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Room and User Statistics
========================
Synapse maintains room and user statistics (as well as a cache of room state),
in various tables. These can be used for administrative purposes but are also
used when generating the public room directory.
# Synapse Developer Documentation
## High-Level Concepts
### Definitions
* **subject**: Something we are tracking stats about – currently a room or user.
* **current row**: An entry for a subject in the appropriate current statistics
table. Each subject can have only one.
* **historical row**: An entry for a subject in the appropriate historical
statistics table. Each subject can have any number of these.
### Overview
Stats are maintained as time series. There are two kinds of column:
* absolute columns – where the value is correct for the time given by `end_ts`
in the stats row. (Imagine a line graph for these values)
* They can also be thought of as 'gauges' in Prometheus, if you are familiar.
* per-slice columns – where the value corresponds to how many of the occurrences
occurred within the time slice given by `(end_ts − bucket_size)…end_ts`
or `start_ts…end_ts`. (Imagine a histogram for these values)
Stats are maintained in two tables (for each type): current and historical.
Current stats correspond to the present values. Each subject can only have one
entry.
Historical stats correspond to values in the past. Subjects may have multiple
entries.
## Concepts around the management of stats
### Current rows
Current rows contain the most up-to-date statistics for a room.
They only contain absolute columns
### Historical rows
Historical rows can always be considered to be valid for the time slice and
end time specified.
* historical rows will not exist for every time slice – they will be omitted
if there were no changes. In this case, the following assumptions can be
made to interpolate/recreate missing rows:
- absolute fields have the same values as in the preceding row
- per-slice fields are zero (`0`)
* historical rows will not be retained forever – rows older than a configurable
time will be purged.
#### Purge
The purging of historical rows is not yet implemented.
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