nickva commented on issue #4448:
URL: https://github.com/apache/couchdb/issues/4448#issuecomment-1714673445
The callback API for each scanner module might look like:
```erlang
{ok, Ctx} = start_scan(#{session_id => Uuid, start_timestamp => UnixTs})
{ok, Ctx1} = start_db(Ctx, DbName)
{ok, Ctx1} = ddoc(Ctx, DbName, DDoc = #doc{})
{ok, Ctx1} = shard(Ctx, Db)
{ok, Ctx1} = end_db(Ctx, DbName)
ok = end_scan(Ctx)
```
This flow would be initialized and kept for each module individually. The
scanner server process would hold a context that looks like:
```erlang
#state{modstates = #{Module1 => Ctx1, Module2 => Ctx2} ....}
```
The scan would be run on all the nodes. During scanning only the dbs with
the first shard copy on that node would be scanned. The API doesn't call
per-document callback. The idea would be that each plugin them may choose to
sample only some docs or process all the docs or simply return `{ok, Ctx}` and
move on.
A few events might stop or pause scanning:
* cluster membership changes (or just nodelist changes?)
* configuration change (should .ini configuration changes stop and reset
the scan?)
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