27/02/2017

Report from the battlefield #8 - always remember about the context

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Title: Sunrise seen from the top of Mount Fuji, , Source: own resources, Authors: Agnieszka and Michał Komorowscy

I decided to change a little bit a character of Report from the battlefield series. Initially, in this series, I was describing my observations from my work as a reviewer for a recruitment company. Now, I'll be also writing about my findings from my day-to-day work. To start, I'll give you a tip how to log useful information.

I worked with an application that is responsible for monitoring folders. If it detects any new files, they are processed and copied somewhere else. The application logs information like the number of files to be processes, the file that is currently being processed etc. This information are logged with severity Information or Debug. It happens that a given file cannot be copied for example because the file with the same name already exists in the destination directory. In that case .NET throws System.IO.IOException. This exception is caught and logged with the severity Error. The simplified version of the log could look in this way:

INFO - 10 files have been found
INFO - Processing file started
...
INFO - Processing file ended
INFO - Processing file started
ERROR - An error occurred while processing a file: Cannot create a file when that file already exists.
...
INFO - Processing file ended
...

It's good that many important information are logged. However, there is a major issue with this log. The lack of the context! For example we know that some files have been processed but we don't know which exactly. This log should look as follows (I used red color to mark changes):

INFO - 10 files have been found in the directory 'C:\Input'
INFO - Processing file 'C:\Input\a.txt' started
...
INFO - Processing file 'C:\Input\a.txt' ended
INFO - Processing file 'C:\Input\b.txt' started
ERROR - An error occurred while processing a file: Cannot create a file when that file already exists.
...
INFO - Processing file 'C:\Input\b.txt' ended
...

It looks much better. Based on the log we can figure out which directory was monitored, which files have been processed successfully and which not. However, it's not everything. There is one more subtle problem here. What if messages with the severity Information or lower won't be logged (for example because of performance issues) and an error will be reported. In this case we'll get the following log:

...
ERROR - An error occurred while processing a file: Cannot create a file when that file already exists.
...

It's better than nothing but again we don't know processing of which file has actually failed. The expected result is:

...
ERROR - An error occurred while processing a file 'C:\Input\b.txt': Cannot create a file when that file already exists.
...

To sum up, always remember about the context when logging.

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