Presently the components of OpenVisualizer operate within a single process. We wish to experiment with extending this operation across processes and machines, as shown in the diagram below.
Messaging
Presently we implement EventBus messaging with PyDispatch, which provides communication between threads in a single process. However, we must split the GUI from the LBR when the LBR is headless, like an embedded Linux device. In addition to host-to-host communication, we also may wish to segment the components in the LBR into separate processes, for example to reimplement serial communication in C.
Remote GUI host and LBR
We wish to minimize the processing required by the LBR due to its limited resources, particularly for a sub-Linux implementation. Therefore, the LBR pushes information to the Remote GUI. However, the GUI also may signal to the LBR, for example to toggle DAGroot status.
Although the LBR is the messaging client, the GUI must make the initial contact to the LBR to establish a tunnel for communication. It simply is not feasible to expect a PC user to open a port for access, even ports 80/443. However, we plan to use a reverse SSH tunnel, as shown in the diagram above. The reverse nature of the tunnel allows the LBR to access the server port on the GUI host.
WebSockets (RFC 6455), as a bi-directional messaging protocol, supports our requirements. As a standard, it also supports OpenWSN's approach.
Tools
LBR
| Role | Tool | Notes |
|---|---|---|
| WebSocket client | websocket-client | v0.11; Installed with pip. |
GUI Host
| Role | Tool | Notes |
|---|---|---|
| WebSocket server | gevent-websocket | v0.3.6; Includes dependencies below |
| server dependency | gevent | v0.13.8; Python network library; statically linked to libevent |
| gevent dependency | greenlet | v0.4.1; Python concurrency library |
| Tunnel to LBR | PuTTY | Command line: |
For reference, installed on Windows 7 Home Professional. Also installed pip, v1.3.1, which also required setuptools, and we used v0.7.7.
Messaging Between Processes
On an embedded LBR, we plan to use the native POSIX message queue due to its built-in availability. The posix_ipc module for Python provides access to this functionality.
Alternative Messaging Approaches
We researched the projects below, but they are not our first choice. We may investigate further at a later date.
Nanomsg and ZeroMQ provides a common messaging API for a variety of endpoints – threads, processes, and machines. They are open source, but not standards. However, it would be efficient to use a single tool for messaging across all domains.
Nanomsg is developed by Martin Sústrik, the original author of ZeroMQ. Sústrik left ZeroMQ due to a dispute over trademarks, as described in a Linux Weekly News article.
Celery provides a distributed message queue, but is more heavy-duty and requires a message broker.