Sonali & Janata Bank Ltd. Assistant Database Administrator Exam: 2022প্রশ্ন ও সমাধান

A Distributed Database (DDB) is a single logical database whose data is physically stored across multiple, geographically dispersed computers (sites) connected by a network, but which is managed by a Distributed DBMS (DDBMS) so that users see it as one integrated database rather than many separate ones.

• Fragmentation: the data is divided — horizontally (rows split across sites, e.g., customers by region) or vertically (columns split, e.g., sensitive columns kept at head office).
• Replication: copies of the same data (or fragments) may be stored at more than one site to improve availability and read performance.
• Transparency: users issue queries without needing to know where the data physically resides (location transparency) or whether it is fragmented/replicated.

graph TD
    subgraph HQ["Head Office Site"]
        DB1[(Local DB - Head Office)]
    end
    subgraph B1["Branch Site 1"]
        DB2[(Local DB - Branch 1)]
    end
    subgraph B2["Branch Site 2"]
        DB3[(Local DB - Branch 2)]
    end
    DB1 <--> NET((Communication Network))
    DB2 <--> NET
    DB3 <--> NET

Advantages:

• Reflects the real organizational structure (data kept close to where it is generated/used).
• Higher reliability and availability — no single point of failure; a site crash does not take down the whole system.
• Improved local performance and parallel query processing.
• Easier incremental expansion (add a new site without rebuilding the whole system).

Disadvantages:

• Complex design, query optimization, and concurrency control (e.g., needing two-phase commit for distributed transactions).
• Higher software/communication cost and greater security exposure across the network.

Exam takeaway: a distributed database is defined by physical distribution + logical unity — if the data is scattered but users cannot query it as one system, it is not a true distributed database.

A Database Administrator (DBA) is the person (or team) responsible for the overall control, security, integrity, and performance of an organization's database system, acting as the bridge between the DBMS and its users.

Core responsibilities:

Responsibility	What it involves
Schema definition	Creates the original database schema using DDL (tables, relationships, constraints).
Storage & access method definition	Decides how data is physically stored and indexed for efficient retrieval.
Schema/organization modification	Alters schema or physical structure as requirements evolve, without disrupting existing applications.
Granting authorization	Controls who can read/write/modify which data (GRANT/REVOKE, roles, user privileges).
Routine maintenance	Periodic backups, monitoring disk space/logs, applying patches and upgrades.
Ensuring integrity & security	Enforces constraints (PK, FK, CHECK), encryption, and audit trails to prevent invalid or unauthorized data changes.
Performance tuning	Monitors slow queries, builds/rebuilds indexes, adjusts configuration for optimal throughput.
Backup & recovery planning	Designs and tests disaster-recovery procedures so data can be restored after failure.
Capacity planning	Forecasts storage/hardware growth needs as data volume increases.

graph TD
    DBA((Database\nAdministrator)) --> S[Schema & Design]
    DBA --> Sec[Security & Authorization]
    DBA --> Bak[Backup & Recovery]
    DBA --> Perf[Performance Tuning]
    DBA --> Maint[Routine Maintenance]

Exam takeaway: the DBA's job spans the full lifecycle of the database — from designing the schema, to protecting and tuning it in production, to guaranteeing it can be recovered after any failure.