final report draft 1 for iskm
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IBRAHIM_MKUSA.md
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IBRAHIM_MKUSA.md
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# Interface to Google Drive in Racket
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# Hermes - A chat server and client written in Racket
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## Fred Martin
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### April 22, 2017
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## Ibrahim Mkusa
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### April 30, 2017
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# Overview
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This set of code provides an interface to searching through one's Google Drive account.
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Its most important feature is that it provides a *folder-delimited search*.
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Hermes is a chat server and client written in Racket. One can run the Hermes
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server on any machine that is internet accessible. The Hermes clients then
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connect to the server from anywhere on the internet. It's inspired by chat
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systems and clients like irc.
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The essential model of files in Google Drive is that they are in one big “pile.” So you can't directly find a file in a
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given folder.
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The goal in building Hermes was to expose myself to several concepts integral to
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systems like networking, synchronization, and multitasking.
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This code recursively collects all folders found within a given folder, and then
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construct a search query that includes a list of all the subfolders (flattened into a single list).
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This then allows you to perform a folder-delimited search.
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**Authorship note:** All of the code described here was written by myself.
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# Libraries Used
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The code uses four libraries:
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Most libraries and utilities used are part of base Drracket installation and
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therefore do not need to be imported.
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The date and time modules were used for various time related queries.
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The tcp module was used for communication via Transmission Control Protocol.
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Concurrency and synchronization modules that provide threads, and semaphores
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were also used.
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Below are libraries that were not part of base system:
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```
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(require net/url)
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(require (planet ryanc/webapi:1:=1/oauth2))
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(require json)
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(require net/uri-codec)
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(require racket/gui/base)
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(require math/base)
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```
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* The ```net/url``` library provides the ability to make REST-style https queries to the Google Drive API.
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* Ryan Culpepper's ```webapi``` library is used to provide the ```oauth2``` interface required for authentication.
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* The ```json``` library is used to parse the replies from the Google Drive API.
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* The ```net/uri-codec``` library is used to format parameters provided in API calls into an ASCII encoding used by Google Drive.
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* The ```racket/gui/base``` library used to build graphical user interface.
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* The ```math/base``` library was used for testing purposes. It was used to
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generated random numbers.
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# Key Code Excerpts
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@ -39,116 +42,187 @@ UMass Lowell's COMP.3010 Organization of Programming languages course.
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Five examples are shown and they are individually numbered.
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## 1. Initialization using a Global Object
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## 1. Tracking client connections using an object and closures.
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The following code creates a global object, ```drive-client``` that is used in each of the subsequent API calls:
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The following code defines and creates a global object, ```make-connections```
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that abstracts client connections. It also creates a semaphore to control access
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to ```make-connections``` object.
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```
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(define drive-client
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(oauth2-client
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#:id "548798434144-6s8abp8aiqh99bthfptv1cc4qotlllj6.apps.googleusercontent.com"
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#:secret "<email me for secret if you want to use my API>"))
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(define (make-connections connections)
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(define (null-cons?)
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(null? connections))
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(define (add username in out)
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(set! connections (append connections (list (list username in out))))
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connections)
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(define (cons-list)
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connections)
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(define (remove-ports in out)
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(set! connections
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(filter
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(lambda (ports)
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(if (and (eq? in (get-input-port ports))
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(eq? out (get-output-port ports)))
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#f
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#t))
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connections)))
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(define (dispatch m)
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(cond [(eq? m 'null-cons) null-cons?]
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[(eq? m 'cons-list) cons-list]
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[(eq? m 'remove-ports) remove-ports]
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[(eq? m 'add) add]))
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dispatch)
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(define c-connections (make-connections '()))
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(define connections-s (make-semaphore 1)) ;; control access to connections
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```
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When the tcp-listener accepts a connection from a client, the associated input
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output ports along with username are added as an entry in ```make-connections``` via ```add``` function.
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External functions can operate on the connections by securing the semaphore,
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and then calling ```cons-list``` to expose the underlying list of connections.
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```remove-ports``` method is also available to remove input output ports from
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managed connections.
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While using global objects is not a central theme in the course, it's necessary to show this code to understand
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the later examples.
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## 2. Selectors and Predicates using Procedural Abstraction
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## 2. Tracking received messages via objects and closures.
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A set of procedures was created to operate on the core ```drive-file``` object. Drive-files may be either
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actual file objects or folder objects. In Racket, they are represented as a hash table.
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```folder?``` accepts a ```drive-file```, inspects its ```mimeType```, and returns ```#t``` or ```#f```:
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The code below manages broadcast messages from one client to the rest. It wraps
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a list of strings inside an object that has functions similar to ```make-connections``` for
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exposing and manipulating the list from external functions. The code creates
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```make-messages``` global object and a semaphore to control access to it from
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various threads of execution.
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```
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(define (folder? drive-file)
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(string=? (hash-ref drive-file 'mimeType "nope") "application/vnd.google-apps.folder"))
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(define (make-messages messages)
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(define (add message)
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(set! messages (append messages (list message)))
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messages)
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(define (mes-list)
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messages)
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(define (remove-top)
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(set! messages (rest messages))
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messages)
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(define (dispatch m)
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(cond [(eq? m 'add) add]
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[(eq? m 'mes-list) mes-list]
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[(eq? m 'remove-top) remove-top]))
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dispatch)
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(define c-messages (make-messages '()))
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(define messages-s (make-semaphore 1)) ;; control access to messages
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```
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Another object produced by the Google Drive API is a list of drive-file objects ("```drive#fileList```").
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When converted by the JSON library,
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this list appears as hash map.
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## 3. Using map to broadcast messages from client to clients
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```get-files``` retrieves a list of the files themselves, and ```get-id``` retrieves the unique ID
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associated with a ```drive#fileList``` object:
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The ```broadcast``` function is called repeatedly in a loop to extract a message
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from ```make-messages``` object, and send it to every other client. It uses the
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```make-connections``` objects to extract output port of a client. The ```map```
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routine is called on every client in the connections object to send it
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a message.
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```
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(define (get-files obj)
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(hash-ref obj 'files))
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(define (get-id obj)
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(hash-ref obj 'id))
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(define broadcast
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(lambda ()
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(semaphore-wait messages-s)
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(cond [(not (null? ((c-messages 'mes-list))))
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(map
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(lambda (ports)
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(if (not (port-closed? (get-output-port ports)))
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(begin
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(displayln (first ((c-messages 'mes-list))) (get-output-port ports))
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(flush-output (get-output-port ports)))
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(displayln-safe "Failed to broadcast. Port not open." error-out-s error-out)))
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((c-connections 'cons-list)))
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(displayln-safe (first ((c-messages 'mes-list))) convs-out-s convs-out)
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;; remove top message from "queue" after broadcasting
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((c-messages 'remove-top))
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; debugging displayln below
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; (displayln "Message broadcasted")
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]) ; end of cond
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(semaphore-post messages-s)))
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```
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## 3. Using Recursion to Accumulate Results
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After the message is send, the message is removed from the "queue" via the
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```remove-top```.
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The low-level routine for interacting with Google Drive is named ```list-children```. This accepts an ID of a
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folder object, and optionally, a token for which page of results to produce.
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The code snippet below creates a thread that iteratively calls ```broadcast```
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every interval, where interval(in secs) is defined by ```sleep-t```.
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A lot of the work here has to do with pagination. Because it's a web interface, one can only obtain a page of
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results at a time. So it's necessary to step through each page. When a page is returned, it includes a token
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for getting the next page. The ```list-children``` just gets one page:
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** note ** : ```sleep``` is very important for making Hermes behave gracefully
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in a system. Without it, it would be called at the rate derived from cpu clock
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rate. This raises cpu temperatures substantially, and make cause a pre-mature
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system shutdown.
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```
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(define (list-children folder-id . next-page-token)
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(read-json
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(get-pure-port
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(string->url (string-append "https://www.googleapis.com/drive/v3/files?"
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"q='" folder-id "'+in+parents"
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"&key=" (send drive-client get-id)
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(if (= 1 (length next-page-token))
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(string-append "&pageToken=" (car next-page-token))
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"")
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; "&pageSize=5"
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))
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token)))
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```
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The interesting routine is ```list-all-children```. This routine is directly invoked by the user.
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It optionally accepts a page token; when it's used at top level this parameter will be null.
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The routine uses ```let*``` to retrieve one page of results (using the above ```list-children``` procedure)
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and also possibly obtain a token for the next page.
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If there is a need to get more pages, the routine uses ```append``` to pre-pend the current results with
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a recursive call to get the next page (and possibly more pages).
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Ultimately, when there are no more pages to be had, the routine terminates and returns the current page.
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This then generates a recursive process from the recursive definition.
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```
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(define (list-all-children folder-id . next-page-token)
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(let* ((this-page (if (= 0 (length next-page-token))
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(list-children folder-id)
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(list-children folder-id (car next-page-token))))
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(page-token (hash-ref this-page 'nextPageToken #f)))
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(if page-token
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(append (get-files this-page)
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(list-all-children folder-id page-token))
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(get-files this-page))))
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(thread (lambda ()
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(displayln-safe "Broadcast thread started!")
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(let loopb []
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(sleep sleep-t) ;; wait 0.2 ~ 0.5 secs before beginning to broadcast
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(broadcast)
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(loopb))))
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```
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## 4. Filtering a List of File Objects for Only Those of Folder Type
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## 4. Filtering a List of connections to find recipient of a whisper
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I implemented a whisper functionality, where a user can whisper to any user in
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the chat room. The whisper message is only sent to specified user. To implement
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this i used ```filter``` over the connections, where the predicate tested whether the
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current list item matched that of a specific user.
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'''
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(define whisper (regexp-match #px"(.*)/whisper\\s+(\\w+)\\s+(.*)" evt-t0))
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[whisper
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(semaphore-wait connections-s)
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; get output port for user
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; this might be null
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(define that-user-ports
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(filter
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(lambda (ports)
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(if (string=? (whisper-to whisper) (get-username ports))
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#t
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#f))
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((c-connections 'cons-list))))
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; try to send that user the whisper
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(if (and (null? that-user-ports)
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#t) ; #t is placeholder for further checks
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(begin
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(displayln "User is unavailable. /color blue" out)
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(flush-output out))
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(begin
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(displayln (string-append "(whisper) "
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(whisper-info whisper) (whisper-message whisper))
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(get-output-port (car that-user-ports)))
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(flush-output (get-output-port (car that-user-ports)))))
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(semaphore-post connections-s)]
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'''
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The snippet above is part of cond statement that tests contents of input from
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clients to determine what the client is trying wants/trying to do. The top-line
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is using regexes to determine whether the received message is a whisper or not.
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## 5. Selectors for dealing with content of a whisper from clients
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Below are are three selectors that help abstract the contents of a whisper
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message.
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The ```list-all-children``` procedure creates a list of all objects contained within a given folder.
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These objects include the files themselves and other folders.
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The ```filter``` abstraction is then used with the ```folder?``` predicate to make a list of subfolders
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contained in a given folder:
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```
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(define (list-folders folder-id)
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(filter folder? (list-all-children folder-id)))
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```
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; whisper selector for the username and message
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(define (whisper-info exp)
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(cadr exp))
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## 5. Recursive Descent on a Folder Hierarchy
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(define (whisper-to exp)
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(caddr exp))
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These procedures are used together in ```list-all-folders```, which accepts a folder ID and recursively
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obtains the folders at the current level and then recursively calls itself to descend completely into the folder
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hierarchy.
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```map``` and ```flatten``` are used to accomplish the recursive descent:
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```
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(define (whisper-message exp)
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(cadddr exp))
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(define (list-all-folders folder-id)
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(let ((this-level (list-folders folder-id)))
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(begin
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@ -156,3 +230,7 @@ hierarchy.
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(append this-level
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(flatten (map list-all-folders (map get-id this-level)))))))
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```
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```whisper-info``` retrieves the date-time and username info.
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```whisper-to``` retrieves the username of the intented recipient of a whisper.
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```whisper-message``` retrieves the actual whisper.
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