In an Ivy file system with multiple logs, a participant's Ivy server consults all the logs to find relevant information. This means that Ivy must decide how to order the records from different logs. The order should obey causality, and all participants with the same view should choose the same order. Ivy orders the records using a version vector [27] contained in each log record.
When an Ivy participant generates a new log record, it includes two pieces of information that are later used to order the record. The seq field contains a numerically increasing sequence number; each log separately numbers its records from zero. The version vector field contains a tuple U:V for each log in the view (including the participant's own log), summarizing the participant's most recent knowledge of that log. U is the DHash key of the log-head of the log being described, and V is the DHash key of that log's most recent record. In the following discussion, a numeric V value refers to the sequence number contained in the record pointed to by a tuple.
Ivy orders log records by comparing the records' version vectors. For example, Ivy considers a log record with version vector (A:5 B:7) to be earlier in time than a record with version vector (A:6 B:7): the latter vector implies that its creator had seen the record with (A:5 B:7). Two version vectors u and v are comparable if and only if or or . Otherwise, u and v are concurrent. For example, (A:5 B:7) and (A:6 B:6) are concurrent.
Simultaneous operations by different participants will result in equal or concurrent version vectors. Ivy orders equal and concurrent vectors by comparing the public keys of the two logs. If the updates affect the same file, perhaps due to a partition, the application may need to take special action to restore consistency; Section 3 explores Ivy's support for application-specific conflict resolution.
Ivy could have used a simpler method of ordering log records, such as a Lamport clock [17]. Version vectors contain more precise information than Lamport clocks about causality; Ivy uses that information to help fix conflicting updates after a partition. Version vectors help prevent a malicious participant from retroactively changing its log by pointing its log-head at a newly-constructed log; other participants' version vectors will still point to the old log's records. Finally, version vectors from one log could be used to help repair another log that has been damaged.