wangkang created FLINK-40397:
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Summary: Add currentBinlogPositionLag metric for MySQL binlog
reader
Key: FLINK-40397
URL: https://issues.apache.org/jira/browse/FLINK-40397
Project: Flink
Issue Type: Improvement
Components: Flink CDC
Reporter: wangkang
## Problem
The existing `currentFetchEventTimeLag` metric (FetchTime - messageTimestamp)
has a fundamental flaw:
it only updates when data flows through the reader.
For example, if the reader consumed a record from 1:00 AM at 10:00 AM, the
metric shows 9 hours of
lag. Even after the reader has fully caught up (no new binlog events on
master), the metric remains
frozen at 9 hours until the next record arrives.
This causes:
- False alerts: reader has caught up but metric still shows high lag
- Cannot distinguish "caught up" from "still catching up"
## Proposal
Add a new gauge metric `currentBinlogPositionLag` that reflects the binlog
position gap between the
reader's current consumed offset and the MySQL master's latest offset.
Semantics:
- Value = 0: fully caught up
- Value > 0: unconsumed binlog data exists
- Value = -1 (UNDEFINED): still in snapshot phase
### GTID Mode
Sum of (master_max_txn_id - current_max_txn_id) for each server UUID.
Note: GtidSet.subtract() cannot be used because CDC may resume from a
checkpoint midpoint
(e.g., uuid:1774595494-1775564172), causing subtract to incorrectly count
historical
transactions as lag.
### Non-GTID Mode
- Same file: master.position - current.position (byte difference)
- Cross-file: (file_sequence_gap) × scale_factor + master.position (synthetic
weight
indicating severity — not actual byte count)
### Design
- Reuse existing JDBC connection in BinlogSplitReader to query SHOW MASTER
STATUS every ~10s
- Share master offset to RecordEmitter via AtomicReference (zero extra
threads/connections)
- RecordEmitter calculates and reports lag on data change records and
heartbeat events
- No impact on data processing path
## Compatibility
- No breaking API changes to public interfaces
- New metric is purely additive
- Works with both GTID and non-GTID MySQL configurations
- Compatible with MySQL 5.7, 8.0, and 8.4+
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