EVM, ONS, FAN

Event Manager (EVM)

The background process that publishes Oracle Clusterware events. EVM scans the designated callout directory and runs all scripts in that directory when an event occurs.

Event Manager Daemon (EVMD)

A UNIX-based event manager daemon that starts the racgevt process to manage callouts.

Event Management (EVM)—A background process that publishes events that crs creates.

evmd—Event manager daemon. This process also starts the racgevt process to manage FAN server callouts.

Fast Application Notification (FAN)

Applications can use FAN to enable rapid failure detection, balancing of connection pools after failures, and re-balancing of connection pools when failed components are repaired. The FAN notification process uses system events that Oracle publishes when cluster servers become unreachable or if network interfaces fail.

FAN has two methods for publishing events to clients, the Oracle Notification Service (ONS), which is used by Java Database Connectivity (JDBC) clients including the Oracle Application Server 10g, and Oracle Streams, Advanced Queueing which is used by Oracle Call Interface (OCI) and Oracle Data Provider for .NET (ODP.NET) clients. When using Advanced Queueing, you must enable the service to use the queue by setting AQ_HA_NOTIFICATIONS to true. (FAN的实现原理：evm, ons/advanced queueing)

Fast Connection Failover

Fast Connection Failover provides high availability to FAN integrated clients, such as clients that use JDBC, OCI, or ODP.NET. If you configure the client to use fast connection failover, then the client automatically subscribes to FAN events and can react to database UP and DOWN events. In response, Oracle gives the client a connection to an active instance that provides the requested database service.

Oracle Notification Service (ONS)—A publish and subscribe service for communicating Fast Application Notification (FAN) events.

上面的内容摘录自：Oracle Database Oracle Clusterware and Oracle Real Application Clusters Administration and Deployment Guide
10g Release 2 (10.2)

FAN：可以实现快速故障检测，连接的负载均衡，连接的快速故障转移等等。他实现的原理是，当cluster不可用或者网络错误时，oracle的crs的后台进程之一evmd会发布相关的事件。

EVMD: 发布crs的各种事件。并启动racgevt，来处理那些需要回调的脚本。

ONS: 提供各种事件的发布和注册服务。所以evmd应该是向ons发布事件，然后ons查看谁订阅了该事件，然后通知 evmd 调用racgevt进行相应的处理。