136 lines
4.3 KiB
Markdown
136 lines
4.3 KiB
Markdown
---
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layout: default
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title: "Communication"
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parent: "Core Abstraction"
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nav_order: 3
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---
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# Communication
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Nodes and Flows **communicate** in two ways:
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1. **Shared Store** – A global data structure (often an in-mem dict) that all nodes can read from and write to. Every Node’s `prep()`, `exec()`, and `post()` methods receive the **same** `shared` store.
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2. **Params** – Each node and Flow has a `params` dict assigned by the **parent Flow**. Params mostly serve as identifiers, letting each node/flow know what task it’s assigned.
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If you know memory management, **Shared Store** is like a **heap** shared across function calls, while **Params** is like a **stack** assigned by parent function calls.
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### Why Not Message Passing?
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**Message passing** can work for simple DAGs (e.g., for data pipelines), but with **nested graphs** (Flows containing Flows, repeated or cyclic calls), routing messages becomes hard to maintain. A shared store keeps the design simpler and easier.
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---
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## 1. Shared Store
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### Overview
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A shared store is typically an in-mem dictionary, like:
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```python
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shared = {"data": {}, "summary": {}, "config": {...}, ...}
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```python
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It can also contain local file handlers, DB connections, or a combination for persistence.
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We recommend deciding the data structure or DB schema in advance based on your app requirements.
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### Example
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```python
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class LoadData(Node):
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def prep(self, shared):
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# Suppose we read from disk or an API
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shared["data"]["my_file.txt"] = "Some text content"
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return None
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class Summarize(Node):
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def prep(self, shared):
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# We can read what LoadData wrote
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content = shared["data"].get("my_file.txt", "")
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return content
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def exec(self, shared, prep_res):
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prompt = f"Summarize: {prep_res}"
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summary = call_llm(prompt)
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return summary
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def post(self, shared, prep_res, exec_res):
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shared["summary"]["my_file.txt"] = exec_res
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return "default"
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load_data = LoadData()
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summarize = Summarize()
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load_data >> summarize
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flow = Flow(start=load_data)
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shared = {}
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flow.run(shared)
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```
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Here:
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- `LoadData` writes to `shared["data"]`.
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- `Summarize` reads from the same location.
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No special data-passing—just the same `shared` object.
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---
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## 2. Params
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**Params** let you store **per-Node** or **per-Flow** config that doesn’t need to live in the global store. They are:
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- **Immutable** during a Node’s run cycle (i.e., they don’t change mid-`prep`, `exec`, `post`).
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- **Set** via `set_params()`.
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⚠️ Only set the uppermost Flow params because others will be overwritten by the parent Flow. If you need to set child node params, see [Batch](./batch.md).
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- **Cleared** and updated each time a parent Flow calls it.
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Typically, **Params** are identifiers (e.g., file name, page number). Use them to fetch the task you assigned or write to a specific part of the shared store.
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### Example
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```python
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# 1) Create a Node that uses params
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class SummarizeFile(Node):
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def prep(self, shared):
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# Access the node's param
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filename = self.params["filename"]
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return shared["data"].get(filename, "")
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def exec(self, shared, prep_res):
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prompt = f"Summarize: {prep_res}"
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return call_llm(prompt)
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def post(self, shared, prep_res, exec_res):
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filename = self.params["filename"]
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shared["summary"][filename] = exec_res
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return "default"
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# 2) Set params
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node = SummarizeFile()
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# 3) Set Node params directly (for testing)
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node.set_params({"filename": "doc1.txt"})
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node.run(shared)
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# 4) Create Flow
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flow = Flow(start=node)
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# 5) Set Flow params (overwrites node params)
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flow.set_params({"filename": "doc2.txt"})
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flow.run(shared) # The node summarizes doc2, not doc1
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```
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---
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## 3. Shared Store vs. Params
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Think of the **Shared Store** like a heap and **Params** like a stack.
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- **Shared Store**:
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- Public, global.
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- You can design and populate ahead, e.g., for the input to process.
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- Great for data results, large content, or anything multiple nodes need.
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- Keep it tidy—structure it carefully (like a mini schema).
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- **Params**:
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- Local, ephemeral.
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- Passed in by parent Flows. You should only set it for the uppermost flow.
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- Perfect for small values like filenames or numeric IDs.
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- Do **not** persist across different nodes and are reset. |