LB方案分類
目前主流的LB方案可分成兩類：一種是集中式LB, 即在服務的消費方和提供方之間使用獨立的LB設施(可以是硬件，如F5, 也可以是軟件，如nginx), 由該設施負責把訪問請求通過某種策略轉發至服務的提供方；另一種是進程內LB，將LB邏輯集成到消費方，消費方從服務注冊中心獲知有哪些地址可用，然后自己再從這些地址中選擇出一個合適的服務器。Ribbon就屬於后者，它只是一個類庫，集成於消費方進程，消費方通過它來獲取到服務提供方的地址。
1. Ribbon是什么？它解決了什么問題？
Ribbon is a client side IPC library that is battle-tested in cloud. It provides the following features
- Load balancing
- Fault tolerance
- Multiple protocol (HTTP, TCP, UDP) support in an asynchronous and reactive model
- Caching and batching
2.它的模塊有哪些？功能分別是什么？
- ribbon: APIs that integrate load balancing, fault tolerance, caching/batching on top of other ribbon modules and Hystrix
- ribbon-loadbalancer: Load balancer APIs that can be used independently or with other modules
- ribbon-eureka: APIs using Eureka client to provide dynamic server list for cloud
- ribbon-transport: Transport clients that support HTTP, TCP and UDP protocols using RxNetty with load balancing capability
- ribbon-httpclient: REST client built on top of Apache HttpClient integrated with load balancers (deprecated and being replaced by ribbon module)
- ribbon-example: Examples
- ribbon-core: Client configuration APIs and other shared APIs
3.哪些模塊應用到生產了？
- ribbon-core: deployed at scale in production
- ribbon-eureka: deployed at scale in production
- ribbon-evcache: not used
- ribbon-guice: not used
- ribbon-httpclient: we use everything not under com.netflix.http4.ssl. Instead, we use an internal solution developed by our cloud security team
- ribbon-loadbalancer: deployed at scale in production
- ribbon-test: this is just an internal integration test suite
- ribbon-transport: not used
- ribbon: not used
4.主要類及其層次結構
說明：
DynamicServerListLoadBalancer：A LoadBalancer that has the capabilities to obtain the candidate list of servers using a dynamic source. i.e. The list of servers can potentially be changed at Runtime. It also contains facilities wherein the list of servers can be passed through a Filter criteria to filter out servers that do not meet the desired criteria.
IClientConfig：Defines the client configuration used by various APIs to initialize clients or load balancers and for method execution. The default implementation is {@link DefaultClientConfigImpl}
IRule：Interface that defines a "Rule" for a LoadBalancer. A Rule can be thought of as a Strategy for loadbalacing. Well known loadbalancing strategies include Round Robin, Response Time based etc.
IPing：Interface that defines how we "ping" a server to check if its alive
ServerList:Interface that defines the methods sed to obtain the List of Servers
ServerListFilter:This interface allows for filtering the configured or dynamically obtained List of candidate servers with desirable characteristics.
ServerListUpdater:strategy for {@link com.netflix.loadbalancer.DynamicServerListLoadBalancer} to use for different ways of doing dynamic server list updates
與Eureka結合使用
當與Eureka組合使用Ribbon時，
server使用DiscoveryEnabledServer實現，Servers that were obtained via Discovery and hence contain meta data in the form of InstanceInfo。
ServerList接口會使用DiscoveryEnabledNIWSServerList實現，The server list class that fetches the server information from Eureka client. ServerList is used by {@link DynamicServerListLoadBalancer} to get server list dynamically.
ServerListFilter使用DefaultNIWSServerListFilter實現，The Default NIWS Filter - deals with filtering out servers based on the Zone affinity and other related properties。
ServerListUpdater使用EurekaNotificationServerListUpdater，A server list updater for the {@link com.netflix.loadbalancer.DynamicServerListLoadBalancer} that utilizes eureka's event listener to trigger LB cache updates。封裝了LegacyEurekaClientProvider：A legacy class to provide eurekaclient via static singletons。
IPing使用NIWSDiscoveryPing實現，"Ping" Discovery Client，i.e. we dont do a real "ping". We just assume that the server is up if Discovery Client says so。
參考文章
【1】https://github.com/Netflix/ribbon
【2】http://blog.csdn.net/neosmith/article/details/53967330