I developed an application (in Python language) that is started by
systemd at startup and stay running forever (until the box is rebooted
or shutdown).
The user can get some info of the system by a web application, as a
typical router or NAS. The user points his web browser to the local IP address of my Linux box and a graphical interface appears. This web application is written in Python (WSGI) deployed on the box thanks to
nginx + gunicorn + Flask.
Now the question is: how to get infos from a running service such that
they can be shown on the web application?
The infos are specific of my application, they aren't standard. I'm in control of the running service and WSGI application, so I can use
whatever solution is better.
Of course this is a typical IPC scenario: one process is WSGI and the
other is the running service.
I can use shared memory, message queues, named pipes, unix sockets,
Internet sockets, D-Bus and many other mechanisms.
Is there one that is better to use in my case? After some reading, maybe D-Bus can be a good way. I understood systemd already uses D-Bus to
exchange infos about its services and units.
However its implementation is not straightforward as a unix socket with custom messages.
What do you suggest?
PS: In the past I read only a few posts regarding Linux development,
even if it's for embedded devices. However I don't know how to ask
questions related to linux development, I noticed Usenet linux groups
are somewhat dead.
On 07/08/2023 23:28, pozz wrote:[...]
What do you suggest?
PS: In the past I read only a few posts regarding Linux development,
even if it's for embedded devices. However I don't know how to ask
questions related to linux development, I noticed Usenet linux groups
are somewhat dead.
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.
Il 08/08/2023 12:27, David Brown ha scritto:
On 07/08/2023 23:28, pozz wrote:[...]
What do you suggest?
PS: In the past I read only a few posts regarding Linux development,
even if it's for embedded devices. However I don't know how to ask
questions related to linux development, I noticed Usenet linux groups
are somewhat dead.
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.
On 08/08/2023 14:59, pozz wrote:
Il 08/08/2023 12:27, David Brown ha scritto:
On 07/08/2023 23:28, pozz wrote:[...]
What do you suggest?
PS: In the past I read only a few posts regarding Linux development,
even if it's for embedded devices. However I don't know how to ask
questions related to linux development, I noticed Usenet linux
groups are somewhat dead.
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.
That would be the normal choice, yes.
Another issue I see is synchronization. Without a sync mechanism, the
reader could read bad data, because the writer is writing to it.
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!
Il 08/08/2023 16:31, David Brown ha scritto:
On 08/08/2023 14:59, pozz wrote:
Il 08/08/2023 12:27, David Brown ha scritto:
On 07/08/2023 23:28, pozz wrote:[...]
What do you suggest?
PS: In the past I read only a few posts regarding Linux
development, even if it's for embedded devices. However I don't
know how to ask questions related to linux development, I noticed
Usenet linux groups are somewhat dead.
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.
That would be the normal choice, yes.
Another issue I see is synchronization. Without a sync mechanism, the
reader could read bad data, because the writer is writing to it.
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?
On 08/08/2023 17:14, pozz wrote:
Il 08/08/2023 16:31, David Brown ha scritto:
On 08/08/2023 14:59, pozz wrote:
Il 08/08/2023 12:27, David Brown ha scritto:
On 07/08/2023 23:28, pozz wrote:[...]
What do you suggest?
PS: In the past I read only a few posts regarding Linux
development, even if it's for embedded devices. However I don't
know how to ask questions related to linux development, I noticed >>>>>> Usenet linux groups are somewhat dead.
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.
That would be the normal choice, yes.
Another issue I see is synchronization. Without a sync mechanism,
the reader could read bad data, because the writer is writing to it.
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?
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.
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, as long as you hold the file descriptor open.
Il 08/08/2023 18:54, David Brown ha scritto:
On 08/08/2023 17:14, pozz wrote:
Il 08/08/2023 16:31, David Brown ha scritto:
On 08/08/2023 14:59, pozz wrote:
Il 08/08/2023 12:27, David Brown ha scritto:
On 07/08/2023 23:28, pozz wrote:[...]
What do you suggest?
PS: In the past I read only a few posts regarding Linux
development, even if it's for embedded devices. However I don't >>>>>>> know how to ask questions related to linux development, I noticed >>>>>>> Usenet linux groups are somewhat dead.
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.
That would be the normal choice, yes.
Another issue I see is synchronization. Without a sync mechanism,
the reader could read bad data, because the writer is writing to it. >>>>>
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?
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).
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?
[1] Ghost because it can't be read by any other process.
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.
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, as long as you hold the file descriptor open.
On 09/08/2023 09:27, pozz wrote:
[1] Ghost because it can't be read by any other process.
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.
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