IERG 4080 Building Scalable Internet-based Services

Transcription

1 Department of Information Engineering, CUHK Term 1, 2015/16 IERG 4080 Building Scalable Internet-based Services Lecture 4 Load Balancing Lecturer: Albert C. M. Au Yeung 30 th September, 2015

2 Web Server + Application Server 2

3 Python Application Server Gunicorn WSGI Server Client HTTP Request HTTP Response Nginx Web Server Server Application developed using Python + Flask 3

4 WSGI WSGI refers to Web Server Gateway Interface Specify the interface through which a server and an application communicate If an application is written according to the specification, it will be able to run on any server developed according to the same specification Applications and servers that use the WSGI interface are said to be WSGI compliant WSGI applications can be stacked Ref: 4

5 WSGI Why WSGI? Web servers are not capable of running Python applications For Apache, there is a module named mod_python, which enables Apache to execute Python codes However, mod_python is - not a standard specifications - no longer under active development Hence, the Python community came up with WSGI as a standard interface for Python Web applications 5

6 WSGI The Web server is configured to forward certain requests to the WSGI server (specified by a URL) Web Server WSGI Interface (callable objects) WSGI Server WSGI Application / Framework Ref: 6

7 WSGI Server vs. WSGI Application Why do we have WSGI servers and WSGI applications? It is just another example of de-coupling: Applications focus on how to get things done (e.g. business logic, updating databases, serving dynamic content, etc.) Servers focus on how to route requests, handle simultaneous connections, optimise computing resources, etc. As a Web application developer, you can focus on developing the functions and features, without worrying about how to interface with the Web server 7

8 Communication between Server and App When a new request comes to the WSGI sever: 1. The server invokes the corresponding function in the application 2. Parameters are passed to the application using environment variables 3. The server also provides a callback function to the application 4. The application processes the request 5. The application returns the response to the server using the callback function provided by the server 8

9 Example A simple WSGI-compatible application that returns Hello World def application(environ, start_response): start_response( 200 OK, [( Content-Type, text/plain )]) yield Hello World\n environ contains parameters that the server passes to the application (e.g. parameters in the query string) start_response is a callback function provided by the server, the application uses it to return the HTTP response 9

10 Developing WSGI Applications You do not need to directly implementing the WSGI interface in your application, as there are many frameworks that will help you development an application more easily In Assignment 1, you will try to build one using the Flask framework ( Relatively easy to pick up Debug mode that assists your development Many plugins and modules 10

11 Developing WSGI Applications Other options: Django ( A comprehensive Web framework following the model-view-controller (MVC) architectural pattern Bottle ( A micro-framework like Flask, but more lightweight and requires no dependencies on other modules For more, see 11

12 WSGI Middleware WSGI Applications can be stacked. (Why do we want to do that?) WSGI Application WSGI Server WSGI Middleware WSGI Application WSGI Application 12

13 Load Balancing 13

14 Load Balancing The act of distributing workloads across multiple computing nodes Why? Avoid overloading a single node Maximise utilization of different nodes Optimise usage of resources Increase reliability and availability 14

15 Load Balancing - Overview How should we implement this? Load Balancer Clients Servers 15

16 Load Balancing There are different ways to implement a load balancer: DNS / Hardware / Software Implement it on different layers Random / Round-robin / dynamic scheduling 16

17 Load Balancing Algorithms Random Round-Robin Distribute load equally by using a rotating scheme Weighted Round-Robin A performance weight is assigned to each server Least Connections Sends requests to a server with the fewest number of connections Fastest Response Time Select the server that responded in the shortest time 17

18 Server Health Checking A load balancer might need to check whether the servers are operating normally and are able to give responses. How can we check if one of the servers has died? 1. Observe the network traffic 2. Probe the server for a quick response What are the pros and cons of these two? How can health checking be done? 18

19 Server Health Checking Ping Send an ICMP message to the server and check for response TCP Connection Attempt to establish a TCP connection to the server (on port 80) HTTP GET (Header) Check the status code of a GET request HTTP GET (Content) Check the content returned by the server for a GET request Question: What are the limitations of the first three methods? 19

20 DNS Load Balancing 20

21 DNS Load Balancing DNS - Domain Name System A directory service of the Internet DNS Server Send HTTP request to Has an A record mapping to CUHK Web Server 21

22 DNS Load Balancing A simple way of implementing load balancing: Create two or more A records in the DNS zone The DNS server sends the client a list of records in random order or in a round-robin fashion The client attempts to connect to the application server using the first IP address in the list 22

23 DNS What are A records? Address record: mapping a domain name to an IPv4 address Other records: AAAA: domain name to IPv6 address CNAME: alias of a domain name MX: mail exchange (identifies the mail server of the domain) NS: name server record 23

24 DNS Load Balancing Round-robin DNS Load Balancing Has two A records mapping to and Client A CUHK Web Server DNS Server Client B CUHK Web Server

25 DNS Load Balancing $ dig ; <<>> DiG ubuntu0.5-Ubuntu <<>> ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 0, ADDITIONAL: 1 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 512 ;; QUESTION SECTION: ; IN A ;; ANSWER SECTION: IN CNAME youtube-ui.l.google.com. youtube-ui.l.google.com. 146 IN A youtube-ui.l.google.com. 146 IN A youtube-ui.l.google.com. 146 IN A youtube-ui.l.google.com. 146 IN A

26 DNS Load Balancing Simple, but a few limitations: Stickiness: It takes time to propagate changes in DNS records adding or removing servers can be slow Loading can not be balanced accurately due to DNS caching (e.g. when a lot of users are from the same ISP) It does not take into account transaction time, server load, network congestions, etc. No fault tolerance (DNS server does not know if a server is operating or not) 26

27 DNS Load Balancing While having some limitations, DNS load balancing is an important method for achieving availability The only way to divert traffic to different data centres 27

28 Hardware / Software Load Balancing 28

29 Load Balancing on Different Layers Load balancing can be done at the different layers in the OSI model Layer 4 (Transport Layer) Load Balancing Relatively simple Balancing server load without inspecting the content of messages Distribute traffic based on servers response time Routing is based on inspecting the first packet of the data stream 29

30 Layer 4 Load Balancing The load balancer forwards requests to one of the servers, and the servers will send responses via the load balancer Layer 4 Load Balancer 30

31 Layer 4 Load Balancing The client sends requests to an IP address (identifying the load balancer) The load balancer translate the destination address of the request to one of the servers IP address, and translate it back in the response. (Bi-directional NAT) Layer 4 Load Balancer 31

32 Layer 4 Load Balancing What are the advantages of Layer 4 Load Balancing? Simple: easy to implement Efficient: load balancer only inspect the first packet However It cannot maintain application session information It cannot route requests to different servers based on their content (e.g. requesting static content vs. dynamic content) 32

33 Layer 7 Load Balancing Layer 7 (Application Layer) Load Balancing Application-level load balancing Parse requests in application layer (e.g. HTTP), and distribute them to servers based on the content of the request (e.g. the URL or the cookie) Relatively high overhead in parsing the metadata Mostly HTTP (because of the popularity of Web apps) 33

34 Layer 7 Load Balancing HTTP requests are terminated at the Layer-7 switch Ref: Cardellini, Casalicchio, Colajanni, and Yu The state of the art in locally distributed Webserver systems. ACM Computer. Survey, 34, 2 (June 2002),

35 Layer 7 Load Balancing Characteristics of Layer 7 Load Balancing More CPU-intensive (e.g. for parsing HTTP content) Can also apply compression, encryption or caching on the content to be delivered Does not require all servers in the backend to have serve the same content (compare with Layer 4 load balancing) Layer 7 load balancers are also called Application Delivery Controllers 35

36 Reverse Proxy Layer 7 load balancing is also called reverse proxy. Why? What is a proxy? A proxy server (It sends request to another server on behalf of the client) 36

37 Reverse Proxy Reverse proxy retrieves resources from different servers on behalf of a client. A reverse proxy (load balancer) Retrieves resources from different servers at the backend 37

38 Abilities of Layer 7 Load Balancers One important ability of a layer 7 load balancer is SSL termination The load balancer has to be able to decrypt the request in order to inspect the content It therefore must be configured with a valid SSL certificate HTTPS requests Unencrypted requests Load balancer 38

39 Common Load Balancers Many solutions available in the market nowadays Hardware LBs Products of F5, Citrix and Cisco, etc. Software LBs Apache, Nginx, HAProxy, Squid, etc. Cloud-based Services Amazon Elastic Load Balancer (load balancing as a service) 39

40 Load Balancing With Nginx 40

41 Load Balancing using Nginx Nginx is a Web server but can also be configured as a reverse proxy server It can proxy requests to another HTTP server or a non-http server It supports the following non-http protocol: FastCGI, uwsgi, SCGI, memcached It can buffer responses from servers to improve performance (when the client is slow) 41

42 Configuring Nginx Nginx can be configured by editing the configuration files In Ubuntu, configuration files are usually stored under /etc/nginx/ A main configuration file named nginx.conf One or more configuration files for each of the sites hosted by the server (see /etc/nginx/site-available and /etc/nginx/site-enabled ) Ref: 42

43 Configuring Nginx The main context Basic structure of a configuration file for Nginx The event context user www-data; worker_processes 4; pid /run/nginx.pid; events {... } http { server { location {... } }... } The http, server and location contexts 43

44 Configuring Nginx A simple setup for a Web server serving content from a particular directory server { listen 80 default_server; listen [::]:80 default_server ipv6only=on; location / { root /data/www; } } Assume that the server is up at then a request to will retrieve an image named x.jpg from the directory /data/www/images 44

45 Configuring Nginx Serving files from different directories server { listen 80 default_server; listen [::]:80 default_server ipv6only=on; location / { root /data/www; } location /images/ { root /data/images; } } 45

46 Configuring Nginx Configuring Nginx as a reverse proxy load balancer is simple. However, before that we introduce the term upstream Upstream Servers Load Balancer Downstream Upstream 46

47 Configuring Nginx Load balancing example in Nginx Note: When no load balancing method is specified, the roundrobin method will be used http { upstream myservers { server s1.myserver.com; server s2.myserver.com; server s3.myserver.com; } } server { listen 80; } location / { proxy_pass } 47

48 Configuring Nginx The example in the previous slide is illustrated in the following diagram: s1 s2 Nginx s3 48

49 Configuring Nginx Load balancing using the least connection method http { upstream myservers { least_conn; server s1.myserver.com; server s2.myserver.com; server s3.myserver.com; } server { listen 80; } } location / { proxy_pass } 49

50 Persistence Persistence (Stickiness) Very often, we need the same server to serve the same client for a series of requests (why?) Round-robin and least connected methods do NOT guarantee that the same client will be served by the same server Persistence (or stickiness) refers to the ability of the load balancing to forward requests to the same server 50

51 Configuring Nginx Use IP hashing as the load balancing method to achieve persistence in Nginx http { upstream myservers { ip_hash; server s1.myserver.com; server s2.myserver.com; server s3.myserver.com; } server { listen 80; } } location / { proxy_pass } 51

52 Configuring Nginx Other functions include: Health checks of servers Buffering server response Routing requests to applications (e.g. to a Python Web app) You will explore these features in assignments. Ref: 52

53 Assignments & Project 53

54 Assignment Plan There will be a total of 5 assignments 1. Building a simple Web application using Nginx, Python and Flask 2. Implementing a load balancer and experimenting with different settings 3. Supporting your application with databases and caches 4. Building an Asynchronous task queue 5. Load testing your application 54

55 Assignment Plan 4 Asynchronous Task Queue Client 5 Load Balancer Nginx Web Server Python Application 3 Database Server 55

56 Project In the project, you will extend the system you build in the assignments to a functional service. You can choose from the following two topics: Personal notebook For storing notes, articles, messages, etc. for users Personal album For storing photos or images for users You have the flexibility on what functions you want to implement (more on next slide) 56

57 Project In the project, we will have a list of required functions and a list of optional functions You must implement the required functions (e.g. user account, searching, add/remove) You get bonus points for implementing the optional functions (or functions not listed in the project specification) In addition, pick one function of your application, and discuss the potential scalability problem, and implement a solution for it 57

58 End of Lecture 4 58