LLM Application Frameworks: LangChain & LangGraph: sections and modules

T4.2

LCEL, Runnables & Chains

Compose Runnables with the pipe operator and run them with invoke, batch and stream.

Lessons

7 lessons

Deliverable: Chain rewrite: rebuild one legacy chain as LCEL, add a branch that routes short and long inputs differently, and compare outputs.
Assessment: Answer the five Interview questions without notes, then run the lab.
Interview probe: Larkspur's nightly job runs `triage_chain.batch()` over 2,000 support tickets. Last night it died at ticket 1,412 with a rate-limit error and wrote nothing. What do you change?
Common mistakes: Using RunnableParallel and losing the question a later step needs
A RunnableBranch default that returns a fixed apology instead of a handoff
Treating no build-time warning from load_summarize_chain as proof it is safe

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