zhyyu opened a new issue #6803:
URL: https://github.com/apache/skywalking/issues/6803
Please answer these questions before submitting your issue.
- Why do you submit this issue?
- [x] Question or discussion
- [ ] Bug
- [ ] Requirement
- [ ] Feature or performance improvement
___
### Question
- What do you want to know?
- I reviewed the code and found that, if there are two L2 aggregators,
when they call the "MetricsPersistentWorker#flushDataToStorage" at the same
time, the final cpm data maybe not accurate. Example steps: two L2 aggregator,
backend A and B
```mermaid
sequenceDiagram
participant L2_A
participant L2_A_context_cache
participant L2_B
participant L2_B_context_cache
participant persistent_layer
L2_A ->> persistent_layer: MetricsPersistentWorker.loadFromStorage return
metrics (demoservice value 2)
L2_A ->> L2_A_context_cache: load persistent metrics to cache
L2_B ->> persistent_layer: MetricsPersistentWorker.loadFromStorage return
metrics (demoservice value 2)
L2_B ->> L2_B_context_cache: load persistent metrics to cache
L2_A ->> L2_A_context_cache: get cache metrics
L2_A ->> L2_A: combine current metrics(value 2) with cache metrics,
calculate it (value 4) 4 = 2 + 2
L2_A ->> persistent_layer: update metrics to (demoservice value 4)
L2_B ->> L2_B_context_cache: get cache metrics
L2_B ->> L2_B: combine current metrics(value 2) with cache metrics,
calculate it (value 4) 4 = 2 + 2
L2_B ->> persistent_layer: update metrics to (demoservice value 4)
L2_A ->> L2_A_context_cache: get cache metrics
L2_A ->> L2_A: combine current metrics(value 3) with cache metrics,
calculate it (value 7) 7 = 4 + 3
L2_A ->> persistent_layer: update metrics to (demoservice value 7)
L2_B ->> L2_B_context_cache: get cache metrics
L2_B ->> L2_B: combine current metrics(value 3) with cache metrics,
calculate it (value 7) 7 = 4 + 3
L2_B ->> persistent_layer: update metrics to (demoservice value 7)
```
- above steps, when L2_A L2_B PersistenceTimer.extractDataAndSave are
executed intersect, and enableDatabaseSession = true, the final result will be
7, but accurate result should be 12 = (2 + (2 + 3) L2_A + (2 + 3)L2_B)
___
### Requirement or improvement
- Please describe your requirements or improvement suggestions.
- If I deploy multi backends in mixed role (multi L2), is it right to turn
enableDatabaseSession off to avoid service cpm not accurate?
- Even though switch enableDatabaseSession off try best avoid cpm not
accurate, but when multi L2 query persistent metrics the same time, the cpm
will still not accurate (like multi-thread no safe)
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