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HubUnit 1 — Introduction to DBMS & Entity-Relationship ModelAdvantages and Disadvantages of DBMSAggregationCharacteristics of DBMSData IndependenceData ModelsDatabase Users and AdministratorsDBMS LanguagesER DiagramsEntity-Relationship ER ModelIntroduction to DBMSKeys in DBMSRelationships and CardinalitySchema and InstanceSpecialization and GeneralizationUnit 1 Summary — Introduction to DBMS & ER ModelThree-Level Architecture ANSI/SPARC ArchitectureWeak Entity SetUnit 2 — Relational Model & SQLConstraints in SQLCursorsDCL and TCL CommandsDDL CommandsDML CommandsDomain Relational Calculus DRCFunctions and Stored ProceduresJoins in SQLNested Queries SubqueriesRelational AlgebraRelational CalculusRelational ModelSQL IntroductionUnit 2 Summary — Relational Model & SQLTriggers in SQLTuple Relational Calculus TRCViews in SQLArmstrong's Axioms & Closure of AttributesBoyce-Codd Normal Form BCNFClosure of AttributesDependency PreservationFifth Normal Form 5NFFirst Normal Form 1NFFourth Normal Form 4NFFunctional DependencyLossless DecompositionMultivalued DependencyNormal Forms — OverviewNormalization — IntroductionSecond Normal Form 2NFUnit 3 Summary — NormalizationThird Normal Form 3NFUnit 4 — Transaction Management & Concurrency ControlACID PropertiesCheckpointsConcurrency Control & SerializabilityDeadlock in DBMSLock-Based Protocols & Two-Phase LockingLog-Based RecoveryRecovery SystemSerializabilityShadow PagingUnit 4 Summary — Transaction Management & Concurrency ControlTimestamp-Based ProtocolTransaction ManagementTransaction StatesTwo-Phase Locking 2PLUnit 5 — Storage & IndexingB+ TreeB-TreeCost EstimationDynamic HashingFile OrganizationHashing TechniquesIndexing — IntroductionMulti-Level IndexingQuery OptimizationQuery ProcessingSingle-Level IndexingStatic HashingUnit 5 Summary — Storage & IndexingUnit 6 — Advanced Topics in DBMSBig Data and DBMSCloud DatabaseData MiningData WarehouseDatabase SecurityDistributed DatabaseDistributed Query ProcessingDistributed DBMS Architecture, Fragmentation & ReplicationMongoDB — IntroductionNoSQL DatabaseReplication in Distributed DatabasesUnit 6 Summary — Advanced Topics in DBMSViva Voce — DBMSImportant Viva Questions — DBMSInterview Questions — DBMSViva Answers — DBMSDBMS Multiple Choice Questions MCQsDBMS Lab Programs
Replication in Distributed Databases
Last Updated : 21 May, 2026
Replication is maintaining identical copies of data fragments or entire relations at more than one site in a distributed database system.
Unit 681 words8 headingsExamples included
Definition
Replication is maintaining identical copies of data (fragments or entire relations) at more than one site in a distributed database system.
Why Replication?
| Problem 1 | Availability |
| If data is at only one site and that site fails | data unavailable. |
| Problem 2 | Performance |
| Users in Chennai querying data stored only in Delhi | slow (network latency). |
| Replicate frequently-read data near users | fast local access. |
Replication Models
Synchronous Replication (Eager)
All replicas are updated simultaneously as part of the same transaction.
Code exampleWoHoTech
Transaction UPDATE Employee WHERE emp_id = 101:
1. Update replica at Site A
2. Update replica at Site B ← must complete before commit
3. Update replica at Site C ← must complete before commit
4. COMMIT (only if ALL sites confirm)
Advantage: All replicas always in sync (strong consistency)
Disadvantage: High latency; transaction blocked if any site is slow/downAsynchronous Replication (Lazy)
The primary site is updated first; replicas are updated later in the background.
| Transaction commits at primary site | DONE |
| Background process propagates update to replicas (delay | milliseconds to minutes) |
| Advantage | Fast commit; high availability |
| Disadvantage | Replicas temporarily stale (eventual consistency) |
| Used in | MySQL replication, read replicas in cloud databases |
Replication Topologies
| Used in | MySQL, PostgreSQL streaming replication |
| Used in | CouchDB, Cassandra, MySQL Group Replication |
| Used in | Cassandra (replication factor N) |
Conflict Resolution
In multi-master replication, two sites may update the same record simultaneously:
| Site A updates Employee E001 salary: 75000 | 80000 |
| Site B updates Employee E001 salary: 75000 | 78000 (simultaneously) |
| - Last-Write-Wins (LWW) | timestamp determines winner |
| - Application-level | application logic decides |
| - Manual | DBA resolves conflicts |
| - Vector clocks | track causal order of updates |
Replication in SQL (MySQL Example)
-- On Primary (master) server:
-- Enable binary logging in my.cnf:
-- [mysqld]
-- log_bin = mysql-bin
-- server_id = 1
-- Create replication user
CREATE USER 'replicator'@'%' IDENTIFIED BY 'replpass';
GRANT REPLICATION SLAVE ON *.* TO 'replicator'@'%';
-- On Replica (slave) server:
CHANGE MASTER TO
MASTER_HOST='192.168.1.10',
MASTER_USER='replicator',
MASTER_PASSWORD='replpass',
MASTER_LOG_FILE='mysql-bin.000001',
MASTER_LOG_POS=154;
START SLAVE;
SHOW SLAVE STATUS\G
Exam Focus
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