Add data flow diagram for various ETL steps in pipelines

documentation/meta/media_properties/etls.md

@@ -0,0 +1,218 @@
+# Extract-Transform-Load flows
+
+The diagrams below show how data flows through Openverse, and the various
+transformation steps that might happen at each stage.
+
+**Guide**:
+
+- **Blue boxes** represent places where data sits at rest.
+- **Purple boxes** represent transformation steps. Each box links to a longer
+  description below.
+
+## Current flow
+
+```{mermaid}
+flowchart TD
+
+%% Graph definitions
+    subgraph AF [Airflow]
+    P{{Provider}} --> M(MediaStore):::ETL
+    M --> T{{TSV}}
+    T --> IC1(Index constraints):::ETL
+    IC1 --> C[(Catalog DB)]
+    C --> B(Batched update):::ETL --> C
+    end
+
+    subgraph IS [Ingestion Server]
+    C --> ING(Deleted media removal):::ETL
+    ING --> TT[(Temp Table)]
+    TT --> CF(Cleaning & filtering):::ETL --> TT
+    TT --> A[(API DB)]
+    A --> IC2(Index constraints):::ETL --> A
+    A --> ESF(Index creation & deleted media filtering):::ETL
+    ESF --> ES[(Elasticsearch)]
+    end
+
+    A --> DL(Dead link filtering):::ETL
+    DL --> CL[Client]
+    CL --> F[Frontend]
+
+%% Style definitions
+    style IS fill:#ffc83d
+    style AF fill:#00c7d4,margin-left:12em
+    classDef ETL fill:#fa7def
+
+%% Reference definitions
+    click M "#mediastore-transformations"
+    click IC1 "#index-constraints-catalog"
+    click B "#batched-update"
+    click ING "#deleted-media-removal"
+    click CF "#cleaning-filtering"
+    click IC2 "#index-constraints-api"
+    click ESF "#es-index-creation-filtering"
+    click DL "#dead-link-filtering"
+```
+
+### MediaStore transformations
+
+These transformations occur as part of the provider ingestion scripts as records
+are being saved from the provider's API response into a TSV. The exhaustive list
+of shared transformations can be found
+[in the `MediaStore` definition](https://github.com/WordPress/openverse/tree/main/catalog/dags/common/storage/media.py),
+as well as individual transformations for
+[images](https://github.com/WordPress/openverse/tree/main/catalog/dags/common/storage/image.py)
+and
+[audio](https://github.com/WordPress/openverse/tree/main/catalog/dags/common/storage/image.py).
+
+A non-exhaustive list includes:
+
+- Filtering out tags based on licenses or tags which add no useful search
+  information
+- Removing integer values that would exceed the Postgres integer maximum
+- Validating filetype and collapsing common types
+- Formatting tags appropriately (see
+  [the media properties definition](catalog.md#tags))
+- Enriching `meta_data` the license URLs
+- Validating URLs
+- Determining `source` is not explicitly provided
+
+### Index constraints (catalog)
+
+As the data from a saved provider TSV gets inserted into the database, it also
+must be compliant with the existing indices. The indices for each primary media
+table can be found here:
+[image](https://github.com/WordPress/openverse/tree/main/docker/upstream_db/0003_openledger_image_schema.sql),
+[audio](https://github.com/WordPress/openverse/tree/main/docker/upstream_db/0006_openledger_audio_schema.sql).
+In general, these include:
+
+- Uniqueness on `provider` + `foreign_identifier` combination
+- Uniqueness on `identifier`
+- Uniqueness on `url`
+
+### Batched update
+
+Updates against the catalog database may also be initiated using the
+[batched update DAG](/catalog/reference/DAGs.md#batched-update-dag). These
+updates may be automated (such as the popularity refresh process) or manual
+(such as data correction).
+
+### Deleted media removal
+
+Presently, we
+[filter out deleted media from each of the primary media tables](https://github.com/WordPress/openverse/blob/b4ab20cdb220f442949d9c06a99ced5e80c1e1e1/ingestion_server/ingestion_server/queries.py#L173-L182)
+while the `SELECT {columns} FROM {upstream_table}` query is being run.
+
+### Cleaning & filtering
+
+We also iterate through the copied data in batches and update the values in the
+new table based on several cleaning/filtering steps (all of which can be seen in
+the ingestion server's
+[cleanup definitions](https://github.com/WordPress/openverse/tree/main/ingestion_server/ingestion_server/cleanup.py)).
+At present, these operations include:
+
+- Ensuring URLs have a scheme
+- Filtering tags out (similar to
+  [the `MediaStore` transformations](#mediastore-transformations))
+- Filtering machine-generated tags below a certain confidence value
+
+### Index constraints (API)
+
+Once the ingestion server data is copied and cleaned/filtered, the API indices
+are applied to the new table. There are several more indices on the API than on
+the catalog database, many of which are for query optimization. These include
+(all are `btree` unless specified):
+
+- Category
+- Foreign identifier
+- Last synced with source
+- Provider
+- Source
+
+### ES index creation & filtering
+
+An additional filtering step happens when the records are indexed from the API
+database into Elasticsearch. In addition to the filtering of
+[sensitive terms using two separate indices](/catalog/reference/DAGs.md#create_filtered_media_type_index)
+and
+[deleted media](https://github.com/WordPress/openverse/blob/b1a1f6eb711a3d6999b2d2bbe4b8cc1d1435c3fa/ingestion_server/ingestion_server/indexer.py#L121),
+there are also transformations that occur when constructing the ES documents.
+The
+[exhaustive list can be seen in the `elasticsearch_models` module](https://github.com/WordPress/openverse/tree/main/ingestion_server/ingestion_server/elasticsearch_models.py),
+which includes:
+
+#### Common
+
+- Computing the authority boost
+- Computing the popularity
+- Parsing a description from the media, if available
+- Parsing tags
+- Determining the `mature` field
+
+#### Images
+
+- Computing the aspect ratio
+- Computing the size
+
+#### Audio
+
+- Computing the duration
+
+### Dead link filtering
+
+When records are retrieved from Elasticsearch, they also go through a process
+called "dead link filtering". This operation attempts to remove links to media
+which the API may have trouble accessing in order to prevent dead or invalid
+links from being surfaced in API results. The full set of dead link logic can be
+found in the
+[`check_dead_link` module](https://github.com/WordPress/openverse/tree/main/api/api/utils/check_dead_links/__init__.py).
+
+## Proposed
+
+The projects #430, #431, and #3925 all intent to modify the above process. Below
+is the diagram of what this process might look like after all steps are taken.
+Most blocks reference the same sections above, with the exception being
+[Deleted media & tag filtering](#deleted-media--tag-filtering).
+
+```{mermaid}
+flowchart TD
+
+%% Graph definitions
+    subgraph AF [Airflow]
+    P{{Provider}} --> M(MediaStore):::ETL
+    M --> T{{TSV}}
+    T --> IC1(Index constraints):::ETL
+    IC1 --> C[(Catalog DB)]
+    C --> B(Batched update):::ETL --> C
+    C --> ING(Deleted media & tag filtering):::ETL
+    ING --> A[(API DB)]
+    A --> IC2(Index constraints):::ETL --> A
+    A --> ESF(Index creation & deleted media filtering):::ETL
+    ESF --> ES[(Elasticsearch)]
+    end
+
+    A --> DL(Dead link filtering):::ETL
+    DL --> CL[Client]
+    CL --> F[Frontend]
+
+%% Style definitions
+    style AF fill:#00c7d4,margin-left:12em
+    classDef ETL fill:#fa7def
+
+%% Reference definitions
+    click M "#mediastore-transformations"
+    click IC1 "#index-constraints-catalog"
+    click B "#batched-update"
+    click ING "#deleted-media-tag-filtering"
+    click IC2 "#index-constraints-api"
+    click ESF "#es-index-creation-filtering"
+    click DL "#dead-link-filtering"
+```
+
+### Deleted media & tag filtering
+
+The data normalization project (#430) intends to remove the
+[cleanup steps](#cleaning--filtering) from the ingestion server. Subsequent
+ingestion server removal project (#3925) will remove the ingestion server
+entirely. This will be fleshed out more as part of #4456, but the _filtering_
+steps (e.g. demographic tags, tags below a confidence level, etc.) will be
+present within the pipeline prior to the API indexing values into Elasticsearch.

documentation/meta/media_properties/index.md

@@ -1,12 +1,14 @@
 # Media properties
 
 This part of the documentation documents media properties at each layer of the
-Openverse stack.
+Openverse stack. It also includes a document describing the entire data flow and
+the various Extract-Transform-Load (ETL) steps that happen at each touch point.
 
 ```{toctree}
 :titlesonly:
 
 catalog
 api
 frontend
+etls
 ```