Linux Embedded: how to get info from a running service

I don't know what kind of information you are needing, but an easy option might be to have the python service regularly write out a json format file with the current status or other information. The web app can have Javascript that regularly reads that file and handles it on the user's web browser. And if you want to go the other way, your Python code can use "inotify" waits to see file writes from the web server.

Sincerely I don't like your solution. First of all, you are writing regularly on a normal file in the filesystem. Ok, maybe I can use a tmpfs filesystem in RAM.

Another issue I see is synchronization. Without a sync mechanism, the reader could read bad data, because the writer is writing to it.

There is a trivial way to deal with that which has been used since time immemorial on Unix:

Write to a temporary file, then close and reanme it. The open() and rename() system calls are atomic with respect to each other. The task calling open() will get either the old file or the new file, never something "in between".

That would be the normal choice, yes.

You typically handle this by writing to "status.tmp", then renaming (moving) it to "status.json", or whatever names you are using. Renaming a file like this is guaranteed atomic on Linux - anything attempting to open a handle to "status.json" will either get the old file (which is kept alive while the file descriptor is open) or the new file. This is not the first situation in which people wanted to avoid reading half-written files!

Good thing to know.

Just to better understand what happens. If reader opens status.json just before the writer rename status.tmp to status.json, we will have a process (the reader) that reads from the old version of "status.json" instead of the new version that is really on the filesystem?

Consider that the reader could keep open the old status.json for a long time. Does the OS guarantee that old data (maybe 1GB) can be read even if a new file with new data is available?

Yes, exactly.

A file in Linux exists independently from filenames. There can be many things pointing to a file, and the file exists until there are no more pointers. Usually these "pointers" are directory entries, but they can also be open file descriptors (which are actually visible as pseudofiles in the /proc filesystem).

So when you open the "status.json" file, you get that file, and it stays in existence at least until the file is closed. The new "status.tmp" is a different file. The rename just makes a new pointer to the new file, and erases the old pointer to the old file.

Yes, as long as you hold the file descriptor open.

Is this behaviour the same for whatever filesystem (ext2, fat, ...)?

What I don't understand is what exactly happens under the hood.

Consider the following sequences:

- process W (writer) write version A to status.tmp

- process W rename status.tmp to status.json

- process W write version B to status.tmp

- process R (reader) open file status.json (version A)

- process W rename status.tmp to status.json [Now all new open operations on status.json will get new version of data] [process W could write/rename status.tmp/json 1000 times]

- after one hour (just to say), process R starts reading from the file

From what I understand, process R will get the full contents of version A (even if it restarts reading changing file position many times). The OS takes care of data A, because this "ghost file"[1] is in use.

Most probably, if the file size is small, the OS copy its contents in a cache in RAM when process R open the file, so process R will read from RAM and this explains why it will get the original version A content.

Anyway, in general the file could be any size, maybe 1GB. So I assume at least some parts of version A data still remains in the HDD, even when process W write/rename a new version. Until process R doesn't close the file, version A data are phisically on the HDD, consuming part of its memory. Is it correct?

Yes (as far as I understand it), though some filesystems (like fat) don't support multiple directory links to the same file. All *nix suitable filesystems do, because hard links are a key feature. (In use-cases like yours, you would - as you suggested - put the file on a tmpfs mount.)

Yes.

It's easy to try all this from two Python interactive sessions. It's hard to test race conditions of doing things quickly, but waiting an hour should be easy to simulate :-)

Yes. Just don't close the file descriptor between accesses. (You can duplicate the file descriptor, close the first one, and carry on using the duplicate to access the old file.)

It is all just like normal files. But since version A is marked for immediate deletion upon closure, the OS knows it never needs to survive a reboot. So if the data was not written out to disk, but was still in the write cache, then it will never be written out to the disk unless the system is desperate for memory.

This is similar to the trick for temporary files. Create a new file for writing. Open a read/write handle to the file. Then delete the file. You can use the handle (and duplicates of it) to access the file, pass on the handle to child processes, etc., but the OS knows it does not need to actually write anything to the disk. (Windows has special API calls to get a similar effect for immediately deletable files.)

Yes. You have understood perfectly.

It can, through a duplicate file descriptor. In practice this is commonly done by child processes - when you fork(), the child gets a copy of all the open file descriptors.

You can also pass filedescriptors to non-related processes via Unix-domain sockets.