JanusGraph/janusgraph

Transaction commits sensitive to system time differences of janusgraph clients

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#1,552 opened on May 5, 2019

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area/corehelp wanted

Description

Reported by Shrikant Pachauri on the user list and reproduced and investigated in more detail by HadoopMarc: if two JanusGraph clients on two different hosts see a different system time, transaction commits from the delayed client can fail silently if the normal client did any commits during the time difference. More strikingly, committed transactions from the delayed client can be rolled-back if the normal client does any transaction before the delayed client caught up in time. This relates to the specific case where two JanusGraph clients try to modify the same property on the same vertex. The ref docs in 31.1.1.2 indeed state that this can "result in a conflict", so maybe this behaviour is only to be expected. But even then silently ignoring transaction commits is unwanted. Also, the impact of system time seems to deserve some attention on the ref docs.

Contributor guide

Tech stack
java
Domain
databasebackenddocumentation
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
needs investigation
Prerequisites
JanusGraph basicsdistributed systems concepts
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Investigate the transaction commit handling in JanusGraph, particularly how system time differences affect conflict detection. Look at the code for commit logic and the documentation section 31.1.1.2. Consider whether silent failures or rollbacks can be prevented, and if the documentation should be updated to warn users about time synchronization requirements.