Our teamlead said that many files much worse than a single, despite of we are working on a big project.He argued that our customers could more easily send logs to us if there will be just a single file.To understand our log-file he wants to develope our own log-analyzer which will analyze a single log-file separating it to many 
Is it normal to write all logs into a single file?
programming practices;patterns and practices;design;logging
Please note that best practice questions are discouraged, because they are too easily answered as if there were a single way of doing something that is superior to all others. Almost always, there isn't such a way.When deciding whether to use one log stream or different ones simultaneously, you have to weigh what the cost and benefit is in the expected use case. Writing everything to one stream is easy when programming, but makes for more effort when you have to retrieve or analyse logs. Pre-sorting all log output takes more effort when programing every log message but saves some effort when reading logs.In the situations I'm familiar with, log messages are programmed more often than they are reported by customers or searched by support staff, so it makes sense to make the common case (developing) easy at the cost of more effort in the rarer case (troubleshooting). Analysing a log involves searching anyway, and searching a longer file is not much more effort than searching one of several shorter files because the computer does the main work, not your brain.But that is just my project and my worldview. If (lucky you!) you have millions of customers and a huge traffic in support requests, then logs will be analysed much more often than they are written. In that case it makes sense to make reporting, analysing and correlating logs as easy as possible at the cost of making it more difficult to program a new log message. Then you should put more effort into making the reporting and analysis workflow efficient. (For instance, if customers have trouble gathering all relevant logs, it's a good idea to program log retrieval into your software so that the admin user need only press a button to send everything relevant.)