> [!WALSenderTimelineHistoryRead] > <table><tr><td rowspan="4" width="25%"><img src="assets/wl_io_walsendertimelinehistoryread.png"></td><td><b>Wait Category</b></td><td>IO</td></tr><tr><td><b>Mechanism</b></td><td><code>WalSender</code></td></tr><tr><td><b>Operations</b></td><td><a href="Operations/Distributed/Redistribute">Redistribute</a></td></tr><tr><td><b>Resources</b></td><td><a href="Resources/disk IO">disk IO</a>, <a href="Resources/CPU">CPU</a></td></tr><tr><td colspan="3"><b>Replication Handshake Retrieval stall</b>. When a standby connects, it often asks for the primary's history to ensure they are on compatible branches of the transaction log.</td></tr></table>