A good picture about linux process life cycle

The differnce between TCP and UDP

Notes to the blog:

http://www.javacodegeeks.com/2014/07/9-differences-between-tcp-and-udp-protocol-java-network-interview-question.html

This question what is the difference between TCP and UDP is often asked during the interviews. It is also important for sysadmin and program to understand for dairy work. Then we will explain what is the major differences we can see between the two protocols.

Connection-oriented vs Connectionless:

this is the essential differnce between TCP and UDP. The differnce also explains the other different points in the blog. when we say TCP is connection-oriented, it means the protocol itself controls and manages the connection session. It needs so-call 3-ways handshake and 4-ways termination. TCP also has many control packages such as SYN,ACK to for flow control. Meanwhile, UDP does not provide a 'connection' session for communication parties.

Reliability:

TCP is reliable as the application can assume TCP provides a reliable communication while UDP does not provide any  delivery guaruntee

Ordering:

TCP also guarantees order of message as it can get the sequence number from all the packages. while UDP does not care about the package sequence.

Data Boundary:

TCP does not preserve data boundary, UDP does.In TCP, data is sent as a byte stream, and no distinguishing indications are transmitted to signal message (segment) boundaries. On UDP, Packets are sent individually and are checked for integrity only if they arrived. Packets have definite boundaries which are honoured upon receipt, meaning a read operation at the receiver socket will yield an entire message as it was originally sent.

Speed:

In one word, TCP is slow and UDP is fast.

Heavy weight vs Light weight:

Because of the overhead mentioned above, Transmission control protocol is considered as heavy weight as compared to light weight UDP protocol. TCP header is longer than UDP headers.

Congestion or Flow control:

TCP does Flow Control. TCP requires three packets to set up a socket connection, before any user data can be sent. TCP handles reliability and congestion control. On the other hand, UDP does not have an option for flow control.

Different Applications:

because TCP and UDP provides different behaviour or characters, different application use either TCP or UDP as their transmission protocal. for example, HTTP and FTP, they use TCP while SIP, TFTP use UDP.

some tips about MySQL query Optimization

1. use the index: for every SQL, use the explain statement to verify the execution plan.
2. dont use index for computing substatement
3. fine the search critieria as much as possible
4. create hash index on fields for min/max/count
5. do manual calculation on some nequalities

Python STL - OS

OS module.

os is a very important standard library (STL) in python for system admin,  sysadmin can use it to interact with OS.

Common os functions

#list the directory’s content

>os.listdir(“pathname”)

 #call shell script or external binary

>os.system(“commands”)

>>> to be continue 

very good summary about linux system administrator interview questions

this is I prepared for the interview for Linux admin/System Engineer Technical questions:

1)      how to install Digital Certificates on apache/nginx

2)      Apache MPM types

3)      Linux symlink vs hardlink

4)      Linux run level (RHEL/Ubuntu)

5)      Linux static library link vs dynamic library link

6)      How to install software from source code and the meaning of each steps

7)      Files in /boot and their function

8)      The content in /etc/grub.conf

9)      Linux booting procedure

10)   MYSQL database creation and deletion

11)   MySQL primary key, unique key, foreign key

12)   MySQL b+ tree and hash index

13)   MySQL index advantage and disadvantage, how to create keys

14)   Select statement, left join, right join, inner join

15)   How to shutdown a linux box

16)   DHCP requests procedure

17)   What is a proxy and reverse proxy

18)   ASymmetric and Symmetric encryption method

19)   How to use ssh key to login to the system without a password (what’s the advantage)

20)   Some remote login security rules

21)   How to execute shell commands in MySQL

22)   Name some options of MySQLadmin

23)   What GUI tools you have used for MySQL admin

24)   How to generate a service for Linux

25)   RAID 0, 1, 5

26)   Name the method to copy a file from a linux server to another server

27)   Rsync working mode. How to setup an rsync environment

28)   Linux – how to enter the single user mode

29)   How kill works in linux, name a few common Signals in LINUX

30)   Describe the memory layout of a procedure

31)   Ext2 special file permission and ACL

32)   HDD layout? How about SSD

33)   What is the function of VFS

34)   List common Linux environment variables

35)   How does CDN work

36)   How to forbid the root user to login directly

37)   How to grant/revoke permissions to MySQL user

38)   How to change mysql user’s password

39)   OSI mode and TCPIP mode

40)   What is VLAN and it’s function

41)   Common HTTP method

42)   Common HTTP response code

43)   How to setup MySQL replication

44)   What is file’s magic number

45)   Linux file types

46)   Linux super lock content

47)   Linux inode content

48)   How ping works

49)   How tracert works

50)   DNS server type, records type

51)   How DNS query works

52)   TCP establishment and termination method

53)   TCP flags

54)   What is TCP half close

55)   TCP 2MSL function

56)   List the TCP timers

57)   The format of a standard URL

58)   How can you design an Web Server

59)   Web Server’s keepalive advantage and disadvantage

60)   The responsibility of a system administrator

61)   The difference between procedure and thread

62)   Procedure life cycle

63)   The priority of the process, nice value and RT

64)   How to track down a process

65)   How does strace/ltrace work

66)   Linux cache/buffer

67)   /etc/passwd, /etc/shadow difference

68)   How to lock a user

69)   How to add a disk to linux system and use it in production environment

70)   How to create file system (for example ext4)

71)   Explain the output of top command and the meaning of each field

72)   The difference between router and switch, what is layer 3 switch

73)   how to make the OS secure

74)   myISAM vs InnoDB

75)   what is ACID

76)   what is the isolation level

77)   explain the backup and restore for MySQL

78)   how to create user in mysql

79)   mysql log explaination

80)   explain what is PKI

81)   SSL establishment procedure

82)   List the file system Linux supports

83)   List the linux disk blocks

84)   What is init process and it’s function

85)   Explain the disk block types in linux

86)   LVS working method and common scheduler

87)   Linux should at least contains which file partitions

88)   How to add modules in Apache

89)   How to integrate Apache and Tomcat

90)   What’s the function of firewall

91)   NTFS vs FAT

92)   Hsrp vs vrrp

93)   Mysqldump common options

94)   Mysqlbinlog

95)   Common redundant method for network

96)   Apache’s tuning

97)   Windows build-in group and users

98)   File system common commands

99)   Yum configuration file and repo.d sample

100) The process status and flag

101) The baseline for linux performance (4 sub system)

102) Linux performance checking tools

103) iptables tables and chains

104) collion domain and broadcast domain

105) NAT types

106) IP private sections

107) how OSPF is working

108) swap related commands. How to setup swap partitions on the fly

109) which software Apache rely on?

110) How Apache and Nginx configure a virtual host