Merge pull request #1575 from puma/schneems/reactor-docs

Document the Puma::Reactor

lib/puma/reactor.rb

@@ -2,15 +2,51 @@ require 'puma/util'
 require 'puma/minissl'
 
 module Puma
+  # Internal Docs, Not a public interface.
+  #
+  # The Reactor object is responsible for ensuring that a request has been
+  # completely received before it starts to be processed. This may be known as read buffering.
+  # If read buffering is not done, and no other read buffering is performed (such as by an application server
+  # such as nginx) then the application would be subject to a slow client attack.
+  #
+  # For a graphical representation of how the reactor works see [architecture.md](https://github.com/puma/puma/blob/master/docs/architecture.md#connection-pipeline).
+  #
+  # ## Reactor Flow
+  #
+  # A request comes into a `Puma::Server` instance, it is then passed to a `Puma::Reactor` instance.
+  # The reactor stores the request in an array and calls `IO.select` on the array in a loop.
+  #
+  # When the request is written to by the client then the `IO.select` will "wake up" and
+  # return the references to any objects that caused it to "wake". The reactor
+  # then loops through each of these request objects, and sees if they're  complete. If they
+  # have a full header and body then the reactor passes the request to a thread pool.
+  # Once in a thread pool, a "worker thread" can run the the application's Ruby code against the request.
+  #
+  # If the request is not complete, then it stays in the array, and the next time any
+  # data is written to that socket reference, then the loop is woken up and it is checked for completeness again.
+  #
+  # A detailed example is given in the docs for `run_internal` which is where the bulk
+  # of this logic lives.
   class Reactor
     DefaultSleepFor = 5
 
+    # Creates an instance of Puma::Reactor
+    #
+    # The `server` argument is an instance of `Puma::Server`
+    # this is used to write a response for "low level errors"
+    # when there is an exception inside of the reactor.
+    #
+    # The `app_pool` is an instance of `Puma::ThreadPool`.
+    # Once a request is fully formed (header and body are received)
+    # it will be passed to the `app_pool`.
     def initialize(server, app_pool)
       @server = server
       @events = server.events
       @app_pool = app_pool
 
       @mutex = Mutex.new
+
+      # Read / Write pipes to wake up internal while loop
       @ready, @trigger = Puma::Util.pipe
       @input = []
       @sleep_for = DefaultSleepFor
@@ -21,6 +57,64 @@ module Puma
 
     private
 
+
+    # Until a request is added via the `add` method this method will internally
+    # loop, waiting on the `sockets` array objects. The only object in this
+    # array at first is the `@ready` IO object, which is the read end of a pipe
+    # connected to `@trigger` object. When `@trigger` is written to, then the loop
+    # will break on `IO.select` and return an array.
+    #
+    # ## When a request is added:
+    #
+    # When the `add` method is called, an instance of `Puma::Client` is added to the `@input` array.
+    # Next the `@ready` pipe is "woken" by writing a string of `"*"` to `@trigger`.
+    #
+    # When that happens, the internal loop stops blocking at `IO.select` and returns a reference
+    # to whatever "woke" it up. On the very first loop, the only thing in `sockets` is `@ready`.
+    # When `@trigger` is written-to, the loop "wakes" and the `ready`
+    # variable returns an array of arrays that looks like `[[#<IO:fd 10>], [], []]` where the
+    # first IO object is the `@ready` object. This first array `[#<IO:fd 10>]`
+    # is saved as a `reads` variable.
+    #
+    # The `reads` variable is iterated through. In the case that the object
+    # is the same as the `@ready` input pipe, then we know that there was a `trigger` event.
+    #
+    # If there was a trigger event, then one byte of `@ready` is read into memory. In the case of the first request,
+    # the reactor sees that it's a `"*"` value and the reactor adds the contents of `@input` into the `sockets` array.
+    # The while then loop continues to iterate again, but now the `sockets` array contains a `Puma::Client` instance in addition
+    # to the `@ready` IO object. For example: `[#<IO:fd 10>, #<Puma::Client:0x3fdc1103bee8 @ready=false>]`.
+    #
+    # Since the `Puma::Client` in this example has data that has not been read yet,
+    # the `IO.select` is immediately able to "wake" and read from the `Puma::Client`. At this point the
+    # `ready` output looks like this: `[[#<Puma::Client:0x3fdc1103bee8 @ready=false>], [], []]`.
+    #
+    # Each element in the first entry is iterated over. The `Puma::Client` object is not
+    # the `@ready` pipe, so the reactor checks to see if it has the fully header and body with
+    # the `Puma::Client#try_to_finish` method. If the full request has been sent,
+    # then the request is passed off to the `@app_pool` thread pool so that a "worker thread"
+    # can pick up the request and begin to execute application logic. This is done
+    # via `@app_pool << c`. The `Puma::Client` is then removed from the `sockets` array.
+    #
+    # If the request body is not present then nothing will happen, and the loop will iterate
+    # again. When the client sends more data to the socket the `Puma::Client` object will
+    # wake up the `IO.select` and it can again be checked to see if it's ready to be
+    # passed to the thread pool.
+    #
+    # ## Time Out Case
+    #
+    # In addition to being woken via a write to one of the sockets the `IO.select` will
+    # periodically "time out" of the sleep. One of the functions of this is to check for
+    # any requests that have "timed out". At the end of the loop it's checked to see if
+    # the first element in the `@timeout` array has exceed it's allowed time. If so,
+    # the client object is removed from the timeout aray, a 408 response is written.
+    # Then it's connection is closed, and the object is removed from the `sockets` array
+    # that watches for new data.
+    #
+    # This behavior loops until all the objects that have timed out have been removed.
+    #
+    # Once all the timeouts have been processed, the next duration of the `IO.select` sleep
+    # will be set to be equal to the amount of time it will take for the next timeout to occur.
+    # This calculation happens in `calculate_sleep`.
     def run_internal
       sockets = @sockets
 
@@ -163,6 +257,16 @@ module Puma
       end
     end
 
+    # The `calculate_sleep` sets the value that the `IO.select` will
+    # sleep for in the main reactor loop when no sockets are being written to.
+    #
+    # The values kept in `@timeouts` are sorted so that the first timeout
+    # comes first in the array. When there are no timeouts the default timeout is used.
+    #
+    # Otherwise a sleep value is set that is the same as the amount of time it
+    # would take for the first element to time out.
+    #
+    # If that value is in the past, then a sleep value of zero is used.
     def calculate_sleep
       if @timeouts.empty?
         @sleep_for = DefaultSleepFor
@@ -177,6 +281,31 @@ module Puma
       end
     end
 
+    # This method adds a connection to the reactor
+    #
+    # Typically called by `Puma::Server` the value passed in
+    # is usually a `Puma::Client` object that responds like an IO
+    # object.
+    #
+    # The main body of the reactor loop is in `run_internal` and it
+    # will sleep on `IO.select`. When a new connection is added to the
+    # reactor it cannot be added directly to the `sockets` aray, because
+    # the `IO.select` will not be watching for it yet.
+    #
+    # Instead what needs to happen is that `IO.select` needs to be woken up,
+    # the contents of `@input` added to the `sockets` array, and then
+    # another call to `IO.select` needs to happen. Since the `Puma::Client`
+    # object can be read immediately, it does not block, but instead returns
+    # right away.
+    #
+    # This behavior is accomplished by writing to `@trigger` which wakes up
+    # the `IO.select` and then there is logic to detect the value of `*`,
+    # pull the contents from `@input` and add them to the sockets array.
+    #
+    # If the object passed in has a timeout value in `timeout_at` then
+    # it is added to a `@timeouts` array. This array is then re-arranged
+    # so that the first element to timeout will be at the front of the
+    # array. Then a value to sleep for is derived in the call to `calculate_sleep`
     def add(c)
       @mutex.synchronize do
         @input << c