add description of changes

CHANGES.md

@@ -2,6 +2,24 @@
 All notable changes to this project will be documented in this file.
 
 
+## Add response passthrough to ranking endpoints
+Previously the STELLA infrastructure demanded a fixed response schema for rankings. The ranking systems were expected to return the documents or items in a certain format and the STELLA app would pass the results after the interleaving als in a certain format. This was not flexible and all content needed to be loaded afterwards from external sources based on the returned ID.  
+
+Improving on that, the STELLA App now supports a passthrough mode for the ranking endpoint. This means that the ranking systems can return the documents in any format they like and the STELLA App will return the same format after interleaving. This allows to return the full content of the documents.
+
+To make use of this feature, the experimental systems need additional configurations to tell the STELLA App the JSON Path to the document ranking in the response and the key of the document ID. This can be configured through the `SYSTEMS_CONFIG` environment variable in the docker compose file.
+
+Example:
+```
+SYSTEMS_CONFIG: |
+        {
+            "ranker_base": {"type": "ranker", "base": true, "docid": "id", "hits_path": "$.hits.hits"},
+            "ranker_exp": {"type": "ranker", "docid": "id", "hits_path": "$.hits.hits"}
+        }
+```
+
+The results are still saved to the database in the base schema of the STELLA app and the original response will not be saved to the database. This is to ensure fast responses and minimize latency. However therefore a new caching mechanism was needed. Therefore, `Flask-Caching` is used. By default, `FileSystemCache` is used, but this can be changed in the `config.py` file.
+
 
 
 ## Allow system config as JSON
@@ -18,12 +36,12 @@ RANKSYS_BASE: gesis_rank_pyserini_base
 After:
 ```
 SYSTEMS_CONFIG: |
-          '[
+          [
             {"name": "gesis_rec_pyterrier", "type": "recommender", "base": true},
             {"name": "gesis_rec_pyserini", "type": "recommender"},
             {"name": "gesis_rank_pyserini_base", "type": "ranker", "base": true},
             {"name": "gesis_rank_pyserini", "type": "recommender"}
-          ]'
+          ]
 ```