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