Causeway: Support for Controlling and Analyzing the Execution of Web-Accessible Applications

Causeway provides runtime support for the development of distributed meta-applications. These meta-applications control or analyze the behavior of multi-tier distributed applications such as multi-tier web sites or web services. Examples of meta-applications include multi-tier debugging, fault diagnosis, resource tracking, prioritization, and security enforcement. Efficient online implementation of these meta-applications requires meta-data to be passed between the different program components. Examples of metadata corresponding to the above meta-applications are request identifiers, priorities or security principal identifiers. Causeway provides the infrastructure for injecting, destroying, reading, and writing such metadata. The key functionality in Causeway is forwarding the metadata associated with a request at so-called transfer points, where the execution of that request gets passed from one component to another. This is done automatically for system-visible channels, such as pipes or sockets. An API is provided to implement the forwarding of metadata at system-opaque channels such as shared memory. We describe the design and implementation of Causeway, and we evaluate its usability and performance. Causeway’s low overhead allows it to be present permanently in production systems. We demonstrate its usability by showing how to implement, in 150 lines of code and without modification to the application, global priority enforcement in a multi-tier dynamic web server.