when the streaming fails after reading 50 lines of a file. Will it start from 51st line of the file when started next time Or will it again read from the start of the file.
Either the entire file is fully processed or none at all. That's how FileFormat works in general in Spark SQL and has little to do with Spark Structured Streaming in particular (since they share the underlying execution infrastructure).
In short, the engine "will it again read from the start of the file."
That also to say that there is no concept of a single line while processing files in Spark Structured Streaming. You process a streaming DataFrame that is an entire file (or even a couple of files) all at once and whether you want to process the dataset line by line or in its entirety is up to you, a Spark developer.
Also, will we have any issues if we use checkpointing in structured streaming when there is any upgrade or any change in the code.
In theory, you should not. The purpose of the new checkpointing mechanism in Spark Structured Streaming (as compared to the legacy Spark Streaming's) was to allow for restarts and upgrades in a more comfortable way. The checkpointing uses just a little information (usually stored in JSON files) to restart processing from the point of the last successful checkpoint.
