Question

Consider the following scenario: A process on the server is used to handle data from a network connection. Twisted makes this very easy with spawnProcess and you can easily connect the ProcessTransport with your protocol on the network side.

However, I was unable to determine how Twisted handles a situation where the data from the network is available faster than the process performs reads on its standard input. As far as I can see, Twisted code mostly uses an internal buffer (self._buffer or similar) to store unconsumed data. Doesn’t this mean that concurrent requests from a fast connection (eg. over local gigabit LAN) could fill up main memory and induce heavy swapping, making the situation even worse? How can this be prevented?

Ideally, the internal buffer would have an upper bound. As I understand it, the OS’s networking code would automatically stall the connection/start dropping packets if the OS’s buffers are full, which would slow down the client. (Yes I know, DoS on the network level is still possible, but this is a different problem). This is also the approach I would take if implementing it myself: just don’t read from the socket if the internal buffer is full.

Restricting the maximum request size is also not an option in my case, as the service should be able to process files of arbitrary size.

Answer 1

The solution has two parts.

One part is called producers. Producers are objects that data comes out of. A TCP transport is a producer. Producers have a couple useful methods: pauseProducing and resumeProducing. pauseProducing causes the transport to stop reading data from the network. resumeProducing causes it to start reading again. This gives you a way to avoid building up an unbounded amount of data in memory that you haven’t processed yet. When you start to fall behind, just pause the transport. When you catch up, resume it.

The other part is called consumers. Consumers are objects that data goes in to. A TCP transport is also a consumer. More importantly for your case, though, a child process transport is also a consumer. Consumers have a few methods, one in particular is useful to you: registerProducer. This tells the consumer which producer data is coming to it from. The consumer can them call pauseProducing and resumeProducing according to its ability to process the data. When a transport (TCP or process) cannot send data as fast as a producer is asking it to send data, it will pause the producer. When it catches up, it will resume it again.

You can read more about producers and consumers in the Twisted documentation.