微服务之API网关：Kong
Kong 是在客户端和（微）服务间转发API通信的API网关，通过插件扩展功能。Kong 有两个主要组件：
1、Kong Server ：基于nginx的服务器，用来接收 API 请求。
2、Apache Cassandra：用来存储操作数据。
你可以通过增加更多 Kong Server 机器对 Kong 服务进行水平扩展，通过前置的负载均衡器向这些机器分发请求。根据文档描述，两个Cassandra节点就足以支撑绝大多数情况，但如果网络非常拥挤，可以考虑适当增加更多节点。
对于开源社区来说，Kong 中最诱人的一个特性是可以通过插件扩展已有功能，这些插件在 API 请求响应循环的生命周期中被执行。插件使用 Lua 编写，而且Kong还有如下几个基础功能：HTTP 基本认证、密钥认证、CORS（ Cross-origin Resource Sharing，跨域资源共享）、TCP、UDP、文件日志、API 请求限流、请求转发以及 nginx 监控。
Kong Gateway
The world’s most popular open source API gateway. Built for multi-cloud and hybrid, optimized for microservices and distributed architectures.
kong在openresty基础上进行的开发，而openresty基于nginx，所以kong的很多概念和用法都是nginx特色，包括对功能进行描述的词汇，比如upstream services就是nginx设定的upstream。kong很好地封装了nginx，并对外提供了如下的功能接口：
- 云原生：具有平台无关性，kong即可以运行在物理设备也可以运行在kubernetes上
- 动态负载均衡：在多个upstream services基础上进行负载均衡设定
- 基于hash的负载均衡：基于hashing/sticky session的负载均衡
- 断路器：能追踪不健康的upstream services
- 健康检测：对upstream services进行主动或者被动地监控
- 服务发现：可结合consul提供服务注册等功能
- WebSockets：通过WebSockets和upstream service进行通信
- OAuth2.0：可对API方便地添加OAuth2.0进行授权
- 日志：通过HTTP/TCP/UDP等方式进行日志相关操作
- 安全：ACL，机器人检测，黑白名单IP等
- 系统日志：日志可输入到系统日志中
- 监控：提供实时监控功能
- 认证：HMAC/JWT以及基本认证方式
- 限流（Rate-Limiting）：基于多变量对请求进行阻塞或者限制
- 转换：对TTP请求和相应进行添加/删除/操纵等操作
- 缓存：在代理层进行缓存和响应处理
- CLI：通过命令行对kong的集群进行控制
- REST API：可灵活地通过RESTful API对kong进行操作
- 失败检测与恢复：Cassandra某一节点停止也不会影响kong的功能
- 集群：所有的kong节点都能自动加入集群，并保证配置在整个节点间得到更新
- 可扩展性：kong可以通过添加节点很容易地实现横向扩展
- 性能：使用nginx作为内核，kong具有nginx的高性能
- plugin：可以对kong和API进行扩展
Features
- Cloud-Native: Platform agnostic, Kong can run from bare metal to Kubernetes.
- Dynamic Load Balancing: Load balance traffic across multiple upstream services.
- Hash-based Load Balancing: Load balance with consistent hashing/sticky sessions.
- Circuit-Breaker: Intelligent tracking of unhealthy upstream services.
- Health Checks: Active and passive monitoring of your upstream services.
- Service Discovery: Resolve SRV records in third-party DNS resolvers like Consul.
- Serverless: Invoke and secure AWS Lambda or OpenWhisk functions directly from Kong.
- WebSockets: Communicate to your upstream services via WebSockets.
- gRPC: Communicate to your gRPC services and observe your traffic with logging and observability plugins
- OAuth2.0: Easily add OAuth2.0 authentication to your APIs.
- Logging: Log requests and responses to your system over HTTP, TCP, UDP, or to disk.
- Security: ACL, Bot detection, whitelist/blacklist IPs, etc...
- Syslog: Logging to System log.
- SSL: Setup a Specific SSL Certificate for an underlying service or API.
- Monitoring: Live monitoring provides key load and performance server metrics.
- Forward Proxy: Make Kong connect to intermediary transparent HTTP proxies.
- Authentications: HMAC, JWT, Basic, and more.
- Rate-limiting: Block and throttle requests based on many variables.
- Transformations: Add, remove, or manipulate HTTP requests and responses.
- Caching: Cache and serve responses at the proxy layer.
- CLI: Control your Kong cluster from the command line.
- REST API: Kong can be operated with its RESTful API for maximum flexibility.
- Geo-Replicated: Configs are always up-to-date across different regions.
- Failure Detection & Recovery: Kong is unaffected if one of your Cassandra nodes goes down.
- Clustering: All Kong nodes auto-join the cluster keeping their config updated across nodes.
- Scalability: Distributed by nature, Kong scales horizontally by simply adding nodes.
- Performance: Kong handles load with ease by scaling and using NGINX at the core.
- Plugins: Extendable architecture for adding functionality to Kong and APIs.