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Konstantin Knizhnik
2024-04-13 21:47:01 +03:00
parent 1b2cfc0259
commit de3fdf9860
1 changed files with 17 additions and 0 deletions
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17
pageserver/src/page_service.rs
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@@ -847,6 +847,23 @@ impl PageServerHandler {
/// In either case, if the page server hasn't received the WAL up to the
/// requested LSN yet, we will wait for it to arrive. The return value is
/// the LSN that should be used to look up the page versions.
///
/// Compute needs to specify:
/// 1. "desired" LSN - which LSN compute expects to be acceptable
/// 2. Upper boundary LSN - PS should not send page with greater LSN to preserver consistency
///
/// In case of primary node then upper boundary is always +inf: nobody except this node can produce more recent version of the page.
/// In case of replica it is not true: replica can lag from primary node and PS and should not receive pages newer than its last_replay_lsn.
/// But it is not good always to request pages at `last_replay_lsn` because replica can be ahead PS and so it has to wait
/// until PS caught up (while for this particular page it is not needed).
///
/// We actually need to handle just three cases:
/// [page_last_written_lsn, +inf] - primary node
/// [page_last_written_lsn, last_replay_lsn] - hot-standby replica (receiving WAL from primary)
/// [snapshot_lsn, snapshot_lsn] - static RO replica (not receiving WAL fro primary)
///
/// Case [0, lsn] is not actually needed and added mostly for convenience as alias for [lsn,lsn]
async fn wait_or_get_last_lsn(
timeline: &Timeline,
lsn: Lsn,