Hi Andrus I know you have been looking at this offline, but I thought I'd raise this here in public to see if others can also help shed light on a difficult area to completely understand in Cayenne. Perhaps this conversation can also help us improve the docs. I'd like to better understand the interactions between the DataRowStore and the ObjectStore.
Here is how I understand the Cayenne object cache: 1. There is a checkbox in the modeler at the top level for "use shared cache". If that checkbox is ticked, then a single DataRowStore (a LRU map) is created, shared between all contexts. If not, a separate DataRowStore is created every time you create a new context. 2. Each context has its own ObjectStore for keeping the actual objects (the snapshot), and another copy of the object with any changes made in that context. Any queries to the database fill the DataRowStore first, and then the objects are constructed in to the ObjectStore. 3. When a commit is made to Context A, the ObjectStore linked to context A has a bunch of changes in it. Those changes are pushed into the DataRowStore, and then into the database. Events are are sent to update the snapshot copy of objects held in all other ObjectStores. 4. These events can even be sent to another Java VM, but we'll ignore that for now. 5. The modeler has a field called "Size of object cache". This refers to the size of the DataRowStore map as a count of objects. Because the rows could be 1kB or 1Mb each, it is impossible to predict how large this cache will grow to, but it will get bigger and smaller over time. 6. If "use shared cache" is NOT ticked, then this size is used for *each* of the DataRowStore maps. This means that the memory usage will grow very rapidly as you add new contexts and fill them with data. 100 contexts, each loaded with a query that fetches the same 10Mb of data, will contain 1000Mb of DataRowStore memory usage. 6. If a DataRow is not held in the DataRowStore (eg, because it was pushed out of the map by newer data) then an event is NOT sent to the other contexts to notify them that the object was updated when context A is committed. Instead stale data is left in those other contexts. 7. Assuming the query cache is disabled, a query against the database will fetch brand new objects into the DataRowStore and potentially update all the snapshots of all ObjectStores which contain a copy of those records. 8. A relationship join (painting.getArtist()) might find a record in the DataRowStore and pull it into the ObjectStore without ever going back to the database. 9. If the query cache is hit, then the results may already be found in the DataRowStore. If so, the the ObjectStore snapshot is updated from these DataRows if any of those objects are not already found in the ObjectStore. So, my questions: A. What is the use case of separate DataRowStores per context? It appears to be a worse possible scenario in every case I can think of, with no upside. The label "use shared cache" gives the impression that you are trading memory usage for staleness of data, but that does not appear to be the case. B. Is it an intrinsic feature of this system that a cache miss in the DataRowStore causes no event to be sent to other ObjectStores, or is that a bug? This means that setting the 'size of object cache' in the modeler to 0 results in a lot of stale data. C. Will a cache miss in the DataRowStore cause Cayenne to refetch that object from the database before it then tries to commit the changes to that object. Have I captured the above notes correctly? Cheers Ari -- --------------------------> Aristedes Maniatis ish http://www.ish.com.au Level 1, 30 Wilson Street Newtown 2042 Australia phone +61 2 9550 5001 fax +61 2 9550 4001 GPG fingerprint CBFB 84B4 738D 4E87 5E5C 5EFA EF6A 7D2E 3E49 102A
