ricxappframe 3.0.2

Reactive Xapps

A reactive Xapp acts on messages that are delivered (pushed) via RMR. The Xapp only takes action upon receipt of an RMR message. The Xapp never takes action at another time.

This type of application is constructed by creating callback functions and registering them with the framework by message type. When an RMR message arrives, the appropriate callback is invoked based on the message type. An Xapp may define and register a separate callback for each expected message type. Every Xapp must define a default callback function, which is invoked when a message arrives for which no type-specific callback was registered. An analogy of this is AWS Lambda: “execute this code every time an event comes in” (the code to execute can depend on the type of event).

General Xapps

A general Xapp acts according to its own criteria, which may include receipt of RMR messages.

This type of application is constructed by creating a single function that is invoked by the framework after initialization. Typically that function contains a while (something) event loop. When the function returns, the Xapp stops. In this usage, the Xapp must fetch its own data, either from RMR, SDL or other source. The framework does less work for a general application compared to a reactive application; the framework only sets up an RMR thread and an SDL connection before invoking the client-provided function.

Threading in the Framework

RMR interactions are processed in a thread started by the framework. This implementation detail is documented here for transparency, but most users will not have to worry about this.

In both types of Xapp, the framework launches a separate thread whose only job is to read from RMR and deposit all messages (and their summaries) into a thread-safe queue. When the client Xapp reads from RMR using the framework (this read is done by the framework itself in the RMR Xapp, but by the client in a general Xapp), the read is done from the framework-managed queue. The framework is implemented this way so that a long-running client function (e.g., consume) will not block RMR reads. This is important because RMR is not a persistent message bus; if an RMR client does not read fast enough, messages can be lost. So in this framework the client code is not in the same thread as the RMR reads, to ensure that long-running client code will not cause message loss.

In the case of RMR Xapps, there are currently 3 potential threads; the thread that reads from RMR directly, and the user can optionally have the RMR queue read run in a thread, returning execution back to the user thread. The default is only two threads however, where .run does not return back execution and the user code is finished at that point.

Healthchecks

The framework provides a default RMR healthcheck probe handler for reactive Xapps. When an RMR healthcheck message arrives, this handler checks that the RMR thread is healthy (of course the Xapp cannot even reply if the thread is not healthy!), and that the SDL connection is healthy. The handler responds accordingly via RMR. The Xapp can override this probe handler by registering a new callback for the healthcheck message type.

The framework provides no healthcheck handler for general Xapps. Those applications must handle healthcheck probe messages appropriately when they read their RMR mailboxes.

There is no http service in the framework, so there is no support for HTTP-based healthcheck probes, such as what a deployment manager like Kubernetes may use.

Examples

Two sample Xapps using this framework are provided in the examples directory of the git repository. The first, ping, is a general Xapp that defines a main function that reads its RMR mailbox in addition to other work. The second, pong, is a reactive Xapp that only takes action when a message is received.

To run a demonstration, build the Docker images for both examples using the supplied Dockerfiles. Then start the Pong container (the listener) followed by the Ping container (the sender). The Ping application sends a message, the pong application receives the message and use RMR’s return-to-sender feature to reply. Ping then reads its own mailbox and demonstrates other functionality.