.

wcfs/internal/zdata/δftail.go

@@ -61,8 +61,35 @@ package zdata
 //
 // Concurrency
 //
-// XXX
-// XXX note about SliceByFileRev Zinblk snapshotting
+// In order to allow multiple Track and queries requests to be served in
+// parallel, ΔFtail bases its concurrency promise on ΔBtail guarantees +
+// snapshot-style access for vδE and ztrackInBlk in queries:
+//
+// 1. Track calls ΔBtail.Track and quickly updates .byFile, .byRoot and
+//    _RootTrack indices under a lock.
+//
+// 2. BlkRevAt queries ΔBtail.GetAt and then combines retrieved zblk object
+//    with vδE and δZ.
+//
+// 3. SliceByFileRev queries ΔBtail.SliceByRootRev and then merges retrieved
+//    vδT data with vδZ, vδE and ztrackInBlk.
+//
+// 4. In queries vδE is retrieved/build in snapshot style similarly to how vδT
+//    is built in ΔBtail. Note that vδE needs to be build only the first time,
+//    and does not need to be rebuilt further, so the logic in ΔFtail is simpler
+//    compared to ΔBtail.
+//
+// 5. for ztrackInBlk - that is used by SliceByFileRev query - an atomic
+//    snapshot is retrieved for objects of interest. This allows to hold
+//    δFtail.mu lock for relatively brief time without blocking other parallel
+//    Track/queries requests for long.
+//
+// Combined this organization allows non-overlapping queries/track-requests
+// to run simultaneously. This property is essential to WCFS because otherwise
+// WCFS would not be able to serve several non-overlapping READ requests to one
+// file in parallel.
+//
+// See "Concurrency" in ΔBtail organization for more details.
 
 import (
 	"context"