Network engineering continued ...

 This is a continuation of the earlier posts starting with this one: http://ravinote.blogspot.com/2020/09/best-practice-from-networking.html

430. The availability of a service is improved by adding a cluster instead of a server. On the other hand, processes involved in background loading can use a primary server together with secondary servers. In such cases, a primary server is authoritative but a secondary server can serve the content when the primary is unavailable. 

431. There are two flavors of the release consistency model - the serialization consistency and processor consistency flavors. All of the models in this group allow a processor to read its own write early. However, the two flavors are the only ones whose straightforward implementations allow a read to return the value of another processor's write early. These models distinguish memory operations based on their type and provide stricter ordering constraints for some type of operations. 

432. The Weak ordering model classifies memory operations into two categories: data operations and synchronization operations. Since the programmer is required to identify at least one of the operations as a synchronization operation, the model can reorder memory operations between these synchronization operations without affecting the program's correctness. 

433. The other category of models for relaxing all program orders such as Alpha, RMO, and PowerPC - all provide explicit fence instructions as to their safety nets. The alpha model provides two different fence instructions: the memory barrier and the write memory barrier. The memory barrier (MB) instruction can be used to maintain program order from any memory operation before the MB to any memory instruction after the MB. The write memory barrier instruction provides this guarantee only among write operations. 

434. The PowerPC model provides a single fence instruction: the SYNC instruction. This is like the memory barrier instruction with the exception that when there are two reads to the same location, one may return the value of an older write than the first read. This model, therefore, requires read-modify-write semantics to enforce program order. 

435. A key goal of the programmer centric approach is to define the operations that should be distinguished as synchronization. In other words, a user's program consists of operations that are to be synchronized or otherwise categorized as data operations in an otherwise sequentially consistent program.