A gossip protocol is a style of computer-to-computer communication protocol inspired by the form of gossip seen in social networks. Modern distributed systems often use gossip protocols to solve problems that might be difficult to solve in other ways, either because the underlying network has an inconvenient structure, is extremely large, or because gossip solutions are the most efficient ones available.
Contents
- Gossip communication
- Gossip protocol types
- Examples
- Epidemic algorithms
- Biased gossip
- Code examples
- References
The term epidemic protocol is sometimes used as a synonym for a gossip protocol, because gossip spreads information in a manner similar to the spread of a virus in a biological community.
Gossip communication
The concept of gossip communication can be illustrated by the analogy of office workers spreading rumors. Let's say each hour the office workers congregate around the water cooler. Each employee pairs off with another, chosen at random, and shares the latest gossip. At the start of the day, Alice starts a new rumor: she comments to Bob that she believes that Charlie dyes his mustache. At the next meeting, Bob tells Dave, while Alice repeats the idea to Eve. After each water cooler rendezvous, the number of individuals who have heard the rumor roughly doubles (though this doesn't account for gossiping twice to the same person; perhaps Alice tries to tell the story to Frank, only to find that Frank already heard it from Dave). Computer systems typically implement this type of protocol with a form of random "peer selection": with a given frequency, each machine picks another machine at random and shares any hot rumors.
The power of gossip lies in the robust spread of information. Even if Dave had trouble understanding Bob, he will probably run into someone else soon and can learn the news that way.
Expressing these ideas in more technical terms, a gossip protocol is one that satisfies the following conditions:
Gossip protocol types
It is useful to distinguish three prevailing styles of gossip protocol:
- Event dissemination protocols use gossip to carry out multicasts. They report events, but the gossip occurs periodically and events don’t actually trigger the gossip. One concern here is the potentially high latency from when the event occurs until it is delivered.
- Background data dissemination protocols continuously gossip about information associated with the participating nodes. Typically, propagation latency isn’t a concern, perhaps because the information in question changes slowly or there is no significant penalty for acting upon slightly stale data.
Many protocols that predate the earliest use of the term "gossip" fall within this rather inclusive definition. For example, Internet routing protocols often use gossip-like information exchanges. A gossip substrate can be used to implement a standard routed network: nodes "gossip" about traditional point-to-point messages, effectively pushing traffic through the gossip layer. Bandwidth permitting, this implies that a gossip system can potentially support any classic protocol or implement any classical distributed service. However, such a broadly inclusive interpretation is rarely intended. More typically gossip protocols are those that specifically run in a regular, periodic, relatively lazy, symmetric and decentralized manner; the high degree of symmetry among nodes is particularly characteristic. Thus, while one could run a 2-phase commit protocol over a gossip substrate, doing so would be at odds with the spirit, if not the wording, of the definition.
Frequently, the most useful gossip protocols turn out to be those with exponentially rapid convergence towards a state that "emerges" with probability 1.0. A classic distributed computing problem, for example, involves building a tree whose inner nodes are the nodes in a network and whose edges represent links between computers (for routing, as a dissemination overlay, etc.). Not all tree-building protocols are gossip protocols (for example, spanning tree constructions in which a leader initiates a flood), but gossip offers a decentralized solution that is useful in many situations.
The term convergently consistent is sometimes used to describe protocols that achieve exponentially rapid spread of information. For this purpose, a protocol must propagate any new information to all nodes that will be affected by the information within time logarithmic in the size of the system (the "mixing time" must be logarithmic in system size).
Examples
Suppose that we want to find the object that most closely matches some search pattern, within a network of unknown size, but where the computers are linked to one another and where each machine is running a small agent program that implements a gossip protocol.
If the messages might get large (for example, if many searches are active all at the same time), a size limit should be introduced. Also, searches should "age out" of the network.
It follows that within logarithmic time in the size of the network (the number of agents), any new search string will have reached all agents. Within an additional delay of the same approximate length, every agent will learn where the best match can be found. In particular, the agent that started the search will have found the best match.
For example, in a network with 25,000 machines, we can find the best match after about 30 rounds of gossip: 15 to spread the search string and 15 more to discover the best match. A gossip exchange could occur as often as once every tenth of a second without imposing undue load, hence this form of network search could search a big data center in about 3 seconds.
In this scenario, searches might automatically age out of the network after, say, 10 seconds. By then, the initiator knows the answer and there is no point in further gossip about that search.
Gossip protocols have also been used for achieving and maintaining distributed database consistency or with other types of data in consistent states, counting the number of nodes in a network of unknown size, spreading news robustly, organizing nodes according to some structuring policy, building so-called overlay networks, computing aggregates, sorting the nodes in a network, electing leaders, etc.
Epidemic algorithms
Gossip protocols can be used to propagate information in a manner rather similar to the way that a viral infection spreads in a biological population. Indeed, the mathematics of epidemics are often used to model the mathematics of gossip communication. The term epidemic algorithm is sometimes employed when describing a software system in which this kind of gossip-based information propagation is employed.
Biased gossip
Above, a purely random peer-selection scheme for gossip was described: when agent A decides to run a gossip round, it picks some peer B uniformly and at random within the network as a whole (or launches a message on a random walk that will terminate at a random agent). More commonly, gossip algorithms are designed so that agents interact mostly with nearby agents, and only sometimes with agents that are far away (in terms of network delay). These biased gossip protocols need to ensure a sufficient degree of connectivity to avoid the risk of complete disconnection of one side of a network from the other, but if care is taken, can be faster and more efficient than protocols that are purely random. Moreover, as a purely practical question, it is much easier to maintain lists of peers in ways that might be somewhat biased.
Code examples
There are three known libraries that implement a Gossip algorithm to discover nodes in a peer-to-peer network: