Checkpointers and threads

Ark’s continuity is hand-rolled: session.json, a writeback hook, an incident log. LangGraph ships this as first-class primitives. This module puts a Postgres checkpointer under the module 3 dice graph, kills the process mid-run, resumes, and walks the checkpoint lineage.

A checkpointer persists graph state after every step. Compile the graph with one and every node transition writes a snapshot. If the process dies, the thread resumes from the last snapshot instead of the start.

import { PostgresSaver } from "@langchain/langgraph-checkpoint-postgres";

const checkpointer = PostgresSaver.fromConnString("postgresql://…");
await checkpointer.setup(); // creates the tables, run once

const graph = builder.compile({ checkpointer });

For dev, MemorySaver from @langchain/langgraph is the in-memory equivalent, gone when the process exits. The Postgres and SQLite savers live in their own packages: @langchain/langgraph-checkpoint-postgres and -sqlite.

A thread is a conversation’s identity. You pass a thread_id in config; all checkpoints for that thread are its history. Same thread_id resumes; a new thread_id starts fresh.

const config = { configurable: { thread_id: "impulse-2026-07-04" } };
await graph.invoke(inputs, config);
// process dies here…
// …new process, same config:
await graph.invoke(null, config); // resumes from last checkpoint