1e41f4b71Sopenharmony_ci# Introduction to Common Events 2e41f4b71Sopenharmony_ci 3e41f4b71Sopenharmony_ci 4e41f4b71Sopenharmony_ciCommon Event Service (CES) is provided for applications to subscribe to, publish, and unsubscribe from common events. 5e41f4b71Sopenharmony_ci 6e41f4b71Sopenharmony_ci 7e41f4b71Sopenharmony_ciCommon events are classified into system common events and custom common events. 8e41f4b71Sopenharmony_ci 9e41f4b71Sopenharmony_ci 10e41f4b71Sopenharmony_ci- System common events: defined in CES. Currently, only system applications and system services can publish system common events, such as HAP installation, update, and uninstall. For details about the supported system common events, see [System Common Events](../reference/apis/commonEventManager-definitions.md). 11e41f4b71Sopenharmony_ci- Custom common events: customized by applications to implement cross-process event communication. 12e41f4b71Sopenharmony_ci 13e41f4b71Sopenharmony_ci 14e41f4b71Sopenharmony_ciCommon events are also classified into unordered, ordered, and sticky common events. 15e41f4b71Sopenharmony_ci 16e41f4b71Sopenharmony_ci 17e41f4b71Sopenharmony_ci- Unordered common events: common events that CES forwards regardless of whether subscribers receive the events and when they subscribe to the events. 18e41f4b71Sopenharmony_ci- Ordered common events: common events that CES forwards based on the subscriber priority. CES preferentially forwards an ordered common event to the subscriber with higher priority, waits until the subscriber receives the event, and then forwards the events to the subscriber with lower priority. Subscribers with the same priority receive common events in a random order. 19e41f4b71Sopenharmony_ci- Sticky common events: common events that can be sent to a subscriber before or after they initiate a subscription. Only system applications and system services can send sticky common events, which remain in the system after being sent. The sends must first request the **ohos.permission.COMMONEVENT_STICKY** permission. For details, see [Declaring Permissions](../../security/AccessToken/declare-permissions.md). 20e41f4b71Sopenharmony_ci 21e41f4b71Sopenharmony_ci 22e41f4b71Sopenharmony_ciEach 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. 23e41f4b71Sopenharmony_ci 24e41f4b71Sopenharmony_ci**Figure 1** Common events 25e41f4b71Sopenharmony_ci 26e41f4b71Sopenharmony_ci 27e41f4b71Sopenharmony_ci 28e41f4b71Sopenharmony_ci <!--no_check-->
