---
layout: default
title: "Communication"
parent: "Core Abstraction"
nav_order: 3
---

# Communication

Nodes and Flows **communicate** in two ways:

1. **Shared Store (recommended)** 

   - A global data structure (often an in-mem dict) that all nodes can read and write by `prep()` and `post()`.  
   - Great for data results, large content, or anything multiple nodes need.
   - You shall design the data structure and populate it ahead.

2. **Params (only for [Batch](./batch.md))** 
   - Each node has a local, ephemeral `params` dict passed in by the **parent Flow**, used as an identifier for tasks. Parameter keys and values shall be **immutable**.
   - Good for identifiers like filenames or numeric IDs, in Batch mode.

If you know memory management, think of the **Shared Store** like a **heap** (shared by all function calls), and **Params** like a **stack** (assigned by the caller).

> Use `Shared Store` for almost all cases. It's flexible and easy to manage. It separates *Data Schema* from *Compute Logic*, making the code easier to maintain. `Params` is more a syntax sugar for [Batch](./batch.md).
{: .best-practice }

---

## 1. Shared Store

### Overview

A shared store is typically an in-mem dictionary, like:
```python
shared = {"data": {}, "summary": {}, "config": {...}, ...}
```

It can also contain local file handlers, DB connections, or a combination for persistence. We recommend deciding the data structure or DB schema first based on your app requirements.

### Example

```python
class LoadData(Node):
    def post(self, shared, prep_res, exec_res):
        # We write data to shared store
        shared["data"] = "Some text content"
        return None

class Summarize(Node):
    def prep(self, shared):
        # We read data from shared store
        return shared["data"]

    def exec(self, prep_res):
        # Call LLM to summarize
        prompt = f"Summarize: {prep_res}"
        summary = call_llm(prompt)
        return summary

    def post(self, shared, prep_res, exec_res):
        # We write summary to shared store
        shared["summary"] = exec_res
        return "default"

load_data = LoadData()
summarize = Summarize()
load_data >> summarize
flow = Flow(start=load_data)

shared = {}
flow.run(shared)
```

Here:
- `LoadData` writes to `shared["data"]`.
- `Summarize` reads from `shared["data"]`, summarizes, and writes to `shared["summary"]`.

---

## 2. Params

**Params** let you store *per-Node* or *per-Flow* config that doesn't need to live in the shared store. They are:
- **Immutable** during a Node's run cycle (i.e., they don't change mid-`prep->exec->post`).
- **Set** via `set_params()`.
- **Cleared** and updated each time a parent Flow calls it.

> 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).
{: .warning }

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.

### Example

```python
# 1) Create a Node that uses params
class SummarizeFile(Node):
    def prep(self, shared):
        # Access the node's param
        filename = self.params["filename"]
        return shared["data"].get(filename, "")

    def exec(self, prep_res):
        prompt = f"Summarize: {prep_res}"
        return call_llm(prompt)

    def post(self, shared, prep_res, exec_res):
        filename = self.params["filename"]
        shared["summary"][filename] = exec_res
        return "default"

# 2) Set params
node = SummarizeFile()

# 3) Set Node params directly (for testing)
node.set_params({"filename": "doc1.txt"})
node.run(shared)

# 4) Create Flow
flow = Flow(start=node)

# 5) Set Flow params (overwrites node params)
flow.set_params({"filename": "doc2.txt"})
flow.run(shared)  # The node summarizes doc2, not doc1
```