shak's Help 

  1. System Concepts
  2. Managing Programming Contests
  3. Mooshak interfaces
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Mooshak's Help: Reliability


A single Mooshak node - one Web server accessible through a set of Web clients on users machines - is sufficient for running a small programming contest (i.e. a contest with up to 20 teams) where reliability is not at premium. Running an official contest, with a concern for reliability and larger number of teams, distributed in several sites, and a simultaneous online contest, requires a more complex setup, with a network of interconnected nodes.

A link from a node X towards a node Y, represents the direction in which contest data must be replicated (from server X to server Y). The main reasons for replicating contest data between Mooshak servers are to support:

  • System Backup: replication is used to maintain a backup system, with an updated version of the contest data, so that it can replace one of the servers in case of hardware failure.

  • Online Contest: replication propagates the contest data to a server with Internet access used to maintain an online contest simultaneously with an official local contest.

  • Load balancing: several servers distribute load among them and replicate their data to the others. In this case each server is assigned to a set of users, for instance, contestants to a server and jury to another, or contestants in different rooms to different servers.

  • Multi-site contest: This case is similar to the previous but servers are in distant locations.

The Mooshak network configuration for a particular contest may contain several of these links. The following figure represents the network for a contest taking place simultaneously in two sites, A and B, the first using two servers (Server A1 and Server A2) for load balancing and the last using just one server (Server B). Each site has a backup with an updated version of the contest data, capable of replacing any of the main servers in case of failure. Site A maintains also an online version of the contest where anyone on the Internet can compete against the official contestants physically located at either site A or at site B. Some nodes are connected in unidirectional links, such as those connecting servers with the backup nodes or online-contest servers, and other are bidirectional, such as those connecting contest servers among them.

The Mooshak replication uses the rsync remote-update protocol. This protocol updates differences between two sets of files over a network link, using an efficient checksum-search algorithm. The replication procedure is invoked frequently to propagate changes to other servers, typically every 60 seconds, and copies only the data that has been changed since the last replication. The object files produced by the compilation of programs are not replicated, just the evaluation reports. If necessary the programs may be reevaluated in a different machine.

The main issue with replication is the consistency of contest data, namely that no data fails to be replicated or is overwritten by replicated data. To guarantee that no data fails to be replicated we must ensure that there is a replication path connecting all servers interfacing with official contestants.

To address the problem of data being overwritten, we must differentiate between contest definition data (such teams, problems, programming languages) and contest transactions (such as submissions, questions and printouts). Of these two, contest transactions, specially submissions, are particularly important. To guarantee uniqueness all transaction data is keyed by a timestamp, the team ID and the problem ID. Thus, if team ID is unique in the system, and transactions from the same team are consistently sent to the same server, then there is no danger of losing transactions due to overwritten data since each transaction key is also unique.

Contest data is not, in principle, changed after the beginning of the contest. It should be updated in a single node for consistency sake, and that node must have a path to every other node in the network. The only exception to this case is the creation of teams for online-contest servers, as we allow contestants to register during the contest. In case of using load balancing for online-contest servers it is important to assign team creation to a single server. Otherwise, two teams with the same name, and same group, registering at same time in different servers could (although not very likely) share the same record.

For the above setup to work properly, all servers clocks must be synchronized. This can be achieved using the Network Time Protocol (NTP).