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Kubernetes
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</div>
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<h1>如何从外部访问Kubernetes集群中的应用？</h1>
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<h2 class="subheading"></h2>
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Posted by
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赵化冰
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on
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Tuesday, November 28, 2017
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post-container">
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<h2 id="前言">前言</h2>
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<p>我们知道，kubernetes的Cluster Network属于私有网络，只能在cluster Network内部才能访问部署的应用，那如何才能将Kubernetes集群中的应用暴露到外部网络，为外部用户提供服务呢？本文探讨了从外部网络访问kubernetes cluster中应用的几种实现方式。</p>
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||
<blockquote>
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||
<p>本文尽量试着写得比较容易理解，但要做到“深入浅出”，把复杂的事情用通俗易懂的语言描述出来是非常需要功力的，个人自认尚未达到此境界，唯有不断努力。此外，kubernetes本身是一个比较复杂的系统，无法在本文中详细解释涉及的所有相关概念，否则就可能脱离了文章的主题，因此假设阅读此文之前读者对kubernetes的基本概念如docker，container，pod已有所了解。</p>
|
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</blockquote>
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||
<p>另外此文中的一些内容是自己的理解，由于个人的知识范围有限，可能有误，如果读者对文章中的内容有疑问或者勘误，欢迎大家指证。</p>
|
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<h2 id="pod和service">Pod和Service</h2>
|
||
<p>我们首先来了解一下Kubernetes中的Pod和Service的概念。</p>
|
||
<p>Pod(容器组),英文中Pod是豆荚的意思，从名字的含义可以看出，Pod是一组有依赖关系的容器，Pod包含的容器都会运行在同一个host节点上，共享相同的volumes和network namespace空间。Kubernetes以Pod为基本操作单元，可以同时启动多个相同的pod用于failover或者load balance。</p>
|
||
<p>
|
||
<img src="/img/access-application-from-outside/pod.PNG" alt="Pod">
|
||
|
||
</p>
|
||
<p>Pod的生命周期是短暂的，Kubernetes根据应用的配置，会对Pod进行创建，销毁，根据监控指标进行缩扩容。kubernetes在创建Pod时可以选择集群中的任何一台空闲的Host，因此其网络地址是不固定的。由于Pod的这一特点，一般不建议直接通过Pod的地址去访问应用。</p>
|
||
<p>为了解决访问Pod不方便直接访问的问题，Kubernetes采用了Service的概念，Service是对后端提供服务的一组Pod的抽象，Service会绑定到一个固定的虚拟IP上，该虚拟IP只在Kubernetes Cluster中可见，但其实该IP并不对应一个虚拟或者物理设备，而只是IPtable中的规则，然后再通过IPtable将服务请求路由到后端的Pod中。通过这种方式，可以确保服务消费者可以稳定地访问Pod提供的服务，而不用关心Pod的创建、删除、迁移等变化以及如何用一组Pod来进行负载均衡。</p>
|
||
<p>Service的机制如下图所示，Kube-proxy监听kubernetes master增加和删除Service以及Endpoint的消息，对于每一个Service，kube proxy创建相应的iptables规则，将发送到Service Cluster IP的流量转发到Service后端提供服务的Pod的相应端口上。
|
||
|
||
<img src="/img/access-application-from-outside/services-iptables-overview.png" alt="Pod和Service的关系">
|
||
|
||
</p>
|
||
<blockquote>
|
||
<p>备注：虽然可以通过Service的Cluster IP和服务端口访问到后端Pod提供的服务，但该Cluster IP是Ping不通的，原因是Cluster IP只是iptable中的规则，并不对应到一个网络设备。</p>
|
||
</blockquote>
|
||
<h2 id="service的类型">Service的类型</h2>
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||
<p>Service的类型(ServiceType)决定了Service如何对外提供服务，根据类型不同，服务可以只在Kubernetes cluster中可见，也可以暴露到Cluster外部。Service有三种类型，ClusterIP，NodePort和LoadBalancer。其中ClusterIP是Service的缺省类型，这种类型的服务会提供一个只能在Cluster内才能访问的虚拟IP，其实现机制如上面一节所述。</p>
|
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<h2 id="通过nodeport提供外部访问入口">通过NodePort提供外部访问入口</h2>
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||
<p>通过将Service的类型设置为NodePort，可以在Cluster中的主机上通过一个指定端口暴露服务。注意通过Cluster中每台主机上的该指定端口都可以访问到该服务，发送到该主机端口的请求会被kubernetes路由到提供服务的Pod上。采用这种服务类型，可以在kubernetes cluster网络外通过主机IP：端口的方式访问到服务。</p>
|
||
<blockquote>
|
||
<p>注意：官方文档中说明了Kubernetes clusterIp的流量转发到后端Pod有Iptable和kube proxy两种方式。但对Nodeport如何转发流量却语焉不详。该图来自网络，从图来看是通过kube proxy转发的，我没有去研究过源码。欢迎了解的同学跟帖说明。</p>
|
||
</blockquote>
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||
<p>
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<img src="/img/access-application-from-outside/nodeport.PNG" alt="Pod和Service的关系">
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||
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</p>
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||
<p>下面是通过NodePort向外暴露服务的一个例子，注意可以指定一个nodePort，也可以不指定。在不指定的情况下，kubernetes会从可用的端口范围内自动分配一个随机端口。</p>
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<pre tabindex="0"><code>kind: Service
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apiVersion: v1
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metadata:
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name: influxdb
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spec:
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type: NodePort
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ports:
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- port: 8086
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nodePort: 30000
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selector:
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name: influxdb
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</code></pre><p>通过NodePort从外部访问有下面的一些问题，自己玩玩或者进行测试时可以使用该方案，但不适宜用于生产环境。</p>
|
||
<ul>
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||
<li>
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||
<p>Kubernetes cluster host的IP必须是一个well-known IP，即客户端必须知道该IP。但Cluster中的host是被作为资源池看待的，可以增加删除，每个host的IP一般也是动态分配的，因此并不能认为host IP对客户端而言是well-known IP。</p>
|
||
</li>
|
||
<li>
|
||
<p>客户端访问某一个固定的host IP存在单点故障。假如一台host宕机了，kubernetes cluster会把应用 reload到另一节点上，但客户端就无法通过该host的nodeport访问应用了。</p>
|
||
</li>
|
||
<li>
|
||
<p>该方案假设客户端可以访问Kubernetes host所在网络。在生产环境中，客户端和Kubernetes host网络可能是隔离的。例如客户端可能是公网中的一个手机APP，是无法直接访问host所在的私有网络的。</p>
|
||
</li>
|
||
</ul>
|
||
<p>因此，需要通过一个网关来将外部客户端的请求导入到Cluster中的应用中，在kubernetes中，这个网关是一个4层的load balancer。</p>
|
||
<h2 id="通过load-balancer提供外部访问入口">通过Load Balancer提供外部访问入口</h2>
|
||
<p>通过将Service的类型设置为LoadBalancer，可以为Service创建一个外部Load Balancer。Kubernetes的文档中声明该Service类型需要云服务提供商的支持，其实这里只是在Kubernetes配置文件中提出了一个要求，即为该Service创建Load Balancer，至于如何创建则是由Google Cloud或Amazon Cloud等云服务商提供的，创建的Load Balancer不在Kubernetes Cluster的管理范围中。kubernetes 1.6版本中，WS, Azure, CloudStack, GCE and OpenStack等云提供商已经可以为Kubernetes提供Load Balancer.下面是一个Load balancer类型的Service例子：</p>
|
||
<pre tabindex="0"><code>kind: Service
|
||
apiVersion: v1
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||
metadata:
|
||
name: influxdb
|
||
spec:
|
||
type: LoadBalancer
|
||
ports:
|
||
- port: 8086
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||
selector:
|
||
name: influxdb
|
||
</code></pre><p>部署该Service后，我们来看一下Kubernetes创建的内容</p>
|
||
<pre tabindex="0"><code>$ kubectl get svc influxdb
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NAME CLUSTER-IP EXTERNAL-IP PORT(S) AGE
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||
influxdb 10.97.121.42 10.13.242.236 8086:30051/TCP 39s
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||
</code></pre><p>Kubernetes首先为influxdb创建了一个集群内部可以访问的ClusterIP 10.97.121.42。由于没有指定nodeport端口，kubernetes选择了一个空闲的30051主机端口将service暴露在主机的网络上，然后通知cloud provider创建了一个load balancer，上面输出中的EEXTERNAL-IP就是load balancer的IP。</p>
|
||
<p>测试使用的Cloud Provider是OpenStack，我们通过neutron lb-vip-show可以查看创建的Load Balancer详细信息。</p>
|
||
<pre tabindex="0"><code>$ neutron lb-vip-show 9bf2a580-2ba4-4494-93fd-9b6969c55ac3
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||
+---------------------+--------------------------------------------------------------+
|
||
| Field | Value |
|
||
+---------------------+--------------------------------------------------------------+
|
||
| address | 10.13.242.236 |
|
||
| admin_state_up | True |
|
||
| connection_limit | -1 |
|
||
| description | Kubernetes external service a6ffa4dadf99711e68ea2fa163e0b082 |
|
||
| id | 9bf2a580-2ba4-4494-93fd-9b6969c55ac3 |
|
||
| name | a6ffa4dadf99711e68ea2fa163e0b082 |
|
||
| pool_id | 392917a6-ed61-4924-acb2-026cd4181755 |
|
||
| port_id | e450b80b-6da1-4b31-a008-280abdc6400b |
|
||
| protocol | TCP |
|
||
| protocol_port | 8086 |
|
||
| session_persistence | |
|
||
| status | ACTIVE |
|
||
| status_description | |
|
||
| subnet_id | 73f8eb91-90cf-42f4-85d0-dcff44077313 |
|
||
| tenant_id | 4d68886fea6e45b0bc2e05cd302cccb9 |
|
||
+---------------------+--------------------------------------------------------------+
|
||
|
||
$ neutron lb-pool-show 392917a6-ed61-4924-acb2-026cd4181755
|
||
+------------------------+--------------------------------------+
|
||
| Field | Value |
|
||
+------------------------+--------------------------------------+
|
||
| admin_state_up | True |
|
||
| description | |
|
||
| health_monitors | |
|
||
| health_monitors_status | |
|
||
| id | 392917a6-ed61-4924-acb2-026cd4181755 |
|
||
| lb_method | ROUND_ROBIN |
|
||
| members | d0825cc2-46a3-43bd-af82-e9d8f1f85299 |
|
||
| | 3f73d3bb-bc40-478d-8d0e-df05cdfb9734 |
|
||
| name | a6ffa4dadf99711e68ea2fa163e0b082 |
|
||
| protocol | TCP |
|
||
| provider | haproxy |
|
||
| status | ACTIVE |
|
||
| status_description | |
|
||
| subnet_id | 73f8eb91-90cf-42f4-85d0-dcff44077313 |
|
||
| tenant_id | 4d68886fea6e45b0bc2e05cd302cccb9 |
|
||
| vip_id | 9bf2a580-2ba4-4494-93fd-9b6969c55ac3 |
|
||
+------------------------+--------------------------------------+
|
||
|
||
$ neutron lb-member-list
|
||
+--------------------------------------+--------------+---------------+--------+----------------+--------+
|
||
| id | address | protocol_port | weight | admin_state_up | status |
|
||
+--------------------------------------+--------------+---------------+--------+----------------+--------+
|
||
| 3f73d3bb-bc40-478d-8d0e-df05cdfb9734 | 10.13.241.89 | 30051 | 1 | True | ACTIVE |
|
||
| d0825cc2-46a3-43bd-af82-e9d8f1f85299 | 10.13.241.10 | 30051 | 1 | True | ACTIVE |
|
||
+--------------------------------------+--------------+---------------+--------+----------------+--------
|
||
</code></pre><p>可以看到OpenStack使用VIP 10.13.242.236在端口8086创建了一个Load Balancer，Load Balancer对应的Lb pool里面有两个成员10.13.241.89 和 10.13.241.10，正是Kubernetes的host节点，进入Load balancer流量被分发到这两个节点对应的Service Nodeport 30051上。</p>
|
||
<p>但是如果客户端不在Openstack Neutron的私有子网上，则还需要在load balancer的VIP上关联一个floating IP，以使外部客户端可以连接到load balancer。</p>
|
||
<p>部署Load balancer后，应用的拓扑结构如下图所示（注：本图假设Kubernetes Cluster部署在Openstack私有云上）。
|
||
|
||
<img src="/img/access-application-from-outside/load-balancer.PNG" alt="外部Load balancer">
|
||
|
||
</p>
|
||
<blockquote>
|
||
<p>备注：如果kubernetes环境在Public Cloud上，Loadbalancer类型的Service创建出的外部Load Balancer已经带有公网IP地址，是可以直接从外部网络进行访问的，不需要绑定floating IP这个步骤。例如在AWS上创建的Elastic Load Balancing (ELB)，有兴趣可以看一下这篇文章：<a href="http://docs.heptio.com/content/tutorials/aws-qs-services-elb.html">Expose Services on your AWS Quick Start Kubernetes cluster</a>。</p>
|
||
</blockquote>
|
||
<p>如果Kubernetes Cluster是在不支持LoadBalancer特性的cloud provider或者裸机上创建的，可以实现LoadBalancer类型的Service吗？应该也是可以的。Kubernetes本身并不直接支持Loadbalancer，但我们可以通过对Kubernetes进行扩展来实现，可以监听kubernetes Master的service创建消息，并根据消息部署相应的Load Balancer（如Nginx或者HAProxy），来实现Load balancer类型的Service。</p>
|
||
<p>通过设置Service类型提供的是四层Load Balancer，当只需要向外暴露一个服务的时候，可以直接采用这种方式。但在一个应用需要对外提供多个服务时，采用该方式会为每一个服务（IP+Port）都创建一个外部load balancer。如下图所示
|
||
|
||
<img src="/img/access-application-from-outside/multiple-load-balancer.PNG" alt="创建多个Load balancer暴露应用的多个服务">
|
||
|
||
|
||
一般来说，同一个应用的多个服务/资源会放在同一个域名下，在这种情况下，创建多个Load balancer是完全没有必要的，反而带来了额外的开销和管理成本。直接将服务暴露给外部用户也会导致了前端和后端的耦合，影响了后端架构的灵活性，如果以后由于业务需求对服务进行调整会直接影响到客户端。可以通过使用Kubernetes Ingress进行L7 load balancing来解决该问题。</p>
|
||
<h2 id="采用ingress作为七层load-balancer">采用Ingress作为七层load balancer</h2>
|
||
<p>首先看一下引入Ingress后的应用拓扑示意图（注：本图假设Kubernetes Cluster部署在Openstack私有云上）。
|
||
|
||
<img src="/img/access-application-from-outside/ingress.PNG" alt="采用Ingress暴露应用的多个服务">
|
||
|
||
|
||
这里Ingress起到了七层负载均衡器和Http方向代理的作用，可以根据不同的url把入口流量分发到不同的后端Service。外部客户端只看到foo.bar.com这个服务器，屏蔽了内部多个Service的实现方式。采用这种方式，简化了客户端的访问，并增加了后端实现和部署的灵活性，可以在不影响客户端的情况下对后端的服务部署进行调整。</p>
|
||
<p>下面是Kubernetes Ingress配置文件的示例，在虚拟主机foot.bar.com下面定义了两个Path，其中/foo被分发到后端服务s1，/bar被分发到后端服务s2。</p>
|
||
<pre tabindex="0"><code>apiVersion: extensions/v1beta1
|
||
kind: Ingress
|
||
metadata:
|
||
name: test
|
||
annotations:
|
||
ingress.kubernetes.io/rewrite-target: /
|
||
spec:
|
||
rules:
|
||
- host: foo.bar.com
|
||
http:
|
||
paths:
|
||
- path: /foo
|
||
backend:
|
||
serviceName: s1
|
||
servicePort: 80
|
||
- path: /bar
|
||
backend:
|
||
serviceName: s2
|
||
servicePort: 80
|
||
</code></pre><p>注意这里Ingress只描述了一个虚拟主机路径分发的要求，可以定义多个Ingress，描述不同的7层分发要求，而这些要求需要由一个Ingress Controller来实现。Ingress Contorller会监听Kubernetes Master得到Ingress的定义，并根据Ingress的定义对一个7层代理进行相应的配置，以实现Ingress定义中要求的虚拟主机和路径分发规则。Ingress Controller有多种实现，Kubernetes提供了一个<a href="https://github.com/kubernetes/ingress-nginx">基于Nginx的Ingress Controller</a>。需要注意的是，在部署Kubernetes集群时并不会缺省部署Ingress Controller，需要我们自行部署。</p>
|
||
<p>下面是部署Nginx Ingress Controller的配置文件示例，注意这里为Nginx Ingress Controller定义了一个LoadBalancer类型的Service，以为Ingress Controller提供一个外部可以访问的公网IP。</p>
|
||
<pre tabindex="0"><code>apiVersion: v1
|
||
kind: Service
|
||
metadata:
|
||
name: nginx-ingress
|
||
spec:
|
||
type: LoadBalancer
|
||
ports:
|
||
- port: 80
|
||
name: http
|
||
- port: 443
|
||
name: https
|
||
selector:
|
||
k8s-app: nginx-ingress-lb
|
||
---
|
||
apiVersion: extensions/v1beta1
|
||
kind: Deployment
|
||
metadata:
|
||
name: nginx-ingress-controller
|
||
spec:
|
||
replicas: 2
|
||
revisionHistoryLimit: 3
|
||
template:
|
||
metadata:
|
||
labels:
|
||
k8s-app: nginx-ingress-lb
|
||
spec:
|
||
terminationGracePeriodSeconds: 60
|
||
containers:
|
||
- name: nginx-ingress-controller
|
||
image: gcr.io/google_containers/nginx-ingress-controller:0.8.3
|
||
imagePullPolicy: Always
|
||
//----omitted for brevity----
|
||
</code></pre><blockquote>
|
||
<p>备注：Google Cloud直接支持Ingress资源，如果应用部署在Google Cloud中，Google Cloud会自动为Ingress资源创建一个7层load balancer，并为之分配一个外部IP，不需要自行部署Ingress Controller。</p>
|
||
</blockquote>
|
||
<h2 id="结论">结论</h2>
|
||
<p>采用Ingress加上Load balancer的方式可以将Kubernetes Cluster中的应用服务暴露给外部客户端。这种方式比较灵活，基本可以满足大部分应用的需要。但如果需要在入口处提供更强大的功能，如有更高的效率要求，需求进行安全认证，日志记录，或者需要一些应用的定制逻辑，则需要考虑采用微服务架构中的API Gateway模式，采用一个更强大的API Gateway来作为应用的流量入口。</p>
|
||
<h2 id="参考">参考</h2>
|
||
<ul>
|
||
<li>
|
||
<p><a href="http://alesnosek.com/blog/2017/02/14/accessing-kubernetes-pods-from-outside-of-the-cluster/">Accessing Kubernetes Pods from Outside of the Cluster</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://daemonza.github.io/2017/02/13/kubernetes-nginx-ingress-controller/">Kubernetes nginx-ingress-controller</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://docs.openstack.org/magnum/ocata/dev/kubernetes-load-balancer.html">Using Kubernetes external load balancer feature</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="http://docs.heptio.com/content/tutorials/aws-qs-services-elb.html">Expose Services on your AWS Quick Start Kubernetes cluster</a></p>
|
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|
||
o.src = u;
|
||
if (c) { o.addEventListener('load', function (e) { c(null, e); }, false); }
|
||
s.parentNode.insertBefore(o, s);
|
||
}
|
||
</script>
|
||
|
||
|
||
|
||
|
||
|
||
|
||
<script>
|
||
|
||
if($('#tag_cloud').length !== 0){
|
||
loadAsync("/js/jquery.tagcloud.js",function(){
|
||
$.fn.tagcloud.defaults = {
|
||
|
||
color: {start: '#bbbbee', end: '#0085a1'},
|
||
};
|
||
$('#tag_cloud a').tagcloud();
|
||
})
|
||
}
|
||
</script>
|
||
|
||
|
||
<script>
|
||
(function() {
|
||
function updateTagcloudColors() {
|
||
const isDark = document.documentElement.getAttribute('data-theme') === 'dark';
|
||
const startColor = isDark ? '#808080' : '#bbbbee';
|
||
if($('#tag_cloud').length !== 0 && $.fn.tagcloud) {
|
||
$.fn.tagcloud.defaults = {
|
||
color: {start: startColor, end: '#0085a1'},
|
||
};
|
||
$('#tag_cloud a').tagcloud();
|
||
}
|
||
}
|
||
|
||
|
||
$(document).ready(function() {
|
||
updateTagcloudColors();
|
||
});
|
||
|
||
|
||
const observer = new MutationObserver(function(mutations) {
|
||
mutations.forEach(function(mutation) {
|
||
if (mutation.attributeName === 'data-theme') {
|
||
updateTagcloudColors();
|
||
}
|
||
});
|
||
});
|
||
|
||
observer.observe(document.documentElement, {
|
||
attributes: true,
|
||
attributeFilter: ['data-theme']
|
||
});
|
||
})();
|
||
</script>
|
||
|
||
|
||
<script>
|
||
loadAsync("https://cdn.jsdelivr.net/npm/fastclick@1.0.6/lib/fastclick.min.js", function(){
|
||
var $nav = document.querySelector("nav");
|
||
if($nav) FastClick.attach($nav);
|
||
})
|
||
</script>
|
||
|
||
|
||
<script src="/js/theme-toggle.js"></script>
|
||
|
||
|
||
|
||
|
||
|
||
|
||
<script type="text/javascript">
|
||
function generateCatalog(selector) {
|
||
|
||
|
||
|
||
|
||
|
||
_containerSelector = 'div.post-container'
|
||
|
||
|
||
|
||
var P = $(_containerSelector), a, n, t, l, i, c;
|
||
a = P.find('h1,h2,h3,h4,h5,h6');
|
||
|
||
|
||
$(selector).html('')
|
||
|
||
|
||
a.each(function () {
|
||
n = $(this).prop('tagName').toLowerCase();
|
||
i = "#" + $(this).prop('id');
|
||
t = $(this).text();
|
||
c = $('<a href="' + i + '" rel="nofollow" title="' + t + '">' + t + '</a>');
|
||
l = $('<li class="' + n + '_nav"></li>').append(c);
|
||
$(selector).append(l);
|
||
});
|
||
return true;
|
||
}
|
||
|
||
generateCatalog(".catalog-body");
|
||
|
||
|
||
$(".catalog-toggle").click((function (e) {
|
||
e.preventDefault();
|
||
$('.side-catalog').toggleClass("fold")
|
||
}))
|
||
|
||
|
||
|
||
|
||
loadAsync("\/js\/jquery.nav.js", function () {
|
||
$('.catalog-body').onePageNav({
|
||
currentClass: "active",
|
||
changeHash: !1,
|
||
easing: "swing",
|
||
filter: "",
|
||
scrollSpeed: 700,
|
||
scrollOffset: 0,
|
||
scrollThreshold: .2,
|
||
begin: null,
|
||
end: null,
|
||
scrollChange: null,
|
||
padding: 80
|
||
});
|
||
});
|
||
</script>
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
</body>
|
||
</html>
|