What is an Oracle database Server ? A Back-To-Basics Guide

30 Mar What is an Oracle database Server ? A Back-To-Basics Guide

If you’re stepping into the world of Oracle or want a refresher, understanding what’s under the hood of an Oracle Database Server is a crucial first step.

This post simplifies the key concepts and architecture of it.

Think of it as two key components working together:

• Database: A set of physical files that store data and metadata on disk.

• Database Instance: A set of memory structures and background processes that interact with those files.

You can technically have one without the other, but to use the database, you need both. An active database needs at least one associated instance.

To better understand what a database is, it has to be understood that there is physical and logical viewpoint, and each have their own structures.

• Physical
  • Control Files. – Binary file that is normally multiplexed due to the importance of its contents. It contains the metadata which specifies the physical organization of your database, as well it holds the following information:
    • Database Name
    • Checkpoint information, which it indicates the System Change Number in the redo stream where recovery needs to begin. Every committed change before a SCN checkpoint is guaranteed to be written to disk.
    • Current Online Redo Log Sequence
    • RMAN Backup Information
  • Online Redo Logs. – Set of files that record altered data that has not been written to the datafiles. Oracle requires that there be a minimum of two. The online redo log for a database instance is called Redo Thread, and each Database Instance will have it’s own Redo thread. Only one multiplexed online redo log file is used at a time per instance. A redo log is made up of redo records and these records include the following metadata:
    • SCN and time stamp of the change
    • Transaction ID of the transaction that generated the change
    • SCN and time stamp when the transaction committed (if it committed)
    • Type of operation that made the change
    • Name and type of the modified data segment
  • Data files. – File that contains system and user data, the space used in these files are broken down into the following:
    • Data File Header. –Contains metadata, as SCN checkpoint and file size, as well as a unique file identifier that relates it to the database, a tablespace identifier that relates it to a tablespace and a container id, which relates it to the correct container.
    • Used
    • Free (Formatted,Never Used)
    • Free (Previously Used,Currently Unused)

• Logical
  • Data block. –It is the smallest logical unit; data is stored in data blocks. One data block corresponds to a specific number of bytes. It is structured in the following way:
    • Block header. – General information about the block, including disk address and segment type
    • Table directory. – Metadata about tables whose rows are stored in this block
    • Row directory. – Describes the location of rows in the data portion of the block.
    • Row Data. –Contains user data/system data. Data is stored in rows and is formatted in the following way:
      • Row Header
      • Column Data

The Database uses a rowid to uniquely identify a row, and the rowid has a four-piece format that is the following:

• Data Object number
• Tablespace-Identifies the data file that contains the row

• Block number that contains the row
• Identifies the row in the block
  • A single allocation of a number of logically bordering data blocks. An extent can exist in only one data file.
  • A set of extents allocated for user, undo or temporary data. The first data block of every segment contains a directory of the extents in the segment.
  • This is the container of segments, defined as a storage unit that groups related logical structures. One tablespace needs at least one datafile. A database must have the SYSTEM and SYSAUX tablespaces. Below you will find the tablespace types:
    • The SYSTEM tablespace has the following:
      • Data dictionary
      • Compiled stored objects
    • The SYSAUX tablespace is an auxiliary to the SYSTEM tablespace and is the default tablespace for many Oracle Database features.
    • Permanent Tablespaces are recommended to store application data.
    • The UNDO tablespace is a locally managed tablespace reserved for system-managed undo tablespaces. Undo data is the copy of the original, premodified , data and is retained until at least the transaction is ended.
    • The temporary contains data that persists only for the duration of a session; it is used for multiple sort operations that can’t be fitted into memory.

Conclusion

If you’re just getting started with Oracle, here’s the big picture:

• Think of an Oracle database like a team—you’ve got files on disk (the database) and the memory/processes (the instance) that keep everything running smoothly. Both are essential, and they work hand-in-hand.
• There are two ways to look at how data is organized: the physical side (actual files and logs) and the logical side(how Oracle makes sense of that data internally). Knowing both helps you understand what’s really going on under the hood.