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Symmetric vs. Asymmetric Comet

by Alessandro AlinoneDecember 7th, 2007

Kris Zyp recently categorized Comet applications into event-based and data-centric Comet. I will try to expand on this by mapping such definitions to two general types of publish/subscribe architectures. Then I will describe two models of publishing.

Comet Pub/Sub: Symmetric vs. Asymmetric

The typical representation of a Comet-based pub/sub architecture is shown in Figure 1. Two actors are involved: the Comet Client and the Comet Server. The Client can act both as a publisher and a subscriber. The Comet Server is the hub that manages the subscriptions and the message routing. This architecture can be mapped to Kris’ Event-based Comet and I will refer to it as symmetric Comet pub/sub.

Symmetric Comet
Figure 1

But there are well known situations where the publishers and the subscribers are completely separated, from both a logical and a physical perspective. Figure 2 shows this topology, where Clients act only as subscribers while the role of the publisher is played by a new actor: the Data Feed. This architecture can be mapped to Kris’ Data-centric Comet and I will refer to it as asymmetric Comet pub/sub.

Asymmetric Comet
Figure 2

Asymmetry between publishers and subscribers is a natural property for many Comet applications. For example, in the Finance industry the most successful application of push-technology is Market Data Distribution (or Dissemination). Each market (or exchange) produces a high volume of real-time data that characterizes the state of a financial security or instrument. Each state is made up of a set of fields (e.g. last price, timestamp, quantity, volume, bid, ask, etc) that can number in the hundreds. There exist some subsets of the state that can change at a very fast pace (the “market depth” can produce hundreds or thousands of updates per second for a single security). These state changes are driven by the orders (buy and sell) submitted by the traders. Collecting, organizing and selling this real-time data flow is the business of several specialized companies (the Financial Information Providers) such as Reuters and Bloomberg. They provide specific platforms together with APIs to make the real-time data flow available to their customers. This infrastructure is usually referred to as the Data Feed.

In a Comet scenario, the data generated by an exchange is delivered from an Information Provider to the Comet Server through its Data Feed. On the other side, there are Web browsers that access this data through JavaScript libraries and display it via HTML. This implies that the API for publishing and the API for subscribing could be radically different. For example, the Data Feed APIs are usually based on Java or C, while Comet has JavaScript-based subscribers. Furthermore, the communication between the Data Feed and the Comet Server should be highly efficient and based on specialized protocols rather than on HTTP.

Different Comet implementations are more oriented towards one or the other type of pub/sub. For example, Cometd is based on symmetric pub/sub, while Lightstreamer is based on asymmetric pub/sub. That does not mean that an asymmetric pub/sub solution does not enable client-side publishing, rather that the exposed interface is ready and optimized for server-side publishing (client-side publishing is usually obtained by having the client behave like a feed through a server-side Adapter).

Adapters

Asymmetric solutions should offer a way to integrate the Comet Server with any Data Feed. This is often accomplished by an Adapter-based architecture, borrowed from Enterprise Application Integration (EAI) systems. An Adapter (also referred to as a Connector) is a software component that performs two-way communication and implements the interfaces exposed by the two ends (the Server and the Feed). In other words, the Adapter adapts the Feed’s protocol to the Server’s interface.

The Comet Server should expose an interface that is flexible and efficient at the same time (this is a trade-off), so that third parties can develop Adapters for any Data Feeds. For example, Lightstreamer exposes three types of Adapter interfaces, based on a Java API, a .NET API, and a socket-level protocol.

This kind of architecture has some business implications too. In fact, off-the-shelf Adapters are beginning to appear on the market (e.g. Migratory Data Systems is a European company that ships a ready-made Reuters RMDS Adapter for Lightstreamer) and some system integrators are specializing in the development of Comet Adapters. The Comet space is becoming “structured”. That is, multiple parts are potentially involved in an enterprise Comet project: the Comet Server vendor, the Adapter vendor, the Information Provider, the AJAX toolkit vendor, and the AJAX system integrator.

Publish vs. “Publish On-Demand”

Another property that naturally applies to the two families of Comet architectures (Event-based/Symmetric and Data-centric/Asymmetric) regards how the publishing process takes place over time.

In symmetric Comet, publishers usually deliver data to the Server totally at their own discretion. When they have something to publish, they send it. The publishing life cycle for each topic usually corresponds to the life cycle of the publisher itself.

In asymmetric Comet, the Comet Server can often be seen as a Web-oriented add-on to a messaging system that exists on the server side (basically, the infrastructure within and behind the Data Feed). The role of the Comet Server is to dispatch the messages received from the publisher to all the users subscribed to a certain topic. If no user is subscribed to the topic to which a newly received message pertains, that message should be discarded. For performance reasons (that is, to avoid processing potentially high volumes of unnecessary data), the Comet Server should not receive messages for which there is no subscriber. When a client subscribes to a topic that is not currently subscribed to by any other client, the Comet Server activates the same subscription on the Data Feed and initializes a “channel” to start receiving updates on that topic. If a second client subscribes to the same topic, no action takes place towards the Data Feed. When all the clients unsubscribe from that topic, the Comet Server will deactivate that subscription on the Data Feed and stop delivering data on that topic. This model can be defined as publish on-demand (be aware that this is not fully compliant with a purist’s definition of pub/sub) and is reflected by the interface exposed by some asymmetric Comet Server implementations.

Conclusion

The architectural notions of symmetric and asymmetric Comet, and of publish-on-demand, are useful to understand which solution better fits the application requirements. Although the implementations of the two Comet models can be arranged to behave as the alternative model (or as a mix of the two models), it is important to understand the very architecture of a Comet solution in order to verify that it matches the actual needs.

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9 Responses to “Symmetric vs. Asymmetric Comet”

  1. Martin Tyler Says:

    Good article Allesandro, clearly describes the main difference between the way people like Lightstreamer, Caplin (and probably some others) do things compared to the various projects out there.

    Caplin supports both models, but in practice when clients are publishing, trading for example, the messages are passing through the comet server to something at the data feed level to handle. This is because trade messages are generally private and need to be fed to a trading system.

    As an open question.. are any of the open comet projects developing backend APIs?

  2. Jacob Rus Says:

    Martin: what kind of back-end APIs do you mean, exactly?

  3. Martin Tyler Says:

    Well, the current Comet projects out there seem to involve messages from comet clients and probably some code in the server itself (as you would if you were writing more traditional servlets or something). So the data clients subscribe to is either directly from other clients publishing, or from the custom code in the server.

    Caplin Liberator is more of a black box, you write your custom code as a separate process using an API to communicate with Liberator. Similarly, I believe Lightstreamer has an API to allow you to write adapters that run in process or as a separate process.

    So I guess these are messaging APIs which allow you to treat the Comet server as a black box rather than something you extend, which means you can be more language neutral. It also means your application (the server side logic) can be run in your favoured environment.

  4. Kris Zyp Says:

    I don’t think that the event-based/data-based dichotomy directly maps to the symmetric/asymmetric dichotomy. The symmetry relates to the origin and destination of messages, where Data-based Comet deals with how messages have are affected by data changes and the implications of the meaning of these messages. I think you can have symmetric Comet with data-based messages where the message recipient clients are also data contributors and therefore message affectors (this is a particular area that I am working on right now).
    That being said, your dichotomy is very valuable and insightful. You certainly have demonstrated how the distinction in symmetry is extremely important issue in Comet architecture. Great article!

  5. Alessandro Alinone Says:

    Kris, I agree that the two dichotomies are orthogonal, and you can have all the four combinations. I tend to see some natural mapping between an asymmetric architecture and data-centric Comet. But my mind is probably stereotyped by the typical financial applications for market data distribution.

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