An XCS network is a variant of the HIPLEX-MSCF functionality which is available in BS2000. In particular, it permits the use of distributed applications and comprises the following functionality:
automatic failure detection and reconfiguration
lock management for distributed applications (DLM)
uniform time throughout the network
The synchronization of the system time in the network is performed by the XCS TIME subsystem, which is initialized on XCS startup and is based on the XCS time, which represents a second time alongside the system time. The XCS time is supplied via GTIME (with SVC however) in TODR format and is based on UTC.
XCS time
Alongside a lock function, distributed applications often require a network-wide, monotonic time, e.g. to permit the correct chronological compilation of locally generated logging files. XCS time guarantees this monotonic time in the event of two successive queries to any systems, provided that the time is determined under the same DLM lock:
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The two GTIME calls provide genuine monotonic time values in which the logging information for manipulations performed to data block x may be chronologically different.
Each system maintains its own XCS time as follows:
In the case of exclusive lock allocation by system A, which is the administrator of a DLM lock, to a second system B, the maximum from the XCS time and system time, incremented by the value “one microsecond”, is sent to the lock requester together with an acknowledgment of allocation.
System B receives the lock allocation from system A
The maximum of the time on system A supplied with the lock allocation and the XCS time on system B, together with its current system time, forms the new current XCS time for system B.
GTIME call for the reception of the XCS time by an application
The maximum, incremented by the value “one microsecond”, of the current system time and the XCS time becomes the current XCS time and is made available to the caller.
This procedure keeps the XCS time of all the participating systems close to the system time of the system with the most advanced time.
> | The corresponding information can be found in the SYSTEM-TIME-PARAMETER section of the SHOW-SYSTEM-INFORMATION command’s output: |
To keep the system times as close together as possible (e.g. to keep the chronology of decentralized system logging data as correct as possible), the XCS-TIME subsystem synchronizes the system time with the XCS time of the system in every participating system. The system time and the XCS time of the individual servers are therefore only approximately equal; no maximum divergence can be guaranteed.
No server can be accepted into a network until its system time had been adapted to that of the partners which are already active. If the time difference on entry is greater than 15 minutes, entry is denied.
External timers and XCS networks
An XCS network can also be synchronized with an external timer.
You are recommended to connect an external time either to all participants of the XCS network or only to one of them.
Explanation
If not all participants of the network are connected to this timer, the connected participants monitor the times of the participants who are not connected and, when required, send time adjustment requests to them.
On the recipient side, adjustment requests from different senders can appear contradictory owing to different runtimes. This can lead to increased CPU time requirements and to time deviations.
Notes for use under VM2000
A VM2000 guest system (including the monitor system) can participate in an XCS network independently of the remaining VMs of the server. If the monitor system is a participant in such a network, the emulation of the SVP time (Store Real Clock) supplied to the guest systems is based on the actual SVP time and not on the NTP-synchronized system time of the monitor system.