The following is a question-answer on examples of programs that highly depend on time values and may be affected if an abnormal time jump is noticed.
Question
The chrony documentation warns
BE WARNED: Certain software will be seriously affected by such jumps in the system time. (That is the reason why chronyd uses slewing normally.) Documentation
But the documentation gives no examples. What are examples of software that will be seriously affected? Is the OS or any background processes at risk?
Answer
This is a bit of open question but let me give some examples:
- databases - most of them rely a lot of precise time for storing records, indexes, etc
- security - precise time is very important for security to map action to time and gaps or time duplication is not accepted
- digital signing - usually part of signed document is the timestamp so wrong time may invalidate the signature
- scheduling software - may skip or repeat twice jobs depend of time jump direction.
- clustering software - probably any cluster will need to be in sync and any jump of one or more nodes may have unpredictable result.
Extra
I recently got bit by a bug that dates back to 1999 and affects both the JVM and Android Runtime: https://bugs.java.com/bugdatabase/view_bug.do?bug_id=4290274
… two extra executions are fired (unexpectedly) when the system clock is set ahead one minute after the task is scheduled using scheduleAtFixedRate().
I work on a device that starts with the 1970 epoch as the current time, then receives the correct network time a little later. Occasionally a 3rd party library would initialize before the time was set, causing it to experience a 50 year time jump.
The result was scheduleAtFixedRate attempting to catch up on ~50 years worth of invocations… which was about 27 million back-to-back invocations with no delay between them.
That would cause the GC to go haywire and generally bog down the system until it was restarted
#reads #stack overflow #romeo ninov #unix #linux #real-time #chrony