Suppose you have multiple processes appending output to the same file without explicit synchronization. These processes might be working in parallel on different parts of the same problem, or these might be threads blocked individually reading different external inputs. There are two concerns that come into play:
1) The append must be atomic such that it doesn’t clobber previous appends by other threads and processes. For example, suppose a write requires two separate operations: first moving the file pointer to the end of the file, then performing the write. There would be a race condition should another process or thread intervene in between with its own write.
2) The output will be interleaved. The primary solution is to design the data format as atomic records, where the ordering of records is unimportant — like rows in a relational database. This could be as simple as a text file with each line as a record. The concern is then ensuring records are written atomically.