Updating the SAP kernel
It is important for an SAP system that all application server instances use the same kernel level. For this reason, SAP implemented a checking mechanism to ensure consistent kernels. In this section, this mechanism is described in detail to help you understand why kernel updates must follow a specific sequence.
- Each application server instance registers at the message server. The connection is established by the dispatcher. The dispatcher informs the message server about the platform type (Linux®, AIX®, and Windows) and its own patch level.
- The message server stores the patch level of the application server instance that connected first, but separately for each platform type. The value pairs, platform type plus corresponding patch level, are kept in memory as long as the message server is running. The values are never reset.
- When another instance registers later, the stored patch level for the corresponding platform is
returned by the message server. If the dispatcher of that application server instance detects a
mismatch, it stops.
An exception to this rule is the SAP rolling kernel switch mechanism (see Updating the SAP kernel using the rolling kernel switch), which tolerates different kernel levels on the application servers.
Although SAP strongly recommends that the patch levels of all application server instances are identical, the checking mechanism enforces this rule only among instances of the same platform type. The reason for this is that sometimes a patch level is not available for all platforms.
If you are using the old central instance concept, this mechanism is very reasonable. The message server is started and stopped with the central instance.
The stored patch level is that of the central instance. However, with the new concept, the application server instances might connect in an arbitrary order. Furthermore, the instances are started and stopped independently of the message server. A new patch level for the application server instance (disp+work) usually does not require a new patch level of the message server or the enqueue server. Nevertheless it is recommended to keep the patch level of central services instance on a current level.
The SAP program sapcpe ensures automatic synchronization of centrally and
locally installed executables. Starting an SAP instance, the program sapstart calls
program sapcpe. During the system start, sapcpe copies the SAP
kernel from the central exe directory (indicated by the profile parameter
DIR_CT_RUN) to the instance-specific exe directories
(indicated by the profile parameter DIR_EXECUTABLE). After installing a new SAP
kernel in the central exe directory, you must restart all SAP instances that
use this kernel.
For SAP systems where a downtime is acceptable, this can be done in a disruptive way, as described in Updating the SAP kernel using a disruptive procedure. For productive SAP systems or systems that require higher levels of availability, the preferred method, which avoids a complete SAP system downtime is described in Updating the SAP kernel using the rolling kernel switch.
The SAP rolling kernel switch procedure comes in two different flavors, depending on the SAP kernel level:
- With SAP 7.2x kernels, the rolling kernel switch needs some manual actions to be performed by the SAP administrator.
- With 7.4x kernels, the rolling kernel switch is available as automated procedure. The SAP administrator just starts the update procedure, which then updates the kernels of all SAP instances in the correct order.