Presently the components of OpenVisualizer operate within a single process. We wish to experiment with extending this operation However, in a realistic sensor networking implementation, we do not wish to deploy a PC as an LBR! This page presents the first steps toward segmenting the components of OpenVisualizer across processes and machines, using an architecture as shown in the diagram below.
| Gliffy | ||
|---|---|---|
|
As a first step toward this goal, we plan to separate the GUI to run on a PC, and an LBR to run on an inexpensive, low-power, embedded Linux device.
| Gliffy | ||
|---|---|---|
|
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.
...
| Role | Tool | Notes |
|---|---|---|
| WebSocket server | gevent-websocket | v0.3.6; Includes dependencies below |
| server dependency | gevent | v0.13.8; Python network library; statically linked with libevent |
| gevent dependency | greenlet | v0.4.1; Python concurrency library |
| Tunnel to LBR | PuTTY | Command line: For production use, replace password with public key authentication. |
For reference, installed the GUI host on Windows 7 Home Professional. Also installed pip, v1.3.1 for component setup, which required setuptools, where we used v0.7.7..
Booting WebSocket Connection
The GUI host must signal to the LBR when it wishes to connect. This signal must trigger the LBR to connect to the GUI hosts's WebSocket server. We leverage the POSIX signal facility to accomplish this goal.
| Remote GUI | LBR |
|---|---|
| On startup, create a signal handler on SIGUSR1 | |
At some later time...
| |
Signal received, and handler connects to WebSocket server on forwarded portws://localhost:8000/lbr |
The signal above assumes the LBR is in the form of the openVisualizerCli module, which probably is the simplest solution.
JSON Messages
JSON provides a convenient and widely accepted mechanism for implementation of the structured messages we must pass between LBR and GUI host.
Messages include:
| Message | Description |
|---|---|
| mote-inventory | A list An array of the connected motes. Sent just after establishing the connection to the GUI host. |
| mote-status | A dictionary of An object with the elements of a Mote Status report. Like OpenVisualizer, these reports are generated at some defined rate. |
| event-count | A dictionary of An object with the current count of eventseach event. |
Messaging Between Processes
As future work on an for the embedded LBR, we plan to use the native POSIX message queue for inter-process messaging, 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.
...