Chains

Using an LLM in isolation is fine for simple applications, but more complex applications require chaining LLMs - either with each other or with other components.

LangChain provides two high-level frameworks for “chaining” components. The legacy approach is to use the Chain interface. The updated approach is to use the LangChain Expression Language (LCEL). When building new applications we recommend using LCEL for chain composition. But there are a number of useful, built-in Chain’s that we continue to support, so we document both frameworks here. As we’ll touch on below, Chain’s can also themselves be used in LCEL, so the two are not mutually exclusive.

LCEL

The most visible part of LCEL is that it provides an intuitive and readable syntax for composition. But more importantly, it also provides first-class support for:

• streaming,
• async calls,
• batching,
• parallelization,
• retries,
• fallbacks,
• tracing,
• and more.

As a simple and common example, we can see what it’s like to combine a prompt, model and output parser:

from langchain.chat_models import ChatAnthropic
from langchain.prompts import ChatPromptTemplate
from langchain.schema import StrOutputParser

model = ChatAnthropic()
prompt = ChatPromptTemplate.from_messages(
    [
        (
            "system",
            "You're a very knowledgeable historian who provides accurate and eloquent answers to historical questions.",
        ),
        ("human", "{question}"),
    ]
)
runnable = prompt | model | StrOutputParser()

for chunk in runnable.stream({"question": "How did Mansa Musa accumulate his wealth?"}):
    print(chunk, end="", flush=True)

 Mansa Musa was the emperor of the Mali Empire in West Africa during the 14th century. He accumulated immense wealth through several means:

- Gold mining - Mali contained very rich gold deposits, especially in the region of Bambuk. Gold mining and gold trade was a major source of wealth for the empire.

- Control of trade routes - Mali dominated the trans-Saharan trade routes connecting West Africa to North Africa and beyond. By taxing the goods that passed through its territory, Mali profited greatly.

- Tributary states - Many lands surrounding Mali paid tribute to the empire. This came in the form of gold, slaves, and other valuable resources.

- Agriculture - Mali also had extensive agricultural lands irrigated by the Niger River. Surplus food produced could be sold or traded. 

- Royal monopolies - The emperor claimed monopoly rights over the production and sale of certain goods like salt from the Taghaza mines. This added to his personal wealth.

- Inheritance - As an emperor, Mansa Musa inherited a wealthy state. His predecessors had already consolidated lands and accumulated riches which fell to Musa.

So in summary, mining, trade, taxes,

For more head to the LCEL section.

[Legacy] Chain interface

Chain’s are the legacy interface for “chained” applications. We define a Chain very generically as a sequence of calls to components, which can include other chains. The base interface is simple:

class Chain(BaseModel, ABC):
    """Base interface that all chains should implement."""

    memory: BaseMemory
    callbacks: Callbacks

    def __call__(
        self,
        inputs: Any,
        return_only_outputs: bool = False,
        callbacks: Callbacks = None,
    ) -> Dict[str, Any]:
        ...

We can recreate the LCEL runnable we made above using the built-in LLMChain:

from langchain.chains import LLMChain

chain = LLMChain(llm=model, prompt=prompt, output_parser=StrOutputParser())
chain.run(question="How did Mansa Musa accumulate his wealth?")

" Mansa Musa was the emperor of the Mali Empire in West Africa in the early 14th century. He accumulated his vast wealth through several means:\n\n- Gold mining - Mali contained very rich gold deposits, especially in the southern part of the empire. Gold mining and trade was a major source of wealth.\n\n- Control of trade routes - Mali dominated the trans-Saharan trade routes connecting West Africa to North Africa and beyond. By taxing and controlling this lucrative trade, Mansa Musa reaped great riches.\n\n- Tributes from conquered lands - The Mali Empire expanded significantly under Mansa Musa's rule. As new lands were conquered, they paid tribute to the mansa in the form of gold, salt, and slaves.\n\n- Inheritance - Mansa Musa inherited a wealthy empire from his predecessor. He continued to build the wealth of Mali through the factors above.\n\n- Sound fiscal management - Musa is considered to have managed the empire and its finances very effectively, including keeping taxes reasonable and promoting a robust economy. This allowed him to accumulate and maintain wealth.\n\nSo in summary, conquest, trade, taxes, mining, and inheritance all contributed to Mansa Musa growing the M"

For more specifics check out: - How-to for walkthroughs of different chain features - Foundational to get acquainted with core building block chains - Document to learn how to incorporate documents into chains