01A stream receives two entries in the same millisecond. What part of the ID preserves ordering?introScenarioA burst of producer traffic lands two XADD calls on the same stream within a single millisecond.AThe millisecond portion is extended with extra nanosecond digits whenever a collision happens.BArrival order inside the underlying structure preserves it, and the two IDs may simply tie, since range queries return entries in insertion order anyway.CThe consumer group's delivery counter assigns the order at the moment the entries are read.DThe sequence portion of the stream ID preserves order when multiple entries share the same millisecond timestamp.
02Why does `XACK` affect consumer group state but not necessarily remove the stream entry?introScenarioA worker finishes processing an entry and acknowledges it, then someone checks the stream's length.AXACK removes or updates pending-entry tracking for a group, but the stream entry remains until trimming or deletion removes it.BXACK deletes the entry once every consumer group on the stream has acknowledged it, a reference count maintained alongside each entry for exactly this purpose.CXACK marks the entry as hidden, and a later XDEL makes that removal permanent.DXACK moves the entry out of the stream and into the consumer group's private archive.
03Compare `XREAD` from an ID with Pub/Sub subscription after reconnect.appliedScenarioA consumer crashed, restarted, and needs to continue from where it stopped receiving messages.ABoth behave the same after reconnecting to the same node, since each tracks the client's position server-side.BXREAD can resume from a stored ID after reconnect, while Pub/Sub only receives messages published while subscribed and connected.CPub/Sub recovers better after reconnect, because the channel backlog replays automatically on resubscribe.DXREAD loses its position on reconnect unless a consumer group holds it, so plain reads always restart from the newest entry.
04What can go wrong if a stream is never trimmed?appliedScenarioA stream has been collecting entries for months with no XTRIM and no MAXLEN on any XADD.AOld entries expire after a default retention period, so the only real risk is stream IDs being reused.BThe stream eventually stops accepting XADD once it reaches a built-in maximum length.CAn untrimmed stream can grow without bound and consume memory or slow range scans.DNothing goes wrong, since streams store only entry IDs while payloads live in ordinary keys with TTLs.
05Why does a pending entries list matter before a consumer acknowledges work?advancedScenarioA worker took an entry from its group and crashed before acknowledging that the work was done.AIt is a performance cache of recent reads, so losing it merely slows the consumers down.BThe pending entries list records delivered but unacknowledged work so another consumer can inspect, retry, or claim it.CIt prevents other consumer groups from reading the same entry until the acknowledgement arrives.DIt stores each consumer's results so XACK can verify the work was performed correctly.
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