Performance

In this final section, we will look at discovery performance implications.

Bandwidth and Scalability

This chapter has emphasized that uncontrolled discovery (broadcast) is only appropriate for small work groups. Balancing geographic reach while limiting excessive bandwidth consumption is a critical requirement of any scalable P2P network. There are a number of solutions that can be applied to optimize the discovery process:

• Use diversity to reach a broader range of the virtual space more efficiently.

• Minimize routing table size by seeding the table with special-purpose peers.

• Throttle discovery requests and minimize heartbeat polling.

• Segment the routing table by operational and environmental metadata.

• Constrain total bandwidth allocated to discovery processing.

• Monitor peer consumption, including bandwidth used by incoming packets, peak bandwidth used, and a quality value associated with the responses received.

Searching

The quality value of the response received can be used to determine future query and discovery scenarios. If you record the quality of a peer response, you can begin to maintain a search priority matrix by topic or category. Peers with high values for a given category will be queried first. You'll query lower-grade peers only if the higher-quality peers fail.

One outcome of this technique is that you may organize peers into peer clusters, or groups of peers capable of satisfying discovery requests by type or class of information. Additionally, peer clusters often have the overall effect of limiting the number of packets used, and thus the network bandwidth required to search for a desired response.

Transport

As this chapter has highlighted, discovery and searching have huge implications on network bandwidth requirements. Most implementations currently reside on either TCP (to improve reliability), or UDP (to improve performance). In addition, the topology of the discovery network is critical to understanding bandwidth and performance requirements.

A decentralized discovery topology relies on peers to propagate discovery requests across the network. Defining special discovery roles is minimized, and the failure of any one node does not necessitate widespread network failure. Often, UDP can be used because of the inherent redundancy built into the discovery (mesh) topology.

Contrasted with a decentralized topology is a centralized discovery platform. Some would argue that centralized discovery, or super nodes, are in direct contradiction to basic P2P network formation. However, in corporate environments, where large enterprise systems are needed for scalability, administration, and security, centralized servers using TCP might be appropriate.

The emerging middle ground (hybrids) uses a combination of centralized and decentralized discovery processing. Special broadcast roles, such as rendezvous and relay peers, effectively bridge the network to minimize the number of concurrent connections that must be supported.

Fault Tolerance

Providing an adequate level of fault tolerance in P2P networks will be critical to their widespread adoption.

Redundancy

P2P technology solves problems inherent in a dynamic network, including dynamically assigned addresses, routing changes, and firewalls. Redundancy is key to reducing failure points and providing extra value for a P2P network.

Load Balancing

Techniques such as round-robin processing, workload queries, and node diversity (geographic distribution) are required to scale P2P networks. This should be implemented without requiring any global coordination.

Storage

Data caching of routes, peer groups, and peers will radically improve performance. Subsequent lookup requests whose paths intersect can be served the cached copy. Initialization of peers from cached information can quickly transfer a large amount of virtual space knowledge from one peer to another.

Distributed caching offloads the peers that hold the primary replicas of data, and minimizes delays and network traffic by dynamically caching copies near interested clients.

Communication

A key design issue is how to efficiently and dynamically maintain the routing table in the presence of peer failures, peer recoveries, and new peer arrivals. Special broadcast peers can periodically exchange keep-alive messages. If a peer is unresponsive for a period, it is presumed failed. All members of the failed peer's group are notified and update their group membership.

Routing table entries that refer to failed peers can be repaired lazily; in other words only when explicitly requested or addressed to do so.

Applications can perform efficient multicast on subnets to repair or recover from localized damage or failures.