compose file to spin up everything.
+
+## Prerequisites
+
+- Docker
+- Docker Compose
+
+## Running the example
+
+First, lets build the example. From this directory, execute:
+
+```
+$ docker-compose build
+```
+
+This pulls all the necessary Docker images (StateFun and Kafka), and also builds the functions service image. This can
+take a few minutes as it also needs to build the function's Java project.
+
+Afterward the build completes, start running all the services:
+
+```
+$ docker-compose run
+```
+
+## Play around!
+
+You can take a look at what messages are being sent to the Kafka egress:
+
+```
+$ docker-compose exec kafka kafka-console-consumer \
+      --bootstrap-server kafka:9092 \
+      --topic greetings \
+      --from-beginning
+```
+
+You can also try modifying the function code in the `src/main/java` directory, and do a zero-downtime upgrade of the
+functions. Some ideas you can try out:
+- Add some more state to be persisted by the `UserFn`. For example, let it additionally keep track of the user's previous login location.
+- Or maybe just simply changing the greetings message generated by the `GreetingsFn`!
+
+After you've finished changing the function code, you can do a hot redeploy of your functions service:
+
+```
+$ docker-compose up -d --build greeter-functions
+```
+
+This rebuilds the functions service image with the updated code, and restarts the service with the new image.

java/greeter/docker-compose.yml

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+################################################################################
+#  Licensed to the Apache Software Foundation (ASF) under one
+#  or more contributor license agreements.  See the NOTICE file
+#  distributed with this work for additional information
+#  regarding copyright ownership.  The ASF licenses this file
+#  to you under the Apache License, Version 2.0 (the
+#  "License"); you may not use this file except in compliance
+#  with the License.  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+# limitations under the License.
+################################################################################
+version: "2.1"
+
+services:
+
+  ###############################################################
+  #    Functions service
+  ###############################################################
+
+  greeter-functions:
+    build:
+      dockerfile: Dockerfile
+      context: .
+    expose:
+      - "1108"
+
+  ###############################################################
+  #    StateFun runtime
+  ###############################################################
+
+  statefun-manager:
+    image: flink-statefun:2.3-SNAPSHOT
+    expose:
+      - "6123"
+    ports:
+      - "8081:8081"
+    environment:
+      ROLE: master
+      MASTER_HOST: statefun-manager
+    volumes:
+      - ./module.yaml:/opt/statefun/modules/greeter/module.yaml
+
+  statefun-worker:
+    image: flink-statefun:2.3-SNAPSHOT
+    expose:
+      - "6121"
+      - "6122"
+    depends_on:
+      - statefun-manager
+      - kafka
+      - greeter-functions
+    links:
+      - "statefun-manager:statefun-manager"
+      - "kafka:kafka"
+      - "greeter-functions:greeter-functions"
+    environment:
+      ROLE: worker
+      MASTER_HOST: statefun-manager
+    volumes:
+      - ./module.yaml:/opt/statefun/modules/greeter/module.yaml
+
+  ###############################################################
+  #    Kafka for ingress and egress
+  ###############################################################
+
+  zookeeper:
+    image: confluentinc/cp-zookeeper:5.4.3
+    environment:
+      ZOOKEEPER_CLIENT_PORT: "2181"
+    ports:
+      - "2181:2181"
+
+  kafka:
+    image: confluentinc/cp-kafka:5.4.3
+    ports:
+      - "9092:9092"
+    depends_on:
+      - zookeeper
+    links:
+      - "zookeeper:zookeeper"
+    environment:
+      KAFKA_ZOOKEEPER_CONNECT: zookeeper:2181
+      KAFKA_ADVERTISED_LISTENERS: PLAINTEXT://kafka:9092
+      KAFKA_AUTO_CREATE_TOPICS_ENABLE: "true"
+      KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR: 1
+
+  ###############################################################
+  #    Simple Kafka JSON producer to simulate ingress events
+  ###############################################################
+
+  user-logins-producer:
+    image: ververica/statefun-playground-producer:latest
+    depends_on:
+      - kafka
+      - statefun-worker
+    links:
+      - "kafka:kafka"
+    environment:
+      APP_PATH: /mnt/user-logins.txt
+      APP_KAFKA_HOST: kafka:9092
+      APP_KAFKA_TOPIC: user-logins
+      APP_JSON_PATH: user_id
+    volumes:
+      - ./user-logins.txt:/mnt/user-logins.txt

java/greeter/module.yaml

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+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements.  See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.
+# The ASF licenses this file to You under the Apache License, Version 2.0
+# (the "License"); you may not use this file except in compliance with
+# the License.  You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+version: "3.0"
+
+module:
+  meta:
+    type: remote
+  spec:
+    endpoints:
+      - endpoint:
+          meta:
+            kind: http
+          spec:
+            typename:
+              namespace: greeter.fns
+            urlPathTemplate: http://greeter-functions:1108/
+    ingresses:
+      - ingress:
+          meta:
+            type: statefun.kafka.io/routable-protobuf-ingress
+            id: greeter.io/user-logins
+          spec:
+            address: kafka:9092
+            consumerGroupId: greeter
+            startupPosition:
+              type: earliest
+            topics:
+              - topic: user-logins
+                typeUrl: greeter.types/org.apache.flink.statefun.playground.java.greeter.types.UserLogin
+                targets:
+                  - greeter.fns/user
+    egresses:
+      - egress:
+          meta:
+            type: statefun.kafka.io/generic-egress
+            id: greeter.io/user-greetings
+          spec:
+            address: kafka:9092
+            deliverySemantic:
+              type: at-least-once