The document discusses distributed database systems. It defines distributed databases as consisting of loosely coupled sites that share no physical components but appear as a single system to users. Processing can occur at sites other than where requests originate. The key aspects covered are: - Distributed databases connect multiple independent databases that cooperate to process requests. - Heterogeneous distributed databases can use different schemas and software, complicating query and transaction processing. - Data can be distributed through replication of copies across sites or fragmentation of relations into portions stored at different sites. - Commit protocols like two-phase commit are used to ensure atomicity of transactions occurring across multiple sites in distributed databases.

1. R.BOOMADEVI.,M.C.A.,M.E
[A/P] CSE DEPARTMENT
CHRIST THE KING ENGINEERING COLLEGE
COIMBATORE

2.  A distributed database system consists of
loosely coupled sites that share no physical
component
 Appears to user as a single system
 Database systems that run on each site are
independent of each other
 Processing maybe done at a site other than the
initiator of request

3.  All sites have identical software
 They are aware of each other and agree to cooperate in
processing user requests
 It appears to user as a single system

4.  A distributed system connects three databases: hq, mfg, and sales
 An application can simultaneously access or modify the data in several
databases in a single distributed environment.

5.  In a heterogeneous distributed
database system, at least one of the
databases uses different schemas and
software.
 A database system having different schema may cause a
major problem for query processing.
 A database system having different software may cause a
major problem for transaction processing.

6.  Replication
◦ System maintains multiple copies of data, stored in
different sites, for faster retrieval and fault tolerance.
 Fragmentation
◦ Relation is partitioned into several fragments stored in
distinct sites
Replication and fragmentation can be combined
• Relation is partitioned into several fragments: system
maintains several identical replicas of each such
fragment.

7. Availability: failure of site containing relation r does
not result in unavailability of r is replicas exist.
Parallelism: queries on r may be processed by several
nodes in parallel.
Reduced data transfer: relation r is available locally
at each site containing a replica of r.

8. Increased cost of updates: each replica of relation r
must be updated.
Increased complexity of concurrency control:
concurrent updates to distinct replicas may lead to
inconsistent data unless special concurrency control
mechanisms are implemented.
 One solution: choose one copy as primary copy and apply
concurrency control operations on primary copy.

9.  Data can be distributed by storing individual
tables at different sites
 Data can also be distributed by decomposing a
table and storing portions at different sites –
called Fragmentation
 Fragmentation can be horizontal or vertical

10.  Usage - in general applications use views so it’s appropriate to
work with subsets
 Efficiency - data stored close to where it is most frequently used
 Parallelism - a transaction can divided into several sub-queries to
increase degree of concurrency
 Security - data more secure - only stored where it is needed
Disadvantages:
Performance - may be slower
Integrity - more difficult

11.  Each fragment, Ti , of table T contains a
subset of the rows
 Each tuple of T is assigned to one or more
fragments.
 Horizontal fragmentation is lossless

12.  A bank account schema has a relation
Account-schema = (branch-name, account-number, balance).
 It fragments the relation by location and stores each fragment
locally: rows with branch-name = `Hillside` are stored in the Hillside
in a fragment

13.  Each fragment, Ti, of T contains a subset of the
columns, each column is in at least one fragment,
and each fragment includes the key:
Ti = Πattr_listi
(T)
T = T1 T2 ….. Tn
All schemas must contain a common candidate key (or
superkey) to ensure lossless join property.
A special attribute, the tuple-id attribute may be added to
each schema to serve as a candidate key.

14.  A employee-info schema has a relation
employee-info schema = (designation, name,
Employee-id, salary).
 It fragments the relation to put information in two
tables for security concern.

15.  Commit protocols are used to ensure
atomicity across sites
 Atomicity states that database modifications must
follow an “all or nothing” rule.
 a transaction which executes at multiple sites must
either be committed at all the sites, or aborted at all the
sites.

16.  What is this?
 Two-phase commit is a transaction protocol designed
for the complications that arise with distributed
resource managers.
 Two-phase commit technology is used for hotel and
airline reservations, stock market transactions, banking
applications, and credit card systems.
 With a two-phase commit protocol, the distributed
transaction manager employs a coordinator to manage
the individual resource managers. The commit process
proceeds as follows:

17.  Step 1  Coordinator asks all participants to
prepare to commit transaction Ti.
 Ci adds the records <prepare T> to the log and forces
log to stable storage (a log is a file which maintains a
record of all changes to the database)
 sends prepare T messages to all sites where T
executed

18.  Step 2  Upon receiving message, transaction
manager at site determines if it can commit the
transaction
 if not:
add a record <no T> to the log and send abort T
message to Ci
 if the transaction can be committed, then:
1). add the record <ready T> to the log
2). force all records for T to stable storage
3). send ready T message to Ci

19.  Step 1  T can be committed of Ci received a ready T
message from all the participating sites: otherwise T
must be aborted.
 Step 2  Coordinator adds a decision record,
<commit T> or <abort T>, to the log and forces record
onto stable storage. Once the record is in stable storage,
it cannot be revoked (even if failures occur)
 Step 3  Coordinator sends a message to each
participant informing it of the decision (commit or abort)
 Step 4  Participants take appropriate action locally.

21.  There have been two performance issues with two
phase commit:
◦ If one database server is unavailable, none of the
servers gets the updates.
◦ This is correctable through network tuning and correctly
building the data distribution through database
optimization techniques.

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