Device publishers are hardware-support components that publish information on attached devices through the Persistent Publish/Subscribe (PPS) service. Applications use this information to access the content on devices, which allows them to display file information in browsers or to invoke multimedia services that can read and play media files from those devices.
Applications could send commands to device drivers to read device information but this design requires developers to learn hardware interfaces, which is inconvenient. Device publishers offer a reliable source of device information by allowing applications to subscribe to PPS objects to receive device updates. See the Persistent Publish/Subscribe Developer's Guide for details on implementing a subscriber.
Device publishers don't monitor the physical connection of devices. Other system services monitor hardware events and respond when users attach or detach storage devices (such as USB sticks) or when they insert or remove mediastores (such as SD cards) from physical slots. For instance, the io-usb-otg manager monitors the hardware and reacts to each USB device attachment or detachment by notifying the usblauncher_otg service.
Device publishers run as persistent background processes. Typically, they're launched during bootup. You can restart publishers if they unexpectedly terminate.