Exploring the database architecture(探索数据库架构)
certification objectives:
- Describe the Single-Instance Architecture
- Explain the Memeory Structures
- Describe the Process Structures
- Summarize(总结) the Storage Structures
An oralce server consists of two entities:the instance and the database.The instance is memeory structures and processes;the database is file on disk.They are separate(分离的),but connected(有联系的).During the creation process,the instance is created first,and then database during the startup process first(优先于) the instance is started,and then the database is opened.In a typical single-instance environmnet,the relationship of instance to database is one-to-one,a single instance connected to a single database,but always bear in mind(考虑到) that there are more complex possibilities for distributed(分布式) environments.
Within the Oracle server,there is complete abstraction of logical storage from physical storage(从物理存储到逻辑存储的完全抽象).The logical structures programmers see (such as tables) are not directly related to the physical structures (datafiles) that system adminsistrators see(程序员看到的逻辑结构（比如表）和系统管理员看到的物理结构（比如数据文件）并没有直接的关系).The relationship between the two is maintained by structures within the controlfile and the data dictionary(两者的关系通过控制文件和数据字典之间的结构维持).
Describe the Single-Instance Architecture
For the most(大多数) part in this book,you will be dealing with the most common database environment:one instance on one computer,opening a database stored on local disks.The more complex distributed architectures,involving(涉及) multiple instances and multiple databases,are beyond(超出...范围) the scope of the OCP examination(though not the OCM qualification(资格，条件，限制)) ,but you may relistically expect to see high-level summary questions on distributed architecture(但实际上你可能期望看到更高级的分布式架构问题).
Single Instance Database Architecture
The instance consists of memory structures and processes.Its existence is transient(短暂的),in your RAM and on your CPU(s).When you shutdown the running instance,all trace(痕迹) of its existence goes away at the same time.The database consists of physical files on disk,Whether running or stopped,these remian(保持，残存).Thus the lifetime of the instance is only as long as it exists in memory:it can be started and stopped.By contrast(对比,相比之下),the database,once created,persists indefinitely(不确定的)--until you deliberately(故意的，慎重的) delete the files that are associated with the database.
The process that make up the instance are known as background processes(实例产生的进程被认为是后台进程),because they are present and running at all time while the instance is active(因为当实例是活动状态时，这些进程一直存在并运行).These processes are for the most part completely self-administering,though in some cases it is possible for the DBA to influence(影响，改变) the number of them and their operation(尽管在一些情况下，DBA可能会改变这些进程的序号或操作，但很大程度上这些进程是自我管理的).
The memory structures,which are implemented in shared memory segments provided by the operating system,are known as the system global area,or SGA(内存结构式通过操作系统提供的共享内存段实现的，常被称作系统全局区，或SGA).This is allocated at instance startup and released on shutdown.Within certain limits,the SGA in the 11g instance and the components within it can be resized while the instance is running,either automatically or in response the DBA's instructions.(SGA在实例启动分配，在实例关闭时释放。在一定范围内，当实例运行时，11g实例中的SGA和它内部的组件是可以自动调整或根据DBA的指令调整。)
User sessions consist of a user process running locally to the user machine connecting to a server process running locally to the instance on the server machine.(一个运行在用户机器上的用户进程连接到一个运行在服务器实例上的服务进程构成了用户会话)。The technique for launching the server processes,which are started on demand for each session,is covered in Chapter 6.(每个会话都需要启动服务器进程，启动服务器进程的知识，会在第6章介绍)。