docs: Add docs for POSTGRES_URI_CUSTOM environment variable (#2951)

docs/docs/cloud/reference/env_var.md

@@ -1,6 +1,6 @@
 # Environment Variables
 
-The LangGraph Cloud API supports specific environment variables for configuring a deployment.
+The LangGraph Cloud Server supports specific environment variables for configuring a deployment.
 
 ## `LANGCHAIN_TRACING_SAMPLING_RATE`
 
@@ -10,10 +10,34 @@ See <a href="https://docs.smith.langchain.com/how_to_guides/tracing/sample_trace
 
 ## `LANGGRAPH_AUTH_TYPE`
 
-Type of authentication for the LangGraph Cloud API deployment. Valid values: `langsmith`, `noop`.
+Type of authentication for the LangGraph Cloud Server deployment. Valid values: `langsmith`, `noop`.
 
 For deployments to LangGraph Cloud, this environment variable is set automatically. For local development or deployments where authentication is handled externally (e.g. self-hosted), set this environment variable to `noop`.
 
 ## `N_JOBS_PER_WORKER`
 
 Number of jobs per worker for the LangGraph Cloud task queue. Defaults to `10`.
+
+## `POSTGRES_URI_CUSTOM`
+
+For [Bring Your Own Cloud (BYOC)](../../concepts/bring_your_own_cloud.md) deployments only.
+
+Specify `POSTGRES_URI_CUSTOM` to use an externally managed Postgres instance. The value of `POSTGRES_URI_CUSTOM` must be a valid [Postgres connection URI](https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNSTRING-URIS).
+
+Postgres:
+
+- Version 15.8 or higher.
+- An initial database must be present and the connection URI must reference the database.
+
+Control Plane Functionality:
+
+- If `POSTGRES_URI_CUSTOM` is specified, the LangGraph Control Plane will not provision a database for the server.
+- If `POSTGRES_URI_CUSTOM` is removed, the LangGraph Control Plane will not provision a database for the server and will not delete the externally managed Postgres instance.
+- If `POSTGRES_URI_CUSTOM` is removed, deployment of the revision will not succeed. Once `POSTGRES_URI_CUSTOM` is specified, it must always be set for the lifecycle of the deployment.
+- If the deployment is deleted, the LangGraph Control Plane will not delete the externally managed Postgres instance.
+- The value of `POSTGRES_URI_CUSTOM` can be updated. For example, a password in the URI can be updated.
+
+Database Connectivity:
+
+- The externally managed Postgres instance must be accessible by the LangGraph Server service in the ECS cluster. The BYOC user is responsible for ensuring connectivity.
+- For example, if an AWS RDS Postgres instance is provisioned, it can be provisioned in the same VPC (`langgraph-cloud-vpc`) as the ECS cluster with the `langgraph-cloud-service-sg` security group to ensure connectivity.

docs/docs/concepts/bring_your_own_cloud.md

@@ -39,6 +39,7 @@ LangChain has no direct access to the resources created in your cloud account, a
             - Read CloudWatch metrics/logs to monitor your instances/push deployment logs
         - https://docs.aws.amazon.com/aws-managed-policy/latest/reference/AmazonRDSFullAccess.html
             - Provision `RDS` instances for your LangGraph Cloud instances
+            - Alternatively, an externally managed Postgres instance can be used instead of the default `RDS` instance. LangChain does not monitor or manage the externally managed Postgres instance. See details for [`POSTGRES_URI_CUSTOM` environment variable](../cloud/reference/env_var.md#postgres_uri_custom).
     2. Either
         - Tags an existing vpc / subnets as `langgraph-cloud-enabled`
         - Creates a new vpc and subnets and tags them as `langgraph-cloud-enabled`