diff --git a/README.md b/README.md
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--- a/README.md
+++ b/README.md
@@ -1,4 +1,88 @@
 # mxpic_EDA
 The EDA coding for the layout for optihk
 # requirements
-flask
\ No newline at end of file
+flask
+
+# Frontend - backend GDS bridge
+Each canvas in the fronend should create a same name **.yaml** file.
+This **.yaml** should contain the following parts.
+
+- basic description : name, type, version
+- ports : the output connection ports for this canvas object
+- instances : the instance objects from **PDK** or other **canvas**, together with their **x, y, a, mirror** informations.
+- bundles : 1-1 or N-N conenctions that used for auto routing. "***This***"  refers to the canvas ports. 
+
+```
+[ Project Canvas ]
+     │  (Contains instances of Component A and Component B)
+     ▼
+[ Component A Canvas ]
+     │  (Contains instances of DC_2x2 and Ubend)
+     ▼
+[ PDK Primitives ] ──> (Loads raw .gds + physical port data .yml)
+```
+
+``` yaml
+# ==========================================
+# mxPIC Cell/Project Definition File
+# ==========================================
+name: MMI_Splitter_Module # Name of this cell/component/project
+type: composite          # "primitive" (PDK base) or "composite" (hierarchical)
+version: "1.0.0"
+
+# 1. External Ports (How this cell connects to the outside world)
+ports:
+  - name: in0
+    layer: WG_CORE
+    x: 0.0
+    y: 0.0
+    angle: 180.0
+    width: 0.5
+  - name: out0
+    layer: WG_CORE
+    x: 100.0
+    y: 10.0
+    angle: 0.0
+    width: 0.5
+  - name: out1
+    layer: WG_CORE
+    x: 100.0
+    y: -10.0
+    angle: 0.0
+    width: 0.5
+
+# 2. Instances (The sub-components dropped onto this canvas)
+instances:
+  mmi_inst1:
+    component: MMI_2x2     # References another PDK cell or composite YAML
+    x: 20.0                 # Placement origin X
+    y: 0.0                 # Placement origin Y
+    rotation: 0.0          # Rotation angle in degrees
+    mirror: false          # True/False for layout mirroring
+    settings:              # Local parameter overrides if parametric
+      length: 25.5
+      
+  ubend_top:
+    component: Ubend
+    x: 60.0
+    y: 5.0
+    rotation: 0.0
+    mirror: false
+
+  ubend_bottom:
+    component: Ubend
+    x: 60.0
+    y: -5.0
+    rotation: 0.0
+    mirror: true           # Flipped for symmetrical routing
+
+# 3. Bundles (Grouped links for multi-bus/parallel routing)
+bundles:
+  output_bus:
+    routing_type: euler_bend # Metadata for the backend router
+    links:
+      - from: ubend_top:out0
+        to: this:out0
+      - from: ubend_bottom:out0
+        to: this:out1
+```
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