Memory efficient way of reading and downloading a large file in Ruby

To read a large file from the disk

File.foreach method reads the file line by line; that is why it is safe to use for large files.
It can accept the block to execute each line of a file.

Example:

File.foreach('example.jsonl') { |line| JSON.parse(line) }

However, in the Ruby documentation, you will not find this method defined on the File class. It is defined on IO, which is a superclass of File.

To download the large files from the Internet

We can use IO.copy_stream to download the large files from the Internet. It will create a stream instead of loading the whole file into memory before writing.

Example:

IO.copy_stream(open('http://example.com/file.jsonl'), 'file.jsonl')

To sum it up…

Memory friendly methods:
File.foreach, IO.copy_stream

Other methods to read a file:
File.read, File.readlines