2.2.3 Hybrid Architectures (Distributed Systems)

1. Introduction to Hybrid Architectures

Hybrid architectures combine elements of client-server and peer-to-peer (P2P) systems.

Goal:

• Take advantage of both models (client-server manageability + P2P scalability).
• Solve the problems of purely decentralized systems (for example, low reliability or "freeloaders" in P2P).

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2. Edge-Server Systems (Systems with edge servers)

2.1. Definition and location in the network

• Edge servers are located at the junction of networks:
  • Between the corporate network and the Internet (for example, for Internet service providers — ISPs).
  • Between users and the core of the Internet (for example, CDN servers).
• Examples:
• Cloudflare and Akamai servers.
  • Local caching proxies at the ISP.

2.2. Architecture and operating principle

1. Origin Server
2. Stores original content (for example, the company's main website).
3. Edge Servers
4. Replicate content from the origin server.
   • Serve user requests closer to their location (reducing delays).
5. Edge server functions: -Caching (saving copies of pages, videos).
   • Filtering (blocking DDoS attacks).
   • Transcoding (adapting content to devices, for example, video compression for mobile).

2.3. Advantages

• Speed: Content is delivered faster (fewer network hops).
• Reliability: If one edge server crashes, the others continue to work.
• Traffic savings: Less load on the origin server.

2.4. Usage Examples

• CDN (Content Delivery Networks):
• Services like Netflix, YouTube use edge servers for video streaming.
• Cloud Computing:
• AWS Lambda@Edge executes code closer to users.

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3. Collaborative Distributed Systems

3.1. General concept

• Hybridity: -Centralized components for management (for example, trackers in BitTorrent).
  • Decentralized components for interaction (P2P data exchange).
• Goal:
  • Combine the efficiency of P2P with the control of client-server systems.

3.2. Example 1: BitTorrent

Architecture

1. Centralized components:
2. Trackers (Tracker): Servers that store information about:
3. Which nodes are involved in the distribution.
   • What parts of the file they have.
     • Directories (.torrent files):
   • Links to trackers and file metadata (hashes, size of parts).
4. Decentralized components:
5. Peers: Nodes that exchange parts of a file directly (P2P).

The principle of operation

1. The user downloads a .torrent file from a directory site.
2. The client (for example, Mktorrent) connects to the tracker to get a list of peers.
3. The download of the file ** begins piece by piece (chunks)** from different nodes.
4. Tit-for-Tat ("you—to-me, I—to-you" exchange):
5. If node A downloads data from node B, then A must also give data to B.
   • Otherwise it reduces the recoil rate (anti-freeloader mechanism).

Problems

• Vulnerability of trackers: If the tracker shuts down, new nodes will not be able to connect.
• Uneven load distribution: Popular files load faster, rare files load slower.

3.3. Example 2: Globule (Collaborative CDN)

Architecture

1. The decentralized part:
2. Users' voluntary web servers (for example, Alice and Bob's server).
   • Each server can:
3. Replicate other people's web pages.
   • Redirect requests.
4. Centralized part:
5. Broker:
   • Registers servers.
   • Coordinates content replication.

The principle of operation

1. A request to Alice's website first goes to her origin server.
2. The server analyzes the load and can:
3. Handle the request itself.
   • Redirect to another server (for example, Bob's), where there is a copy of the page.
4. Broker helps servers find each other.

Advantages

• Flexibility: It is possible to scale without expensive data centers.
• Fault tolerance: If one server crashes, the others continue to work.

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4. Comparison of hybrid architectures

System	Centralized components	Decentralized components	Usage Examples
Edge Servers	Origin server	Replicated edge Servers	CDN (Cloudflare, Akamai)
BitTorrent	Trackers, .torrent directories	Peers (P2P)	File sharing (mktorrent)
Globule	Broker	Voluntary Web Servers	Decentralized CDNs

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5. Conclusions

1. Hybrid systems combine the best of client-server and P2P architectures.
2. Edge-Server Systems optimize content delivery (less delays, higher reliability).
3. Collaborative systems (BitTorrent, Globule) use centralized management to start, but work in a decentralized manner.
4. Trends:
5. Development of CDN and edge computing (for example, AWS Edge Locations).
   • Hybrid blockchains (Ethereum 2.0 with different types of nodes).

*The result: Hybrid architectures are a compromise between manageability and scalability that is critical for modern distributed systems.