reupload snapshot engine page

Author: Cary Huang

docs/en/user-guide/configure_snapshot_engine.md

@@ -25,7 +25,63 @@ SELECT synchdb_start_engine_bgw('olrconn', 'schemasync');
 
 ## **Build and Install oracle_fdw**
 
+If "fdw" is selected as snapshot engine, you need to ensure the corresponding Foreign Data Wrapper has been installed or available for synchdb. For your reference, below is a brief procedure to build and install oracle_fdw from source code.
 
+### **Install OCI**
 
+OCI is required to build and run oracle_fdw. For this example, I am using version 23.9.0:
 
+```bash
+# Get the pre-build packages:
+wget https://download.oracle.com/otn_software/linux/instantclient/2390000/instantclient-basic-linux.x64-23.9.0.25.07.zip
+wget https://download.oracle.com/otn_software/linux/instantclient/2390000/instantclient-sdk-linux.x64-23.9.0.25.07.zip
+
+# unzip them: Select 'Y' to overwrite metadata files
+unzip instantclient-basic-linux.x64-23.9.0.25.07.zip
+unzip instantclient-sdk-linux.x64-23.9.0.25.07.zip
+```
+
+Update environment variables so the system knows where to find OCI headers and libraries.
+
+```bash
+export OCI_HOME=/home/$USER/instantclient_23_9
+export OCI_LIB_DIR=$OCI_HOME
+export OCI_INC_DIR=$OCI_HOME/sdk/include
+export LD_LIBRARY_PATH=$OCI_HOME:${LD_LIBRARY_PATH}
+export PATH=$OCI_HOME:$PATH
+
+```
+
+You could add the above commands to the end of `~/.bashrc` to automatically set the PATHs every time you login to your system.
+
+You can also add $OCI_HOME to `/etc/ld.so.conf.d/x86_64-linux-gnu.conf` (ubuntu example), as a new path to find shared libraries.
+
+You should do a ldconfig so your linker knows where to find shared libraries:
+
+### **Build oracle_fdw 2.8.0**
+
+```bash
+git clone https://github.com/laurenz/oracle_fdw.git --branch ORACLE_FDW_2_8_0
+
+```
+
+Ensure oracle_fdw's Makefile can find OCI includes and libraries by adjusting these 2 lines in Makefile:
+
+```bash
+FIND_INCLUDE := $(wildcard /usr/include/oracle/*/client64 /usr/include/oracle/*/client /home/$USER/instantclient_23_9/sdk/include)
+FIND_LIBDIRS := $(wildcard /usr/lib/oracle/*/client64/lib /usr/lib/oracle/*/client/lib /home/$USER/instantclient_23_9)
+
+```
+
+Build and install oracle_fdw:
+
+```bash
+make PG_CONFIG=/usr/local/pgsql/bin/pg_config
+sudo make install PG_CONFIG=/usr/local/pgsql/bin/pg_config
+
+```
+
+Oralce_fdw is ready to go. 
+
+**<<IMPORTANT>>** You do not have to run `CREATE EXTENSION oracle_fdw` prior to using FDW based initial snapshot, nor do you have to `CREATE SERVER` or `CREATE USER MAPPING`. SynchDB takes care of all of these when it performs the snapshot..