AWS placement group ,ENI ,EIP学习笔记

Placement Groups（Single Availability Zone）

a logical grouping of instances within a single Availability Zone. Using placement groups enables applications to participate in a low-latency , 10 Gbps network.

A placement group can't span multiple Availability Zones.

ENI (需选择subnet)

包含一个primary private ip;一个或者多个 secondary private ip address;一个EIP/per private ip address; 一个public ip address(will be auto-assigned to the ENI for eth0).

图中：eth0 是primary interface，因为在instance状态为stopped or running 下 ， primary interface 是不支持deattach。所以将EIP attach 到eth1中，在instance 意外fail之后，可迅速将其所使用eth1 attach到standby instance上。

To ensure failover capabilities, consider using a secondary private IP for incoming traffic on a network interface. In the event of an instance failure, you can move the interface and/orsecondary private IP address to a standby instance(manually).

实际操作中： default vpc下建的instance，它default primary interface， private ip addr 已经对应了一个 public ip。如果需要再attach EIP 需要再加sendary private ip,这之后才能进行相应操作。

实际应用场景：

you can use an ENI as your primary or secondary network interface to a critical service such as a database instance or a NA T instance.比如RDS primary down了，将其使用的ENI attach 到standbby，可以通过手动或者代码的方式（代码的方式还有待研究），reattach 期间会损失一些用户请求，但是极大的保证了系统的available.

EIP

An Elastic IP address(EIP) is a static IP address designed for dynamic cloud computing. With an EIP , you can mask the failure of an instance or software by rapidly remapping the address to another instance in your account. ENI 很好的体现了一个思想 design for failure.

一种情况： 当你disassociate an EIP address from an instance launched in default VPC, the instance 在几分钟之内会自动assign一个public ip，但是如果你attach了一个eth1，就不会assign一个新的public ip。  （不明白为什么）

EIP属于稀缺资源，AWS建议use an EIP primarily f or load balancing use cases, and use DNS hostnames for all other inter-node communication.