Shantanu's Blog

Corporate Consultant

March 26, 2015

 

Managing sockets using python

Here is an example to understand how to open port and get the data from client machines.

This script will open the port 400007 on the server and start listening.
When a client sends data, it simply prints it. And it will also send the same data packet back to the client with the word "Echo =>" appended at the beginning.

from socket import *
myHost = ''
myPort = 40007

sockobj = socket(AF_INET, SOCK_STREAM)
sockobj.bind((myHost, myPort))
sockobj.listen(5)

while True:
    connection, address = sockobj.accept()
    print('Server connected by', address)
    while True:
        data = connection.recv(1024)
        print data
        if not data: break
        connection.send(b'Echo=>' + data)
    connection.close()

The client should also have python installed and it the OS can be Windows or Linux.
The following script will send data to the server mentioned above.
The host, port and message can also be supplied through command line.

import sys
from socket import *

serverHost = '23.21.167.60'
serverPort = 50007
message = [b'Hello network world by shantanu from Windows PC']

if len(sys.argv) > 1:      
    serverHost = sys.argv[1]
    if len(sys.argv) > 2:
        message = (x.encode() for x in sys.argv[2:]) 

sockobj = socket(AF_INET, SOCK_STREAM)
sockobj.connect((serverHost, serverPort))

for line in message:
    sockobj.send(line)
    data = sockobj.recv(1024)
    print('Client received:', data)
   
sockobj.close()

It creates a TCP/IP socket and then connects to the server/ port mentioned.
The object (remember this is python!) is supplied each line through send method.
The response received from the server is simply printed next to "Client Received:"

_____

The server does not handle simultaneous requests. Instead it lines up a queue of 5 clients using the following line....
sockobj.listen(5)
If the server gets more than 5 requsets, it simply denies them with an error. So we need a better solution that will handle multiple concurrent connections.

import os, time, sys
from socket import *
myHost = ''
myPort = 50007

sockobj = socket(AF_INET, SOCK_STREAM)
sockobj.bind((myHost, myPort))
sockobj.listen(5)

def now():
    return time.ctime(time.time())

activeChildren = []
def reapChildren():
        while activeChildren:
            pid, stat = os.waitpid(0, os.WNOHANG)
            if not pid: break
            activeChildren.remove(pid)

def handleClient(connection):
    time.sleep(5)
    while True:
        data = connection.recv(1024)
        if not data: break
        reply = 'Echo=>%s at %s' % (data, now())
        connection.send(reply.encode())
    connection.close()
    os._exit(0)

def dispatcher():
    while True:
        connection, address = sockobj.accept()
        print('Server connected by', address)
        print('at', now())
        reapChildren()
        childPid = os.fork()
        if childPid == 0:
            handleClient(connection)
        else:
            activeChildren.append(childPid)

dispatcher()

Labels:


February 24, 2015

 

python map reduce moedule

Here is how we can use Map Reduce jobs using Amazon EMR

python wordc.py big_file.txt  -r emr --conf=mrjob.conf

The configuration file looks like this...

# cat mrjob.conf

runners:
  emr:
    aws_access_key_id: xxx
    aws_secret_access_key: xxx
    ec2_instance_type: m1.small
    num_ec2_instances: 5

# cat wordc.py
"""The classic MapReduce job: count the frequency of words.
"""
from mrjob.job import MRJob
import re

WORD_RE = re.compile(r"[\w']+")

class MRWordFreqCount(MRJob):

    def mapper(self, _, line):
        for word in WORD_RE.findall(line):
            yield (word.lower(), 1)

    def combiner(self, word, counts):
        yield (word, sum(counts))

    def reducer(self, word, counts):
        yield (word, sum(counts))

if __name__ == '__main__':
     MRWordFreqCount.run()

Labels: , ,


February 15, 2015

 

Restrict access to S3 bucket

Let's assume I have a S3 bucket named "mybest". I want to restrict (especially download) access from all IP addresses except 182.72.128.206 that is my work IP address.

I can use this policy to control the access of the bucket from any other IP address other than my office address.

{
    "Statement": [
        {
            "Effect": "Deny",
            "Principal": {
                "AWS": "*"
            },
            "Action": "s3:*",
            "Resource": "arn:aws:s3:::mybest/*",
            "Condition": {
                "NotIpAddress": {
                    "aws:SourceIp": [
                        "182.72.128.206/32"
                    ]
                }
            }
        }
    ]
}

Labels: ,


January 30, 2015

 

Using string templates in python

$to and $from in the docstring can be used as a template so that the values from dictionary can replace the placeholder using substitute method of string.template object. neat !

import string
tmpl = string.Template('''
Hi ${to},
welcome to the club
Regards,
$from
''')
dct={'to': 'Rick', 'from':'Stuart'}

tmpl.substitute(dct)

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lambda functions in python

def lambda_adder(a, b):
    return a+b

This function can also be written as lambda function as shown below:

lambda_adder = lambda a,b: a+b

Call either of the functions above using:
lambda_adder(1,2)

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