# Specification: Roadmap Phase VII - Modern Replication & GTID Mastery

## Context

MySQL and MariaDB replication have evolved significantly with GTID-based failover, parallel applier enhancements, and binary log compression. Phase VII focuses on optimizing these distributed data flows for durability and throughput.

## Proposed Replication Indicators

### 1. Binary Log Compression & Efficiency

* **Metric**: `binlog_transaction_compression` (MySQL 8.0.20+).
* **Indicator**: Measure compression ratio and CPU overhead using Performance Schema.
* **Recommendation**: Enable compression for high-volume write workloads on WAN/Cloud links.

### 2. GTID Integrity & Consistency Audit

* **Metric**: `gtid_executed` holes and gaps.
* **Indicator**: Detect missing transactions or non-contiguous GTID sets on replicas.
* **Check**: Ensure `enforce_gtid_consistency=ON` and `gtid_mode=ON`.

### 3. Advanced Parallel Replication (MTR)

* **Metric**: `performance_schema.replication_applier_status_by_worker`.
* **Indicator**: Analyze worker thread busy-wait distribution.
* **Check**: Dependency tracking type (`COMMIT_ORDER` vs `WRITESET` in MySQL).
* **Recommendation**: Suggest increasing `slave_parallel_workers` if threads are saturated or switching to `WRITESET` for higher concurrency.

### 4. Semi-Synchronous Performance & Safety

* **Metric**: `Rpl_semi_sync_master_no_times` and `Rpl_semi_sync_master_wait_sessions`.
* **Indicator**: Identify how often semi-sync falls back to asynchronous replication.
* **Check**: `rpl_semi_sync_master_wait_point` (AFTER_SYNC vs AFTER_COMMIT).

### 5. Multi-Source Replication Channel Health

* **Logic**: Expand current logic to support multiple replication channels in Performance Schema.
* **Indicator**: Per-channel lag and error reporting.

### 6. Binlog Cache Deep-Dive

* **Metric**: `Binlog_cache_disk_use` ratio.
* **Indicator**: Detect large transactions causing disk-based binary logging.
* **Recommendation**: Increase `binlog_cache_size` to prevent I/O stalls during large commits.

## Expected Value

* **Data Integrity**: Ensuring GTID consistency across the topology.
* **Throughput**: Maximizing parallel applier performance.
* **Resilience**: Better observability of semi-synchronous failure modes.
