# Internal Load Balancer as Next Hop

This example bootstraps a minimal infrastructure for testing [ILB as next hop](https://cloud.google.com/load-balancing/docs/internal/ilb-next-hop-overview),  using simple Linux gateway VMS between two VPCs to emulate virtual appliances.

The following diagram shows the resources created by this example

![High-level diagram](diagram.png "High-level diagram")

Two ILBs are configured on the primary and secondary interfaces of gateway VMs with active health checks, but only a single one is used as next hop by default to simplify testing. The second (right-side) VPC has default routes that point to the gateway VMs, to also use the right-side ILB as next hop set the `ilb_right_enable` variable to `true`.

## Testing

Since ILBs as next hops only forward TCP and UDP traffic, simple tests use `curl` on clients to send HTTP requests. To make this practical, test VMs on both VPCs have `nginx` pre-installed and active on port 80.

On the gateways, `iftop` is installed by default to quickly monitor traffic passing forwarded across VPCs.

Session affinity on the ILB backend services can be changed using `gcloud compute backend-services update` on each of the ILBs, or by setting the `ilb_session_affinity` variable to update both ILBs.

Simple `/root/start.sh` and `/root/stop.sh` scripts are pre-installed on both gateways to configure `iptables` so that health check requests are rejected and re-enabled, to quickly simulate removing instances from the ILB backends.

Some scenarios to test:

- short-lived connections with session affinity set to the default of `NONE`, then to `CLIENT_IP`
- long-lived connections, failing health checks on the active gateway while the connection is active

### Useful commands (adjust names and addresses to match)

Create a large file on a destination VM (eg `ilb-test-vm-right-1`) to test long-running connections.

```bash
dd if=/dev/zero of=/var/www/html/test.txt bs=10M count=100 status=progress
```

Run curl from a source VM (eg `ilb-test-vm-left-1`) to send requests to a destination VM artifically slowing traffic.

```
curl -0 --output /dev/null --limit-rate 10k 10.0.1.3/test.txt
```

Monitor traffic from a source VM (eg `ilb-test-vm-left-1`) on the gateways.

```bash
iftop -n -F 10.0.0.3/32
```

Poll summary health status for a backend.

```bash
watch '\
  gcloud compute backend-services get-health ilb-test-ilb-right \
    --region europe-west1 \
    --flatten status.healthStatus \
    --format "value(status.healthStatus.ipAddress, status.healthStatus.healthState)" \
'
```

A sample testing session using `tmux`:

<a href="https://raw.githubusercontent.com/terraform-google-modules/cloud-foundation-fabric/master/networking/ilb-next-hop/test_session.png" title="Test session screenshot"><img src="./test_session.png" width="640px" alt="Test session screenshot"></img>

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## Variables

| name | description | type | required | default |
|---|---|:---: |:---:|:---:|
| project_id | Existing project id. | <code title="">string</code> | ✓ |  |
| *ilb_right_enable* | Route right to left traffic through ILB. | <code title="">bool</code> |  | <code title="">false</code> |
| *ilb_session_affinity* | Session affinity configuration for ILBs. | <code title="">string</code> |  | <code title="">CLIENT_IP</code> |
| *ip_ranges* | IP CIDR ranges used for VPC subnets. | <code title="map&#40;string&#41;">map(string)</code> |  | <code title="&#123;&#10;left  &#61; &#34;10.0.0.0&#47;24&#34;&#10;right &#61; &#34;10.0.1.0&#47;24&#34;&#10;&#125;">...</code> |
| *prefix* | Prefix used for resource names. | <code title="">string</code> |  | <code title="">ilb-test</code> |
| *project_create* | Create project instead of using an existing one. | <code title="">bool</code> |  | <code title="">false</code> |
| *region* | Region used for resources. | <code title="">string</code> |  | <code title="">europe-west1</code> |

## Outputs

<!-- END TFDOC -->