Tiering service Deep Dive part 2#10
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| Now customers is next in the commit order. The enumerator generates 12 splits (since this is round 1 of a PK table, these are snapshot splits reading the full KV state). 12 readers can work on its splits at once; the 4 spare readers, again, pick up whatever's next in the queue rather than idling. The round takes roughly 100 seconds, and customers' commit lands at **T=220s**, well inside the configured 5-minute freshness. | ||
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| But notice one thing here. clicks committed at **T=30s**; with a 1-minute freshness, its next round was scheduled for **T=90s**, so it's been waiting in the queue since **T=90s**. Its reads may well start earlier (spare readers can pick up clicks round 2 while customers is still being read), but its *commit* can't land until orders and customers have committed ahead of it. |
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The article says commits happen “in queue order”, but the current implementation doesn't guarantees that.
The commit operator is single-parallelism, so commits are serialized, but TieringCommitOperator commits a table once it has collected all write results for that table. It does not appear to maintain the coordinator pending-queue order across in-flight tables. A later-assigned but faster table could therefore reach the committer first and be committed before an earlier table.
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