Distributed Content Generation

This page covers the serialization mechanisms that enable content generation (LLM structured output, image generation, PDF extraction) to run across Temporal worker processes. For the broader Temporal architecture — LibraryCrate propagation, deferred hydration, per-workflow scoping — see Temporal Integration.

The Two Serialization Challenges

Content generation introduces two cross-cutting problems that don't exist in single-process mode:

Dynamic Class Propagation

Schema embedding

When a pipe produces structured output, the caller captures the Pydantic model's JSON schema into the assignment — a serializable Temporal argument that carries everything the worker needs:

class ObjectAssignment(BaseModel):
    object_class_name: str
    object_class_schema: dict[str, Any]
    llm_assignment_for_object: LLMAssignment

    @staticmethod
    def make_for_class(
        object_class: type[BaseModel],
        llm_assignment: LLMAssignment,
    ) -> "ObjectAssignment":
        return ObjectAssignment(
            object_class_name=object_class.__name__,
            object_class_schema=object_class.model_json_schema(),
            llm_assignment_for_object=llm_assignment,
        )

The schema is a plain dict — fully serializable, visible in the Temporal dashboard, and self-contained. No class reference crosses the process boundary.

Schema-to-model reconstruction

On the worker, model_class_from_json_schema() rebuilds a live Pydantic class from the embedded schema:

source_code = _generate_source_from_schema(schema)        # datamodel-code-generator
reconstructed_class = _exec_and_extract_class(source_code, class_name)  # exec()
reconstructed_class.__kajson_class_source__ = source_code  # attached for downstream use

The reconstruction pipeline:

1. Generate — datamodel-code-generator converts the JSON schema into Python source code defining a BaseModel subclass.
2. Execute — exec() compiles the source in an isolated namespace and extracts the named class.
3. Cache — A thread-safe SHA-256 hash cache (with double-check locking) avoids redundant generation for the same schema.
4. Tag — The generated source code is attached as __kajson_class_source__ on the class, enabling Kajson to carry it through Temporal payloads.

Source code injection into Temporal payloads

When the activity returns a BaseModel whose class has __kajson_class_source__, the custom BaseModelPayloadConverter injects it into the Temporal payload metadata:

Serialization (to_payload):

class_source = getattr(type(value), "__kajson_class_source__", None)
if class_source is not None:
    metadata["kajson_class_source"] = class_source.encode()

Deserialization (from_payload):

source_bytes = payload.metadata.get("kajson_class_source")
class_source_code = source_bytes.decode() if source_bytes else None
pydantic_gizmo = kajson.loads(
    data,
    class_registry=get_class_registry(),
    class_source_code=class_source_code,
)
# Re-attach source so it survives further Temporal hops
if class_source_code is not None:
    if isinstance(pydantic_gizmo, BaseModel):
        type(pydantic_gizmo).__kajson_class_source__ = class_source_code
    elif isinstance(pydantic_gizmo, list):
        for item in pydantic_gizmo:
            if isinstance(item, BaseModel):
                type(item).__kajson_class_source__ = class_source_code

The source code rides in Temporal metadata (not the payload data itself), keeping the JSON payload clean and the class reconstruction transparent. After kajson.loads() reconstructs the object, the converter re-attaches __kajson_class_source__ to the deserialized class — this ensures the source survives if the object crosses another Temporal boundary (e.g. workflow returning a result to a parent activity that forwards it elsewhere).

The type bridge

The class reconstructed on the worker is a structural match to the original, but it is a different Python class in memory. Standard isinstance() checks would fail. The caller bridges this gap with a model_validate round-trip:

raw_obj = await llm_gen_object(object_assignment=object_assignment)
return object_class.model_validate(raw_obj.model_dump(serialize_as_any=True))

model_dump(serialize_as_any=True) produces a plain dict from the reconstructed object, and model_validate() rebuilds it as a proper instance of the original class. This restores type safety for downstream code.

End-to-end flow

sequenceDiagram
    participant S as Submitter
    participant W as Worker Activity
    participant LLM as LLM Provider

    S->>S: object_class.model_json_schema()
    S->>W: ObjectAssignment (class_name + schema)

    Note over W: model_class_from_json_schema()
    W->>W: datamodel-code-generator → source
    W->>W: exec() → class + __kajson_class_source__

    W->>LLM: gen_object(schema=reconstructed_class)
    LLM-->>W: structured JSON

    W-->>S: BaseModel result<br/>(metadata: kajson_class_source)

    Note over S: kajson.loads(class_source_code=...)
    S->>S: model_validate() → original type

Large Payload Management

The payload size constraint

Temporal payloads have a practical limit around 2MB. A single generated image can exceed this in base64 encoding. PDF extraction produces multiple page images. Sending raw binary through Temporal would either fail with a DataConverterError or severely degrade workflow history performance.

The store-then-reference pattern

Activities handle the full binary lifecycle internally — generate, store, return a lightweight reference:

@activity.defn
async def act_img_gen_images(img_gen_assignment: ImgGenAssignment) -> list[ImageContent]:
    """Generate images and store them, returning lightweight ImageContent references.

    Large binary data (base64/bytes) is stored within the activity and never crosses
    the Temporal workflow boundary — only URLs are returned.
    """
    storage_provider = get_storage_provider()
    generated_content_factory = GeneratedContentFactory(storage_provider=storage_provider)
    return await img_gen_image_list_and_store(
        img_gen_assignment=img_gen_assignment,
        generated_content_factory=generated_content_factory,
    )

GeneratedContentFactory uploads binary data via the configured StorageProviderAbstract (S3, local filesystem, etc.) and returns ImageContent or PageContent — Pydantic models carrying URIs, MIME types, and metadata, but never raw bytes.

Content type scenarios

Kajson Class Resolution

When kajson.loads() receives both a class_registry and class_source_code, the resolution follows a priority chain:

if class_source_code is not None:
    source_registry = _build_registry_from_source(class_source_code)
    if class_registry is not None:
        # Source-derived classes fill gaps; explicit registry takes priority
        for name, cls in source_registry.root.items():
            if not class_registry.has_class(name):
                class_registry.register_class(cls, name=name, ...)
    else:
        class_registry = source_registry

Resolution order:

1. Explicit class_registry — the per-workflow scoped registry (see Temporal Integration)
2. Source-derived classes — from class_source_code, filling gaps in the explicit registry
3. sys.modules — already-imported classes
4. Dynamic import — last resort, importing the module path from __module__ metadata

File Reference

Next Steps

• Temporal Integration — LibraryCrate propagation, deferred hydration, per-workflow scoping
• Pipe Routing & Execution — how PipeJobs travel through the system
• Architecture Overview — the two-layer design and how components fit together