A few months ago we added the implicit tracker:modified property to all resources. This property is an auto-increment. It used to be that the property was incremented on ~ each SQL update-query that happens. The value is stored per resource.
We are now changing this to be per transaction. A transaction in Tracker is one set of SPARQL-Update INSERT or DELETE queries. You can do inserts and deletes about multiple resources in one such sentence (a sentence can contain multiple space delimited Update queries). An exception is everything related to ontology changes. These ontology changes get the first increment as their value for tracker:modified. This is also for ontology changes that happen after the initial ontology transaction (at the first start, is this first transaction made). The exception is made for supporting future ontology changes and the possibly needed data conversions.
The per-resource tracker:modified value is useful for application’s synchronization purposes: you can test your application’s stored tracker:modified value against the always increasing (w. exception at int. overflow) Tracker’s tracker:modified value to know whether or not your version is older.
The reason why we are changing this to per-transaction is because this way we can guarantee that the value will be restored after a journal replay and/or a backup’s restore without having to store it in either the journal nor the backup. This means that we now guarantee the value being restored without having to change either the backup’s format nor the journal’s format.
Having a persistent journal we actually make a simple copy of the journal to deliver you a backup in a fast file-copy. But let this deception be known only by the people who care about the implementation. Sssht!
We’re already rotating and compressing the rotated chunks for reducing the journal size. We’re working on not journaling data that is embedded in local files this week. A re-index of that local file will re-insert the data anyway. This will significantly reduce the size of the journal too.