LangChain Expression Language (LCEL) is the composable, streaming-first framework for building LLM pipelines. Understanding the Runnable interface, pipe operator, parallel/branch primitives, and the callback system unlocks the full power of LangChain's chain composition.
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What does the pipe operator (|) do when used between two LangChain LCEL components?
In LCEL, the | operator is syntactic sugar for RunnableSequence. It chains two Runnables so the output of the left flows as input to the right, building a composable data pipeline.
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You need to pass the same input to two different chains and merge their outputs into a dict. Which LCEL primitive should you use?
RunnableParallel (also constructable with a plain dict of Runnables) executes multiple Runnables with the same input concurrently and returns a dict mapping each key to its corresponding output. It is the standard fan-out primitive in LCEL.
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What is the purpose of RunnableBranch in LCEL?
RunnableBranch implements conditional logic: it evaluates a list of (condition, Runnable) pairs against the input and executes the Runnable for the first truthy condition, with a default fallback. It is LCEL's equivalent of an if-elif-else chain.
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LCEL's Runnable interface exposes invoke, batch, and stream. What advantage does the unified interface provide?
Because every LCEL component implements the same Runnable interface, chains are composable and interchangeable. Calling stream() on a chain automatically propagates streaming through all components that support it, without any extra wiring.
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When you pass a callback handler to an LCEL chain's invoke() call, at what points does it fire events?
LCEL's callback system is hierarchical: a handler passed to the outermost invoke() automatically propagates into every nested Runnable. Events fire at the start, end, and on errors of each component, enabling detailed tracing and logging throughout the chain.