Add Scan Data Log documentation

Document how the scan log data is stored

Author: kasemir

services/scan-server/ScanDataLog.md

@@ -0,0 +1,55 @@
+Scan Data Log
+=============
+
+The `Log` command to log samples and the associated `scan/{id}/data` REST interface to fetch logged data
+are based on an Apache Derby database.
+It is stored in the `<data_log>` folder provided in the `scan_config.xml`,
+with the example value of `/tmp/scan_log_db`.
+
+It’s a relational database. To access the RDB, the Derby “ij” command line tool is included in the scan server.
+In addition to the Derby jar files, the scan server jar is required because the scan sample "value"
+is stored as a serialized `org.csstudio.scan.server.log.derby.SampleValue`.
+
+Example for starting "ij" and accessing the basic tables:
+
+```
+$ cd scan-server/lib
+$ java -Dderby.system.home=/tmp/scan_log_db  \
+       -cp "service-scan-server-4.7.4-SNAPSHOT.jar:lib/derby-10.16.1.1.jar:lib/derbyshared-10.16.1.1.jar:lib/derbytools-10.16.1.1.jar” \
+        org.apache.derby.impl.tools.ij.Main
+ij version 10.16
+ij> CONNECT 'jdbc:derby:scan';
+ij> SELECT * FROM scans;
+ID |NAME    |CREATED                      
+---------------------------------------
+1  |Example |2025-10-31 11:22:55.760598
+
+ij> SELECT * FROM devices;
+ID  |NAME                                                                                                
+---------------
+1   |loc://x(0)                                                                                          
+
+ij> SELECT * FROM scans;
+ID  |NAME         |CREATED                      
+---------------------------------------------
+1   |Example      |2025-10-31 11:22:55.760598   
+
+ij> SELECT * from samples;
+SCAN_ID|DEVICE_ID |SERIAL  |TIMESTAMP                  |VALUE          
+-------------------------------------------------------------
+1      |1         |0       |2025-10-31 11:22:55.82952  |1.0            
+```
+
+Joining the tables together as is tradition with RDBs:
+
+```
+ij> SELECT c.name, s.serial, s.timestamp, d.name, s.value
+    FROM samples s
+    JOIN scans c on c.ID = s.SCAN_ID
+    JOIN devices d ON d.ID = s.DEVICE_ID
+    ORDER BY c.name, s.serial;
+
+NAME     |SERIAL    |TIMESTAMP                    |NAME          |VALUE          
+-----------------------------------------------------------------------
+Example  |0         |2025-10-31 11:22:55.82952    |loc://x(0)    |1.0            
+```