Document how the NGINX Ingress controller build nginx.conf

docs/how-it-works.md

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+# How it works
+
+The objective of this document explains how the NGINX Ingress controller works, in particular how the NGINX model is built and why we need a one.
+
+## NGINX configuration
+
+The goal of this Ingress controller is the assembly of a configuration file (nginx.conf). The main implication of this requirement is the need to reload NGINX after any change in the configuration file.
+
+## NGINX model
+
+Usually, a Kubernetes Controller utilizes the [synchronization loop pattern](1) to check if the desired state in the controller is updated or a change is required. To this purpose, we need to build a model using different objects from the cluster, in particular (in no special order) Ingresses, Services, Endpoints, Secrets, and Configmaps to generate a point in time configuration file that reflects the state of the cluster.
+
+To get this object from the cluster, we use [Kubernetes Informers](2), in particular, `FilteredSharedInformer`. This informers allows reacting to changes in using [callbacks](3) to individual changes when a new object is added, modified or removed. Unfortunately, there is no way to know if a particular change is going to affect the final configuration file. Therefore on every change, we have to rebuild a new model from scratch based on the state of cluster and compare it to the current model. If the new model equals to the current one, then we avoid generating a new NGINX configuration and [trigger a reload](7). Otherwise, we create a new NGINX configuration based on the new model, replace the current model and [trigger a reload](7).
+
+One of the uses of the model is to avoid unnecessary reloads when there's no change in the state and to detect conflicts in definitions.
+
+The final representation of the NGINX configuration is generated from a [Go template](6) using the new model as input for the variables required by the template.
+
+## Building the NGINX model
+
+Building a model is an expensive operation, for this reason, the use of the synchronization loop is a must. By using a [work queue](4) it is possible to not lose changes and remove the use of [sync.Mutex](5) to force a single execution of the sync loop and additionally it is possible to create a time window between the start and end of the sync loop that allows us to discard unnecessary updates. It is important to understand that any change in the cluster could generate events that the informer will send to the controller and one of the reasons for the [work queue](4).
+
+Operations to build the model:
+
+- Order Ingress rules by `ResourceVersion` field, i.e., old rules first.
+  - If the same path for the same host is defined in more than one Ingress, the oldest rule wins.
+  - If more than one Ingress contains a TLS section for the same host, the oldest rule wins.
+  - If multiple Ingresses define an annotation that affects the configuration of the Server block, the oldest rule wins.
+
+- Create a list of NGINX Servers (per hostname)
+- Create a list of NGINX Upstreams
+- If multiple Ingresses define different paths for the same host, the ingress controller will merge the definitions.
+- Annotations are applied to all the paths in the Ingress.
+- Multiple Ingresses can define different annotations. These definitions are not shared between Ingresses.
+
+## When a reload is required
+
+The next list describes the scenarios when a reload is required:
+
+- New Ingress Resource Created.
+- TLS section is added to existing Ingress.
+- Change in Ingress annotations.
+- A path is added/removed from an Ingress.
+- An Ingress, Service, Secret is removed.
+- Some missing referenced object from the Ingress is available, like a Service, Secret or Endpoint.
+- A Secret is updated.
+
+## Avoiding reloads
+
+In some cases, it is possible to avoid reloads, in particular when there is a change in the endpoints, i.e., a pod is started or replaced. It is out of the scope of this Ingress controller to remove reloads completely. This would require an incredible amount of work and at some point makes no sense. This can change only if NGINX changes the way new configurations are read, basically, new changes do not replace worker processes.
+
+[0]: https://github.com/openresty/lua-nginx-module/pull/1259
+[1]: https://coreos.com/kubernetes/docs/latest/replication-controller.html#the-reconciliation-loop-in-detail
+[2]: https://godoc.org/k8s.io/client-go/informers#NewFilteredSharedInformerFactory
+[3]: https://godoc.org/k8s.io/client-go/tools/cache#ResourceEventHandlerFuncs
+[4]: https://github.com/kubernetes/ingress-nginx/blob/master/internal/task/queue.go#L38
+[5]: https://golang.org/pkg/sync/#Mutex
+[6]: https://github.com/kubernetes/ingress-nginx/blob/master/rootfs/etc/nginx/template/nginx.tmpl
+[7]: http://nginx.org/en/docs/beginners_guide.html#control