LCEL Demo 2

Before diving into the runnable interface and pass-through components, this lesson reviews three primary ways to invoke a chain in LangChain:

• Synchronous invocation (invoke)
• Streaming output (stream)
• Batch processing (batch)

1. Synchronous Invocation (invoke)

Execute the entire chain in one call and receive the complete response:

from langchain_openai import ChatOpenAI
from langchain_core.prompts import ChatPromptTemplate
from langchain_core.output_parsers import StrOutputParser

# 1. Construct prompt, LLM, and parser
prompt = ChatPromptTemplate.from_template(
    "You are a helpful assistant.\nAnswer the following question: {question}"
)
llm = ChatOpenAI()
output_parser = StrOutputParser()

# 2. Build the chain
chain = prompt | llm | output_parser

# 3. Invoke synchronously
result = chain.invoke({"question": "Tell me about The Godfather Movie"})
print(result)

2. Streaming Output (stream)

When you need a “typing” effect or to process tokens on the fly, use streaming:

for chunk in chain.stream({"question": "Tell me about The Godfather Movie"}):
    # Print each chunk without extra newlines
    print(chunk, end="")

Example of streamed chunks:

The God
father is
a classic
American
crime
film
directed
by Francis...

3. Batch Processing (batch)

Process multiple inputs in parallel to improve throughput:

questions = [
    {"question": "What is LangChain?"},
    {"question": "Explain chain-of-thought prompting."},
    {"question": "How does streaming work?"}
]
results = chain.batch(questions)
for response in results:
    print(response)

Batch execution is ideal for high-volume workloads and will be explored further alongside parallelism patterns.

Invocation Methods Comparison

What’s Next?

In the following sections, we’ll explore:

• The Runnable Interface: How to wrap custom logic into reusable components
• Pass-Through Components: Techniques for chaining without transforming the data

Links and References

• LangChain Documentation
• langchain-openai on GitHub
• ChatPromptTemplate API
• StrOutputParser Guide