1# Common Event and Notification 2 3## Introduction 4 5OpenHarmony provides a Common Event Service (CES) for applications to subscribe to, publish, and unsubscribe from common events. 6 7Common events in OpenHarmony are classified into system common events and custom common events. 8 9- System common event: sent by the system based on system policies to the applications that have subscribed to the event. This type of event is typically system events published by key system services, such as HAP installation, update, and uninstallation. 10 11- Custom common events: customized by applications to implement cross-application event communication. 12 13Each application can subscribe to common events as required. After your application subscribes to a common event, the system sends it to your application every time the event is published. Such an event may be published by the system, other applications, or your own application. 14 15### Architecture 16 17 18 19## Directory Structure 20 21``` 22/base/notification/ces_standard/ 23│── frameworks # Component directory 24│ |── common/log # Logs 25│ |── core # Internal implementation code of native APIs 26│ ├── native # Implementation code of native APIs 27│── interface # External APIs 28| |── innerkits # Definition of native APIs 29| |── kits/napi # NAPI implementation 30├── sa_profile # Service configuration profile 31├── services # Service implementation 32├── tools # Tools 33│── ohos.build # Build script 34 35``` 36 37## How to Use 38 39For details, see [Common Event Development](../application-dev/application-models/common-event-overview.md). 40 41## Repositories Involved 42 43Common Event and Notification 44 45**notification_ces_standard**
