[Spark Streaming] Checkpoint backup (.bk) file purpose

[Bartosz Konieczny <ba...@gmail.com>]

Hello,

Actually I'm studying metadata checkpoint implementation in Spark Streaming
and I was wondering the purpose of so called "backup files":

CheckpointWriter snippet:

> // We will do checkpoint when generating a batch and completing a batch.
> When the processing
> // time of a batch is greater than the batch interval, checkpointing for
> completing an old
> // batch may run after checkpointing of a new batch. If this happens,
> checkpoint of an old
> // batch actually has the latest information, so we want to recovery from
> it. Therefore, we
> // also use the latest checkpoint time as the file name, so that we can
> recover from the
> // latest checkpoint file.
> //
> // Note: there is only one thread writing the checkpoint files, so we
> don't need to worry
> // about thread-safety.
> val checkpointFile = Checkpoint.checkpointFile(checkpointDir,
> latestCheckpointTime)
> val backupFile = Checkpoint.checkpointBackupFile(checkpointDir,
> latestCheckpointTime)
>
> // ... some lines further
> // If the checkpoint file exists, back it up
> // If the backup exists as well, just delete it, otherwise rename will fail
> if (fs.exists(checkpointFile)) {
>   fs.delete(backupFile, true) // just in case it exists
>   if (!fs.rename(checkpointFile, backupFile)) {
>     logWarning(s"Could not rename $checkpointFile to $backupFile")
>   }
> }
>

What is the role of this *backupFile* ? I understand that they are
generated if checkpoint file for given timestamp already exists. But how it
could be produced ? Is it a protection against checkpointing of different
Spark applications to the same directory ? Or it's adapted to case
described above (old batch terminated after new batch start) ?

Best regards,
Bartosz.