Consumer groups
Objection: that lag is hearsay.
Select a group under Consumer groups to open its screen. Marshall reads it through the admin API and an offset fetch. He never joins the group, so looking at it never shows up in its rebalance.
The verdict first
Where it stands opens with one sentence, before any number:
| Verdict | Means |
|---|---|
| Caught up. | Every partition it reads is read to the end |
| Behind. | It has messages waiting; the worst partition and its share are named |
| Nobody home. | The group has no members right now |
| Never committed. | It has members but has not committed a position, so its lag is unknown, not zero |
| Mid-rebalance. | Members are being reassigned |
| Can't see. | There is no end to measure against: the topic is not listed, or its watermarks would not read |
A group with no members and nothing committed anywhere is reported as exactly that: the broker remembers its name and nothing else.
The numbers
Per partition: the committed offset, the end, and how far behind it is. A position past the end of a partition is pointed out as past the end.
Who's reading lists the members: client, host, the partitions each holds, static membership ids when they are set, and the group's coordinator.
The topics a group reads come from what it actually committed, not from its name. If half of the information cannot be read (the members or the offsets), the other half is still shown and the missing part is listed under Can't see.
Moving its position
Move the position… resets the group's committed offsets on a topic. It is refused while the group has members. See Producing and resetting offsets.