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# Context & Configuration
**Part 1: Foundation - Lesson 4**
> Passing state, callbacks, and metadata through Runnable chains
## Overview
You've learned Runnables (Lesson 1), Messages (Lesson 2), and LLM Wrappers (Lesson 3). Now we tackle a critical question: **How do we pass configuration, callbacks, and state through complex chains without cluttering our code?**
The answer is **RunnableConfig** - a powerful pattern that threads context through every step of a pipeline, enabling logging, debugging, authentication, and more without changing your core logic.
## Why Does This Matter?
### The Problem: Configuration Chaos
Without a proper context system:
```javascript
// Bad: Configuration everywhere
async function complexPipeline(input, temperature, callbacks, debug, userId) {
const result1 = await step1(input, temperature, debug);
callbacks.onStep('step1', result1);
const result2 = await step2(result1, userId, debug);
callbacks.onStep('step2', result2);
const result3 = await step3(result2, temperature, callbacks, debug);
callbacks.onStep('step3', result3);
return result3;
}
// Every function needs to know about every configuration option!
```
Problems:
- Every function signature becomes huge
- Adding new config requires changing every function
- Hard to add features like logging or metrics
- Impossible to intercept at specific points
- Can't pass user context through chains
### The Solution: RunnableConfig
With RunnableConfig:
```javascript
// Good: Config flows automatically
const config = {
temperature: 0.7,
callbacks: [loggingCallback, metricsCallback],
metadata: { userId: 'user_123', sessionId: 'sess_456' },
tags: ['production', 'api-v2']
};
const result = await pipeline.invoke(input, config);
// Every Runnable in the pipeline receives config automatically
// No need to pass it manually at each step!
```
Much cleaner! And infinitely extensible.
## Learning Objectives
By the end of this lesson, you will:
- ✅ Understand the RunnableConfig pattern
- ✅ Implement a callback system for monitoring
- ✅ Add metadata and tags for tracking
- ✅ Build configurable Runnables
- ✅ Create custom callbacks for logging and metrics
- ✅ Debug chains with visibility into each step
- ✅ Understand how LangChain's callbacks work
## Core Concepts
### What is RunnableConfig?
RunnableConfig is an object that flows through your entire pipeline, carrying:
1. **Callbacks** - Functions called at specific points (logging, metrics, debugging)
2. **Metadata** - Arbitrary data (user IDs, session info, request context)
3. **Tags** - Labels for filtering and organization
4. **Recursion Limit** - Prevent infinite loops
5. **Runtime Configuration** - Override default settings (temperature, max tokens)
**📍 Where This Lives in the Framework:**
Looking back at our framework structure from the main README, RunnableConfig is part of the **Core** module:
```javascript
// Core module (what we're building now)
export {
Runnable, // ← Lesson 1
RunnableSequence, // ← Lesson 1
BaseMessage, // ← Lesson 2
HumanMessage, // ← Lesson 2
AIMessage, // ← Lesson 2
SystemMessage, // ← Lesson 2
RunnableConfig // ← THIS LESSON (Lesson 4)
} from './core/index.js';
```
RunnableConfig isn't a separate feature you add later - it's **foundational infrastructure** built into the Core module that every other part of the framework depends on. The callback system we're about to build is how this config becomes useful for observability.
### The Flow
```
User calls: runnable.invoke(input, config)
↓
┌───────────────┴─────────────┐
│ │
Config passed to every step │
│ │
┌─────────┼─────────┬──────────┬────────┼────┐
│ │ │ │ │ │
Step1 Step2 Step3 Step4 Step5 ...
│ │ │ │ │
└─────────┴─────────┴──────────┴────────┘
│
All use same config
All trigger callbacks
All have access to metadata
```
### Key Benefits
1. **Separation of concerns**: Logic separate from monitoring
2. **Composability**: Add features without changing code
3. **Observability**: See what's happening at every step
4. **Flexibility**: Runtime configuration override
5. **Extensibility**: Easy to add new capabilities
## Implementation Deep Dive
### Step 1: The RunnableConfig Object
**Location:** `src/core/context.js`
```javascript
/**
* RunnableConfig - Configuration passed through chains
*/
export class RunnableConfig {
constructor(options = {}) {
// Callbacks for monitoring
this.callbacks = options.callbacks || [];
// Metadata (arbitrary data)
this.metadata = options.metadata || {};
// Tags for filtering/organization
this.tags = options.tags || [];
// Recursion limit (prevent infinite loops)
this.recursionLimit = options.recursionLimit ?? 25;
// Runtime overrides for generation parameters
this.configurable = options.configurable || {};
}
/**
* Merge with another config (child inherits from parent)
*/
merge(other) {
return new RunnableConfig({
callbacks: [...this.callbacks, ...(other.callbacks || [])],
metadata: { ...this.metadata, ...(other.metadata || {}) },
tags: [...this.tags, ...(other.tags || [])],
recursionLimit: other.recursionLimit ?? this.recursionLimit,
configurable: { ...this.configurable, ...(other.configurable || {}) }
});
}
/**
* Create a child config with additional settings
*/
child(options = {}) {
return this.merge(new RunnableConfig(options));
}
}
```
**Why this design?**
- Immutable merging (doesn't modify original)
- Child configs inherit parent settings
- Easy to add new fields without breaking existing code
### Step 2: The Callback System
**📍 How Callbacks Relate to the Framework:**
Callbacks are the **mechanism** that makes RunnableConfig useful. They're not a separate module - they're the "hooks" that get triggered as your Runnables execute. Think of them as event listeners built into the Core module.
**In the framework structure, callbacks support observability:**
- They live in Core (used by every module)
- Later modules like **Agents** and **Chains** use them for tracing
- The **Utils** module's `CallbackManager` (which we'll see) is a helper for managing them
Here's the base callback class:
**Location:** `src/utils/callbacks.js`
```javascript
/**
* BaseCallback - Abstract callback handler
*/
export class BaseCallback {
/**
* Called when a Runnable starts
*/
async onStart(runnable, input, config) {
// Override in subclass
}
/**
* Called when a Runnable completes successfully
*/
async onEnd(runnable, output, config) {
// Override in subclass
}
/**
* Called when a Runnable errors
*/
async onError(runnable, error, config) {
// Override in subclass
}
/**
* Called for LLM token streaming
*/
async onLLMNewToken(token, config) {
// Override in subclass
}
/**
* Called when a chain step completes
*/
async onChainStep(stepName, output, config) {
// Override in subclass
}
}
```
**Callback Lifecycle**:
```
invoke() called
↓
onStart() ← Before execution
↓
[execution] ← Your _call() method runs
↓
onEnd() ← After success
or
onError() ← After failure
```
**Key insight:** Callbacks are optional observers - your code works fine without them, but they let you see what's happening.
### Step 3: CallbackManager
Manages multiple callbacks and ensures they all get called:
**Location:** `src/utils/callback-manager.js`
```javascript
/**
* CallbackManager - Manages multiple callbacks
*/
export class CallbackManager {
constructor(callbacks = []) {
this.callbacks = callbacks;
}
/**
* Add a callback
*/
add(callback) {
this.callbacks.push(callback);
}
/**
* Call onStart for all callbacks
*/
async handleStart(runnable, input, config) {
await Promise.all(
this.callbacks.map(cb =>
this._safeCall(() => cb.onStart(runnable, input, config))
)
);
}
/**
* Call onEnd for all callbacks
*/
async handleEnd(runnable, output, config) {
await Promise.all(
this.callbacks.map(cb =>
this._safeCall(() => cb.onEnd(runnable, output, config))
)
);
}
/**
* Call onError for all callbacks
*/
async handleError(runnable, error, config) {
await Promise.all(
this.callbacks.map(cb =>
this._safeCall(() => cb.onError(runnable, error, config))
)
);
}
/**
* Call onLLMNewToken for all callbacks
*/
async handleLLMNewToken(token, config) {
await Promise.all(
this.callbacks.map(cb =>
this._safeCall(() => cb.onLLMNewToken(token, config))
)
);
}
/**
* Call onChainStep for all callbacks
*/
async handleChainStep(stepName, output, config) {
await Promise.all(
this.callbacks.map(cb =>
this._safeCall(() => cb.onChainStep(stepName, output, config))
)
);
}
/**
* Safely call a callback (don't let one callback crash others)
*/
async _safeCall(fn) {
try {
await fn();
} catch (error) {
console.error('Callback error:', error);
// Don't throw - callbacks shouldn't break the pipeline
}
}
}
```
**Key insight**: Callbacks can fail without breaking the pipeline.
### Step 4: Integrating with Runnable
Update the Runnable base class to use config:
**Location:** `src/core/runnable.js`
```javascript
export class Runnable {
constructor() {
this.name = this.constructor.name;
}
/**
* Execute with config support
*/
async invoke(input, config = {}) {
// Normalize config to RunnableConfig instance
const runnableConfig = config instanceof RunnableConfig
? config
: new RunnableConfig(config);
// Create callback manager
const callbackManager = new CallbackManager(runnableConfig.callbacks);
try {
// Notify callbacks: starting
await callbackManager.handleStart(this, input, runnableConfig);
// Execute the runnable
const output = await this._call(input, runnableConfig);
// Notify callbacks: success
await callbackManager.handleEnd(this, output, runnableConfig);
return output;
} catch (error) {
// Notify callbacks: error
await callbackManager.handleError(this, error, runnableConfig);
throw error;
}
}
async _call(input, config) {
throw new Error(
`${this.name} must implement _call() method`
);
}
// ... stream(), batch(), pipe() methods remain the same ...
}
```
Now every Runnable automatically:
- ✅ Receives config
- ✅ Triggers callbacks
- ✅ Handles errors properly
- ✅ Passes config to nested Runnables
### Step 5: Useful Built-in Callbacks
```javascript
/**
* ConsoleCallback - Logs to console
*/
export class ConsoleCallback extends Callbacks {
constructor(options = {}) {
super();
this.verbose = options.verbose ?? true;
this.colors = options.colors ?? true;
}
async onStart(runnable, input, config) {
if (this.verbose) {
console.log(`\n▶ Starting: ${runnable._name}`);
console.log(` Input:`, this._format(input));
}
}
async onEnd(runnable, output, config) {
if (this.verbose) {
console.log(`✓ Completed: ${runnable._name}`);
console.log(` Output:`, this._format(output));
}
}
async onError(runnable, error, config) {
console.error(`✗ Error in ${runnable._name}:`, error.message);
}
async onLLMNewToken(token, config) {
process.stdout.write(token);
}
_format(value) {
if (typeof value === 'string') {
return value.length > 100 ? value.substring(0, 97) + '...' : value;
}
return JSON.stringify(value, null, 2);
}
}
```
```javascript
/**
* MetricsCallback - Tracks timing and counts
*/
export class MetricsCallback extends Callbacks {
constructor() {
super();
this.metrics = {
calls: {},
totalTime: {},
errors: {}
};
this.startTimes = new Map();
}
async onStart(runnable, input, config) {
const name = runnable._name;
this.startTimes.set(name, Date.now());
this.metrics.calls[name] = (this.metrics.calls[name] || 0) + 1;
}
async onEnd(runnable, output, config) {
const name = runnable._name;
const startTime = this.startTimes.get(name);
if (startTime) {
const duration = Date.now() - startTime;
this.metrics.totalTime[name] = (this.metrics.totalTime[name] || 0) + duration;
this.startTimes.delete(name);
}
}
async onError(runnable, error, config) {
const name = runnable._name;
this.metrics.errors[name] = (this.metrics.errors[name] || 0) + 1;
}
getReport() {
const report = [];
for (const [name, calls] of Object.entries(this.metrics.calls)) {
const totalTime = this.metrics.totalTime[name] || 0;
const avgTime = calls > 0 ? (totalTime / calls).toFixed(2) : 0;
const errors = this.metrics.errors[name] || 0;
report.push({
runnable: name,
calls,
avgTime: `${avgTime}ms`,
totalTime: `${totalTime}ms`,
errors
});
}
return report;
}
reset() {
this.metrics = {calls: {}, totalTime: {}, errors: {}};
this.startTimes.clear();
}
}
```
```javascript
/**
* FileCallback - Logs to file
*/
export class FileCallback extends Callbacks {
constructor(filename) {
super();
this.filename = filename;
this.logs = [];
}
async onStart(runnable, input, config) {
this.logs.push({
timestamp: new Date().toISOString(),
event: 'start',
runnable: runnable._name,
input: this._serialize(input)
});
}
async onEnd(runnable, output, config) {
this.logs.push({
timestamp: new Date().toISOString(),
event: 'end',
runnable: runnable._name,
output: this._serialize(output)
});
}
async onError(runnable, error, config) {
this.logs.push({
timestamp: new Date().toISOString(),
event: 'error',
runnable: runnable._name,
error: error.message
});
}
async flush() {
const fs = await import('fs/promises');
await fs.writeFile(
this.filename,
JSON.stringify(this.logs, null, 2),
'utf-8'
);
this.logs = [];
}
_serialize(value) {
if (typeof value === 'string') return value;
if (value?.content) return value.content; // Message
return JSON.stringify(value);
}
}
```
## Complete Implementation
Here's the full context system:
```javascript
/**
* Context & Configuration System
*
* @module core/context
*/
/**
* RunnableConfig - Configuration passed through chains
*/
export class RunnableConfig {
constructor(options = {}) {
this.callbacks = options.callbacks || [];
this.metadata = options.metadata || {};
this.tags = options.tags || [];
this.recursionLimit = options.recursionLimit ?? 25;
this.configurable = options.configurable || {};
}
merge(other) {
return new RunnableConfig({
callbacks: [...this.callbacks, ...(other.callbacks || [])],
metadata: { ...this.metadata, ...(other.metadata || {}) },
tags: [...this.tags, ...(other.tags || [])],
recursionLimit: other.recursionLimit ?? this.recursionLimit,
configurable: { ...this.configurable, ...(other.configurable || {}) }
});
}
child(options = {}) {
return this.merge(new RunnableConfig(options));
}
}
/**
* BaseCallback - Base class for callbacks
*/
export class BaseCallback {
async onStart(runnable, input, config) {}
async onEnd(runnable, output, config) {}
async onError(runnable, error, config) {}
async onLLMNewToken(token, config) {}
async onChainStep(stepName, output, config) {}
}
/**
* CallbackManager - Manages multiple callbacks
*/
export class CallbackManager {
constructor(callbacks = []) {
this.callbacks = callbacks;
}
add(callback) {
this.callbacks.push(callback);
}
async handleStart(runnable, input, config) {
await Promise.all(
this.callbacks.map(cb =>
this._safeCall(() => cb.onStart(runnable, input, config))
)
);
}
async handleEnd(runnable, output, config) {
await Promise.all(
this.callbacks.map(cb =>
this._safeCall(() => cb.onEnd(runnable, output, config))
)
);
}
async handleError(runnable, error, config) {
await Promise.all(
this.callbacks.map(cb =>
this._safeCall(() => cb.onError(runnable, error, config))
)
);
}
async handleLLMNewToken(token, config) {
await Promise.all(
this.callbacks.map(cb =>
this._safeCall(() => cb.onLLMNewToken(token, config))
)
);
}
async handleChainStep(stepName, output, config) {
await Promise.all(
this.callbacks.map(cb =>
this._safeCall(() => cb.onChainStep(stepName, output, config))
)
);
}
async _safeCall(fn) {
try {
await fn();
} catch (error) {
console.error('Callback error:', error);
}
}
}
/**
* ConsoleCallback - Logs to console with colors
*/
export class ConsoleCallback extends BaseCallback {
constructor(options = {}) {
super();
this.verbose = options.verbose ?? true;
}
async onStart(runnable, input, config) {
if (this.verbose) {
console.log(`\n▶ Starting: ${runnable._name}`);
console.log(` Input:`, this._format(input));
if (config.metadata && Object.keys(config.metadata).length > 0) {
console.log(` Metadata:`, config.metadata);
}
}
}
async onEnd(runnable, output, config) {
if (this.verbose) {
console.log(`✓ Completed: ${runnable._name}`);
console.log(` Output:`, this._format(output));
}
}
async onError(runnable, error, config) {
console.error(`✗ Error in ${runnable._name}:`, error.message);
}
async onLLMNewToken(token, config) {
process.stdout.write(token);
}
_format(value) {
if (typeof value === 'string') {
return value.length > 100 ? value.substring(0, 97) + '...' : value;
}
if (value?.content) {
return value.content.substring(0, 100);
}
return JSON.stringify(value, null, 2);
}
}
/**
* MetricsCallback - Tracks performance metrics
*/
export class MetricsCallback extends BaseCallback {
constructor() {
super();
this.metrics = {
calls: {},
totalTime: {},
errors: {}
};
this.startTimes = new Map();
}
async onStart(runnable, input, config) {
const key = `${runnable._name}_${Date.now()}_${Math.random()}`;
this.startTimes.set(key, { name: runnable._name, time: Date.now() });
const name = runnable._name;
this.metrics.calls[name] = (this.metrics.calls[name] || 0) + 1;
}
async onEnd(runnable, output, config) {
const name = runnable._name;
// Find the most recent start time for this runnable
let startTime = null;
for (const [key, value] of this.startTimes.entries()) {
if (value.name === name) {
startTime = value.time;
this.startTimes.delete(key);
break;
}
}
if (startTime) {
const duration = Date.now() - startTime;
this.metrics.totalTime[name] = (this.metrics.totalTime[name] || 0) + duration;
}
}
async onError(runnable, error, config) {
const name = runnable._name;
this.metrics.errors[name] = (this.metrics.errors[name] || 0) + 1;
}
getReport() {
const report = [];
for (const [name, calls] of Object.entries(this.metrics.calls)) {
const totalTime = this.metrics.totalTime[name] || 0;
const avgTime = calls > 0 ? (totalTime / calls).toFixed(2) : 0;
const errors = this.metrics.errors[name] || 0;
report.push({
runnable: name,
calls,
avgTime: `${avgTime}ms`,
totalTime: `${totalTime}ms`,
errors,
successRate: calls > 0 ? `${((calls - errors) / calls * 100).toFixed(1)}%` : '0%'
});
}
return report;
}
printReport() {
console.log('\n📊 Performance Report:');
console.log('─'.repeat(80));
console.table(this.getReport());
}
reset() {
this.metrics = { calls: {}, totalTime: {}, errors: {} };
this.startTimes.clear();
}
}
export default {
RunnableConfig,
BaseCallback,
CallbackManager,
ConsoleCallback,
MetricsCallback
};
```
## Real-World Examples
### Example 1: Basic Logging
```javascript
import { ConsoleCallback } from './context.js';
const logger = new ConsoleCallback({ verbose: true });
const config = {
callbacks: [logger]
};
// Every step will log
const result = await chain.invoke(input, config);
```
Output:
```
▶ Starting: PromptTemplate
Input: "Translate to Spanish: Hello"
✓ Completed: PromptTemplate
Output: "Translate the following to Spanish: Hello"
▶ Starting: LlamaCppLLM
Input: "Translate the following to Spanish: Hello"
✓ Completed: LlamaCppLLM
Output: AIMessage("Hola")
```
### Example 2: Performance Monitoring
```javascript
import { MetricsCallback } from './context.js';
const metrics = new MetricsCallback();
const config = {
callbacks: [metrics]
};
// Run multiple times
for (let i = 0; i < 10; i++) {
await chain.invoke(input, config);
}
// Get performance report
metrics.printReport();
```
Output:
```
📊 Performance Report:
┌─────────┬─────────────────┬───────┬──────────┬───────────┬────────┬──────────────┐
│ (index) │ runnable │ calls │ avgTime │ totalTime │ errors │ successRate │
├─────────┼─────────────────┼───────┼──────────┼───────────┼────────┼──────────────┤
│ 0 │ 'PromptTemplate'│ 10 │ '5.20ms' │ '52ms' │ 0 │ '100.0%' │
│ 1 │ 'LlamaCppLLM' │ 10 │ '243.5ms'│ '2435ms' │ 0 │ '100.0%' │
└─────────┴─────────────────┴───────┴──────────┴───────────┴────────┴──────────────┘
```
### Example 3: Metadata Tracking
```javascript
const config = {
metadata: {
userId: 'user_123',
sessionId: 'sess_456',
requestId: 'req_789'
},
tags: ['production', 'api-v2']
};
await agent.invoke(input, config);
// Every callback receives this metadata
// Useful for logging, debugging, billing
```
### Example 4: Multiple Callbacks
```javascript
const logger = new ConsoleCallback();
const metrics = new MetricsCallback();
const fileLogger = new FileCallback('./logs/agent.json');
const config = {
callbacks: [logger, metrics, fileLogger]
};
await chain.invoke(input, config);
// All three callbacks are triggered
await fileLogger.flush(); // Save to file
metrics.printReport(); // Show metrics
```
### Example 5: Runtime Configuration Override
```javascript
const llm = new LlamaCppLLM({
modelPath: './model.gguf',
temperature: 0.7 // default
});
// Override at runtime
const result1 = await llm.invoke(input, {
configurable: { temperature: 0.2 } // more deterministic
});
const result2 = await llm.invoke(input, {
configurable: { temperature: 1.2 } // more creative
});
```
### Example 6: Custom Callback for API Logging
```javascript
class APILoggerCallback extends Callbacks {
constructor(apiKey) {
super();
this.apiKey = apiKey;
}
async onEnd(runnable, output, config) {
// Send to logging API
await fetch('https://api.yourservice.com/logs', {
method: 'POST',
headers: {
'Authorization': `Bearer ${this.apiKey}`,
'Content-Type': 'application/json'
},
body: JSON.stringify({
runnable: runnable._name,
output: this._serialize(output),
metadata: config.metadata,
timestamp: new Date().toISOString()
})
});
}
_serialize(output) {
if (output?.content) return output.content;
return String(output);
}
}
// Usage
const apiLogger = new APILoggerCallback(process.env.API_KEY);
const config = {callbacks: [apiLogger]};
await chain.invoke(input, config);
```
## Advanced Patterns
### Pattern 1: Conditional Callbacks
```javascript
class ConditionalCallback extends Callbacks {
constructor(condition, callback) {
super();
this.condition = condition;
this.callback = callback;
}
async onEnd(runnable, output, config) {
if (this.condition(runnable, output, config)) {
await this.callback.onEnd(runnable, output, config);
}
}
}
// Only log slow operations
const slowLogger = new ConditionalCallback(
(runnable, output, config) => {
// Check if operation took > 1 second
return config.executionTime > 1000;
},
new ConsoleCallback()
);
```
### Pattern 2: Callback Composition
```javascript
class CompositeCallback extends Callbacks {
constructor(callbacks) {
super();
this.callbacks = callbacks;
}
async onStart(runnable, input, config) {
for (const cb of this.callbacks) {
await cb.onStart(runnable, input, config);
}
}
async onEnd(runnable, output, config) {
for (const cb of this.callbacks) {
await cb.onEnd(runnable, output, config);
}
}
async onError(runnable, error, config) {
for (const cb of this.callbacks) {
await cb.onError(runnable, error, config);
}
}
}
// Combine multiple callbacks
const composite = new CompositeCallback([
new ConsoleCallback(),
new MetricsCallback(),
new FileCallback('./logs.json')
]);
```
### Pattern 3: Filtered Logging
```javascript
class FilteredCallback extends Callbacks {
constructor(filter, callback) {
super();
this.filter = filter;
this.callback = callback;
}
async onStart(runnable, input, config) {
if (this.filter(runnable._name, 'start')) {
await this.callback.onStart(runnable, input, config);
}
}
async onEnd(runnable, output, config) {
if (this.filter(runnable._name, 'end')) {
await this.callback.onEnd(runnable, output, config);
}
}
}
// Only log LLM calls
const llmOnly = new FilteredCallback(
(name, event) => name.includes('LLM'),
new ConsoleCallback()
);
```
### Pattern 4: Callback with State
```javascript
class StatefulCallback extends Callbacks {
constructor() {
super();
this.state = {
callCount: 0,
totalTokens: 0,
errors: []
};
}
async onEnd(runnable, output, config) {
this.state.callCount++;
if (output?.additionalKwargs?.usage) {
this.state.totalTokens += output.additionalKwargs.usage.totalTokens;
}
}
async onError(runnable, error, config) {
this.state.errors.push({
runnable: runnable._name,
error: error.message,
timestamp: Date.now()
});
}
getState() {
return {...this.state};
}
}
```
## Common Use Cases
### Use Case 1: Debug Mode
```javascript
const debugConfig = {
callbacks: [new ConsoleCallback({ verbose: true })],
tags: ['debug']
};
// See everything that happens
await agent.invoke(query, debugConfig);
```
### Use Case 2: Production Monitoring
```javascript
const productionConfig = {
callbacks: [
new MetricsCallback(),
new APILoggerCallback(API_KEY)
],
metadata: {
environment: 'production',
version: '1.2.3'
},
tags: ['production']
};
```
### Use Case 3: A/B Testing
```javascript
const configA = {
metadata: { variant: 'A' },
configurable: { temperature: 0.7 }
};
const configB = {
metadata: { variant: 'B' },
configurable: { temperature: 0.9 }
};
// Track which performs better
```
### Use Case 4: User Context
```javascript
const userConfig = {
metadata: {
userId: req.userId,
sessionId: req.sessionId,
plan: req.user.plan // 'free', 'pro', 'enterprise'
}
};
// Different behavior based on user plan
if (userConfig.metadata.plan === 'free') {
userConfig.configurable = { maxTokens: 100 };
} else if (userConfig.metadata.plan === 'pro') {
userConfig.configurable = { maxTokens: 500 };
}
```
## Best Practices
### ✅ DO:
```javascript
// Use config for cross-cutting concerns
const config = {
callbacks: [logger, metrics],
metadata: { userId, sessionId }
};
// Let config flow automatically
await chain.invoke(input, config);
// Create child configs for nested calls
const childConfig = config.child({ tags: ['nested'] });
// Handle callback errors gracefully (already done in CallbackManager)
```
### ❌ DON'T:
```javascript
// Don't pass config manually at each step
const result1 = await step1(input, config);
const result2 = await step2(result1, config); // Unnecessary
// Don't mutate config
config.metadata.foo = 'bar'; // Bad! Create new config instead
// Don't put business logic in callbacks
// Callbacks are for observability, not logic
```
## Debugging Tips
### Tip 1: Add Timestamps
```javascript
class TimestampCallback extends Callbacks {
async onStart(runnable, input, config) {
console.log(`[${new Date().toISOString()}] ${runnable._name} started`);
}
}
```
### Tip 2: Stack Traces in Callbacks
```javascript
class DebugCallback extends Callbacks {
async onError(runnable, error, config) {
console.error('Full stack trace:');
console.error(error.stack);
console.error('Config:', config);
}
}
```
### Tip 3: Callback Filtering
```javascript
// Only show LLM operations
const config = {
callbacks: [
new FilteredCallback(
name => name.includes('LLM'),
new ConsoleCallback()
)
]
};
```
## Exercises
Practice building with context and callbacks:
### Exercise 13: Build a Simple Logging Callback
Understand the basic callback lifecycle.
**Starter code**: [`exercises/13-simple-logger.js`](exercises/13-simple-logger.js)
### Exercise 14: Build a Metrics Tracker with Metadata
Learn to use config metadata and track metrics.
**Starter code**: [`exercises/14-metrics-metadata.js`](exercises/14-metrics-metadata.js)
### Exercise 15: Config Merging and Child Configs
Understand how configs inherit and merge.
**Starter code**: [`exercises/15-retry-callback.js`](exercises/15-retry-callback.js)
### Exercise 16: Runtime Configuration Override
Learn to override LLM settings at runtime.
**Starter code**: [`exercises/16-runtime-config.js`](exercises/16-runtime-config.js)
## Summary
Congratulations! You've mastered the context and configuration system.
### Key Takeaways
1. **RunnableConfig flows automatically**: No need to pass manually at each step
2. **Callbacks enable observability**: See what's happening without changing code
3. **Metadata carries context**: User info, session data, request IDs
4. **Tags enable filtering**: Organize and filter operations
5. **Callbacks don't break pipelines**: Errors in callbacks are caught
6. **Configuration is composable**: Child configs inherit from parents
7. **Runtime overrides are powerful**: Change behavior without changing code
### What You Built
A context system that:
- ✅ Flows config through chains automatically
- ✅ Supports multiple callbacks
- ✅ Tracks metrics and performance
- ✅ Enables runtime configuration
- ✅ Provides observability
- ✅ Handles errors gracefully
- ✅ Is infinitely extensible
### Foundation Complete! 🎉
You've completed Part 1 (Foundation). You now understand:
1. **Runnable** - The composability pattern
2. **Messages** - Structured conversation data
3. **LLM Wrapper** - Integrating real models
4. **Context** - Passing state and observability
These four concepts are the foundation of **every** agent framework.
### What's Next
In **Part 2: Composition**, you'll learn:
- Prompt templates
- Output parsers
- LLM chains
- The pipe operator
- Memory systems
➡️ [Continue to Part 2: Composition](../02-composition/01-prompts.md)
## Additional Resources
- [LangChain Callbacks](https://python.langchain.com/docs/modules/callbacks/)
- [OpenTelemetry](https://opentelemetry.io/) - Industry standard for observability
- [Structured Logging](https://www.structlog.org/en/stable/)
## Questions & Discussion
**Q: Why not just use console.log everywhere?**
A: Callbacks are:
- Composable (turn on/off easily)
- Non-invasive (don't clutter code)
- Centralized (one place to change logging)
- Production-ready (can send to monitoring services)
**Q: What's the performance overhead of callbacks?**
A: Minimal if implemented correctly. The CallbackManager calls them in parallel and catches errors, so one slow callback doesn't slow everything down.
**Q: Can I modify the input/output in a callback?**
A: You *can*, but you *shouldn't*. Callbacks are for observation, not transformation. Transformations belong in the Runnable logic.
**Q: How do I pass authentication through chains?**
A: Use metadata:
```javascript
const config = {
metadata: {
authToken: req.headers.authorization
}
};
```
---
**Built with ❤️ for learners who want to understand AI agents deeply**
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