Flow Execution

A Contextual agent is the host process for the execution of a Node-RED flow. Node-RED runs on a single-threaded event loop (like Node.js) but can handle multiple concurrent operations through non-blocking asynchronous callbacks.

However, the Agent-type (HTTP or Event) determines whether multiple requests can be handled concurrently. HTTP agents can handle multiple concurrent operations, but Event agents handle one message at a time received from the message service.

While the agent process hosts the Node-RED flow, it also interfaces to other Contextual services such as the logging service. This allows you to use Log-tap nodes in your flows which emit data which is then sent to the logging service by the Agent.

Similarly, it is the agent software outside of Node-RED that receives incoming events and injects them into the flow at the apporpriate input node.

And while you can catch errors within your flow using a Catch Node, the agent itself will catch any uncaught errors and log them to the logging service, allowing you to further diagnose problems with your flows.

Flow Editor and deployed runtime

The Flow Editor and a deployed agent use different host environments. Host-provided message properties can differ even when the flow is the same. For example, see HTTP In: request bodies and webhook signatures for the scope of msg.req.rawBody. Verify that msg.req.rawBody is available against the actual deployed endpoint rather than inferring its availability solely from an editor test.