Database interview questions:

Database interview questions:                                                                                                                                                                                                                              

1. What are the components of Physical database structure of Oracle Database?.

ORACLE database is comprised of three types of files. One or more Data files, two are more Redo Log files, and one or more
Control files.

2. What are the components of Logical database structure of ORACLE database?

Tablespaces and the Database's Schema Objects.

3. What is a Tablespace?

A database is divided into Logical Storage Unit called tablespaces. A tablespace is used to grouped related logical structures
together.

4. What is SYSTEM tablespace and When is it Created?

Every ORACLE database contains a tablespace named SYSTEM, which is automatically created when the database is created.
The SYSTEM tablespace always contains the data dictionary tables for the entire database.

5. Explain the relationship among Database, Tablespace and Data file.

Each databases logically divided into one or more tablespaces One or more data files are explicitly created for each tablespace.

6. What is schema?

A schema is collection of database objects of a User.

7. What are Schema Objects ?

Schema objects are the logical structures that directly refer to the database's data. Schema objects include tables, views,
sequences, synonyms, indexes, clusters, database triggers, procedures, functions packages anddatabase links.

8. Can objects of the same Schema reside in different tablespaces.?

Yes.

9. Can a Tablespace hold objects from different Schemes ?

Yes.

10. what is Table ?

A table is the basic unit of data storage in an ORACLE database. The tables of a database hold all of the user accessible data.
Table data is stored in rows and columns.

11. What is a View ?

A view is a virtual table. Every view has a Query attached to it. (The Query is a SELECT statement that identifies the columns and
rows of the table(s) the view uses.)

12. Do View contain Data ?

Views do not contain or store data.

13. Can a View based on another View ?

Yes.

14. What are the advantages of Views ?

Provide an additional level of table security, by restricting access to a predetermined set of rows and columns of a table.
Hide data complexity.
Simplify commands for the user.
Present the data in a different perpecetive from that of the base table.
Store complex queries.

15. What is a Sequence ?

A sequence generates a serial list of unique numbers for numerical columns of a database's tables.

16. What is a Synonym ?

A synonym is an alias for a table, view, sequence or program unit.

17. What are the type of Synonyms?

There are two types of Synonyms Private and Public.

18. What is a Private Synonyms ?

A Private Synonyms can be accessed only by the owner.

19. What is a Public Synonyms ?

A Public synonyms can be accessed by any user on the database.

20. What are synonyms used for ?

Synonyms are used to : Mask the real name and owner of an object.
Provide public access to an object
Provide location transparency for tables,views or program units of a remote database.
Simplify the SQL statements for database users.

21. What is an Index ?

An Index is an optional structure associated with a table to have direct access to rows,which can be created to increase the
performance of data retrieval. Index can be created on one or more columns of a table.

22. How are Indexes Update ?

Indexes are automatically maintained and used by ORACLE. Changes to table data are automatically incorporated into all
relevant indexes.

23. What are Clusters ?

Clusters are groups of one or more tables physically stores together to share common columns and are often used together.

24. What is cluster Key ?

The related columns of the tables in a cluster is called the Cluster Key.

25. What is Index Cluster ?

A Cluster with an index on the Cluster Key.

26. What is Hash Cluster ?

A row is stored in a hash cluster based on the result of applying a hash function to the row's cluster key value. All rows with the
same hash key value are stores together on disk.

27. When can Hash Cluster used ?

Hash clusters are better choice when a table is often queried with equality queries. For such queries the specified cluster key value
is hashed. The resulting hash key value points directly to the area on disk that stores the specified rows.

28. What is Database Link ?

A database link is a named object that describes a path from one database to another.

29. What are the types of Database Links ?

Private Database Link, Public Database Link & Network Database Link.

30. What is Private Database Link ?

Private database link is created on behalf of a specific user. A private database link can be used only when the owner of the link
specifies a global object name in a SQL statement or in the definition of the owner's views or procedures.

31. What is Public Database Link ?

Public database link is created for the special user group PUBLIC. A public database link can be used when any user in the
associated database specifies a global object name in a SQL statement or object definition.

32. What is Network Database link ?

Network database link is created and managed by a network domain service. A network database link can be used when any
user of any database in the network specifies a global object name in a SQL statement or object definition.

33. What is Data Block ?

ORACLE database's data is stored in data blocks. One data block corresponds to a specific number of bytes of physical
database space on disk.

34. How to define Data Block size ?

A data block size is specified for each ORACLE database when the database is created. A database users and allocated free
database space in ORACLE datablocks. Block size is specified in INIT.ORA file and cann't be changed latter.

35. What is Row Chaining ?

In Circumstances, all of the data for a row in a table may not be able to fit in the same data block. When this occurs , the data for
the row is stored in a chain of data block (one or more) reserved for that segment.

36. What is an Extent ?

An Extent is a specific number of contiguous data blocks, obtained in a single allocation, used to store a specific type of
information.

37. What is a Segment ?

A segment is a set of extents allocated for a certain logical structure.

38. What are the different type of Segments ?

Data Segment, Index Segment, Rollback Segment and Temporary Segment.

39. What is a Data Segment ?

Each Non-clustered table has a data segment. All of the table's data is stored in the extents of its data segment. Each cluster has a
data segment. The data of every table in the cluster is stored in the cluster's data segment.

40. What is an Index Segment ?

Each Index has an Index segment that stores all of its data.

41. What is Rollback Segment ?

A Database contains one or more Rollback Segments to temporarily store undo information.

42. What are the uses of Rollback Segment ?

Rollback Segments are used :
To generate read-consistent database information during database recovery to rollback uncommitted transactions for users.

43. What is a Temporary Segment ?

Temporary segments are created by ORACLE when a SQL statement needs a temporary work area to complete execution.
When the statement finishes execution, the temporary segment extents are released to the system for future use.

44. What is a Data File ?

Every ORACLE database has one or more physical data files. A database's data files contain all the database data. The data of
logical database structures such as tables and indexes is physically stored in the data files allocated for a database.

45. What are the Characteristics of Data Files ?

A data file can be associated with only one database.Once created a data file can't change size.
One or more data files form a logical unit of database storage called a tablespace.

46. What is a Redo Log ?

The set of Redo Log files for a database is collectively known as the database's redo log.

47. What is the function of Redo Log ?

The Primary function of the redo log is to record all changes made to data.

48. What is the use of Redo Log Information ?

The Information in a redo log file is used only to recover the database from a system or media failure prevents database data from
being written to a database's data files.

49. What does a Control file Contain ?

A Control file records the physical structure of the database. It contains the following information.
Database Name
Names and locations of a database's files and redolog files.
Time stamp of database creation.

50. What is the use of Control File ?

When an instance of an ORACLE database is started, its control file is used to identify the database and redo log files that must
be opened for database operation to proceed. It is also used in database recovery.

51. What is a Data Dictionary ?

The data dictionary of an ORACLE database is a set of tables and views that are used as a read-only reference about the
database.
It stores information about both the logical and physical structure of the database, the valid users of an ORACLE database,
integrity constraints defined for tables in the database and space allocated for a schema object and how much of it is being used.

52. What is an Integrity Constrains ?

An integrity constraint is a declarative way to define a business rule for a column of a table.

53. Can an Integrity Constraint be enforced on a table if some existing table data does not satisfy the constraint ?

No.

54. Describe the different type of Integrity Constraints supported by ORACLE ?

NOT NULL Constraint - Disallows NULLs in a table's column.
UNIQUE Constraint - Disallows duplicate values in a column or set of columns.
PRIMARY KEY Constraint - Disallows duplicate values and NULLs in a column or set of columns.
FOREIGN KEY Constrain - Require each value in a column or set of columns match a value in a related table's UNIQUE or
PRIMARY KEY.
CHECK Constraint - Disallows values that do not satisfy the logical expression of the constraint.

55. What is difference between UNIQUE constraint and PRIMARY KEY constraint ?

A column defined as UNIQUE can contain NULLs while a column defined as PRIMARY KEY can't contain Nulls.

56. Describe Referential Integrity ?

A rule defined on a column (or set of columns) in one table that allows the insert or update of a row only if the value for the
column or set of columns (the dependent value) matches a value in a column of a related table (the referenced value). It also
specifies the type of data manipulation allowed on referenced data and the action to be performed on dependent data as a result
of any action on referenced data.

57. What are the Referential actions supported by FOREIGN KEY integrity constraint ?

UPDATE and DELETE Restrict - A referential integrity rule that disallows the update or deletion of referenced data.
DELETE Cascade - When a referenced row is deleted all associated dependent rows are deleted.

58. What is self-referential integrity constraint ?

If a foreign key reference a parent key of the same table is called self-referential integrity constraint.

59. What are the Limitations of a CHECK Constraint ?

The condition must be a Boolean expression evaluated using the values in the row being inserted or updated and can't contain
subqueries, sequence, the SYSDATE,UID,USER or USERENV SQL functions, or the pseudo columns LEVEL or ROWNUM.

60. What is the maximum number of CHECK constraints that can be defined on a column ?

No Limit.

61. What constitute an ORACLE Instance ?

SGA and ORACLE background processes constitute an ORACLE instance. (or) Combination of memory structure and
background process.

62. What is SGA ?

The System Global Area (SGA) is a shared memory region allocated by ORACLE that contains data and control information for
one ORACLE instance.

63. What are the components of SGA ?

Database buffers, Redo Log Buffer the Shared Pool and Cursors.

64. What do Database Buffers contain ?

Database buffers store the most recently used blocks of database data. It can also contain modified data that has not yet been
permanently written to disk.

65. What do Redo Log Buffers contain ?

Redo Log Buffer stores redo entries a log of changes made to the database.

66. What is Shared Pool ?

Shared Pool is a portion of the SGA that contains shared memory constructs such as shared SQL areas.

67. What is Shared SQL Area ?

A Shared SQL area is required to process every unique SQL statement submitted to a database and contains information such as
the parse tree and execution plan for the corresponding statement.

68. What is Cursor ?

A Cursor is a handle ( a name or pointer) for the memory associated with a specific statement.

69. What is PGA ?

Program Global Area (PGA) is a memory buffer that contains data and control information for a server process.

70. What is User Process ?

A user process is created and maintained to execute the software code of an application program. It is a shadow process created
automatically to facilitate communication between the user and the server process.

71. What is Server Process ?

Server Process handle requests from connected user process. A server process is in charge of communicating with the user
process and interacting with ORACLE carry out requests of the associated user process.

72. What are the two types of Server Configurations ?

Dedicated Server Configuration and Multi-threaded Server Configuration.

73. What is Dedicated Server Configuration ?

In a Dedicated Server Configuration a Server Process handles requests for a Single User Process.

74. What is a Multi-threaded Server Configuration ?

In a Multi-threaded Server Configuration many user processes share a group of server process.

75. What is a Parallel Server option in ORACLE ?

A configuration for loosely coupled systems where multiple instance share a single physical database is called Parallel Server.

76. Name the ORACLE Background Process ?

DBWR - Database Writer.
LGWR - Log Writer
CKPT - Check Point
SMON - System Monitor
PMON - Process Monitor
ARCH - Archiver
RECO - Recover
Dnnn - Dispatcher, and
LCKn - Lock
Snnn - Server.

77. What Does DBWR do ?

Database writer writes modified blocks from the database buffer cache to the data files.
78.When Does DBWR write to the database ?
DBWR writes when more data needs to be read into the SGA and too few database buffers are free. The least recently used data
is written to the data files first. DBWR also writes when CheckPoint occurs.

79. What does LGWR do ?

Log Writer (LGWR) writes redo log entries generated in the redo log buffer of the SGA to on-line Redo Log File.

80. When does LGWR write to the database ?

LGWR writes redo log entries into an on-line redo log file when transactions commit and the log buffer files are full.

81. What is the function of checkpoint(CKPT)?

The Checkpoint (CKPT) process is responsible for signaling DBWR at checkpoints and updating all the data files and control files
of the database.

82. What are the functions of SMON ?

System Monitor (SMON) performs instance recovery at instance start-up. In a multiple instance system (one that uses the Parallel
Server), SMON of one instance can also perform instance recovery for other instance that have failed SMON also cleans up
temporary segments that are no longer in use and recovers dead transactions skipped during crash and instance recovery because
of file-read or off-line errors. These transactions are eventually recovered by SMON when the tablespace or file is brought back
on-line SMON also coalesces free extents within the database to make free space contiguous and easier to allocate.

83. What are functions of PMON ?

Process Monitor (PMON) performs process recovery when a user process fails PMON is responsible for cleaning up the cache
and Freeing resources that the process was using PMON also checks on dispatcher and server processes and restarts them if
they have failed.

84. What is the function of ARCH ?

Archiver (ARCH) copies the on-line redo log files to archival storage when they are full. ARCH is active only when a database's
redo log is used in ARCHIVELOG mode.

85. What is function of RECO ?

RECOver (RECO) is used to resolve distributed transactions that are pending due to a network or system failure in a distributed
database. At timed intervals,the local RECO attempts to connect to remote databases and automatically complete the commit or
rollback of the local portion of any pending distributed transactions.

86. What is the function of Dispatcher (Dnnn) ?

Dispatcher (Dnnn) process is responsible for routing requests from connected user processes to available shared server processes
and returning the responses back to the appropriate user processes.

87. How many Dispatcher Processes are created ?

Atleast one Dispatcher process is created for every communication protocol in use.

88. What is the function of Lock (LCKn) Process ?

Lock (LCKn) are used for inter-instance locking when the ORACLE Parallel Server option is used.

89. What is the maximum number of Lock Processes used ?

Though a single LCK process is sufficient for most Parallel Server systems
upto Ten Locks (LCK0,....LCK9) are used for inter-instance locking.
DATA ACCESS

90. Define Transaction ?

A Transaction is a logical unit of work that comprises one or more SQL statements executed by a single user.

91. When does a Transaction end ?

When it is committed or Rollbacked.

92. What does COMMIT do ?

COMMIT makes permanent the changes resulting from all SQL statements in the transaction. The changes made by the SQL
statements of a transaction become visible to other user sessions transactions that start only after transaction is committed.

93. What does ROLLBACK do ?

ROLLBACK retracts any of the changes resulting from the SQL statements in the transaction.

94. What is SAVE POINT ?

For long transactions that contain many SQL statements, intermediate markers or savepoints can be declared which can be used
to divide a transaction into smaller parts. This allows the option of later rolling back all work performed from the current point in
the transaction to a declared savepoint within the transaction.

95. What is Read-Only Transaction ?

A Read-Only transaction ensures that the results of each query executed in the transaction are consistant with respect to the same
point in time.

96. What is the function of Optimizer ?

The goal of the optimizer is to choose the most efficient way to execute a SQL statement.

97. What is Execution Plan ?

The combinations of the steps the optimizer chooses to execute a statement is called an execution plan.

98. What are the different approaches used by Optimizer in choosing an execution plan ?

Rule-based and Cost-based.

99. What are the factors that affect OPTIMIZER in choosing an Optimization approach ?

The OPTIMIZER_MODE initialization parameter Statistics in the Data Dictionary the OPTIMIZER_GOAL parameter of the
ALTER SESSION command hints in the statement.

100. What are the values that can be specified for OPTIMIZER MODE Parameter ?

COST and RULE.

101. Will the Optimizer always use COST-based approach if OPTIMIZER_MODE is set to Cost'?

Presence of statistics in the data dictionary for atleast one of the tables accessed by the SQL statements is necessary for the
OPTIMIZER to use COST-based approach. Otherwise OPTIMIZER chooses RULE-based approach.

102. What is the effect of setting the value of OPTIMIZER_MODE to 'RULE' ?

This value causes the optimizer to choose the rule_based approach for all SQL statements issued to the instance regardless of the
presence of statistics.

103. What are the values that can be specified for OPTIMIZER_GOAL parameter of the ALTER SESSION Command ?

CHOOSE,ALL_ROWS,FIRST_ROWS and RULE.

104. What is the effect of setting the value CHOOSE for OPTIMIZER_GOAL, parameter of the ALTER SESSION

Command ?

The Optimizer chooses Cost_based approach and optimizes with the goal of best throughput if statistics for atleast one of the
tables accessed by the SQL statement exist in the data dictionary. Otherwise the OPTIMIZER chooses RULE_based approach.

105. What is the effect of setting the value ALL_ROWS for OPTIMIZER_GOAL parameter of the ALTER SESSION

command ?

This value causes the optimizer to the cost-based approach for all SQL statements in the session regardless of the presence of
statistics and to optimize with a goal of best throughput.

106. What is the effect of setting the value 'FIRST_ROWS' for OPTIMIZER_GOAL parameter of the ALTER SESSION

command ?

This value causes the optimizer to use the cost-based approach for all SQL statements in the session regardless of the presence of
statistics and to optimize with a goal of best response time.

107. What is the effect of setting the 'RULE' for OPTIMIER_GOAL parameter of the ALTER SESSION Command ?

This value causes the optimizer to choose the rule-based approach for all SQL statements in a session regardless of the presence
of statistics.

108. What is RULE-based approach to optimization ?

Choosing an executing planbased on the access paths available and the ranks of these access paths.

109. What is COST-based approach to optimization ?

Considering available access paths and determining the most efficient execution plan based on statistics in the data dictionary for
the tables accessed by the statement and their associated clusters and indexes.
PROGRAMMATIC CONSTRUCTS

110. What are the different types of PL/SQL program units that can be defined and stored in ORACLE database ?

Procedures and Functions,Packages and Database Triggers.

111. What is a Procedure ?

A Procedure consist of a set of SQL and PL/SQL statements that are grouped together as a unit to solve a specific problem or
perform a set of related tasks.

112. What is difference between Procedures and Functions ?

A Function returns a value to the caller where as a Procedure does not.

113. What is a Package ?

A Package is a collection of related procedures, functions, variables and other package constructs together as a unit in the
database.

114. What are the advantages of having a Package ?

Increased functionality (for example,global package variables can be declared and used by any proecdure in the package) and
performance (for example all objects of the package are parsed compiled, and loaded into memory once)

115. What is Database Trigger ?

A Database Trigger is procedure (set of SQL and PL/SQL statements) that is automatically executed as a result of an insert
in,update to, or delete from a table.

116. What are the uses of Database Trigger ?

Database triggers can be used to automatic data generation, audit data modifications, enforce complex Integrity constraints, and
customize complex security authorizations.

117. What are the differences between Database Trigger and Integrity constraints ?

A declarative integrity constraint is a statement about the database that is always true. A constraint applies to existing data in the
table and any statement that manipulates the table.
A trigger does not apply to data loaded before the definition of the trigger, therefore, it does not guarantee all data in a table
conforms to the rules established by an associated trigger.
A trigger can be used to enforce transitional constraints where as a declarative integrity constraint cannot be used.
DATABASE SECURITY

118. What are Roles ?

Roles are named groups of related privileges that are granted to users or other roles.

119. What are the use of Roles ?

REDUCED GRANTING OF PRIVILEGES - Rather than explicitly granting the same set of privileges to many users a database
administrator can grant the privileges for a group of related users granted to a role and then grant only the role to each member of
the group.
DYNAMIC PRIVILEGE MANAGEMENT - When the privileges of a group must change, only the privileges of the role need to
be modified. The security domains of all users granted the group's role automatically reflect the changes made to the role.
SELECTIVE AVAILABILITY OF PRIVILEGES - The roles granted to a user can be selectively enable (available for use) or
disabled (not available for use). This allows specific control of a user's privileges in any given situation.
APPLICATION AWARENESS - A database application can be designed to automatically enable and disable selective roles
when a user attempts to use the application.

120. How to prevent unauthorized use of privileges granted to a Role ?

By creating a Role with a password.

121. What is default tablespace ?

The Tablespace to contain schema objects created without specifying a tablespace name.

122. What is Tablespace Quota ?

The collective amount of disk space available to the objects in a schema on a particular tablespace.

123. What is a profile ?

Each database user is assigned a Profile that specifies limitations on various system resources available to the user.

124. What are the system resources that can be controlled through Profile ?

The number of concurrent sessions the user can establish the CPU processing time available to the user's session the CPU
processing time available to a single call to ORACLE made by a SQL statement the amount of logical I/O available to the user's
session the amout of logical I/O available to a single call to ORACLE made by a SQL statement the allowed amount of idle time
for the user's session the allowed amount of connect time for the user's session.

125. What is Auditing ?

Monitoring of user access to aid in the investigation of database use.

126. What are the different Levels of Auditing ?

Statement Auditing, Privilege Auditing and Object Auditing.

127. What is Statement Auditing ?

Statement auditing is the auditing of the powerful system privileges without regard to specifically named objects.

128. What is Privilege Auditing ?

Privilege auditing is the auditing of the use of powerful system privileges without regard to specifically named objects.

129. What is Object Auditing ?

Object auditing is the auditing of accesses to specific schema objects without regard to user.
DISTRIBUTED PROCESSING AND DISTRIBUTED DATABASES

130. What is Distributed database ?

A distributed database is a network of databases managed by multiple database servers that appears to a user as single logical
database. The data of all databases in the distributed database can be simultaneously accessed and modified.

131. What is Two-Phase Commit ?

Two-phase commit is mechanism that guarantees a distributed transaction either commits on all involved nodes or rolls back on all
involved nodes to maintain data consistency across the global distributed database. It has two phase, a Prepare Phase and a
Commit Phase.

132. Describe two phases of Two-phase commit ?

Prepare phase - The global coordinator (initiating node) ask a participants to prepare (to promise to commit or rollback the
transaction, even if there is a failure)
Commit - Phase - If all participants respond to the coordinator that they are prepared, the coordinator asks all nodes to commit
the transaction, if all participants cannot prepare, the coordinator asks all nodes to roll back the transaction.

133. What is the mechanism provided by ORACLE for table replication ?

Snapshots and SNAPSHOT LOGs

134. What is a SNAPSHOT ?

Snapshots are read-only copies of a master table located on a remote node which is periodically refreshed to reflect changes
made to the master table.

135. What is a SNAPSHOT LOG ?

A snapshot log is a table in the master database that is associated with the master table. ORACLE uses a snapshot log to track
the rows that have been updated in the master table. Snapshot logs are used in updating the snapshots based on the master table.

136. What is a SQL * NET?

SQL *NET is ORACLE's mechanism for interfacing with the communication protocols used by the networks that facilitate
distributed processing and distributed databases. It is used in Clint-Server and Server-Server communications.
DATABASE OPERATION, BACKUP AND RECOVERY

137. What are the steps involved in Database Startup ?

Start an instance, Mount the Database and Open the Database.

138. What are the steps involved in Database Shutdown ?

Close the Database, Dismount the Database and Shutdown the Instance.

139. What is Restricted Mode of Instance Startup ?

An instance can be started in (or later altered to be in) restricted mode so that when the database is open connections are limited
only to those whose user accounts have been granted the RESTRICTED SESSION system privilege.

140. What are the different modes of mounting a Database with the Parallel Server ?

Exclusive Mode If the first instance that mounts a database does so in exclusive mode, only that Instance can mount the database.
Parallel Mode If the first instance that mounts a database is started in parallel mode, other instances that are started in parallel
mode can also mount the database.

141. What is Full Backup ?

A full backup is an operating system backup of all data files, on-line redo log files and control file that constitute ORACLE
database and the parameter.

142. Can Full Backup be performed when the database is open ?

No.

143. What is Partial Backup ?

A Partial Backup is any operating system backup short of a full backup, taken while the database is open or shut down.

144.WhatisOn-lineRedoLog?

The On-line Redo Log is a set of tow or more on-line redo files that record all committed changes made to the database.
Whenever a transaction is committed, the corresponding redo entries temporarily stores in redo log buffers of the SGA are written
to an on-line redo log file by the background process LGWR. The on-line redo log files are used in cyclical fashion.

145. What is Mirrored on-line Redo Log ?

A mirrored on-line redo log consists of copies of on-line redo log files physically located on separate disks, changes made to one
member of the group are made to all members.

146. What is Archived Redo Log ?

Archived Redo Log consists of Redo Log files that have archived before being reused.

147. What are the advantages of operating a database in ARCHIVELOG mode over operating it in NO ARCHIVELOG mode ?

Complete database recovery from disk failure is possible only in ARCHIVELOG mode.
Online database backup is possible only in ARCHIVELOG mode.

148. What is Log Switch ?

The point at which ORACLE ends writing to one online redo log file and begins writing to another is called a log switch.

149. What are the steps involved in Instance Recovery ?

R_olling forward to recover data that has not been recorded in data files, yet has been recorded in the on-line redo log, including
the contents of rollback segments.
Rolling back transactions that have been explicitly rolled back or have not been committed as indicated by the rollback segments
regenerated in step a.
Releasing any resources (locks) held by transactions in process at the time of the failure.
Resolving any pending distributed transactions undergoing a two-phase commit at the time of the instance failure.
Data Base Administration
Introduction to DBA

Os Interview questions

1. Explain the concept of Reentrancy?

It is a useful, memory-saving technique for multiprogrammed timesharing systems. A Reentrant Procedure is one in which multiple users can share a single copy of a program during the same period. Reentrancy has 2 key aspects: The program code cannot modify itself, and the local data for each user process must be stored separately. Thus, the permanent part is the code, and the temporary part is the pointer back to the calling program and local variables used by that program. Each execution instance is called activation. It executes the code in the permanent part, but has its own copy of local variables/parameters. The temporary part associated with each activation is the activation record. Generally, the activation record is kept on the stack.
Note: A reentrant procedure can be interrupted and called by an interrupting program, and still execute correctly on returning to the procedure.

2. Explain Belady's Anomaly?

Also called FIFO anomaly. Usually, on increasing the number of frames allocated to a process virtual memory, the process execution is faster, because fewer page faults occur. Sometimes, the reverse happens, i.e., the execution time increases even when more frames are allocated to the process. This is Belady's Anomaly. This is true for certain page reference patterns.

3. What is a binary semaphore? What is its use?

A binary semaphore is one, which takes only 0 and 1 as values. They are used to implement mutual exclusion and synchronize concurrent processes.

4. What is thrashing?

It is a phenomenon in virtual memory schemes when the processor spends most of its time swapping pages, rather than executing instructions. This is due to an inordinate number of page faults.

5. List the Coffman's conditions that lead to a deadlock.

   1. Mutual Exclusion: Only one process may use a critical resource at a time.
   2. Hold & Wait: A process may be allocated some resources while waiting for others.
   3. No Pre-emption: No resource can be forcible removed from a process holding it.
   4. Circular Wait: A closed chain of processes exist such that each process holds at least one resource needed by another process in the chain.

6. What are short, long and medium-term scheduling?

Long term scheduler determines which programs are admitted to the system for processing. It controls the degree of multiprogramming. Once admitted, a job becomes a process.

Medium term scheduling is part of the swapping function. This relates to processes that are in a blocked or suspended state. They are swapped out of real-memory until they are ready to execute. The swapping-in decision is based on memory-management criteria.

Short term scheduler, also know as a dispatcher executes most frequently, and makes the finest-grained decision of which process should execute next. This scheduler is invoked whenever an event occurs. It may lead to interruption of one process by preemption.
7. What are turnaround time and response time?

Turnaround time is the interval between the submission of a job and its completion. Response time is the interval between submission of a request, and the first response to that request.

8. What are the typical elements of a process image?

User data: Modifiable part of user space. May include program data, user stack area, and programs that may be modified.

User program: The instructions to be executed.

System Stack: Each process has one or more LIFO stacks associated with it. Used to store parameters and calling addresses for procedure and system calls.

Process control Block (PCB): Info needed by the OS to control processes.

9. What is the Translation Lookaside Buffer (TLB)?

In a cached system, the base addresses of the last few referenced pages is maintained in registers called the TLB that aids in faster lookup. TLB contains those page-table entries that have been most recently used. Normally, each virtual memory reference causes 2 physical memory accesses- one to fetch appropriate page-table entry, and one to fetch the desired data. Using TLB in-between, this is reduced to just one physical memory access in cases of TLB-hit.

10. What is the resident set and working set of a process?

Resident set is that portion of the process image that is actually in real-memory at a particular instant. Working set is that subset of resident set that is actually needed for execution. (Relate this to the variable-window size method for swapping techniques.)

11. When is a system in safe state?

The set of dispatchable processes is in a safe state if there exists at least one temporal order in which all processes can be run to completion without resulting in a deadlock.

12. What is cycle stealing?

We encounter cycle stealing in the context of Direct Memory Access (DMA). Either the DMA controller can use the data bus when the CPU does not need it, or it may force the CPU to temporarily suspend operation. The latter technique is called cycle stealing. Note that cycle stealing can be done only at specific break points in an instruction cycle.

13. What is meant by arm-stickiness?

If one or a few processes have a high access rate to data on one track of a storage disk, then they may monopolize the device by repeated requests to that track. This generally happens with most common device scheduling algorithms (LIFO, SSTF, C-SCAN, etc). High-density multisurface disks are more likely to be affected by this than low density ones.

14. What are the stipulations of C2 level security?

C2 level security provides for:

   1. Discretionary Access Control
   2. Identification and Authentication
   3. Auditing
   4. Resource reuse

15. What is busy waiting?

The repeated execution of a loop of code while waiting for an event to occur is called busy-waiting. The CPU is not engaged in any real productive activity during this period, and the process does not progress toward completion.

16. Explain the popular multiprocessor thread-scheduling strategies.

   1. Load Sharing: Processes are not assigned to a particular processor. A global queue of threads is maintained. Each processor, when idle, selects a thread from this queue. Note that load balancing refers to a scheme where work is allocated to processors on a more permanent basis.
   2. Gang Scheduling: A set of related threads is scheduled to run on a set of processors at the same time, on a 1-to-1 basis. Closely related threads / processes may be scheduled this way to reduce synchronization blocking, and minimize process switching. Group scheduling predated this strategy.
   3. Dedicated processor assignment: Provides implicit scheduling defined by assignment of threads to processors. For the duration of program execution, each program is allocated a set of processors equal in number to the number of threads in the program. Processors are chosen from the available pool.
   4. Dynamic scheduling: The number of thread in a program can be altered during the course of execution.

17. When does the condition 'rendezvous' arise?

In message passing, it is the condition in which, both, the sender and receiver are blocked until the message is delivered.

18. What is a trap and trapdoor?

Trapdoor is a secret undocumented entry point into a program used to grant access without normal methods of access authentication. A trap is a software interrupt, usually the result of an error condition.

19. What are local and global page replacements?

Local replacement means that an incoming page is brought in only to the relevant process address space. Global replacement policy allows any page frame from any process to be replaced. The latter is applicable to variable partitions model only.

20. Define latency, transfer and seek time with respect to disk I/O.

Seek time is the time required to move the disk arm to the required track. Rotational delay or latency is the time it takes for the beginning of the required sector to reach the head. Sum of seek time (if any) and latency is the access time. Time taken to actually transfer a span of data is transfer time.

21. Describe the Buddy system of memory allocation.

Free memory is maintained in linked lists, each of equal sized blocks. Any such block is of size 2^k. When some memory is required by a process, the block size of next higher order is chosen, and broken into two. Note that the two such pieces differ in address only in their kth bit. Such pieces are called buddies. When any used block is freed, the OS checks to see if its buddy is also free. If so, it is rejoined, and put into the original free-block linked-list.

22. What is time-stamping?

It is a technique proposed by Lamport, used to order events in a distributed system without the use of clocks. This scheme is intended to order events consisting of the transmission of messages. Each system 'i' in the network maintains a counter Ci. Every time a system transmits a message, it increments its counter by 1 and attaches the time-stamp Ti to the message. When a message is received, the receiving system 'j' sets its counter Cj to 1 more than the maximum of its current value and the incoming time-stamp Ti. At each site, the ordering of messages is determined by the following rules: For messages x from site i and y from site j, x precedes y if one of the following conditions holds....(a) if Ti<Tj or (b) if Ti=Tj and i<j.

23. How are the wait/signal operations for monitor different from those for semaphores?

If a process in a monitor signal and no task is waiting on the condition variable, the signal is lost. So this allows easier program design. Whereas in semaphores, every operation affects the value of the semaphore, so the wait and signal operations should be perfectly balanced in the program.

24. In the context of memory management, what are placement and replacement algorithms?

Placement algorithms determine where in available real-memory to load a program. Common methods are first-fit, next-fit, best-fit. Replacement algorithms are used when memory is full, and one process (or part of a process) needs to be swapped out to accommodate a new program. The replacement algorithm determines which are the partitions to be swapped out.

25. In loading programs into memory, what is the difference between load-time dynamic linking and run-time dynamic linking?

For load-time dynamic linking: Load module to be loaded is read into memory. Any reference to a target external module causes that module to be loaded and the references are updated to a relative address from the start base address of the application module.

With run-time dynamic loading: Some of the linking is postponed until actual reference during execution. Then the correct module is loaded and linked.

26. What are demand-paging and pre-paging?

With demand paging, a page is brought into memory only when a location on that page is actually referenced during execution. With pre-paging, pages other than the one demanded by a page fault are brought in. The selection of such pages is done based on common access patterns, especially for secondary memory devices.

27. Paging a memory management function, while multiprogramming a processor management function, are the two interdependent?

Yes.

28. What is page cannibalizing?

Page swapping or page replacements are called page cannibalizing.

29. What has triggered the need for multitasking in PCs?

   1. Increased speed and memory capacity of microprocessors together with the support fir virtual memory and
   2. Growth of client server computing

30. What are the four layers that Windows NT have in order to achieve independence?

   1. Hardware abstraction layer
   2. Kernel
   3. Subsystems
   4. System Services.

31. What is SMP?

To achieve maximum efficiency and reliability a mode of operation known as symmetric multiprocessing is used. In essence, with SMP any process or threads can be assigned to any processor.

32. What are the key object oriented concepts used by Windows NT?

Encapsulation, Object class and instance.

33. Is Windows NT a full blown object oriented operating system? Give reasons.

No Windows NT is not so, because its not implemented in object oriented language and the data structures reside within one executive component and are not represented as objects and it does not support object oriented capabilities.

34. What is a drawback of MVT?

It does not have the features like

   1. ability to support multiple processors
   2. virtual storage
   3. source level debugging

35. What is process spawning?

When the OS at the explicit request of another process creates a process, this action is called process spawning.

36. How many jobs can be run concurrently on MVT?

15 jobs.
37. List out some reasons for process termination.

   1. Normal completion
   2. Time limit exceeded
   3. Memory unavailable
   4. Bounds violation
   5. Protection error
   6. Arithmetic error
   7. Time overrun
   8. I/O failure
   9. Invalid instruction
  10. Privileged instruction
  11. Data misuse
  12. Operator or OS intervention
  13. Parent termination.

38. What are the reasons for process suspension?

   1. swapping
   2. interactive user request
   3. timing
   4. parent process request

39. What is process migration?

It is the transfer of sufficient amount of the state of process from one machine to the target machine.

40. What is mutant?

In Windows NT a mutant provides kernel mode or user mode mutual exclusion with the notion of ownership.

41. What is an idle thread?

The special thread a dispatcher will execute when no ready thread is found.

42. What is FtDisk?

It is a fault tolerance disk driver for Windows NT.
43. What are the possible threads a thread can have?

   1. Ready
   2. Standby
   3. Running
   4. Waiting
   5. Transition
   6. Terminated

44. What are rings in Windows NT?

Windows NT uses protection mechanism called rings provides by the process to implement separation between the user mode and kernel mode.

45. What is Executive in Windows NT?

In Windows NT, executive refers to the operating system code that runs in kernel mode.

46. What are the sub-components of I/O manager in Windows NT?

   1. Network redirector/ Server
   2. Cache manager.
   3. File systems
   4. Network driver
   5. Device driver

47. What are DDks? Name an operating system that includes this feature.

DDks are device driver kits, which are equivalent to SDKs for writing device drivers. Windows NT includes DDks.

48. What level of security does Windows NT meets?

C2 level security

Yahoo coding problem 2: 2012

Yahoo coding problem 2: 2012

finding common prefix in array of strings

  

testcase1: "india"

                 "indka" 

                  "indus"

                  "indaa"

output:ind 

n strings and n length

timecomplexity :nlogn

solution:

#include<stdio.h>
#include<math.h>
int main()
{
    char a[4][6]={"india" "indka" "indus","indaa"};
        int i,j,k=0,l=0,p=3,m;
      for(m=0;m<log2(4);m++)
      {
          for(i=0;i<=p;)
            {
                for(j=0;j<5;)
                {
                    if(a[i][j]==a[i+1][j])
                    {              
                        a[k][l]=a[i][j];
                        l++;
                        j++;
                     }
                    else  j=5;
                }
                a[k][l]='\0';
                k++;
                i=i+2;
             }
            p=p/2;
            k=0;
        }
        printf("%s",a[0]);
      }

Yahoo coding problem1

Yahoo coding problem1:

Given string contains abcabcc i.e contains only a,b,c replace missing character from non two repeating characters if characters repeating simply move forword

i.e 

testcase1:abc->cc

testcase 2:abca->cca->cb->a 

 solution1:

#include<stdio.h>
#include<string.h>
int main()
{
 char a[]="aabc";
 int b[3]={0,0,0};
 int i,j=0,k=1,m,l=0,flag=1;
  while(a[j+1]!='\0' || k == 1 )
  { 
        k=0;
    getchar();
    printf("%s\n",a);
    printf(" out j = %d\n",j);
         m=0;l=0;k=0;
    for(j=0;j<strlen(a)&&m!=1;)
     {
          b[a[j]-'a']=1;
          b[a[j+1]-'a']=1;
          m=0;
         if(a[j]!=a[j+1])
             {
                  if(b[0]==0) l=0;
                  else if(b[1]==0) l=1;
                  else l=2;

                 a[j]=l+'a';
                 for(l=j+1;l<strlen(a);l++)
                  {
                    a[l]=a[l+1];
                  }
                 a[l]='\0';
                b[0]=0;b[1]=0;b[2]=0;
                m=1;
                ++j;
                printf("j = %d\n",j);
          
               //if(a[j+1] == '\0') flag=0;
          }  
       else
       {
             j++;
             if(a[j+1]=='\0')
                  m=1;
    
            b[0]=0;b[1]=0;b[2]=0;
        }
   
   }//for loop
   for(i=0;i<strlen(a)-1;i++)
        if(a[i] != a[i+1])
            k=1;
 }  //while
printf("%s",a);
}

solution2: 

#include<stdio.h>
#include<string.h>
int  hash[3]={0,};
int main()
{
    char a[]="aabc",miss,b[10]="";
    int i=0,k=0,j=0,t;

    while(a[i]!='\0' && a[i+1]!='\0')
    {  
                if(a[i]!=a[(i+1)])
        {
            hash[a[i]-'a']=1;
            hash[a[i+1]-'a']=1;
            for(i=0;i<3;i++)
            {
                if(hash[i]==0)
                {  
                    miss=i+'a';
                }
            }  
          
            i=k;
            b[k++]=miss;
                      
                         
            for(j=i+2;a[j]!='\0';j++)
            {
                b[k]=a[j];
                                k++;          
                        }
            strcpy(a,b);
            hash[0]=0;hash[1]=0;hash[2]=0;
            for(t=0;t<10;t++)
            {
                b[t]='\0';
            }
                        i=0;
                        k=0;
            j=0;
        }
        else
        {
            if(strlen(a)>2)
            {
                b[k]=a[i];
                    k++;
                i++;
            }
            else
            {
                break;
            }
        }
        //len=strlen(a);
        //printf("%c--%c--i=%d--j=%d--k=%d",a[i],a[i+1],i,j,k);
    }
    printf("\n%s",a);
    return 0;
}