The Use of Linux in an Embedded System
One of the primary questions to ask yourself at the end of the project is: Should we have done it differently? Although there is a lot of pressure to follow the Microsoft flow because people are familiar with that environment, I'd have to answer “No.” The system as we installed it has met or exceeded all of the customer's expectations. The facilities provided with the linux operating system have allowed us to deliver a system that is reliable and is easily serviced and upgraded. For example, cron is used in conjunction with the logrotate utility to ensure that the log files don't fill up all disk space. Yet, at the same time, we can easily review the last four weeks of data. In a similar vein, PostgreSQL provides all of the database services that are required and, along with a quick script, delete stale data. These together keep the system from getting bogged down with bloated files. Other utilities come into play as well, the apcupsd daemon monitors the ups and ensures an orderly shutdown if necessary. As a side benefit, we also have a log of the power in the building. Using mgetty with pppd allows dial-in for any servicing that may be required. So far, all updates to the system have been handled through this path as the current network connection between us and the site is very slow.
The choice of using apache to provide the GUI couldn't have been better. When we started, the idea was to allow any browser on the network to gain access and allow updates to occur from remote locations. As noted above, the network connection is slow so this has not happened to any great extent. Also, by coincidence, the initial site did not have any other local machines. To take care of this situation, the X windows system was put into use and the Netscape browser is used at the machine. A special login was defined that placed the user into Netscape and on exiting X windows, the user is logged out. Security has been retained and the user is none the wiser.
Another thing to ask is: Where might we be headed in the future? The first step is provide a serial link to report activity to the external security system. A second is to have updates to the system database be done from another machine. Initial discussions centered around using a spare serial ports for these functions. No problem! In getting this far, we have all the experience with serial programming to know exactly what's needed for both tasks. Later discussions have moved towards using a TCP/IP link for the update function. Again, no problem with the networking environment provided by linux!
We have also been asked by the customer about setting up a central monitoring facility with the intent of being able report hardware failures. With the networking capabilities, both direct and dial-in, it was fairly simple to propose a system where a central unit could poll all of the units in the field. Alternatively, the field systems could call in and report their health. The system could even be a hybrid of these two approaches! The customer has not yet responded to this proposal, but it is not a big step to see that not only can all systems be monitored, but all program updates could be handled through this mechanism as well.
As you may see from my enthusiasm, I'm all for linux! The biggest barrier we've had is that of the familiar environment that people have come to expect. My expectations are that, given time, there will be a shift in the linux world and there will be 'education' of the masses and it will become more natural to use linux for projects of this type. In anticipation of this type of shift, the system was recently ported over to Red Hat version 6.0. With some minor tweaking for Apache and a recompilation of our code to use the new libraries, the transition was fairly uneventful.
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