unit 8: distributed DBMS reliability

Question 1

Topics included in this topic

Answer 1

reliability concepts
failures and fault tolerance
failures in Distributed DBMS
Local reliability protocol
Distributed reliability protocol
dealing with site failure
dealing with network partitioning

Question 2

what will happen if system has some inputs

Answer 2

system will produce some outputs

Question 3

what is realibality

Answer 3

reliability is nothing but a measure of how far a system is working

OR

It is a measure used to indicate how successful a system is in providing the service it was intended

Question 4

Reliable proper definition

Answer 4

A system is considered a reliable if it functions as per its specifications and produces a correct output values for a given set of input values

Question 5

what is purpose of reliability

Answer 5

purpose of reliability is, ki ham database ki atomicity and durability ko kaise maintain kar sake

jab ki dono, atomicity and durability transaction ki properties he

Question 6

what is reliable DBMS

Answer 6

a reliable DDBMS is the one which continues to process the user requests even when the system is unreliable i.e. even if the components of distributed computing fails, DDBMS should be able to continue execution of user request without violating database consistency

Question 7

what does reliability refers to ?

Answer 7

reliability refers to atomicity and durability of transaction.

Question 8

explain what is an availability ?

Answer 8

• the fraction of the time that a system meet its specification
• the probability that the system is operational at a given time t.

Question 9

which protocols address these issues of reliability

Answer 9

the protocol, which addresses these issues are commit & recovery protocol resp.

Question 10

what is meaning of failure

Answer 10

the failure occurs when system does not function according to specifications of a system

OR

the failure of a system occurs when the system does not function according to its specifications and fails to deliver the service for which it was intended

Question 11

what is erroneous state

Answer 11

the internal state of a system such that there exists circumstances in which further processing, by the normal algorithms of the system, will lead to a failure which is not attributed to a subsequent fault

===

koi internal matter jisse further ki processing nahi hoti he, unn kuch internal faults ya incorrect state ki vajah se aap failure ki taraf jaa sakte he

Question 12

What is an error

Answer 12

an error in the system occurs when a component of the system assumes a state that is not desirable the fact that the state is undesirable is a subjective judgeing

Question 13

what are the types of errors / how errors are distinguished

Answer 13

errors are distinguished as, those which are permanent and those which are non-permanent

Question 14

what is fault

Answer 14

a fault is detected either when an error is propagated from one component to another or failure of the component is observed

Question 15

fault to failure diagram

Answer 15

Fault [causes] Error [results in] Failure

Question 16

types of faults

Answer 16

Hard faults
Soft faults

Question 17

how hard faults are further divided

Answer 17

• permanent
• resulting failures are called hard failures

Question 18

explain soft faults

Answer 18

• transient of intermittent
• account for more than 90% of all failures
• resulting failures are called soft failures

Question 19

faults classification

Answer 19

three type of errors
1. permanent error
2. intermittent error
3. transient error

permanent fault and incorrect design causes, permanent error, leads to system failure

unstable or marginal components causes Internal error leads to system failure

unstable environment causes transient error leads to system failure

operator mistake leads to system failure

Question 20

what is fault-tolerant system

Answer 20

in addition to fault detection scheme, two system has redundant components and sub-system build in

on detection of a fault, these redundant components are used to replace the faulty components

Question 21

full form of MTBF

Answer 21

mean time between failure

Question 22

MTTR full form

Answer 22

mean time to repair

Question 23

types of failure in DBMS

Answer 23

1. hardware failures
2. software failures
3. storage medium failures
4. implementation of stable storage
5. communication failure
6. transaction failure

Question 24

what are hardware failures divided into further

Answer 24

• design errors
• poor quality control
• over utilization and overloading
• wear out

Question 25

system / site failure

Answer 25

- failures of processor, main memory, power supply - main memory contents are lost, but secondary storage contents are safe - partial vs total failure

Question 26

Software failures

Answer 26

- design errors - poor quality control - over utilization and overloading - wear out

Question 27

what are further storage medium failure divided into

Answer 27

- volatile storage - non-volatile storage - permanent or stable storage

Question 28

which type of failures are common in both environment

Answer 28

1. hardware failure 2. software failures 3. system failures this failures are common in both environment

Question 29

which failure is specific to distributed environment

Answer 29

- communication failure is specific to distributed environment only

Question 30

1. explain communication failure 2. what are types of communication failure

Answer 30

all previous failures are common in both environment but communication failure is specific to distributed environment 2. types of communication failure - error in messages - improper ordering of messages - lost messages - line failures

Question 31

which failures are handled by communication network

Answer 31

error in message and improper ordering of messages this errors are handled by communication software

Question 32

what is responsibility of network software and hardware

Answer 32

network hardware and software are responsible for for ensuring that the messages reach from source to destination correctly & in order

Question 33

how messages are lost

Answer 33

messages are lost due to line or site failures if communication link fails, the messages are lost but in addition network might get divided in disjoint parts, called as network partitions

Question 34

what is network partitions

Answer 34

if communication link fails, the messages are lost but in addition network might get divided in disjoint parts, called as network partitions

Question 35

how this network partitions create problem

Answer 35

if network gets partition, then sites in each partition can keep working but if a transaction tries to access data from two or more partitions, it can create problem - thus maintaining mutual consistency is a problem when database is replicated

Question 36

whose responsibility it is to handle loss of messages

Answer 36

handling loss of messages is the responsibility of network software

Question 37

what is performance failure

Answer 37

failure of communication network to deliver & receive messages in certain time periods is called performance Failure

Question 38

media failures

Answer 38

- failures of secondary storage devices such that the stored data is lost - head crash / controller failure

Question 39

communication failures

Answer 39

- lost / undeliverable messages - network partitioning

Question 40

explain implementation of stable storage

Answer 40

- writing same block of data from volatile to stable storage in multiple for two or more times ensuring successful identified block writing

Question 41

what is audit trails

Answer 41

audit trail is one record which is generated for each and every transaction and regarding the transaction it keeps certain information the information it stores like - who has initiated it - when it has been initiated - what is purpose - where it has initiated

Question 42

audit trail

Answer 42

the audit trail records who ( user or the application program and a transaction number ), when ( time and data ), from where ( location of the data affected, as well as a before and after image of that portion of the database that was affected by the update operation ) in addition, a DBMS contains routines that make a backup copy of the data that is modified, this is done by taking a snapshot of the before and after image of that portion of the database that is modified. For obvious reasons, the backup are produced on a separate storage medium.

Question 43

Explain local reliability protocols

Question 44

what is Local Recovery Manager

Answer 44

LRM is a module of DBMS, which exists at each of the site.

Question 45

what is function of Local recovery manager

Answer 45

Its function is to maintain atomicity and durability of local transactions.

Question 46

what are commands handle by local recovery manager

Answer 46

the commands it handles are - begin transaction - read - write - commit - abort

Question 47

architecture of local reliability protocol

Answer 47

secondary storage - stable DB Main memory - local recovery manager [ fetch / flush ] - database buffer manager [ write / read ] - database buffers volatile memory

Question 48

what is Stable Database

Answer 48

the database, stored on stable storage device is known as stable database

Question 49

what is volatile database

Answer 49

the data loaded in internal memory is called as volatile database

Question 50

When does LRM issues fetch command

Answer 50

when Local Recovery Manager ( LRM ) wants to read a page on the behalf of transaction, i tissues Fetch command specifying page number to buffer manager

Question 51

What is use of Flush command

Answer 51

LRM can also force the buffer manager to write the page on to disk. This can be done using Flush command

Question 52

how buffer manager responds to fetch command

Answer 52

buffer manager responds to fetch command in following manner - searches buffer pool for required page - if not found allocates a free buffer page and loads required database page into it - if no free buffer is available, selects a buffer, vacates it & uses to load the page from stable database

Question 53

how allocation of buffer pages is done

Answer 53

allocation of buffer pages is done dynamically

Question 54

how buffer manager allocates pages to process

Answer 54

buffer manager finds out number of pages each process will need and accordingly attempts to allocate those many pages to each process

Question 55

which is best known technique to replace buffer pages

Answer 55

to replace buffer pages the best known technique is Least recently used algorithm

Question 56

what is recovery informatoin

Answer 56

when system fails it lost all volatile data and system must maintain some information about its state at the time of failure to restore consistency this is called recovery information https://www.youtube.com/watch?v=eq2EMu1Mh-w

Question 57

on what methods recovery information depends

Answer 57

- in place update - out of place update

Question 58

reliability issues are simpler if out of place updates are used. but most of the systems use in-place updates because of its efficiency

Question 59

what are in-place update

Question 60

out of place update

Question 61

explain shadowing

Answer 61

- when update is done, this creates duplicate page called shadow page - https://www.youtube.com/watch?v=YA0sXVDoHig

Question 62

what is differential file

Answer 62

https://www.youtube.com/watch?v=1xX68YYAMAM&t=147s

Question 63

execution of LRM commands

Question 64

log based database recovery

Answer 64

https://www.youtube.com/watch?v=0_DnBLn3nqg

Question 65

explain redo and undo in recovery of database

Answer 65

https://www.youtube.com/watch?v=NzQetfezwp0

Question 66

what is primary copy

Answer 66

https://youtu.be/RsudXqML-M8?list=PLV8vIYTIdSnbAW2wj_TiHyrFJId5zkhz2 -

Question 67

what is primary site

Question 68

check pointing

Answer 68

- https://www.youtube.com/watch?v=cQHriQKfA_c

Question 69

what is majority locking

Answer 69

https://youtu.be/RsudXqML-M8?list=PLV8vIYTIdSnbAW2wj_TiHyrFJId5zkhz2

Question 70

what is timestamping

Answer 70

- idea is that each transaction in the system is assigned a unique timestamp to determine the serialization order

Question 71

how timestamping works in centralized scheme

Question 72

how timestamping works in distributed scheme

Question 77

2 Phase commit protocol write a short note on two phase commit protocol

Answer 77

- two phase commit protocol is used in distributed database systems - it is basically used as a recovery system in database - this has two phases 1. voting phase 2. decision phase there are two different type of sites - participant site - coordinator site in voting, participant vote that they want abort or commit in decision phase, coordinator site decides whether the transaction is completed or aborted - suppose we have transaction T1, which is started at site S1 - where transaction is started is known as coordinator site - this transaction is running on 3 different sites, S2, S3 and S4 - all other sites, where transaction is running is known as participating sites Now who will vote and who will participate - S2, S3 and S4 are participating sites, inke voting ke aadhar par S1 decide karega ki isko commit karna he ya abort karna he - kyunki S1 is coordinator site

Question 78

how voting take place

Answer 78

- voting me jo bhi coordinator site he, S1 iss case me, wo apne log record me entry karta he jisko bolte he transaction T prepare [T, prepare] - iska matlab hota he ki this site is ready to commit or abort the transaction [ Ready to commit ] - aaur ye S1 wait kar rahi he ki baki ki sites commit karengi ya abort karengi - jab apne log me entry kar lega S1, tab baki ki sites ko message send karega S2 s3 and s4 ko ki, [T, prepare] - agar S2 ready he to wo apne log record me likh dega, [ T, ready ] - agar S3 ready he to [ T, ready ] - S4 [T, not-ready ] === depending on this votes S1 will decide whethere to commit or abort this transaction

Question 79

decision phase of 2 phase commit

Answer 79

decision phase ke andar do rules hote he, 1. agar sabse ready milta he, [ Ready, T ] message then commit 2. if atleast one [ not-ready, T ] then abort the transaction S1 will abort transaction if any one of participant has voted for not-ready

Question 80

why 2 phase name like this

Answer 80

there are two phases 1. voting phase 2. decision phase