Servers
Microkernel servers are essentially daemon programs like any others, except that the kernel grants some of them privileges to interact with parts of physical memory that are otherwise off limits to most programs. This allows some servers, particularly device drivers, to interact directly with hardware.
A basic set of servers for a general-purpose microkernel includes file system servers, device driver servers, networking servers, display servers, and user interface device servers. This set of servers (drawn from QNX) provides roughly the set of services offered by a Unix monolithic kernel. The necessary servers are started at system startup and provide services, such as file, network, and device access, to ordinary application programs. With such servers running in the environment of a user application, server development is similar to ordinary application development, rather than the build-and-boot process needed for kernel development.
Additionally, many "crashes" can be corrected by simply stopping and restarting the server. However, part of the system state is lost with the failing server, hence this approach requires applications to cope with failure. A good example is a server responsible for TCP/IP connections: If this server is restarted, applications will experience a "lost" connection, a normal occurrence in networked system. For other services, failure is less expected and may require changes to application code. For QNX, restart capability is offered as the QNX High Availability Toolkit.
To make all servers restartable, some microkernels have concentrated on adding various database-like methods such as transactions, replication and checkpointing to preserve essential state across single server restarts. An example is ChorusOS, which was made for high-availability applications in the telecommunications world. Chorus included features to allow any "properly written" server to be restarted at any time, with clients using those servers being paused while the server brought itself back into its original state. However, such kernel features are incompatible with the minimality principle, and are thus not provided in modern microkernels, which instead rely on appropriate user-level protocols.
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