update node doc

docs/node.md

@@ -7,35 +7,38 @@ nav_order: 1
 
 # Node
 
-A **Node** is the smallest building block. Each Node has 3 steps:
+A **Node** is the smallest building block. Each Node has 3 steps `prep->exec->post`:
 
 1. `prep(shared)`
-   - A reliable step for preprocessing data from the `shared` store. 
+   - **Read and preprocess data** from `shared` store. 
    - Examples: *query DB, read files, or serialize data into a string*.
-   - Returns `prep_res`, which is used by `exec()` and `post()`.
+   - Return `prep_res`, which is used by `exec()` and `post()`.
 
 2. `exec(prep_res)`
-   - The **main execution** step, with optional retries and error handling (below).
-   - Examples: *primarily for LLMs, but can also for remote APIs*.
+   - **Execute compute logic**, with optional retries and error handling (below).
+   - Examples: *(mostly) LLM calls, remote APIs, tool use*.
+   - ⚠️ This shall be only for compute and **NOT** access `shared`.
    - ⚠️ If retries enabled, ensure idempotent implementation.
-   - ⚠️ This must **NOT** write to `shared`. If reads are necessary, extract them in `prep()` and pass them in `prep_res`.
-   - Returns `exec_res`, which is passed to `post()`.
+   - Return `exec_res`, which is passed to `post()`.
 
 3. `post(shared, prep_res, exec_res)`
-   - A reliable postprocessing step to write results back to the `shared` store and decide the next Action. 
-   - Examples: *update DB, change states, log results, decide next Action*.
-   - Returns a **string** specifying the next Action (`"default"` if none).
+   - **Postprocess and write data** back to `shared`.
+   - Examples: *update DB, change states, log results*.
+   - **Decide the next action** by returning a *string* (`action = "default"` if *None*).
 
-> All 3 steps are optional. You could run only `prep` if you just need to prepare data without calling the LLM.
+> **Why 3 steps?** To enforce the principle of *separation of concerns*. The data model are operated separately from the business logic on them.
+>
+> All steps are *optional*. E.g., you can only implement `prep` and `post` if you just need to process data.
 {: .note }
 
 
 ### Fault Tolerance & Retries
 
-Nodes can **retry** execution if `exec()` raises an exception. You control this via two parameters when you create the Node:
+You can **retry** `exec()` if it raises an exception via two parameters when define the Node:
 
-- `max_retries` (int): How many times to try running `exec()`. The default is `1`, which means **no** retry.
-- `wait` (int): The time to wait (in **seconds**) before each retry attempt. By default, `wait=0` (i.e., no waiting). Increasing this is helpful when you encounter rate-limits or quota errors from your LLM provider and need to back off.
+- `max_retries` (int): Max times to run `exec()`. The default is `1` (**no** retry).
+- `wait` (int): The time to wait (in **seconds**) before next retry. By default, `wait=0` (no waiting). 
+`wait` is helpful when you encounter rate-limits or quota errors from your LLM provider and need to back off.
 
 ```python 
 my_node = SummarizeFile(max_retries=3, wait=10)
@@ -46,28 +49,36 @@ When an exception occurs in `exec()`, the Node automatically retries until:
 - It either succeeds, or
 - The Node has retried `max_retries - 1` times already and fails on the last attempt.
 
+You can get the current retry times (0-based) from `self.cur_retry`.
+
+```python 
+class RetryNode(Node):
+    def exec(self, prep_res):
+        print(f"Retry {self.cur_retry} times")
+        raise Exception("Failed")
+```
+
 ### Graceful Fallback
 
-If you want to **gracefully handle** the error rather than raising it, you can override:
+To **gracefully handle** the exception (after all retries) rather than raising it, override:
 
 ```python 
 def exec_fallback(self, shared, prep_res, exc):
     raise exc
 ```
 
-By default, it just re-raises `exc`. But you can return a fallback result instead, which becomes the `exec_res` passed to `post()`.
+By default, it just re-raises exception. But you can return a fallback result instead, which becomes the `exec_res` passed to `post()`.
 
 ### Example: Summarize file
 
 ```python 
 class SummarizeFile(Node):
     def prep(self, shared):
-        filename = self.params["filename"]
-        return shared["data"][filename]
+        return shared["data"]
 
     def exec(self, prep_res):
         if not prep_res:
-            raise ValueError("Empty file content!")
+            return "Empty file content"
         prompt = f"Summarize this text in 10 words: {prep_res}"
         summary = call_llm(prompt)  # might fail
         return summary
@@ -77,18 +88,16 @@ class SummarizeFile(Node):
         return "There was an error processing your request."
 
     def post(self, shared, prep_res, exec_res):
-        filename = self.params["filename"]
-        shared["summary"][filename] = exec_res
-        # Return "default" by not returning anything
+        shared["summary"] = exec_res
+        # Return "default" by not returning
 
 summarize_node = SummarizeFile(max_retries=3)
-summarize_node.set_params({"filename": "test_file.txt"})
 
 # node.run() calls prep->exec->post
 # If exec() fails, it retries up to 3 times before calling exec_fallback()
 action_result = summarize_node.run(shared)
 
-print("Action returned:", action_result)  # Usually "default"
-print("Summary stored:", shared["summary"].get("test_file.txt"))
+print("Action returned:", action_result)  # "default"
+print("Summary stored:", shared["summary"])
 ```