Merge pull request #1810 from jszczepkowski/ha-doc

Added user doc for GCE HA master

docs/admin/ha-master-gce.md

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+---
+assignees:
+- jszczepkowski
+
+---
+
+* TOC
+{:toc}
+
+## Introduction
+
+Kubernetes version 1.5 adds alpha support for replicating Kubernetes masters in `kube-up` or `kube-down` scripts for Google Compute Engine.
+This document describes how to use kube-up/down scripts to manage highly available (HA) masters and how HA masters are implemented for use with GCE.
+
+## Starting an HA-compatible cluster
+
+To create a new HA-compatible cluster, you must set the following flags in your `kube-up` script:
+
+* `MULTIZONE=true` - to prevent removal of master replicas kubelets from zones different than server's default zone.
+Required if you want to run master replicas in different zones, which is recommended.
+
+* `ENABLE_ETCD_QUORUM_READS=true` - to ensure that reads from all API servers will return most up-to-date data.
+If true, reads will be directed to leader etcd replica.
+Setting this value to true is optional: reads will be more reliable but will also be slower.
+
+Optionally, you can specify a GCE zone where the first master replica is to be created.
+Set the the following flag:
+
+* `KUBE_GCE_ZONE=zone` - zone where the first master replica will run.
+
+The following sample command sets up a HA-compatible cluster in the GCE zone europe-west1-b:
+
+```shell
+$ MULTIZONE=true KUBE_GCE_ZONE=europe-west1-b  ENABLE_ETCD_QUORUM_READS=true ./cluster/kube-up.sh
+```
+
+Note that the commands above create a cluster with one master;
+however, you can add new master replicas to the cluster with subsequent commands.
+
+## Adding a new master replica
+
+After you have created an HA-compatible cluster, you can add master replicas to it.
+You add master replicas by using a `kube-up` script with the following flags:
+
+* `KUBE_REPLICATE_EXISTING_MASTER=true` - to create a replica of an existing
+master.
+
+* `KUBE_GCE_ZONE=zone` - zone where the master replica will run.
+Must be in the same region as other replicas' zones.
+
+You don't need to set the `MULTIZONE` or `ENABLE_ETCD_QUORUM_READS` flags,
+as those are inherited from when you started your HA-compatible cluster.
+
+The following sample command replicates the master on an existing HA-compatible cluster:
+
+```shell
+$ KUBE_GCE_ZONE=europe-west1-c KUBE_REPLICATE_EXISTING_MASTER=true ./cluster/kube-up.sh
+```
+
+## Removing a master replica
+
+You can remove a master replica from an HA cluster by using a `kube-down` script with the following flags:
+
+* `KUBE_DELETE_NODES=false` - to restrain deletion of kubelets.
+
+* `KUBE_GCE_ZONE=zone` - the zone from where master replica will be removed.
+ 
+* `KUBE_REPLICA_NAME=replica_name` - (optional) the name of master replica to remove.
+If empty: any replica from the given zone will be removed.
+
+The following sample command removes a master replica from an existing HA cluster:
+
+```shell
+$ KUBE_DELETE_NODES=false KUBE_GCE_ZONE=europe-west1-c ./cluster/kube-down.sh
+```
+
+## Handling master replica failures
+
+If one of the master replicas in your HA cluster fails,
+the best practice is to remove the replica from your cluster and add a new replica in the same zone.
+The following sample commands demonstrate this process:
+
+1. Remove the broken replica:
+
+```shell
+$ KUBE_DELETE_NODES=false KUBE_GCE_ZONE=replica_zone KUBE_REPLICA_NAME=replica_name ./cluster/kube-down.sh
+```
+
+2. Add a new replica in place of the old one:
+
+```shell
+$ KUBE_GCE_ZONE=replica-zone KUBE_REPLICATE_EXISTING_MASTER=true ./cluster/kube-up.sh
+```
+
+## Best practices for replicating masters for HA clusters
+
+* Try to place masters replicas in different zones. During a zone failure, all master placed inside the zone will fail.
+To survive zone failure, also place nodes in multiple zones
+(see [multiple-zones](http://kubernetes.io/docs/admin/multiple-zones/) for details).
+
+* Do not use a cluster with two master replicas. Consensus on a two replica cluster requires both replicas running when changing persistent state.
+As a result, both replicas are needed and a failure of any replica turns cluster into majority failure state.
+A two-replica cluster is thus inferior, in terms of HA, to a single replica cluster.
+
+* When you add a master replica, cluster state (etcd) is copied to a new instance.
+If the cluster is large, it may take a long time to duplicate its state.
+This operation may be speed up by migrating etcd data directory, as described [here](https://coreos.com/etcd/docs/latest/admin_guide.html#member-migration) here
+(we are considering adding support for etcd data dir migration in future).
+
+## Implementation notes
+
+![](ha-master-gce.png)
+
+### Overview
+
+Each of master replicas will run the following components in the following mode:
+
+* etcd instance: all instances will be clustered together using consensus;
+
+* API server: each server will talk to local etcd - all API servers in the cluster will be available;
+
+* controllers, scheduler, and cluster auto-scaler: will use lease mechanism - only one instance of each of them will be active in the cluster;
+
+* add-on manager: each manager will work independently trying to keep add-ons in sync.
+
+In addition, there will be a load balancer in front of API servers that will route external and internal traffic to them.
+
+### Load balancing
+
+When starting the second master replica, a load balancer containing the two replicas will be created
+and the IP address of the first replica will be promoted to IP address of load balancer.
+Similarly, after removal of the penultimate master replica, the load balancer will be removed and its IP address will be assigned to the last remaining replica.
+Please note that creation and removal of load balancer are complex operations and it may take some time (~20 minutes) for them to propagate.
+
+### Master service & kubelets
+
+Instead of trying to keep an up-to-date list of Kubernetes apiserver in the Kubernetes service,
+the system directs all traffic to the external IP:
+
+* in one master cluster the IP points to the single master,
+
+* in multi-master cluster the IP points to the load balancer in-front of the masters.
+
+Similarly, the external IP will be used by kubelets to communicate with master.
+
+### Master certificates
+
+Kubernetes generates Master TLS certificates for the external public IP and local IP for each replica.
+There are no certificates for the ephemeral public IP for replicas;
+to access a replica via its ephemeral public IP, you must skip TLS verification.
+
+### Clustering etcd
+
+To allow etcd clustering, ports needed to communicate between etcd instances will be opened (for inside cluster communication).
+To make such deployment secure, communication between etcd instances is authorized using SSL.
+
+## Additional reading
+
+[Automated HA master deployment - design doc](https://github.com/kubernetes/kubernetes/blob/master/docs/design/ha_master.md)
+