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Implementation

We have implemented the Mobile Streams model in a toolkit we call AGNI 3 . AGNI is a multi-threaded TCL extension that uses the thread-safety features of TCL 8.1 and consists of roughly 23,000 lines of C++ code. In this section, we give highlights of the implementation some initial performance results.

Each workstation that wishes to participate in the distributed system runs a copy of an Agent Daemon. A distinguished Agent Daemon houses the Session Leader and is in charge of accepting or rejecting new Agent Daemons. This Daemon also serves as a Location Manager and Failure Manager for all MStreams in the Session. Each Agent Daemon has a unique identifier that it obtains from the Session Leader. Each Agent Daemon maintains a connection with the Session Leader Agent Daemon. Conceptually, the arrangement is as shown in the figure 3.

Each Agent has a TCL interpreter and thread of execution that is used by the Handlers that it registers. These resources are created for an Agent at a Site on its the first visit to the Site and remains allocated until the Agent (or the MStream to which it is attached) is destroyed. When a new Agent is added to an MStream, its code is propagated and initialized on the first move of the Agent to a previously unvisited Site, and remains cached there until it is destroyed. Provided an MStream has visited a Site previously, and no new Agents have been attached since its last visit, MStream movement simply consists of moving the state information in the briefcase (see Section 2) of each Agent of the MStream to the new Site and concurrently invoking each on_stream_arrival Handler.

Figure 3: Each Site runs an Agent Daemon that is connected to the Session Leader. The Agent Daemon is Multi-threaded with one thread per agent. The Session Leader maintains location and cache information.

Except for the case when the MStream is co-located with the Site from where the message originates, all control Events destined for an MStream (e.g. creation, relocation, new agent attachment) are delivered through the Session Leader Agent Daemon via the TCP connection that each Agent Daemon maintains with it. The Session Leader Agent Daemon also acts as a Location Manager, keeping track of where each MStream is located and is hence able to re-direct messages to the location of the MStream. Sending all control Events through the Session Leader is a simple means of achieving a global ordering on control messages. The negative aspect of this design is that the Session Leader has the potential of becoming a bottleneck. However, we expect the number of control messages to be much smaller than the number of data messages (appends) processed by the MStream and hence do not consider this a serious limitation at present. In our future work, we plan to alleviate this problem by replication of the Session Leader. The Session Leader Daemon also manages the tracking information for the code and state cache described previously and is charge of propagating code to previously unvisited locations. As all Agent code is registered at the Session Leader and propagated from there, this simplifies the trust model to pair-wise relationships between each site and the Session Leader. provided all parties trust the Session Leader.

Appended data messages are delivered to the destination MStream directly without going through the Session Leader. Thus the Session Leader is not a bottleneck for data message delivery.

Next: An In-Situ Simulation Environment Up: AGNI: A Multi-threaded Middleware Previous: Handling Failures