Merge pull request #170 from jpetazzo/headless-services

Add headless services

slides/kube/kubectlexpose.md

@@ -138,3 +138,87 @@ Note: please DO NOT call the service `search`. It would collide with the TLD.
 --
 
 Our requests are load balanced across multiple pods.
+
+---
+
+class: extra-details
+
+## If we don't need a load balancer
+
+- Sometimes, we want to access our scaled services directly:
+
+  - if we want to save a tiny little bit of latency (typically less than 1ms)
+
+  - if we need to connect over arbitrary ports (instead of a few fixed ones)
+
+  - if we need to communicate over another protocol than UDP or TCP
+
+  - if we want to decide how to balance the requests client-side
+
+  - ...
+
+- In that case, we can use a "headless service"
+
+---
+
+class: extra-details
+
+## Headless services
+
+- A headless service is obtained by setting the `clusterIP` field to `None`
+
+  (Either with `--cluster-ip=None`, or by providing a custom YAML)
+
+- As a result, the service doesn't have a virtual IP address
+
+- Since there is no virtual IP address, there is no load balancer either
+
+- `kube-dns` will return the pods' IP addresses as multiple `A` records
+
+- This gives us an easy way to discover all the replicas for a deployment
+
+---
+
+class: extra-details
+
+## Services and endpoints
+
+- A service has a number of "endpoints"
+
+- Each endpoint is a host + port where the service is available
+
+- The endpoints are maintained and updated automatically by Kubernetes
+
+.exercise[
+
+- Check the endpoints that Kubernetes has associated with our `elastic` service:
+  ```bash
+  kubectl describe service elastic
+  ```
+
+]
+
+In the output, there will be a line starting with `Endpoints:`.
+
+That line will list a bunch of addresses in `host:port` format.
+
+---
+
+class: extra-details
+
+## Viewing endpoint details
+
+- When we have many endpoints, the previous command truncates the list
+
+- If we want to see the full list, we can use one of the following commands:
+  ```bash
+  kubectl describe endpoint elastic
+  kubectl get endpoint elastic -o yaml
+  ```
+
+- These addresses will show us a list of IP addresses
+
+- These IP addresses should match the addresses of the corresponding pods:
+  ```bash
+  kubectl get pods -l run=elastic -o wide
+  ```