5 ms·

This is so cool! I often wondered how possible would it be to keep a TCP connection alive during a server restart. How is the connection severed and the client

This is so cool! I often wondered how possible would it be to keep a TCP connection alive during a server restart. How is the connection severed and the client not being aware of it works?

pyinstallwoes 4y ago
Probably a layer of indirection. One layer acting as the mailbox for sessions which then communicates with whatever services handles the connection to the user client. Effectively builds up a buffer until the service tree is restarted. Pub/sub abstractions basically. I could be wrong.
ecdavis 4y ago
Depends what you mean by server. Some old game server processes would support a "copyover" function. They would write the file descriptors of connected sockets to a file, restart the process, then read the list of file descriptors back into memory and resume normal use of the socket. Unsophisticated, but it works. It's completely transparent to the client because the connection is never actually closed. The worst the client might experience is some latency as their input gets buffered by the OS while the server process restarts. Of course the connection would not persist over a physical server restart (or something like a pod being killed or whatever). I imagine it's possible to move a connection between nodes, though, with a fairly similar process.
- rramadass 4y ago
  >They would write the file descriptors of connected sockets to a file, restart the process, then read the list of file descriptors back into memory and resume normal use of the socket Nope. When a Process exits for whatever reason, the OS closes and releases all resources connected with all File/Socket descriptors for that Process. You will have to architect your System to explicitly accommodate this scenario. Some techniques here : https://stackoverflow.com/questions/55006657/can-i-allow-my-server-process-to-restart-without-killing-existing-connections https://stackoverflow.com/questions/55006657/can-i-allow-my-...
  - ecdavis 4y ago
    Ah, yeah, "restart the process" was not the right description of what was happening in the cases I was thinking of - they'd exec themselves to hot reload code which I see is the first suggestion on the SA answer you linked.
  - twic 4y ago
    Not quite. Terminating a process releases its filehandles, and if any sockets are now unreferenced, they are closed. But if there is a live filehandle in another process, the socket stays alive. You can send socket file handles over UNIX domain sockets, so you could use this to export your list of sockets to persist over restart.
    - rramadass 4y ago
      Yep (reference counting in play), It is already mentioned in the SO answer i linked to.
rramadass 4y ago
>I often wondered how possible would it be to keep a TCP connection alive during a server restart If you define a "Server" as a logical piece of functionality not directly mapped to a OS Process, then it can be done. See https://news.ycombinator.com/item?id=32067570 https://news.ycombinator.com/item?id=32067570

This is so cool! I often wondered how possible would it be to keep a TCP connection alive during a server restart. How is the connection severed and the client not being aware of it works?

pyinstallwoes 4y ago

Probably a layer of indirection. One layer acting as the mailbox for sessions which then communicates with whatever services handles the connection to the user client. Effectively builds up a buffer until the service tree is restarted. Pub/sub abstractions basically. I could be wrong.

ecdavis 4y ago

Depends what you mean by server. Some old game server processes would support a "copyover" function. They would write the file descriptors of connected sockets to a file, restart the process, then read the list of file descriptors back into memory and resume normal use of the socket. Unsophisticated, but it works. It's completely transparent to the client because the connection is never actually closed. The worst the client might experience is some latency as their input gets buffered by the OS while the server process restarts. Of course the connection would not persist over a physical server restart (or something like a pod being killed or whatever). I imagine it's possible to move a connection between nodes, though, with a fairly similar process.

rramadass 4y ago
>They would write the file descriptors of connected sockets to a file, restart the process, then read the list of file descriptors back into memory and resume normal use of the socket Nope. When a Process exits for whatever reason, the OS closes and releases all resources connected with all File/Socket descriptors for that Process. You will have to architect your System to explicitly accommodate this scenario. Some techniques here : https://stackoverflow.com/questions/55006657/can-i-allow-my-server-process-to-restart-without-killing-existing-connections https://stackoverflow.com/questions/55006657/can-i-allow-my-...
- ecdavis 4y ago
  Ah, yeah, "restart the process" was not the right description of what was happening in the cases I was thinking of - they'd exec themselves to hot reload code which I see is the first suggestion on the SA answer you linked.
- twic 4y ago
  Not quite. Terminating a process releases its filehandles, and if any sockets are now unreferenced, they are closed. But if there is a live filehandle in another process, the socket stays alive. You can send socket file handles over UNIX domain sockets, so you could use this to export your list of sockets to persist over restart.
  - rramadass 4y ago
    Yep (reference counting in play), It is already mentioned in the SO answer i linked to.

rramadass 4y ago

>I often wondered how possible would it be to keep a TCP connection alive during a server restart If you define a "Server" as a logical piece of functionality not directly mapped to a OS Process, then it can be done. See https://news.ycombinator.com/item?id=32067570 https://news.ycombinator.com/item?id=32067570