How can I read file in FileSystem with the FIFO principle ?

[Cass Ted]

Hello  dear Rancher,
I receive a stream of Data each seconde and store it in a Filesystem. Please how can I read this Data with the FIFO principle  ?

Thanks!

[Campbell Ritchie]

Welcome to the Ranch

Have you seen the Java™ Tutorials section about NIO?
Please explain how these data are stored. Are they received as some sort of input stream, or are they written directly to a file? Does your input write them sequentially into your file?

[Stephan van Hulst]

If you're asking how to store a stack in a file, you can do this easily by keeping the offset of the last entry in the file header, and in every entry keep the offset of the previous entry.

It goes roughly like this:

void pushToFile(Path path, InputStream entryData) throws IOException {
  try (
    var channel = Files.newByteChannel(path, CREATE, READ, WRITE);
  ) {
    var offsetOfPreviousEntry = readHeaderOffset(channel);
    var offsetOfNewEntry = channel.size();
    writeHeaderOffset(channel, offsetOfNewEntry);
    appendEntry(channel, entryData, offsetOfPreviousEntry);
  }
}

byte[] popFromFile(Path path) throws IOException {
  try (
    var channel = Files.newByteChannel(path, READ, WRITE);
  ) {
    var offsetOfLastEntry = readHeaderOffset(channel);
    var offsetOfPreviousEntry = readPreviousOffset(channel, offsetOfLastEntry);
    var result = truncateLastEntry(channel);
    writeHeaderOffset(channel, offsetOfPreviousEntry);
    return result;
  }
}

[Cass Ted]

Thank    
The data  are written directly to  a file

[Campbell Ritchie]

Stephan: wouldn't a stack be LIFO/FILO and OP asked for FIFO?
Presumably you can write successive lines to a file and read them in order. But that produces a race condition, so it wouldn't be thread‑safe. It might also not be possible to open a file for reading and writing simultaneously.

[Stephan van Hulst]

Ahhh yes. For some reason I read LIFO and I just went with it.

FIFO is indeed just writing the entries one after the other. I have no idea why one would want to read and write at the same time, maybe OP can share more details about their use case with us.

[Matthew Bendford]

@Campbell I had to look it up as I also get confused from time to time - but you're correct:

FIFO: Queue - what comes in first gets served back first
LIFO: Stack - what comes in last gets served back first

[Campbell Ritchie]

But you can easily use a list as a FIFO collection.