> For the complete documentation index, see [llms.txt](https://stephen-tsoi.gitbook.io/stephen-tsoi-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://stephen-tsoi.gitbook.io/stephen-tsoi-docs/event-driven-architecture/event-driven-architecture-aligning-technology-with-business-outcomes/day-2-7-optimizing-cost-of-change.md).
# Day 2/7 Optimizing Cost of Change
Day 2 of 7: A concrete financial and technical comparison proving how EDA slashes infrastructure and operational costs compared to legacy REST architectures at scale.
**Application Host Count**
* EDA: A single event bus can handle up to 200,000 concurrent connections.
* REST: To manage the same number of connections, 400 web servers are necessary, with each server supporting only 500 connections concurrently.
**Network Infrastructure**
* EDA: The Event Bus's non-exclusive queue enables an Active-Active consumer model, eliminating the need for additional load balancers.
* REST: Separate load balancers are vital for the Web Server, Channel Application Server, and Backend Server within the REST framework.
**Bandwidth**
* EDA: MQTT, a lightweight protocol with persistent connections, is employed.
* REST: The REST solution requires appending authentication data to each message, resulting in message sizes approximately 10 times larger than MQTT.
**Operating Costs** Operational expenses are influenced by hosting and network equipment requirements, including factors like power consumption and data center space. The EDA solution proves more cost-effective than REST, as one event bus is equivalent to 400 web servers and 30-40 application servers.
**Internal Efforts** Project implementation, solution design, development, testing, and interface maintenance constitute internal efforts, varying based on system complexity and network infrastructure. EDA's system decoupling minimizes change scope, while REST's tightly coupled systems demand redundant logic across different channel application servers, leading to increased efforts in architecture design, development, and testing compared to the EDA solution.
Comparing the cost implications for supporting 200,000 concurrent connections in real-time operations using two distinct approaches: REST and EDA.
**Assumption**
* 3 Channels (2 Mobile Apps and one Browser)
* 200,000 concurrent connections
* 10 Operations (e.g. Account Opening, Account Enquiry, etc.)
* REST message size: 100Kb, EDA MQTT message size: 10Kb (10% of REST message)
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