--- title: "Distributed and nested distributed transactions" subject: "Distributed System" chapter: 8 key: "semester_7/distributed_system:distributed-transaction-nested" source: https://www.bctengineer.com/q/semester_7/distributed_system/distributed-transaction-nested site: BCT Engineering --- # Distributed and nested distributed transactions Distributed System · Chapter 8: Transaction and Concurrency Control - Asked in: Model question - Video: [DS37:Distributed Transactions: Flat and nested distributed transactions in distributed sustem](https://www.youtube.com/watch?v=ChjUflFtS6s) — University Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/distributed_system/distributed-transaction-nested - Whole chapter: https://www.bctengineer.com/md/semester_7/distributed_system/chapter-8 ## Question Define distributed transaction and nested distributed transaction with examples. ## Short answer A distributed transaction spans multiple servers, requiring atomic commit to ensure all-or-nothing semantics, e.g., inter-bank transfer. A nested distributed transaction decomposes into sub-transactions at different servers that run concurrently and provisionally commit; only the top-level commit makes them permanent, allowing independent aborts, e.g., trip booking with flight and hotel. ## Answer #### Distributed transaction A **distributed transaction** is a transaction whose operations access objects managed by **several servers**. It can be flat (the client makes requests one after another to different servers) or nested. It must still satisfy ACID, so all servers must commit or all abort, which needs a **coordinator** and an atomic commit protocol (two-phase commit). Example: a funds transfer where account A is at Bank server X and account B is at server Y. ```text Client --begin--> Coordinator (X) T: X.withdraw(A,100) T: Y.deposit(B,100) Client --commit--> 2PC between X and Y ``` When server Y is first used, it joins the transaction by informing the coordinator, which keeps a list of participants. #### Nested distributed transaction A **nested distributed transaction** is a transaction with a tree of sub-transactions in which the sub-transactions run on different servers. Each sub-transaction can run in parallel at its server, can abort independently, and commits provisionally; the real commit happens only when the top-level transaction commits (using a nested two-phase commit that involves all servers). Example: a *book trip* transaction T starts sub-transactions T1 (flight at server F), T2 (hotel at server H), T3 (taxi at server C). T2 may abort if no room, and T can then try another hotel. ```text T (client) / | \ T1 (F) T2 (H) T3 (C) ``` Each server records the sub-transactions it holds; the coordinator of the top level finds out which participants did work (and which provisionally committed or aborted) before running the commit protocol. Rules: if the parent aborts, all children abort; a child's locks pass to the parent when it commits. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What makes a transaction a distributed transaction? A. It has more than one client B. Its operations access objects managed by several servers C. It uses nested sub-transactions D. It runs in parallel threads Answer: B. A distributed transaction spans objects at multiple servers, and must still be atomic across them. (Key point: Distributed transaction) 2. How do the servers of a distributed transaction reach a common commit or abort outcome? A. Each server decides independently B. The coordinator runs an atomic commit protocol such as two-phase commit C. The client commits at each server in turn D. The oldest server decides alone Answer: B. A coordinator and 2PC ensure that all servers commit or all abort. (Key point: Atomic commit) 3. In a nested distributed transaction, when does a sub-transaction's commit become permanent? A. Immediately when the sub-transaction finishes B. Only when the top-level transaction commits C. When the child's server restarts D. When the parent starts Answer: B. Sub-transactions commit provisionally; the real commit happens at the top level. (Key point: Nested sub-transactions) 4. In the book-trip example, what can happen if the hotel sub-transaction T2 aborts? A. T and all other sub-transactions must abort B. T can try another hotel and continue C. The flight booking is automatically permanent D. The coordinator ends the system Answer: B. Sub-transactions can abort independently, letting the parent choose an alternative. (Key point: Independent abort) ## Related in Distributed System - [Nested vs distributed transaction; 2PC](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Anested-vs-distributed-2pc) — Distributed System, chapter 8, asked 2× - [Transaction, nested transaction, atomicity](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Atransaction-nested-atomicity) — Distributed System, chapter 8 - [Flat/nested transactions; optimistic CC](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aflat-nested-optimistic) — Distributed System, chapter 8 - [Flat/nested transactions; concurrency control](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aflat-nested-cc-methods) — Distributed System, chapter 8 - [Nested transactions; optimistic CC advantages](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Anested-optimistic-advantages) — Distributed System, chapter 8 - [Need of transaction; phantom deadlock](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Atransaction-deadlock-phantom) — Distributed System, chapter 8