Add LGP Arch page (#3428)

docs/docs/concepts/index.md

@@ -26,8 +26,8 @@ The conceptual guide does not cover step-by-step instructions or specific implem
 - [Human-in-the-Loop](human_in_the_loop.md): Explains different ways of integrating human feedback into a LangGraph application.
 - [Time Travel](time-travel.md): Time travel allows you to replay past actions in your LangGraph application to explore alternative paths and debug issues.
 - [Persistence](persistence.md): LangGraph has a built-in persistence layer, implemented through checkpointers. This persistence layer helps to support powerful capabilities like human-in-the-loop, memory, time travel, and fault-tolerance.
-- [Memory](memory.md): Memory in AI applications refers to the ability to process, store, and effectively recall information from past interactions. With memory, your agents can learn from feedback and adapt to users' preferences.  
-- [Streaming](streaming.md): Streaming is crucial for enhancing the responsiveness of applications built on LLMs. By displaying output progressively, even before a complete response is ready, streaming significantly improves user experience (UX), particularly when dealing with the latency of LLMs. 
+- [Memory](memory.md): Memory in AI applications refers to the ability to process, store, and effectively recall information from past interactions. With memory, your agents can learn from feedback and adapt to users' preferences.
+- [Streaming](streaming.md): Streaming is crucial for enhancing the responsiveness of applications built on LLMs. By displaying output progressively, even before a complete response is ready, streaming significantly improves user experience (UX), particularly when dealing with the latency of LLMs.
 - [Functional API (beta)](functional_api.md): An alternative to [Graph API (StateGraph)](low_level.md#stategraph) for development in LangGraph.
 - [FAQ](faq.md): Frequently asked questions about LangGraph.
 
@@ -37,7 +37,6 @@ LangGraph Platform is a commercial solution for deploying agentic applications i
 
 The LangGraph Platform offers a few different deployment options described in the [deployment options guide](./deployment_options.md).
 
-
 !!! tip
 
     * LangGraph is an MIT-licensed open-source library, which we are committed to maintaining and growing for the community.
@@ -46,6 +45,7 @@ The LangGraph Platform offers a few different deployment options described in th
 ### High Level
 
 - [Why LangGraph Platform?](./langgraph_platform.md): The LangGraph platform is an opinionated way to deploy and manage LangGraph applications. This guide provides an overview of the key features and concepts behind LangGraph Platform.
+- [Platform Architecture](./platform_architecture.md): A high-level overview of the architecture of the LangGraph Platform.
 - [Deployment Options](./deployment_options.md): LangGraph Platform offers four deployment options: [Self-Hosted Lite](./self_hosted.md#self-hosted-lite), [Self-Hosted Enterprise](./self_hosted.md#self-hosted-enterprise), [bring your own cloud (BYOC)](./bring_your_own_cloud.md), and [Cloud SaaS](./langgraph_cloud.md). This guide explains the differences between these options, and which Plans they are available on.
 - [Plans](./plans.md): LangGraph Platforms offer three different plans: Developer, Plus, Enterprise. This guide explains the differences between these options, what deployment options are available for each, and how to sign up for each one.
 - [Template Applications](./template_applications.md): Reference applications designed to help you get started quickly when building with LangGraph.
@@ -54,7 +54,7 @@ The LangGraph Platform offers a few different deployment options described in th
 
 The LangGraph Platform comprises several components that work together to support the deployment and management of LangGraph applications:
 
-- [LangGraph Server](./langgraph_server.md): The LangGraph Server is designed to support a wide range of agentic application use cases, from background processing to real-time interactions. 
+- [LangGraph Server](./langgraph_server.md): The LangGraph Server is designed to support a wide range of agentic application use cases, from background processing to real-time interactions.
 - [LangGraph Studio](./langgraph_studio.md): LangGraph Studio is a specialized IDE that can connect to a LangGraph Server to enable visualization, interaction, and debugging of the application locally.
 - [LangGraph CLI](./langgraph_cli.md): LangGraph CLI is a command-line interface that helps to interact with a local LangGraph
 - [Python/JS SDK](./sdk.md): The Python/JS SDK provides a programmatic way to interact with deployed LangGraph Applications.
@@ -71,8 +71,7 @@ The LangGraph Platform comprises several components that work together to suppor
 
 ### Deployment Options
 
-
 - [Self-Hosted Lite](./self_hosted.md): A free (up to 1 million nodes executed per year), limited version of LangGraph Platform that you can run locally or in a self-hosted manner
 - [Cloud SaaS](./langgraph_cloud.md): Hosted as part of LangSmith.
 - [Bring Your Own Cloud](./bring_your_own_cloud.md): We manage the infrastructure, so you don't have to, but the infrastructure all runs within your cloud.
-- [Self-Hosted Enterprise](./self_hosted.md): Completely managed by you.
+- [Self-Hosted Enterprise](./self_hosted.md): Completely managed by you.

docs/docs/concepts/platform_architecture.md

@@ -0,0 +1,23 @@
+# LangGraph Platform Architecture
+
+![](img/langgraph_platform_deployment_architecture.png)
+
+## How we use Postgres
+
+Postgres is the persistence layer for all user and run data in LGP. This stores both checkpoints (see more info [here](./persistence.md)) as well as the server resources (threads, runs, assistants and crons).
+
+## How we use Redis
+
+Redis is used in each LGP deployment as a way for server and queue workers to communicate, and to store ephemeral metadata, more details on both below. No user/run data is stored in Redis.
+
+### Communication
+
+All runs in LGP are executed by the pool of background workers that are part of each deployment. In order to enable some features for those runs (such as cancellation and output streaming) we need a channel for two-way communication between the server and the worker handling a particular run. We use Redis to organize that communication.
+
+1. A Redis list is used as a mechanism to wake up a worker as soon as a new run is created. Only a sentinel value is stored in this list, no actual run info. The run information is then retrieved from Postgres by the worker.
+2. A combination of a Redis string and Redis PubSub channel is used for the server to communicate a run cancellation request to the appropriate worker.
+3. A Redis PubSub channel is used by the worker to broadcast streaming output from an agent while the run is being handled. Any open `/stream` request in the server will subscribe to that channel and forward any events to the response as they arrive. No events are stored in Redis at any time.
+
+### Ephemeral metadata
+
+Runs in an LGP deployment may be retried for specific failures (currently only for transient Postgres errors encountered during the run). In order to limit the number of retries (currently limited to 3 attempts per run) we record the attempt number in a Redis string when is picked up. This contains no run-specific info other than its ID, and expires after a short delay.