From b3a4b1853df7cb4d0260b30f053a4d026b3c1ac7 Mon Sep 17 00:00:00 2001 From: Justin Santa Barbara Date: Sat, 19 Sep 2015 12:53:19 -0400 Subject: [PATCH] Changes per reviews --- docs/design/aws_under_the_hood.md | 286 +++++++++++++++++------------- 1 file changed, 160 insertions(+), 126 deletions(-) diff --git a/docs/design/aws_under_the_hood.md b/docs/design/aws_under_the_hood.md index eece5dfb7ff..17ac1543567 100644 --- a/docs/design/aws_under_the_hood.md +++ b/docs/design/aws_under_the_hood.md @@ -31,21 +31,31 @@ Documentation for other releases can be found at -## Peeking under the hood of kubernetes on AWS +# Peeking under the hood of Kubernetes on AWS -We encourage you to use kube-up (or CloudFormation) to create a cluster. But -it is useful to know what is being created: for curiosity, to understand any -problems that may arise, or if you have to create things manually because the -scripts are unsuitable for any reason. We don't recommend manual configuration -(please file an issue and let us know what's missing if there's something you -need) but sometimes it is the only option. +This document provides high-level insight into how Kubernetes works on AWS and +maps to AWS objects. We assume that you are familiar with AWS. -This document sets out to document how kubernetes on AWS maps to AWS objects. -Familiarity with AWS is assumed. +We encourage you to use [kube-up](../getting-started-guides/aws.md) (or +[CloudFormation](../getting-started-guides/aws-coreos.md) to create clusters on +AWS. We recommend that you avoid manual configuration but are aware that +sometimes it's the only option. -### Top-level +Tip: You should open an issue and let us know what enhancements can be made to +the scripts to better suit your needs. + +That said, it's also useful to know what's happening under the hood when +Kubernetes clusters are created on AWS. This can be particularly useful if +problems arise or in circumstances where the provided scripts are lacking and +you manually created or configured your cluster. + +### Architecture overview + +Kubernetes is a cluster of several machines that consists of a Kubernetes +master and a set number of nodes (previously known as 'minions') for which the +master which is responsible. See the [Architecture](architecture.md) topic for +more details. -Kubernetes consists of a single master node, and a collection of minion nodes. Other documents describe the general architecture of Kubernetes (all nodes run Docker; the kubelet agent runs on each node and launches containers; the kube-proxy relays traffic between the nodes etc). @@ -53,171 +63,192 @@ kube-proxy relays traffic between the nodes etc). By default on AWS: * Instances run Ubuntu 15.04 (the official AMI). It includes a sufficiently - modern kernel to give a good experience with Docker, it doesn't require a - reboot. (The default SSH user is `ubuntu` for this and other ubuntu images) + modern kernel that parise well with Docker and doesn't require a + reboot. (The default SSH user is `ubuntu` for this and other ubuntu images.) * By default we run aufs over ext4 as the filesystem / container storage on the nodes (mostly because this is what GCE uses). -These defaults can be changed by passing different environment variables to +You can override these defaults by passing different environment variables to kube-up. ### Storage -AWS does support persistent volumes via EBS. These can then be attached to -pods that should store persistent data (e.g. if you're running a database). +AWS supports persistent volumes by using [Elastic Block Store +(EBS)](../user-guide/volumes.md#awselasticblockstore). These can then be +attached to pods that should store persistent data (e.g. if you're running a +database). -Minions do not have persistent volumes otherwise. In general, kubernetes -containers do not have persistent storage unless you attach a persistent -volume, and so minions on AWS use instance storage. Instance storage is -cheaper, often faster, and historically more reliable. This does mean that you -should pick an instance type that has sufficient instance storage, unless you -can make do with whatever space is left on your root partition. +By default, nodes in AWS use `[instance +storage](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/InstanceStorage.html)' +unless you create pods with persistent volumes +`[(EBS)](../user-guide/volumes.md#awselasticblockstore)`. In general, +Kubernetes containers do not have persistent storage unless you attach a +persistent volume, and so nodes on AWS use instance storage. Instance +storage is cheaper, often faster, and historically more reliable. This does +mean that you should pick an instance type that has sufficient instance +storage, unless you can make do with whatever space is left on your root +partition. -The master _does_ have a persistent volume attached to it. Containers are -mostly run against instance storage, just like the minions, except that we -repoint some important data onto the peristent volume. +Note: Master uses a persistent volume ([etcd](architecture.html#etcd)) to track +its state but similar to the nodes, container are mostly run against instance +storage, except that we repoint some important data onto the peristent volume. -By default we use aufs over ext4. `DOCKER_STORAGE=btrfs` is also a good choice -for a filesystem: it is relatively reliable with Docker; btrfs itself is much -more reliable than it used to be with modern kernels. It can easily span -multiple volumes, which is particularly useful when we are using an instance -type with multiple ephemeral instance disks. +The default storage driver for Docker images is aufs. Passing the environment +variable `DOCKER_STORAGE=btrfs` is also a good choice for a filesystem. btrfs +is relatively reliable with Docker and has improved its reliability with modern +kernels. It can easily span multiple volumes, which is particularly useful +when we are using an instance type with multiple ephemeral instance disks. ### AutoScaling -We run the minions in an AutoScalingGroup. Currently auto-scaling (e.g. based -on CPU) is not actually enabled (#11935). Instead, the auto-scaling group -means that AWS will relaunch any minions that are terminated. +Nodes (except for the master) are run in an +`[AutoScalingGroup](http://docs.aws.amazon.com/AutoScaling/latest/DeveloperGuide/AutoScalingGroup.html) +on AWS. Currently auto-scaling (e.g. based on CPU) is not actually enabled +([#11935](http://issues.k8s.io/11935)). Instead, the auto-scaling group means +that AWS will relaunch any non-master nodes that are terminated. We do not currently run the master in an AutoScalingGroup, but we should -(#11934) +([#11934](http://issues.k8s.io/11934)). ### Networking Kubernetes uses an IP-per-pod model. This means that a node, which runs many -pods, must have many IPs. The way we implement this on AWS is to use VPCs and -the advanced routing support that it allows. Each pod is assigned a /24 CIDR; -then this CIDR is configured to route to an instance in the VPC routing table. +pods, must have many IPs. AWS uses virtual private clouds (VPCs) and advanced +routing support so each pod is assigned a /24 CIDR. Each pod is assigned a /24 +CIDR; the assigned CIDR is then configured to route to an instance in the VPC +routing table. -It is also possible to use overlay networking on AWS, but the default kube-up -configuration does not. +It is also possible to use overlay networking on AWS, but that is not the +configuration of the kube-up script. -### NodePort & LoadBalancing +### NodePort and LoadBalancing -Kubernetes on AWS integrates with ELB. When you create a service with -Type=LoadBalancer, kubernetes (the kube-controller-manager) will create an ELB, -create a security group for the ELB which allows access on the service ports, -attach all the minions to the ELB, and modify the security group for the -minions to allow traffic from the ELB to the minions. This traffic reaches -kube-proxy where it is then forwarded to the pods. +Kubernetes on AWS integrates with [Elastic Load Balancing +(ELB)](http://docs.aws.amazon.com/AutoScaling/latest/DeveloperGuide/US_SetUpASLBApp.html). +When you create a service with `Type=LoadBalancer`, Kubernetes (the +kube-controller-manager) will create an ELB, create a security group for the +ELB which allows access on the service ports, attach all the nodes to the ELB, +and modify the security group for the nodes to allow traffic from the ELB to +the nodes. This traffic reaches kube-proxy where it is then forwarded to the +pods. -ELB requires that all minions listen on a single port, and it acts as a layer-7 -forwarding proxy (i.e. the source IP is not preserved). It is not trivial for -kube-proxy to recognize the traffic therefore. So, LoadBalancer services are -also exposed as NodePort services. For NodePort services, a cluster-wide port -is assigned by kubernetes to the service, and kube-proxy listens externally on -that port on every minion, and forwards traffic to the pods. So for a -load-balanced service, ELB is configured to proxy traffic on the public port -(e.g. port 80) to the NodePort assigned to the service (e.g. 31234), kube-proxy -recognizes the traffic coming to the NodePort by the inbound port number, and -send it to the correct pods for the service. +ELB has some restrictions: it requires that all nodes listen on a single port, +and it acts as a forwarding proxy (i.e. the source IP is not preserved). To +work with these restrictions, in Kubernetes, `[LoadBalancer +services](../user-guide/services.html#type-loadbalancer)` are exposed as +`[NodePort services](../user-guide/services.html#type-nodeport)`. Then +kube-proxy listens externally on the cluster-wide port that's assigned to +NodePort services and forwards traffic to the corresponding pods. So ELB is +configured to proxy traffic on the public port (e.g. port 80) to the NodePort +that is assigned to the service (e.g. 31234). Any in-coming traffic sent to +the NodePort (e.g. port 31234) is recognized by kube-proxy and then sent to the +correct pods for that service. Note that we do not automatically open NodePort services in the AWS firewall (although we do open LoadBalancer services). This is because we expect that NodePort services are more of a building block for things like inter-cluster services or for LoadBalancer. To consume a NodePort service externally, you -will likely have to open the port in the minion security group +will likely have to open the port in the node security group (`kubernetes-minion-`). -### IAM +### Identity and Access Management (IAM) kube-proxy sets up two IAM roles, one for the master called -(kubernetes-master)[cluster/aws/templates/iam/kubernetes-master-policy.json] -and one for the minions called -(kubernetes-minion)[cluster/aws/templates/iam/kubernetes-minion-policy.json]. +[kubernetes-master](../../cluster/aws/templates/iam/kubernetes-master-policy.json) +and one for the non-master nodes called +[kubernetes-minion](../../cluster/aws/templates/iam/kubernetes-minion-policy.json). The master is responsible for creating ELBs and configuring them, as well as setting up advanced VPC routing. Currently it has blanket permissions on EC2, along with rights to create and destroy ELBs. -The minion does not need a lot of access to the AWS APIs. It needs to download -a distribution file, and then it is responsible for attaching and detaching EBS -volumes to itself. +The (non-master) nodes do not need a lot of access to the AWS APIs. They need to download +a distribution file, and then are responsible for attaching and detaching EBS +volumes from itself. -The minion policy is relatively minimal. The master policy is probably overly +The (non-master) node policy is relatively minimal. The master policy is probably overly permissive. The security concious may want to lock-down the IAM policies -further (#11936) +further ([#11936](http://issues.k8s.io/11936)). We should make it easier to extend IAM permissions and also ensure that they -are correctly configured (#???) +are correctly configured ([#14226](http://issues.k8s.io/14226)). ### Tagging -All AWS resources are tagged with a tag named "KuberentesCluster". This tag is -used to identify a particular 'instance' of Kubernetes, even if two clusters -are deployed into the same VPC. (The script doesn't do this by default, but it -can be done.) +All AWS resources are tagged with a tag named "KuberentesCluster", with a value +that is the unique cluster-id. This tag is used to identify a particular +'instance' of Kubernetes, even if two clusters are deployed into the same VPC. +Resources are considered to belong to the same cluster if and only if they have +the same value in the tag named "KubernetesCluster". (The kube-up script is +not configured to create multiple clusters in the same VPC by default, but it +is possible to create another cluster in the same VPC.) Within the AWS cloud provider logic, we filter requests to the AWS APIs to -match resources with our cluster tag. So we only see our own AWS objects. +match resources with our cluster tag. By filtering the requests, we ensure +that we see only our own AWS objects. -If you choose not to use kube-up, you must tag everything with a -KubernetesCluster tag with a unique per-cluster value. +Important: If you choose not to use kube-up, you must pick a unique cluster-id +value, and ensure that all AWS resources have a tag with +`Name=KubernetesCluster,Value=`. - -# AWS Objects +### AWS Objects The kube-up script does a number of things in AWS: -* Creates an S3 bucket (`AWS_S3_BUCKET`) and copy the kubernetes distribution +* Creates an S3 bucket (`AWS_S3_BUCKET`) and then copies the Kubernetes distribution and the salt scripts into it. They are made world-readable and the HTTP URLs -are passed to instances; this is how kubernetes code gets onto the machines. -* Creates two IAM profiles based on templates in `cluster/aws/templates/iam`. - `kubernetes-master` is used by the master node; `kubernetes-minion` is used -by minion nodes. +are passed to instances; this is how Kubernetes code gets onto the machines. +* Creates two IAM profiles based on templates in `cluster/aws/templates/iam`: + * `kubernetes-master` is used by the master node + * `kubernetes-minion` is used by non-master nodes. * Creates an AWS SSH key named `kubernetes-`. Fingerprint here is the OpenSSH key fingerprint, so that multiple users can run the script with -different keys and their keys will not collide (with near-certainty) It will +different keys and their keys will not collide (with near-certainty). It will use an existing key if one is found at `AWS_SSH_KEY`, otherwise it will create one there. (With the default ubuntu images, if you have to SSH in: the user is `ubuntu` and that user can `sudo`) -* Creates a VPC for use with the cluster (with a CIDR of 172.20.0.0/16)., and +* Creates a VPC for use with the cluster (with a CIDR of 172.20.0.0/16) and enables the `dns-support` and `dns-hostnames` options. * Creates an internet gateway for the VPC. * Creates a route table for the VPC, with the internet gateway as the default route * Creates a subnet (with a CIDR of 172.20.0.0/24) in the AZ `KUBE_AWS_ZONE` - (defaults to us-west-2a). Currently kubernetes runs in a single AZ; there -are two philosophies on how to achieve HA: cluster-per-AZ and -cross-AZ-clusters. cluster-per-AZ says you should have an independent cluster -for each AZ, they are entirely separate. cross-AZ-clusters allows a single -cluster to span multiple AZs. The debate is open here: cluster-per-AZ is more -robust but cross-AZ-clusters are more convenient. For now though, each AWS -kuberentes cluster lives in one AZ. + (defaults to us-west-2a). Currently, each Kubernetes cluster runs in a +single AZ on AWS. Although, there are two philosophies in discussion on how to +achieve High Availability (HA): + * cluster-per-AZ: An independent cluster for each AZ, where each cluster + is entirely separate. + * cross-AZ-clusters: A single cluster spans multiple AZs. +The debate is open here, where cluster-per-AZ is discussed as more robust but +cross-AZ-clusters are more convenient. * Associates the subnet to the route table * Creates security groups for the master node (`kubernetes-master-`) - and the minion nodes (`kubernetes-minion-`) -* Configures security groups so that masters & minions can intercommunicate, - and opens SSH to the world on master & minions, and opens port 443 to the -world on the master (for the HTTPS API endpoint) + and the non-master nodes (`kubernetes-minion-`) +* Configures security groups so that masters and nodes can communicate. This + includes intercommunication between masters and nodes, opening SSH publicly +for both masters and nodes, and opening port 443 on the master for the HTTPS +API endpoints. * Creates an EBS volume for the master node of size `MASTER_DISK_SIZE` and type `MASTER_DISK_TYPE` -* Launches a master node with a fixed IP address (172.20.0.9), with the - security group, IAM credentials etc. An instance script is used to pass -vital configuration information to Salt. The hope is that over time we can -reduce the amount of configuration information that must be passed in this way. -* Once the instance is up, it attaches the EBS volume & sets up a manual +* Launches a master node with a fixed IP address (172.20.0.9) that is also + configured for the security group and all the necessary IAM credentials. An +instance script is used to pass vital configuration information to Salt. Note: +The hope is that over time we can reduce the amount of configuration +information that must be passed in this way. +* Once the instance is up, it attaches the EBS volume and sets up a manual routing rule for the internal network range (`MASTER_IP_RANGE`, defaults to 10.246.0.0/24) -* Creates an auto-scaling launch-configuration and group for the minions. The - name for both is `-minion-group`, defaults to -`kubernetes-minion-group`. The auto-scaling group has size min & max both set -to `NUM_MINIONS`. You can change the size of the auto-scaling group to add or -remove minions (directly though the AWS API/Console). The minion nodes -self-configure: they come up, run Salt with the stored configuration; connect -to the master and are assigned an internal CIDR; the master configures the -route-table with the minion CIDR. The script does health-check the minions, -but this is a self-check, it is not required. +* For auto-scaling, on each nodes it creates a launch configuration and group. + The name for both is <*KUBE_AWS_INSTANCE_PREFIX*>-minion-group. The default +name is kubernetes-minion-group. The auto-scaling group has a min and max size +that are both set to NUM_MINIONS. You can change the size of the auto-scaling +group to add or remove the total number of nodes from within the AWS API or +Console. Each nodes self-configures, meaning that they come up; run Salt with +the stored configuration; connect to the master; are assigned an internal CIDR; +and then the master configures the route-table with the assigned CIDR. The +kube-up script performs a health-check on the nodes but it's a self-check that +is not required. + If attempting this configuration manually, I highly recommend following along with the kube-up script, and being sure to tag everything with a @@ -227,29 +258,32 @@ simplify this by having Kubernetes take on more node configuration, and even potentially remove Salt altogether. -## Manual infrastructure creation +### Manual infrastructure creation -While this work is not yet complete, advanced users may choose to create (some) -AWS objects themselves, and still make use of the kube-up script (to configure -Salt, for example). +While this work is not yet complete, advanced users might choose to manually +certain AWS objects while still making use of the kube-up script (to configure +Salt, for example). These objects can currently be manually created: -* `AWS_S3_BUCKET` will use an existing S3 bucket -* `VPC_ID` will reuse an existing VPC -* `SUBNET_ID` will reuse an existing subnet -* If your route table is tagged with the correct `KubernetesCluster`, it will - be reused +* Set the `AWS_S3_BUCKET` environment variable to use an existing S3 bucket. +* Set the `VPC_ID` environment variable to reuse an existing VPC. +* Set the `SUBNET_ID` environemnt variable to reuse an existing subnet. +* If your route table has a matching `KubernetesCluster` tag, it will + be reused. * If your security groups are appropriately named, they will be reused. -Currently there is no way to do the following with kube-up. If these affect -you, please open an issue with a description of what you're trying to do (your -use-case) and we'll see what we can do: +Currently there is no way to do the following with kube-up: -* Use an existing AWS SSH key with an arbitrary name -* Override the IAM credentials in a sensible way (but this is in-progress) -* Use different security group permissions -* Configure your own auto-scaling groups +* Use an existing AWS SSH key with an arbitrary name. +* Override the IAM credentials in a sensible way + ([#14226](http://issues.k8s.io/14226)). +* Use different security group permissions. +* Configure your own auto-scaling groups. -# Instance boot +If any of the above items apply to your situation, open an issue to request an +enhancement to the kube-up script. You should provide a complete description of +the use-case, including all the details around what you want to accomplish. + +### Instance boot The instance boot procedure is currently pretty complicated, primarily because we must marshal configuration from Bash to Salt via the AWS instance script. @@ -260,7 +294,7 @@ When the kube-up script launches instances, it builds an instance startup script which includes some configuration options passed to kube-up, and concatenates some of the scripts found in the cluster/aws/templates directory. These scripts are responsible for mounting and formatting volumes, downloading -Salt & Kubernetes from the S3 bucket, and then triggering Salt to actually +Salt and Kubernetes from the S3 bucket, and then triggering Salt to actually install Kubernetes.