Problem Statement
What are the essential attributes of a scalable web system, and how to design such a system?Solution Abstract
What? What? How?
What is a scalable web application? The basic function of a web server (web-site or web-application) is to serve static content like HTML, JS, CSS, etc., over HTTP/HTTPS channel, using a Request-Response model of communication. For any real-life web there will be added complexity on server-side like - complex logic execution, database access, application security etc. Along with these a scalable system needs to be - responsive, capable of processing large number of requests, and be able to scale to meet ever increasing load.
What are the key challenges? The key concerns being :-
- reduce latency or time taken to serve a request from client - mostly computed in seconds if not sub-seconds;
- improve throughput or the number of requests that can be processed in certain amount of time;
- remove server resource contention like - memory, CPU, IO, bandwidth;
- recover from hardware/software failure;
- handle unexpected surge in load - either genuine or DoS like attack; and
- support frequent deployments with minimal down-time.
How to solve the challenges? The architecture and design need to :-
- have coarse-grained interface for reduced calls over network;
- have specialized fine-grained interfaces for selective data transfer over network;
- implement data-cache to reduce - database reads and CPU cycle;
- implement content-cache to reduce - file-system reads;
- partition data and content-files for improved reads and load-sharing;
- design state-less components and APIs;
- design self-sufficient context-agnostic components;
- have load-balancing and clustering in place for load sharing and fault tolerance;
- design specialized code to detect failure and use backup systems; and
- support session persistence to enable recovery and scale-out;
The Inner-workings
For a system to be scalable, specialized implementation considerations are needed in - logical architecture and deployment architecture. While the logical construct remains same, there can be two key variants of the architectural structure -
- standard - in which contents and calls are served from the same domain;
- x-domain - in which the contents and calls are served/composed from multiple domains, resulting in x-domain (aka cross-domain) calls.
Architectural Flow
Standard Web System
In a
standard web system contents are served to the web-browser from the same domain. Static resources like - css, javascript, images, etc., may be served from different domains/sub-domains over HTTP GET (occasionally using AJAX JSONP calls).
Note: JSONP is a security vulnerability, and not preferred.Logical Architecture
Deployment Topology
x-domain Web System
In a
x-domain web system contents are served to the web-browser from multiple domain/sub-domains, over HTTP GET/POST/PUT/DELETE. This is specifically used in systems serving web-APIs and/or aggregated content. The architectural construct remains same as a
standard web system, with the addition of
reverse proxy.
Logical Architecture
Deployment Topology
Implementation Matrix
The below matrix depicts the implementation essentials and the problems they solve.
Glossary
| RESTful API | Representational State Transition API. A web based simplified API working over HTTP QueryString parameters. |
| Static Content | The contents that do not change and are retrieved from direct file-system reads, like - html, text, script, image files. |
| XHR | XML Http Request. A built-in communication object available in browser over which all HTTP communication occurs. |
| Load Detector | A pass-through component which monitors the incoming requests and detects high-loads based on pre-defined thresholds. This is an optional module built into the web system. |
| Endpoint | The address or URL through which communication occurs. |
| Message Queue | A store-forward asynchronous system which can accept requests, persist it and later forward to the target system for processing. |
References
- Patterns & Prctices - Application Architecture Guide (Microsoft)
Paper Code: TWP_1002.11, Version: 1.1, Author: Biswajit Dash, License: CC-BY-ND, Published: Sep-2016