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io_bottlenecks/main.py

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+import requests
+from bs4 import BeautifulSoup
+from time import time
+
+# This program is going to compare the time it takes
+# to fetch a single page to the time it takes to
+# follow all links on a page using a Web crawler. 
+# It will demonstrate how an I/O Bottleneck can build 
+# up while waiting for responses from Web servers
+
+
+def fetch_webpage(url):
+    return requests.get(url, data=None)
+
+
+def single_fetch_time(url):
+    t0 = time()
+    fetch_webpage(url)
+    t1 = time()
+    return f'{t1-t0:.2f}'
+
+
+def crawl_time(url):
+    req = fetch_webpage(url)
+    soup = BeautifulSoup(req.text, 'html.parser')
+    t0 = time()
+    print(f'\nLinks found crawlling {url}:')
+    for link in soup.find_all('a'):
+        print(link.get('href'))
+    t1 = time()
+    return f'{t1-t0:.2f}'
+
+
+def display_time(url, action, time_str):
+    output = f'Time to {action} {url}: {time_str}s'
+    n = len(output)
+    print('\n' + '=' * n)
+    print(output)
+    print('=' * n)
+
+
+def main():
+    url = 'https://www.google.com'
+    sf_time = single_fetch_time(url)
+    display_time(url, 'fetch', sf_time)
+    c_time = crawl_time(url)
+    display_time(url, 'crawl', c_time)
+    
+
+if __name__ == '__main__':
+    main()

non_deterministic_threading/main.py

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+from threading import Thread
+from random import randint
+import time
+
+# Having multiple Threads running that update a shared resource
+# without proper Thread synchronisation is risky.
+#
+# In this example, 2 worker Threads both try to complete a job
+# updating the counter variable. One is trying to increment it
+# to > 1000, while the other is trying to decrement it to < -1000
+#
+# The progam is non-deterministic and unreliable because the
+# OS Task Scheduler is constantly switching between Threads,
+# with neither taking precedence. This behaviour can lead to
+# switching infinitely back-and-forth between the Threads, plus
+# we have no possible way of predicting the output of the program
+
+counter = 1
+
+
+def worker_a():
+    global counter
+    while counter < 1000:
+        counter += 1
+        print(f'Worker A incremented counter by 1, counter = {counter}')
+        time.sleep(randint(0, 1))
+
+
+def worker_b():
+    global counter
+    while counter > -1000:
+        counter -= 1
+        print(f'Worker B decremented counter by 1, counter = {counter}')
+        time.sleep(randint(0, 1))
+
+
+def start_threads(t1, t2):
+    t1.start()
+    t2.start()
+    t1.join()
+    t2.join()
+
+
+def main():
+    t0 = time.time()
+    thread1 = Thread(target=worker_a, args=())
+    thread2 = Thread(target=worker_b, args=())
+    start_threads(thread1, thread2)
+    t1 = time.time()
+    print(f'Execution time: {t1-t0:.2f}')
+
+
+if __name__ == '__main__':
+    main()