How is InputStream managed in memory?

Question

I am familiar with the concept of InputStream,buffers and why they are useful (when you need to work with data that might be larger then the machines RAM for example).

I was wondering though, how does the InputStream actually carry all that data?. Could a OutOfMemoryError be caused if there is TOO much data being transfered?

Case-scenario

If I connect from a client to a server,requesting a 100GB file, the server starts iterating through the bytes of the file with a buffer, and writing the bytes back to the client with outputStream.write(byte[]). The client is not ready to read the InputStream right now,for whatever reason. Will the server continue sending the bytes of the file indefinitely? and if so, won't the outputstream/inputstream be larger than the RAM of one of these machines?

Answer 1

InputStream and OutputStream implementations do not generally use a lot of memory. In fact, the word "Stream" in these types means that it does not need to hold the data, because it is accessed in a sequential manner -- in the same way that a stream can transfer water between a lake and the ocean without holding a lot of water itself.

But "stream" is not the best word to describe this. It's more like a pipe, because when you transfer data from a server to a client, every stage transfers back-pressure from the client that controls the rate at which data gets sent. This is similar to how your faucet controls the rate of flow through your pipes all the way to the city reservoir:

1. As the client reads data, it's InputStream only requests more data from the OS when its internal (small) buffers are empty. Each request allows only a limited amount of data to be transferred;
2. As data is requested from the OS, its own internal buffer empties, and it notifies the server about how much space there is for new data. The server can send only this much (that's called 'flow control' in TCP: https://en.wikipedia.org/wiki/Transmission_Control_Protocol#Resource_usage)
3. On the server side, the server-side OS sends out data from its own internal buffer when the client has space to receive it. As its own internal buffer empties, it allows the writing process to re-fill it with more data.
4. As the server-side process write()s to its OutputStream, the OutputStream will try to write data to the OS. When the OS buffer is full, it will make the server process wait until the server-side buffer has space to accept new data.

Notice that a slow client can make the server process take a very long time. If you're writing a server, and you don't control the clients, then it's very important to consider this and to ensure that there are not a lot of server-side resources tied up while a long data transfer takes place.