docs: Add README describing Solr custom schema changes for Solr 9.x.x version config (#537)

Author: DhavalGojiya

docker/docker-compose-solr.yml

@@ -36,7 +36,7 @@ services:
       cp -r $$TECHPRODUCTS_CONFIGSET $$SITECORE_CONFIGSET
 
       # Override with pysolr-specific configset configuration
-      cp -r $$PYSOLR_CONFIGSET/. $$SITECORE_CONFIGSET/conf
+      cp $$PYSOLR_CONFIGSET/solrconfig.xml $$SITECORE_CONFIGSET/conf/solrconfig.xml
 
       # Rename managed-schema.xml to schema.xml
       mv "$$SITECORE_CONFIGSET/conf/managed-schema.xml" "$$SITECORE_CONFIGSET/conf/schema.xml"

docker/scripts/solr-init.sh

@@ -5,7 +5,7 @@ echo "Solr init starting ..."
 
 echo "Preparing sitecore_configset..."
 cp -r "$TECHPRODUCTS_CONFIGSET" "$SITECORE_CONFIGSET"
-cp -r "$PYSOLR_CONFIGSET"/. "$SITECORE_CONFIGSET/conf"
+cp "$PYSOLR_CONFIGSET/solrconfig.xml" "$SITECORE_CONFIGSET/conf/solrconfig.xml"
 
 # Rename managed-schema.xml to schema.xml before uploading the configset to ZooKeeper
 mv "$SITECORE_CONFIGSET/conf/managed-schema.xml" "$SITECORE_CONFIGSET/conf/schema.xml"
@@ -19,4 +19,5 @@ curl -fsS \
     "http://solr-node0:8983/solr/admin/collections?action=CREATE&name=$SOLR_COLLECTION_NAME&numShards=1&replicationFactor=2&collection.configName=sitecore_configset&createNodeSet=solr-node0:8983_solr,solr-node1:8983_solr"
 
 echo "Collection '$SOLR_COLLECTION_NAME' created successfully."
+
 echo "Solr init completed."

docker/solr_configs/9.x.x/README.md

@@ -0,0 +1,78 @@
+# Solr 9 Config Notes
+
+**Directory:** `docker/solr_configs/9.x.x`
+
+This directory contains Solr configuration files customized specifically for
+**pysolr test requirements**.
+
+These files are based on Solr 9 defaults, with a small number of intentional
+changes. If this configuration needs to be updated for a future Solr version
+(e.g. Solr 10), please make sure the customizations documented below are
+preserved.
+
+---
+
+## schema.xml
+
+We do **not** maintain a separate `schema.xml` file here.
+
+Originally, `schema.xml` was just a renamed copy of Solr’s default
+`managed-schema.xml`.
+
+Instead of duplicating it, the Docker/compose scripts now **rename**
+`<CORE>/conf/managed-schema.xml` to `<CORE>/conf/schema.xml` before Solr starts.
+
+This avoids maintaining two equivalent schema files and keeps the source of
+truth aligned with upstream Solr defaults.
+
+---
+
+## solrconfig.xml
+
+There are **two intentional changes** compared to the default Solr configuration:
+
+### 1. ClassicIndexSchemaFactory
+
+```xml
+<schemaFactory class="ClassicIndexSchemaFactory" />
+```
+
+This tells Solr to use `schema.xml` instead of `managed-schema.xml`.
+Using `schema.xml` is preferred because it is **immutable at runtime**.
+
+**Source location:**
+[ClassicIndexSchemaFactory configuration](https://github.com/django-haystack/pysolr/blob/fa3457b4b3de44c053128052c9dc61282a2f9021/docker/solr_configs/9.x.x/solrconfig.xml#L37)
+
+**Solr documentation:**
+[Solr 9 Schema Factories – ClassicIndexSchemaFactory](https://solr.apache.org/guide/solr/9_9/configuration-guide/schema-factory.html#schema-factories)
+
+---
+
+### 2. MoreLikeThis request handler
+
+```xml
+<requestHandler name="/mlt" class="solr.MoreLikeThisHandler" />
+```
+
+This request handler is **not enabled by default** in Solr, but several pysolr
+test cases depend on it.
+
+**Source location:**
+[MoreLikeThis request handler configuration](https://github.com/django-haystack/pysolr/blob/fa3457b4b3de44c053128052c9dc61282a2f9021/docker/solr_configs/9.x.x/solrconfig.xml#L615)
+
+**Solr documentation:**
+[Solr 9 MoreLikeThis query handler](https://solr.apache.org/guide/solr/9_9/query-guide/morelikethis.html)
+
+---
+
+## Notes for Future Solr Versions
+
+When upgrading to a new Solr major version (e.g. Solr 10):
+
+- Create a new versioned directory, e.g. `docker/solr_configs/10.x.x`
+- Start from the **upstream default `solrconfig.xml`** for that Solr version
+- Re-apply the two changes documented above
+- Keep schema handling via `schema.xml` + `ClassicIndexSchemaFactory`
+- Ensure the `/mlt` request handler remains enabled
+
+This README exists to avoid losing these details during future upgrades.