Wrap up of day 1 in exam period spring 2021

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Øyvind Skaaden 2021-05-04 22:24:30 +02:00
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course: ttk4145
desc: Sanntidsprogrammering, våren 2021.
updated: 2021-05-04
-
course: tiø4252
desc: Teknologiledelse, våren 2021.
updated: 2021-05-04
# Høsten 2020
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---
title: "Oppsumering av TTK4145"
description: "Lot of theory and discussion, some fomulas, spring 2021."
date: 2021-05-04
math: true
---
## Fault tolerance
Hard to capture faults.
### Bugs
* 1 bug per 50 lines before testing
* 1 bug per 500 at release
* 1 bug per 550 after a year, the constant
1. Make the program work within specs.
2. Run/Tests of the program-
3. Errors happen
4. Locate errors
* Incomplete spec
* Missing handleling of som situation
5. Fix code
### Traditional error handeling
{% highlight c %}
FILE *
openConfigFile(){
FILE * f = fopen("/path/to/config.conf");
if (f == NULL) {
switch(errno){
case ENOMEM: {
...
break;
}
case ENOTDIR: {
...
break;
}
// Do this for all errors
}
}
}
{% endhighlight %}
### Causes of errors
* Incomplete specification
* Software bugs
* HW problems
* Communication problems
### Fault tolerance in real time systems
The problem with traditional errorhandleing is that errors can happen at any possible time.
This is extremely hard to test.
This is some of the error handling real time programming have.
* Handling of unexpected errors
* More threads hanles errors
* Can not test the conventional way
* Can only show extistence of errors
* Can not find errors in specification
* Can not find race conditions
The fault path is shown under.
![Fault tolerance](figures/fault-path.svg)
With fault tolerance the path looks something more like the figure under.
![Fault tolerance](figures/fault-tolarance.svg)
### Error handling
Keep it simple!
The error modes is a part of the module interface.
One way is to handle all errors the same way.
Handle the as if it was the worst error.
Crash and start again.
A different approach is to check that everything is OK.
To test how the systems responds for a unknown error is to insert a failed acceptance test (a not OK signal).
### Redundancy
* If I have $N$ copies of my data, it is possible to handle that one is destroyed.
* Sending $N$ messages, trying $N$ times.
**Static redundancy**
* $N$ active copies. Sending $N$ messages if it is necessary or not.
* Detecting errors is not important.
* Handles cosmic rays easily.
**Dynammic redunancy**
* Relies on detecting the error and recovering
* Resend if timeout and not receiving "ack"
* Go with default if no messages have been received
* The acceptancetest must be good.

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## Oppsummering
[Oppsummering](summary/) av faget TTK4145.
## Prosjekt
## Øvinger