stats: document which stats to index

docs/develop/howtos/backup_search_indices.md

@@ -1,5 +1,4 @@
 # Back up search indices
-
 Starting with InvenioRDM v12, not all search indices can be recreated from the database
 anymore.
 In order to not lose any data when something goes wrong with the search indices, they
@@ -12,9 +11,38 @@ should be backed up regularly.
     (total, from harvesters and users), which produced approximately **10Gb** of
     usage statistics data.
 
+## Indices to back up
+All search indices can be re-created from the database except for the `statistics` indices.
+These indices are not stored in the database, and if lost, there is potentially no way to
+recover this data. This means that view and download statistics may be permanently lost.
+
+**Types of statistics indices**
+
+Unless configured otherwise, there are two main classes of statistics indices:
+
+- **Raw events**: Each individual view or download hit.
+- **Aggregated events**: Aggregated statistics per record or file.
+
+Each of these indices is created on a **monthly basis** for both views and downloads.
+For more details, see the [related documentation](../../reference/statistics.md).
+
+**Examples of index naming**
+
+- `<prefix>-events-stats-file-download-2023-08`: Stores raw file download events for August 2023.
+- `<prefix>-stats-record-view-2023-08`: Stores aggregated record views for August 2023.
+
+Additionally, there is a `<prefix>-stats-bookmarks` index that tracks the latest aggregation created.
+
+**Backup recommendations**
+
+The most critical indices to back up are those matching the pattern:
 
-## Non-exhaustive list of methods
+`<prefix>-events-stats*`
 
+If lost, **aggregated indices** and **bookmarks** can be re-created from the raw event data.
+However, if you have enough backup space, it is recommended to back up all statistics indices.
+
+## Non-exhaustive list of backup methods
 There are several tools and approaches to pick from when it comes to preserving your
 indexed data in Elasticsearch or OpenSearch.
 Here is a small selection:
@@ -32,19 +60,15 @@ Here is a small selection:
 Of course there's more possibilities out there (e.g. [Curator](https://github.com/elastic/curator)
 for Elasticsearch), but this should give you an idea where to start.
 
-
 ## Elasticdump
-
 For the sake of brevity, this guide only deals with `elasticdump` as it is a very simple
 tool to use and works with both Elasticsearch and OpenSearch.
 
 !!! info "Make sure to back up all relevant indices"
     Please note that the given example only deals with backing up and restoring one single index.
     In your instance, you should make sure to back up *all* relevant indices regularly!
 
-
 ### Backup
-
 All it takes to back up a search index are two commands, one for saving the mappings
 and one for saving the data.
 With the following commands, you'll create the files `stats-record-view-2023-04.mappings.json`
@@ -91,9 +115,7 @@ $ curl -X DELETE http://localhost:9200/my-site-stats-record-view-2023-04
 
 And just like that, the record view statistics for all of April 2023 are gone.
 
-
 ### Restore
-
 Luckily, the restore is just as simple as the backup was.
 All you need to do is to restore the mappings first, and the data afterwards.
 The commands are nearly identical to the backup, just with the values for `--input` and
@@ -125,9 +147,7 @@ Tue, 04 Apr 2023 17:01:40 GMT | dump complete
 
 Now, the index is fully restored including the mappings as well as the data!
 
-
 ### Example scripts
-
 Backing up each index manually can quickly become tedious.
 Luckily there is an API endpoint for listing the available indices in both [ES](https://www.elastic.co/guide/en/elasticsearch/reference/current/cat-indices.html) and [OS](https://opensearch.org/docs/latest/api-reference/cat/cat-indices/).
 The following basic `bash` script uses this endpoint to find all `stats` indices and backs them up: