Runnable interface

The Runnable interface is foundational for working with LangChain components, and it's implemented across many of them, such as language models, output parsers, retrievers, compiled LangGraph graphs and more.

This guide covers the main concepts and methods of the Runnable interface, which allows developers to interact with various LangChain components in a consistent and predictable manner.

Related Resources

• The "Runnable" Interface API Reference provides a detailed overview of the Runnable interface and its methods.
• A list of built-in Runnables can be found in the LangChain Core API Reference. Many of these Runnables are useful when composing custom "chains" in LangChain using the LangChain Expression Language (LCEL).

Overview of runnable interface

The Runnable way defines a standard interface that allows a Runnable component to be:

• Invoked: A single input is transformed into an output.
• Batched: Multiple inputs are efficiently transformed into outputs.
• Streamed: Outputs are streamed as they are produced.
• Inspected: Schematic information about Runnable's input, output, and configuration can be accessed.
• Composed: Multiple Runnables can be composed to work together using the LangChain Expression Language (LCEL) to create complex pipelines.

Please review the LCEL Cheatsheet for some common patterns that involve the Runnable interface and LCEL expressions.

Optimized parallel execution (batch)

LangChain Runnables offer a built-in batch (and batch_as_completed) API that allow you to process multiple inputs in parallel.

Using these methods can significantly improve performance when needing to process multiple independent inputs, as the processing can be done in parallel instead of sequentially.

The two batching options are:

• batch: Process multiple inputs in parallel, returning results in the same order as the inputs.
• batch_as_completed: Process multiple inputs in parallel, returning results as they complete. Results may arrive out of order, but each includes the input index for matching.

The default implementation of batch and batch_as_completed use a thread pool executor to run the invoke method in parallel. This allows for efficient parallel execution without the need for users to manage threads, and speeds up code that is I/O-bound (e.g., making API requests, reading files, etc.). It will not be as effective for CPU-bound operations, as the GIL (Global Interpreter Lock) in Python will prevent true parallel execution.

Some Runnables may provide their own implementations of batch and batch_as_completed that are optimized for their specific use case (e.g., rely on a batch API provided by a model provider).

note

The async versions of abatch and abatch_as_completed these rely on asyncio's gather and as_completed functions to run the ainvoke method in parallel.

tip

When processing a large number of inputs using batch or batch_as_completed, users may want to control the maximum number of parallel calls. This can be done by setting the max_concurrency attribute in the RunnableConfig dictionary. See the RunnableConfig for more information.

Chat Models also have a built-in rate limiter that can be used to control the rate at which requests are made.

Asynchronous support

Runnables expose an asynchronous API, allowing them to be called using the await syntax in Python. Asynchronous methods can be identified by the "a" prefix (e.g., ainvoke, abatch, astream, abatch_as_completed).

Please refer to the Async Programming with LangChain guide for more details.

Streaming apis

Streaming is critical in making applications based on LLMs feel responsive to end-users.

Runnables expose the following three streaming APIs:

1. sync stream and async astream: yields the output a Runnable as it is generated.
2. The async astream_events: a more advanced streaming API that allows streaming intermediate steps and final output
3. The legacy async astream_log: a legacy streaming API that streams intermediate steps and final output

Please refer to the Streaming Conceptual Guide for more details on how to stream in LangChain.

Input and output types

Every Runnable is characterized by an input and output type. These input and output types can be any Python object, and are defined by the Runnable itself.

Runnable methods that result in the execution of the Runnable (e.g., invoke, batch, stream, astream_events) work with these input and output types.

• invoke: Accepts an input and returns an output.
• batch: Accepts a list of inputs and returns a list of outputs.
• stream: Accepts an input and returns a generator that yields outputs.

The input type and output type vary by component:

Component	Input Type	Output Type
Prompt	dictionary	PromptValue
ChatModel	a string, list of chat messages or a PromptValue	ChatMessage
LLM	a string, list of chat messages or a PromptValue	String
OutputParser	the output of an LLM or ChatModel	Depends on the parser
Retriever	a string	List of Documents
Tool	a string or dictionary, depending on the tool	Depends on the tool

Please refer to the individual component documentation for more information on the input and output types and how to use them.

Inspecting schemas

note

This is an advanced feature that is unnecessary for most users. You should probably skip this section unless you have a specific need to inspect the schema of a Runnable.

In some advanced uses, you may want to programmatically inspect the Runnable and determine what input and output types the Runnable expects and produces.

The Runnable interface provides methods to get the JSON Schema of the input and output types of a Runnable, as well as Pydantic schemas for the input and output types.

These APIs are mostly used internally for unit-testing and by LangServe which uses the APIs for input validation and generation of OpenAPI documentation.

In addition, to the input and output types, some Runnables have been set up with additional run time configuration options. There are corresponding APIs to get the Pydantic Schema and JSON Schema of the configuration options for the Runnable. Please see the Configurable Runnables section for more information.

Method	Description
get_input_schema	Gives the Pydantic Schema of the input schema for the Runnable.
get_output_chema	Gives the Pydantic Schema of the output schema for the Runnable.
config_schema	Gives the Pydantic Schema of the config schema for the Runnable.
get_input_jsonschema	Gives the JSONSchema of the input schema for the Runnable.
get_output_jsonschema	Gives the JSONSchema of the output schema for the Runnable.
get_config_jsonschema	Gives the JSONSchema of the config schema for the Runnable.

With_types

LangChain will automatically try to infer the input and output types of a Runnable based on available information.

Currently, this inference does not work well for more complex Runnables that are built using LCEL composition, and the inferred input and / or output types may be incorrect. In these cases, we recommend that users override the inferred input and output types using the with_types method (API Reference.

RunnableConfig

Any of the methods that are used to execute the runnable (e.g., invoke, batch, stream, astream_events) accept a second argument called RunnableConfig (API Reference). This argument is a dictionary that contains configuration for the Runnable that will be used at run time during the execution of the runnable.

A RunnableConfig can have any of the following properties defined:

Attribute	Description
run_name	Name used for the given Runnable (not inherited).
run_id	Unique identifier for this call. sub-calls will get their own unique run ids.
tags	Tags for this call and any sub-calls.
metadata	Metadata for this call and any sub-calls.
callbacks	Callbacks for this call and any sub-calls.
max_concurrency	Maximum number of parallel calls to make (e.g., used by batch).
recursion_limit	Maximum number of times a call can recurse (e.g., used by Runnables that return Runnables)
configurable	Runtime values for configurable attributes of the Runnable.

Passing config to the invoke method is done like so:

some_runnable.invoke(
   some_input, 
   config={
      'run_name': 'my_run', 
      'tags': ['tag1', 'tag2'], 
      'metadata': {'key': 'value'}
      
   }
)

Propagation of RunnableConfig

Many Runnables are composed of other Runnables, and it is important that the RunnableConfig is propagated to all sub-calls made by the Runnable. This allows providing run time configuration values to the parent Runnable that are inherited by all sub-calls.

If this were not the case, it would be impossible to set and propagate callbacks or other configuration values like tags and metadata which are expected to be inherited by all sub-calls.

There are two main patterns by which new Runnables are created:

1. Declaratively using LangChain Expression Language (LCEL):

   chain = prompt | chat_model | output_parser

2. Using a custom Runnable (e.g., RunnableLambda) or using the @tool decorator:

   def foo(input):
       # Note that .invoke() is used directly here
       return bar_runnable.invoke(input)
   foo_runnable = RunnableLambda(foo)

LangChain will try to propagate RunnableConfig automatically for both of the patterns.

For handling the second pattern, LangChain relies on Python's contextvars.

In Python 3.11 and above, this works out of the box, and you do not need to do anything special to propagate the RunnableConfig to the sub-calls.

In Python 3.9 and 3.10, if you are using async code, you need to manually pass the RunnableConfig through to the Runnable when invoking it.

This is due to a limitation in asyncio's tasks in Python 3.9 and 3.10 which did not accept a context argument).

Propagating the RunnableConfig manually is done like so:

async def foo(input, config): # <-- Note the config argument
    return await bar_runnable.ainvoke(input, config=config)
    
foo_runnable = RunnableLambda(foo)

caution

When using Python 3.10 or lower and writing async code, RunnableConfig cannot be propagated automatically, and you will need to do it manually! This is a common pitfall when attempting to stream data using astream_events and astream_log as these methods rely on proper propagation of callbacks defined inside of RunnableConfig.

Setting custom run name, tags, and metadata

The run_name, tags, and metadata attributes of the RunnableConfig dictionary can be used to set custom values for the run name, tags, and metadata for a given Runnable.

The run_name is a string that can be used to set a custom name for the run. This name will be used in logs and other places to identify the run. It is not inherited by sub-calls.

The tags and metadata attributes are lists and dictionaries, respectively, that can be used to set custom tags and metadata for the run. These values are inherited by sub-calls.

Using these attributes can be useful for tracking and debugging runs, as they will be surfaced in LangSmith as trace attributes that you can filter and search on.

The attributes will also be propagated to callbacks, and will appear in streaming APIs like astream_events as part of each event in the stream.

Related

• How-to trace with LangChain

Setting run id

note

This is an advanced feature that is unnecessary for most users.

You may need to set a custom run_id for a given run, in case you want to reference it later or correlate it with other systems.

The run_id MUST be a valid UUID string and unique for each run. It is used to identify the parent run, sub-class will get their own unique run ids automatically.

To set a custom run_id, you can pass it as a key-value pair in the config dictionary when invoking the Runnable:

import uuid

run_id = uuid.uuid4()

some_runnable.invoke(
   some_input, 
   config={
      'run_id': run_id
   }
)

# Do something with the run_id

Setting recursion limit

note

This is an advanced feature that is unnecessary for most users.

Some Runnables may return other Runnables, which can lead to infinite recursion if not handled properly. To prevent this, you can set a recursion_limit in the RunnableConfig dictionary. This will limit the number of times a Runnable can recurse.

Setting max concurrency

If using the batch or batch_as_completed methods, you can set the max_concurrency attribute in the RunnableConfig dictionary to control the maximum number of parallel calls to make. This can be useful when you want to limit the number of parallel calls to prevent overloading a server or API.

tip

If you're trying to rate limit the number of requests made by a Chat Model, you can use the built-in rate limiter instead of setting max_concurrency, which will be more effective.

See the How to handle rate limits guide for more information.

Setting configurable

The configurable field is used to pass runtime values for configurable attributes of the Runnable.

It is used frequently in LangGraph with LangGraph Persistence and memory.

It is used for a similar purpose in RunnableWithMessageHistory to specify either a session_id / conversation_id to keep track of conversation history.

In addition, you can use it to specify any custom configuration options to pass to any Configurable Runnable that they create.

Setting callbacks

Use this option to configure callbacks for the runnable at runtime. The callbacks will be passed to all sub-calls made by the runnable.

some_runnable.invoke(
   some_input,
   {
      "callbacks": [
         SomeCallbackHandler(),
         AnotherCallbackHandler(),
      ]
   }
)

Please read the Callbacks Conceptual Guide for more information on how to use callbacks in LangChain.

important

If you're using Python 3.9 or 3.10 in an async environment, you must propagate the RunnableConfig manually to sub-calls in some cases. Please see the Propagating RunnableConfig section for more information.

Creating a runnable from a function

You may need to create a custom Runnable that runs arbitrary logic. This is especially useful if using LangChain Expression Language (LCEL) to compose multiple Runnables and you need to add custom processing logic in one of the steps.

There are two ways to create a custom Runnable from a function:

• RunnableLambda: Use this simple transformations where streaming is not required.
• RunnableGenerator: use this for more complex transformations when streaming is needed.

See the How to run custom functions guide for more information on how to use RunnableLambda and RunnableGenerator.

important

Users should not try to subclass Runnables to create a new custom Runnable. It is much more complex and error-prone than simply using RunnableLambda or RunnableGenerator.

Configurable runnables

note

This is an advanced feature that is unnecessary for most users.

It helps with configuration of large "chains" created using the LangChain Expression Language (LCEL) and is leveraged by LangServe for deployed Runnables.

Sometimes you may want to experiment with, or even expose to the end user, multiple different ways of doing things with your Runnable. This could involve adjusting parameters like the temperature in a chat model or even switching between different chat models.

To simplify this process, the Runnable interface provides two methods for creating configurable Runnables at runtime:

• configurable_fields: This method allows you to configure specific attributes in a Runnable. For example, the temperature attribute of a chat model.
• configurable_alternatives: This method enables you to specify alternative Runnables that can be run during run time. For example, you could specify a list of different chat models that can be used.

See the How to configure runtime chain internals guide for more information on how to configure runtime chain internals.