Add save and load route table feature to nats-client

[domdom82]

• A short explanation of the proposed change:

• Added "save" and "load" functionality to the nats client in routing_utils
• The "save" function stores a gorouter's routing table in a json file
• The "load" function plays back the json file by registering all routes with gorouter again

• An explanation of the use cases your change solves
  The main purpose of this functionality is to be able to easily load scenarios where the route table is in a specific state.
  This is helpful in debugging problems like the ones mentioned here and here or here

Developers can simply craft a route table as desired instead of having to tweak apps and NATS to behave in such a way that the issue is reproduced.

• Instructions to functionally test the behavior change using operator interfaces (BOSH manifest, logs, curl, and metrics)

1. Deploy nats and gorouter as described in the README.md
2. Provide a minimal gorouter.yml config file for the nats-client to read. If you started gorouter without a config file, you may use this one:

---

status:
  port: 8082
  user:
  pass:

nats:
  tls_enabled: false
  hosts:
  - hostname: localhost
    port: 4222

3. Add a few routes. You may use the nats-client or simply curl to send some router.register NATS messages.
4. Save the current route table using

./nats_client gorouter.yml save routes.json
Saving route table to routes.json
Done

5. You should now have a routes.json file with a pretty-printed output of the /routes endpoint of gorouter. It could look like this:

{
  "api.cf.my.domain.com": [
    {
      "address": "10.0.65.66:9024",
      "private_instance_id": "767a8085-3e34-47d6-47b3-1ac87020edad",
      "protocol": "http1",
      "server_cert_domain_san": "api.cf.my.domain.com",
      "tags": {
        "component": "CloudController"
      },
      "tls": true,
      "ttl": 120
    }
  ],
  "cf-app.cf.my.domain.com": [
    {
      "address": "10.0.73.0:61015",
      "private_instance_id": "32e285ab-9b42-4de8-7141-3b87",
      "protocol": "http1",
      "server_cert_domain_san": "32e285ab-9b42-4de8-7141-3b87",
      "tags": {
        "app_id": "e74be186-20f5-4ea8-844b-77895c477c10",
        "app_name": "cf-app",
        "component": "route-emitter",
        "instance_id": "0",
        "organization_id": "bc578e33-bce1-4bce-91c3-a302bcb27ee3",
        "organization_name": "ACME",
        "process_id": "e74be186-20f5-4ea8-844b-77895c477c10",
        "process_instance_id": "32e285ab-9b42-4de8-7141-3b87",
        "process_type": "web",
        "source_id": "e74be186-20f5-4ea8-844b-77895c477c10",
        "space_id": "e646f0e9-1aa3-41f3-b177-9427ec5e7c84",
        "space_name": "dev"
      },
      "tls": true,
      "ttl": 120
    },
(...)
}

6. You may now edit the routes.json file, e.g. add another route or change an existing one, add more endpoints etc. Be aware that this only works well with TLS routes as non-TLS routes will be pruned shortly after.
7. Use the nats-client to upload the changed routes to gorouter:

./nats_client gorouter.yml load routes.json
Loading route table from routes.json
Done

8. You can then test the changed routes by curling them, e.g. if you changed the route to cf-app.cf.my.domain.com:

curl -v http://localhost:8081 -H "Host: cf-app.cf.my.domain.com"
(...)

9. You should see the effects in the behavior of gorouter. It will use the changed route instead. Be aware that your changes may be overwritten by route-emitters of the app you changed. So for best results it is recommended to inject a new route that does not exist on any app.

• Expected result after the change

nats-client can save and restore gorouter route tables using NATS.

• Current result before the change

nats-client can only publish and subscribe to NATS.

• Links to any other associated PRs

• I have viewed signed and have submitted the Contributor License Agreement

• I have made this pull request to the develop branch

• I have run all the unit tests using scripts/run-unit-tests-in-docker

• (Optional) I have run Routing Acceptance Tests and Routing Smoke Tests on bosh lite

• (Optional) I have run CF Acceptance Tests on bosh lite

[ameowlia]

This will be great for debugging.

❓ Can you add these notes for how to use it in a doc in the docs dir?

I'll let you assign a reviewer for a more thorough review. Thanks!

[domdom82]

Can you add these notes for how to use it in a doc in the docs dir?

Good idea! Will do.

[b1tamara]

LGFM