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title	description	date	math
Oppsumering av TTK4145	Lot of theory and discussion, some fomulas, spring 2021.	2021-05-04	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

Make the program work within specs.
Run/Tests of the program-
Errors happen
Locate errors
- Incomplete spec
- Missing handleling of som situation
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.

With fault tolerance the path looks something more like the figure under.

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.

Fault model

Example with storage functions.

Step 1: Failure modes

Find the failure modes: What could go wrong?

Write: May return "I failed". Does not know why it faield
Read: May return "I failed". Does not know why it failed.

Step 2: Detect, Simplify, Inject errors

Write information on where/what/how the process is doing.
All errors --> Fail
Inject errors

Step 3: Handling with redundancy

Have multiple copies of the the information
- Use only the newest

Example with communication function