Geode WAN replication example

This example demonstrates Geode support for asynchronous WAN replication between clusters. WAN replication allows remote Geode clusters to automatically keep their region data consistent through the use of gateway senders and receivers. A gateway sender distributes region events to another, remote Geode cluster. A gateway receiver configures a physical connection for receiving region events from gateway senders in remote Geode clusters. The gateway senders and receivers can be configured in several different topologies based on specific business needs. For more information on example topologies and associated use cases see Geode documentation on Multi-site WAN Configuration

In this example, two clusters are created on your local machine, each with a unique distributed system id and the WAN gateway configured for active-active, bidirectional region updates. The New York cluster (ny) has id=1 and the London cluster (ln) has id=2. Each cluster contains the same partitioned region (example-region) and each has parallel gateway senders, which means each server in the cluster will send data updates for the primary region buckets they hold. Alternately, you can configure serial gateway senders, where only one server in each cluster sends all data updates across the WAN. Serial gateway senders are typically used for replicated regions or when the order of events between different keys in a partitioned region needs to be preserved.

This example runs a single client that connects to the London cluster and puts 10 entries into the example-region and prints them. After the client app has run, both clusters will contain the data.

Special Note The gfsh scripts and gradle tasks for this example do not follow the standard used by other geode-examples in order to create 2 separate clusters. Due to this, you must follow the steps outlined below, as this example will not run using the runAll gradle task.

This example assumes that Java and Geode are installed.

Steps

From the geode-examples/wan directory, build the client app example
```
 $ ../gradlew build
```
Run the script that starts the London cluster (1 locator and 2 servers) and creates the gateway senders and receivers.
```
 $ gfsh run --file=scripts/start-ln.gfsh
```
Run the script that starts the New York cluster (1 locator and 2 servers) and creates the gateway senders and receivers.
```
 $ gfsh run --file=scripts/start-ny.gfsh
```
Run the script that creates the example-region in each cluster and associates the gateway senders to this region.
```
 $ gfsh run --file=scripts/start-wan.gfsh
```
Run the client example app that connects to the London cluster and puts 10 entries into the example-region. The data will be automatically sent to the New York cluster, as well as printed to the console.
```
 $ ../gradlew run
```

In one terminal, run a gfsh command, connect to the New York cluster, and verify the region contents

 $ gfsh
 ...
 Cluster-1 gfsh>connect --locator=localhost[10331]
 Cluster-1 gfsh>query --query="select e.key from /example-region.entries e"
 ...

In another terminal, run a gfsh command, connect to the London cluster, and verify the region contents

 $ gfsh
 ...
 Cluster-2 gfsh>connect --locator=localhost[10332]
 Cluster-2 gfsh>query --query="select e.key from /example-region.entries e"
 ...

Use other gfsh commands to learn statistics about the regions, gateway senders, and gateway receivers for each cluster.
```
 Cluster-1 gfsh>describe region --name=example-region
 Cluster-1 gfsh>list gateways
```

In the terminal connected to the New York cluster, put another entry in the region and verify it is in the region on this cluster.

 Cluster-1 gfsh>put --key=20 --value="value20" --region=example-region
 Cluster-1 gfsh>query --query="select e.key from /example-region.entries e"

In the terminal connected to the London cluster, verify the new entry has also been added to the region on this cluster.
```
Cluster-2 gfsh>query --query="select e.key from /example-region.entries e"
```
Exit gfsh in each terminal and shutdown the cluster using the stop.gfsh script
```
$ gfsh run --file=scripts/stop.gfsh
```
Clean up any generated directories and files.
```
$ ../gradlew cleanServer
```