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๐Ÿงญ Design Goals and Principles

Minimal time/effort-to-value If a user already has an llm app coded in one of the supported libraries, give them some value with the minimal efford beyond that app.

Currently to get going, a user needs to add 4 lines of python:

from trulens_eval import Tru # line 1
tru = Tru() # line 2
with tru.Chain(app): # 3
    app.invoke("some question") # doesn't count since they already had this

tru.start_dashboard() # 4

3 of these lines are fixed so only #3 would vary in typical cases. From here they can open the dashboard and inspect the recording of their app's invocation including performance and cost statistics. This means trulens must do quite a bit of haggling under the hood to get that data. This is outlined primarily in the Instrumentation section below.


App Data

We collect app components and parameters by walking over its structure and producing a json reprensentation with everything we deem relevant to track. The function jsonify is the root of this process.

class/system specific

pydantic (langchain)

Classes inheriting BaseModel come with serialization to/from json in the form of model_dump and model_validate. We do not use the serialization to json part of this capability as a lot of LangChain components are tripped to fail it with a "will not serialize" message. However, we use make use of pydantic fields to enumerate components of an object ourselves saving us from having to filter out irrelevant internals that are not declared as fields.

We make use of pydantic's deserialization, however, even for our own internal structures (see for example).

dataclasses (no present users)

The built-in dataclasses package has similar functionality to pydantic. We use/serialize them using their field information.

dataclasses_json (llama_index)

Placeholder. No present special handling.

generic python (portions of llama_index and all else)

TruLens-specific Data

In addition to collecting app parameters, we also collect:

  • (subset of components) App class information:

    • This allows us to deserialize some objects. Pydantic models can be deserialized once we know their class and fields, for example.
    • This information is also used to determine component types without having to deserialize them first.
    • See Class for details.


Methods and functions are instrumented by overwriting choice attributes in various classes.

class/system specific

pydantic (langchain)

Most if not all LangChain components use pydantic which imposes some restrictions but also provides some utilities. Classes inheriting BaseModel do not allow defining new attributes but existing attributes including those provided by pydantic itself can be overwritten (like dict, for example). Presently, we override methods with instrumented versions.


  • intercepts package (see

    Low level instrumentation of functions but is architecture and platform dependent with no darwin nor arm64 support as of June 07, 2023.

  • sys.setprofile (see

    Might incur much overhead and all calls and other event types get intercepted and result in a callback.

  • langchain/llama_index callbacks. Each of these packages come with some callback system that lets one get various intermediate app results. The drawbacks is the need to handle different callback systems for each system and potentially missing information not exposed by them.

  • wrapt package (see

    This is only for wrapping functions or classes to resemble their original but does not help us with wrapping existing methods in langchain, for example. We might be able to use it as part of our own wrapping scheme though.


The instrumented versions of functions/methods record the inputs/outputs and some additional data (see RecordAppCallMethod). As more than one instrumented call may take place as part of a app invokation, they are collected and returned together in the calls field of Record.

Calls can be connected to the components containing the called method via the path field of RecordAppCallMethod. This class also holds information about the instrumented method.

Call Data (Arguments/Returns)

The arguments to a call and its return are converted to json using the same tools as App Data (see above).


  • The same method call with the same path may be recorded multiple times in a Record if the method makes use of multiple of its versions in the class hierarchy (i.e. an extended class calls its parents for part of its task). In these circumstances, the method field of RecordAppCallMethod will distinguish the different versions of the method.

  • Thread-safety -- it is tricky to use global data to keep track of instrumented method calls in presence of multiple threads. For this reason we do not use global data and instead hide instrumenting data in the call stack frames of the instrumentation methods. See get_all_local_in_call_stack.

  • Generators and Awaitables -- If an instrumented call produces a generator or awaitable, we cannot produce the full record right away. We instead create a record with placeholder values for the yet-to-be produce pieces. We then instrument (i.e. replace them in the returned data) those pieces with (TODO generators) or awaitables that will update the record when they get eventually awaited (or generated).


Threads do not inherit call stacks from their creator. This is a problem due to our reliance on info stored on the stack. Therefore we have a limitation:

  • Limitation: Threads need to be started using the utility class TP or ThreadPoolExecutor also defined in utils/ in order for instrumented methods called in a thread to be tracked. As we rely on call stack for call instrumentation we need to preserve the stack before a thread start which python does not do.


Similar to threads, code run as part of a asyncio.Task does not inherit the stack of the creator. Our current solution instruments asyncio.new_event_loop to make sure all tasks that get created in async track the stack of their creator. This is done in tru_new_event_loop . The function stack_with_tasks is then used to integrate this information with the normal caller stack when needed. This may cause incompatibility issues when other tools use their own event loops or interfere with this instrumentation in other ways. Note that some async functions that seem to not involve Task do use tasks, such as gather.

  • Limitation: Tasks must be created via our task_factory as per task_factory_with_stack. This includes tasks created by function such as asyncio.gather. This limitation is not expected to be a problem given our instrumentation except if other tools are used that modify async in some ways.


  • Threading and async limitations. See Threads and Async .

  • If the same wrapped sub-app is called multiple times within a single call to the root app, the record of this execution will not be exact with regards to the path to the call information. All call paths will address the last subapp (by order in which it is instrumented). For example, in a sequential app containing two of the same app, call records will be addressed to the second of the (same) apps and contain a list describing calls of both the first and second.

TODO(piotrm): This might have been fixed. Check.

  • Some apps cannot be serialized/jsonized. Sequential app is an example. This is a limitation of LangChain itself.

  • Instrumentation relies on CPython specifics, making heavy use of the inspect module which is not expected to work with other Python implementations.


  • langchain/llama_index callbacks. These provide information about component invocations but the drawbacks are need to cover disparate callback systems and possibly missing information not covered.

Calls: Implementation Details

Our tracking of calls uses instrumentated versions of methods to manage the recording of inputs/outputs. The instrumented methods must distinguish themselves from invocations of apps that are being tracked from those not being tracked, and of those that are tracked, where in the call stack a instrumented method invocation is. To achieve this, we rely on inspecting the python call stack for specific frames:

  • Prior frame -- Each instrumented call searches for the topmost instrumented call (except itself) in the stack to check its immediate caller (by immediate we mean only among instrumented methods) which forms the basis of the stack information recorded alongside the inputs/outputs.


  • Python call stacks are implementation dependent and we do not expect to operate on anything other than CPython.

  • Python creates a fresh empty stack for each thread. Because of this, we need special handling of each thread created to make sure it keeps a hold of the stack prior to thread creation. Right now we do this in our threading utility class TP but a more complete solution may be the instrumentation of threading.Thread class.


  • contextvars -- LangChain uses these to manage contexts such as those used for instrumenting/tracking LLM usage. These can be used to manage call stack information like we do. The drawback is that these are not threadsafe or at least need instrumenting thread creation. We have to do a similar thing by requiring threads created by our utility package which does stack management instead of contextvar management.

    NOTE(piotrm): it seems to be standard thing to do to copy the contextvars into new threads so it might be a better idea to use contextvars instead of stack inspection.