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A failure detector is a key building block for fault-tolerant distributed system, which provide a mechanism to collect information of process failure.
The Heartbeat Model
The heartbeat model is used in most distributed systems. Every process p periodically sends an “I am alive” heartbeat message to the process q. The period is the heartbeat interval Δi.
If q does not receive a heartbeat message from p after a timeout delay Δto, p is added to the list of suspected processes. If q receives the heartbeat message from p later, then q removes p from its list of suspected processes, as shown in Fig. 1.
The heartbeat interval Δi:Δi is the time between two emissions of the “I am alive” heartbeat message. The timeout delay Δto:Δto is the time between the last reception of the “I am alive” message from p and the time where q starts suspecting p. The transmission delay Δtr:Δtr is the time between the emission of the heartbeat message and the reception of the heartbeat message.
detects inactive computers by sending and receveing "heartbeats" as UDP packets on the network, and keeping track of how much time passed since each known computer sent its last heartbeat. The concurrency in the server is implemented using threads first, and then again using the Twisted Matrix framework.
How it works
When we have a number of computers, we are often interested in monitoring their working state. It is possible to detect when a computer stops working by using a pair of programs, one client and one server.
The client program, running on any number of computers, periodically sends an UDP packet to the server program, listening on one computer. The server program dinamically builds a dictionary that stores the IP addresses of the client computers, and the time stamp of the last packet received from each one. At the same time it periodically checks the dictionary, checking whether any of the time stamps is older than a defined timeout.
In this kind of application there is no need to use reliable TCP connections, since the loss of a packet now and then does not produce false alarms, given that the server checking timeout is kept suitably larger than the client sending period. On the other hand, if we have hundreds of computers to monitor, it is preferable to keep the bandwith used and the load on the server at a minimum. We obtain this by periodically sending a small UDP packet, instead of setting up a comparably expensive TCP connection each time.
The packets are sent from each client with a period of five seconds, while the server checks the dictionary with a period of twenty seconds, and its timeout is set to fifteen seconds. These parameters, along with the server IP address and port used, may be configured to one's needs.
In the threaded server, one thread listens to the UDP packets coming from the clients, while the main thread periodically checks the recorded heartbeats. The shared data structure, a dictionary, must be locked and released at each access, both while writing and reading, to avoid data corruption on concurrent access. Such data corruption often manifests itself as intermittent, time-dependent bugs that are difficult to reproduce, investigate and correct.
The Twisted server employs an asynchronous, event driven model, being based on the Twisted Matrix framework ( http://www.twistedmatrix.com/ ). The framework is built around a central "reactor" that dispatches events from a queue in a single thread, and monitors network and host resources. The user program is composed of short code fragments invoked by the reactor when dispatching the matching events. Such a working model guarantees that only one user code fragment is being executed at any given time, eliminating at the root all problems of concurrent access to shared data structures.
The server program is composed of an Application and two Services, the UDPServer and the DetectorService. It is invoked by means of the "twistd" command, with the following options:
$ twistd -ony TwistedBeatServer.py
See the Twisted Matrix documentation for further information.
This program has been tested on Python 2.3.4 and Twisted 1.3.0 . It will work on Python 2.2 by substituting the three occurrences of the "super" keyword in the ThreadedBeatServer.py file with the corresponding old form.