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About the Hume Datahub

A key issue in distributed systems is providing a consistent view of shared data throughout the application and doing it in an efficient manner. The usual method has been to have processes query a central database at the point in time when they need to access a data item. Since the using process does not know if the data has changed, the safe course is to read the latest value at every use. This method is intolerably inefficient if the cost per query is high, or the number of queries is high.

The subscription concept offers a superior approach. The user of a data item opens a subscription to the data item with a subscription server. As the data item is updated, the user process receives asynchronous notification from the subscription server. Now there is no polling - the user process accurately tracks the item of interest.

The Datahub is a high performance in-memory database, a subscription server, a configurable Tcl/Tk interpreter, and a Distributed Message Hub (DMH) server. The synergistic combination of these capabilities is the cornerstone of a new architecture for distributed applications.

As a database server, the Datahub provides the familiar programming model of relational database tables using a subset of Structured Query Language (SQL). A graphical user interface is available to display and manipulate table data either remotely as a client, or as part of the Datahub process when it is not running in the background.

As a subscription server, the Datahub provides asynchronous notification to Client processes whenever SQL table data that meets their selection criteria is inserted, updated, or deleted. These notifications can be standard SQL messages which are useful for data replication to other Datahubs, or for data replication to commercially available persistent databases, such as Oracle. Other subscription options enable the developer to execute user defined Tcl procedures within the Datahub process, or to obtain notification messages in a format that is designed for use by Tcl list manipulation commands.

When used as a DMH message server, a Datahub becomes the hub of an efficient, event-driven, distributed application. The Tcl interpreter provides a high-level, dynamically customizable programming environment with comprehensive features. Client processes can easily exchange peer messages, can easily share common data in relational tables, and can subscribe and respond in real-time to changes in the application data. Shared application logic can be executed within the Datahub process, sharing the same address space with application data tables and SQL command processing logic. The requirement for interprocess communication and the overhead that it entails is reduced drastically compared to other approaches.

Because the Datahub is both a database and a subscription server, clients can open "synchronized" subscriptions to existing table data when they are initialized. A shortcoming of other notification mechanisms that are based only on connecting to "broadcast" or "distribution list" message streams, is that the client has no historical data to synchronize with. It is an additional burden on the application developer to design a synchronization mechanism if dynamic client connections are supported.

Better Than Publish and Subscribe

The products we have seen that offer similar publish and subscribe functionality have certain shortcomings that potential users should consider.

  • The application developers have to explicitly choose each data change or event that is published. Typically this is not an easy choice because each published item contributes to loading of the network regardless of whether the event is subscribed to or not, and much discussion has to take place among the developers to make the appropriate tradeoffs.
  • The content and format of published messages has to be agreed upon by all parties since these are not typically controlled by the subscriber. Typically there is anguish over how much data to put in the notification message, and what data is left at the server for clients to query. Again, the developers sit around weighing the tradeoffs.
  • The subscribing clients have to write custom parsing code for each message type since there is typically not a Tcl or SQL parser in use.
  • The traditional approach to the specification of different message types is to declare a union in C code of all of the binary message structures. This technique is sensitive to cross platform issues such as byte ordering and data alignment. Also, it is typical that all of the executables on all platforms need to be recompiled when a new message structure is defined or an existing structure is updated since the included header file is used extensively throughout the source code.
  • Some products are based on UDP broadcasting and will not work across subnets without explicit configuration of the network routing and the participating hosts.

In contrast, when the Datahub is used, any data that is stored in the SQL tables can be subscribed to by clients. There is no overhead of broadcasting changes to the network just in case there is an interested client. Similarly, there is not an issue with data propagation across subnets since reliable point-to-point TCP/IP protocols are used.

With the Datahub, the subscriber has complete control over the content and format of data change notifications. The software development proceeds without the elaborate negotiation between the data producers and the data consumers. For example, a client may open a subscription to a data table where only certain column values are part of the notification, and the notifications occur only for changes to data rows or new data rows where a selection condition, such sensor_id='VIP23' is true. If configured, the notification can execute a user written Tcl procedure which can perform actions such as sending custom formatted messages through the network, or invoking an XML-RPC method on a remote server.

Read Detailed Documentation about the Datahub

The Datahub SDK product page

 


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