Redundancy and errors
A BOINC 'result' abstracts an instance of a computation, possibly not performed yet. Typically, a BOINC server sends 'results' to clients, and the clients perform the computation and replies to the server. But many things can happen to a result:
- The client computes the result correctly and returns it.
- The client computes the result incorrectly and returns it.
- The client fails to download or upload files.
- The application crashes on the client.
- The client never returns anything because it breaks or stops running BOINC.
- The scheduler isn't able to send the result because it requires more resources than any client has.
BOINC provides a form of redundant computing in which each computation is performed on multiple clients, the results are compared, and are accepted only when a 'consensus' is reached. In some cases new results must be created and sent.
BOINC manages most of the details; however, there are two places where the application developer gets involved:
- Validation: This performs two functions. First, when a sufficient number (a 'quorum') of successful results have been returned, it compares them and sees if there is a 'consensus'. The method of comparing results (which may need to take into account platform-varying floating point arithmetic) and the policy for determining consensus (e.g., best two out of three) are supplied by the application. If a consensus is reached, a particular result is designated as the 'canonical' result. Second, if a result arrives after a consensus has already been reached, the new result is compared with the canonical result; this determines whether the user gets credit.
- Assimilation: This is the mechanism by which the project is notified of the completion (success or unsuccessful) of a work unit. It is performed exactly once per work unit. If the work unit was completed successfully (i.e. if there is a canonical result) the project-supplied function reads the output file(s) and handles the information, e.g. by recording it in a database. If the workunit failed, the function might write an entry in a log, send an email, etc.
In the following example, the project creates a workunit with min_quorum = 2 target_nresults = 3 max_delay = 10
BOINC automatically creates three results, which are sent at various times. At time 8, two successful results have returned so the validator is invoked. It finds a consensus, so the work unit is assimilated. At time 10 result 3 arrives; validation is performed again, this time to check whether result 3 gets credit.
time 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
created validate; assimilate
WU x x x
created sent success
result 1 x x---------------x
created sent success
result 2 x x-------------------x
created sent success
result 3 x x-----------------------x
In the next example, result 2 is lost (i.e., there's no reply to the BOINC scheduler). When result 3 arrives a consensus is found and the work unit is assimilated. At time 13 the scheduler 'gives up' on result 2 (this allows it to delete the canonical result's output files, which are needed to validate late-arriving results).
time 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
created validate; assimilate
WU x x x
created sent success
result 1 x x---------------x
created sent lost giveup
result 2 x x-------- x
created sent success
result 3 x x-----------------------x
In the next example, results 2 returns an error at time 5. This reduces the number of outstanding results to 2; because target_nresults is 3, BOINC creates another result (result 4). A consensus is reached at time 9, before result 4 is returned.
time 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
created validate; assimilate
WU x x x
created sent success
result 1 x x---------------x
created sent error
result 2 x x-------x
created sent success
result 3 x x-------------------x
created sent success
result 4 x x----------------------x
