Defining global transactions In a distributed DBMS, a “global transaction” is best described as one where:
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AAll required data are at a single local site and the DDBMS routes requests only internally.
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BSome required data are at one or more nonlocal sites and the DDBMS routes requests across sites as needed.
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CAll required data are at one local site and the DDBMS passes requests only to the local engine.
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DData are at nonlocal sites but the DDBMS never routes beyond the local engine.
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EThe application never needs two-phase commit or coordination.
Answer
Correct Answer: Some required data are at one or more nonlocal sites and the DDBMS routes requests across sites as needed.
Explanation
Introduction / Context:Transactions define units of atomic work. In distributed databases, a transaction may touch data at multiple sites. Such a transaction must be coordinated to ensure ACID properties across nodes, typically through a transaction manager and protocols like two-phase commit (2PC).
Given Data / Assumptions:
- The database is distributed across networked sites.
- The transaction's read/write set may include fragments at remote sites.
- The DDBMS provides routing and commit coordination.
Concept / Approach:
A global (or distributed) transaction accesses at least one nonlocal site. The DDBMS decomposes the query/updates into subtransactions, executes them at the right sites, and coordinates commit/rollback. If everything is local, it is not “global.” If the DDBMS refuses to route, it cannot satisfy cross-site data access, contradicting the definition.
Step-by-Step Solution:
1) Determine whether nonlocal sites are involved → yes for global transactions.2) Ensure the DDBMS routes and coordinates requests across sites.3) Recognize the need for distributed commit to maintain atomicity.4) Choose the option that states nonlocal access with DDBMS routing.Verification / Alternative check:
Most DDBMS textbooks classify transactions as local (single site) or global/distributed (multiple sites), with 2PC ensuring atomic outcomes across participants.
Why Other Options Are Wrong:
- Options a/c: describe local-only transactions.
- Option d: contradicts routing beyond local; then it isn't truly distributed.
- Option e: cross-site commit typically requires coordination (e.g., 2PC).
Common Pitfalls:
- Ignoring partial failures and network partitions in design of global transactions.
Final Answer:
Some required data are at one or more nonlocal sites and the DDBMS routes requests across sites as needed.