The Secret Life of AWS: The Tangled Web
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When to use a simple Bridge (Peering) and when to build a central Hub (Transit Gateway
Part 6 of The Secret Life of AWS
Timothy stood at the chalkboard, drawing lines between three separate rectangles labeled Library, Finance, and HR.
"What is this?" Margaret asked.
"I am connecting our networks," Timothy said. "I need the Library VPC to talk to Finance, and Finance to talk to HR."
He drew another line. "But the diagram is getting messy."
Margaret stepped closer. "You are creating a mess because you are building individual connections for everything. In the cloud, we need to choose the right topology."
VPC Peering (The Direct Line)
Margaret erased the messy lines. She pointed to the Library and Finance boxes.
"If you have two VPCs that need to talk privately," she said, "we use VPC Peering."
She drew a single, clean line connecting the two rectangles.
"This connection uses AWS's private infrastructure. It is secure, fast, and reliable."
"Excellent," Timothy said. "So I will just peer Library to Finance. And Finance to HR. And then Library can talk to HR, right?"
"No," Margaret said firmly. "VPC Peering is Non-Transitive. It is not a chain. If Library peers with Finance, and Finance peers with HR, Library cannot reach HR. You would need to build a direct peering connection between Library and HR too."
The Scaling Problem (The Mesh)
Timothy looked at the board. "So... if I have 3 VPCs, I need 3 connections to link them all."
"Correct."
"And if I have 4 VPCs?"
"6 connections."
"And if I have 100 VPCs?"
"4,950 connections," Margaret calculated. "You would spend your entire life managing route tables. This is called a Full Mesh Topology, and it does not scale."
"So peering is bad?"
"No," Margaret corrected. "Peering is excellent for simple, one-to-one connections. But when you need to connect many-to-many, you need a different tool."
Transit Gateway (The Hub)
Margaret wiped the board clean. She drew the three VPC rectangles (Library, Finance, HR) in a circle. In the center, she drew a large icon.
"This," she said, "is the Transit Gateway."
"Think of this as a central Hub. Instead of building hundreds of direct connections, you simply connect each VPC to the Transit Gateway."
"So Library connects to the Hub," Timothy traced the line. "And HR connects to the Hub."
"And the Hub handles the routing," Margaret finished. "If Library wants to send data to HR, it sends it to the Transit Gateway, which forwards it to HR. You manage one connection per VPC, not hundreds."
Shared Services
"Why would we have so many VPCs?" Timothy asked.
"Security and organization," Margaret said. "Imagine we build a Shared Services VPC—a network just for your Active Directory server or your Logging tools."
She drew a fourth box connected to the Transit Gateway.
"With this Hub, every other department—Finance, HR, Engineering—can instantly use those tools through the Gateway. If we used Peering, we would have to manually link every single department to that Shared Services VPC. The Gateway makes the network scalable."
The Cost of Convenience
Timothy nodded. "This looks much tidier. I should always use the Transit Gateway."
"Not always," Margaret cautioned. "The Transit Gateway is a managed service, and it costs money. VPC Peering is just a connection."
She wrote a comparison on the board:
- VPC Peering: "Think of it as a direct phone line between two offices. It is private, cheap, and perfect for confidential one-on-one conversations. But it doesn't scale."
- Transit Gateway: "Think of it as a corporate switchboard. Everyone has an extension. It is easy to connect new people, but you pay for the system."
"If you are just connecting the Library to Finance," Margaret said, "use Peering. Keep it simple. If you are connecting the Library to fifty other departments... use the Gateway."
The Lesson
Timothy picked up the chalk and drew a single line between Library and Finance. Then he drew a Transit Gateway next to it, ready for future expansion.
"Start with the direct line," he said. "Upgrade to the Hub when the network grows."
"Precisely," Margaret smiled. "It's not about connecting everything to everything. It's about choosing the right connection for the scale you need."
Aaron Rose is a software engineer and technology writer at tech-reader.blog and the author of Think Like a Genius.
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