Wrote a long-form technical post on what’s actually happening under the LangGraph API.

The main insight that surprised me: LangGraph’s StateGraph is a high-level abstraction over a Pregel runtime. The real primitives are actors (PregelNodes) and channels - not nodes and state dicts. Reducers are channel update rules, not just a convenience annotation. Once you see it this way, the parallel execution model, checkpointing behavior, and subgraph boundary problem all make sense as consequences of the same design.

Covers:

•   Actors, channels, and reducers

•   Superstep execution - Plan, Execute, Update, Checkpoin

•   compile() internals - what validation runs before inference starts

•   Checkpointing - the four Postgres tables and the write amplification trap

•   Subgraphs vs subagents - structural organization vs context isolation

•   DeepAgents - middleware stack mapped to failure modes

Link to the article: https://internals.laxmena.com/p/langgraph-internals-how-production