[ZODB4] Backport the way MVCC is handled from ZODB5
This backports to ZODB4 Connection ZODB5's approach to handle MVCC via always calling storage.loadBefore() instead of "load for latest version + loadBefore if we were notified of database changes" approach.
Short answer: because Wendelin.core 2 needs to know at which particular database state application-level ZODB connection is viewing the database, and it is hard to implement such functionality correctly without this backport. Please see appendix for the explanation.
This backports to ZODB4 the minimum necessary part of upstream commit 227953b9 (Simplify MVCC by determining transaction start time using lastTransaction) + follow-up correctness fixes:
- a Connection is always opened with explicitly corresponding to a particular database revision
- Connection uses only loadBefore with that revision to load objects
- every time a Connection is (re)opened, the result of queued invalidations and explicit query to storage.lastTransaction is carefully merged to refresh Connection's idea about which database state it corresponds to.
The "careful" in last point is important. Historically ZODB5 was first reworked in commit 227953b9 (https://github.com/zopefoundation/ZODB/pull/56) to always call lastTransaction to refresh state of Connection view. Since there was no proper synchronisation with respect to process of handling invalidations, that lead to data corruption issue due to race in Connection.open() vs invalidations:
That race and data corruption was fixed in commit b5895a5c (https://github.com/zopefoundation/ZODB/pull/291) by way of avoiding lastTransaction call and relying only on invalidations channel when refreshing Connection view.
This fix in turn led to another data corruption issue because in presence of client-server reconnections, ZODB storage drivers can partly skip notifying client with detailed invalidation messages:
A fix to that issue (https://github.com/zopefoundation/ZODB/pull/307) proposed to change back to query storage for lastTransaction on every Connection refresh, but to implement careful merging of lastTransaction result and data from invalidation channel. However it was found that the "careful merging" can work correctly only if we require from storage drivers a particular ordering of invalidation events wrt lastTransaction return and result:
While ZEO was already complying with that requirements, NEO had to be fixed to support that:
We change Connection._txn_time to be a "before" for the database state to which Connection view corresponds. This state is hooked to be initialized and updated in Connection._flush_invalidations - the function that is called from both explicit Connection (re)open and at transaction boundaries via Connection.afterCompletion hook.
Objects loading is factored into Connection._load which replaces old "load + check invalidated + fallback to loadBefore" game in Connection._setstate.
Connection.open now calls Connection._flush_invalidations unconditionally - even if it was global cache reset event - because besides invalidation flushes the latter is now responsible for querying storage lastTransaction.
TmpStore - a "storage" that provides runtime support for savepoints - is refactored correspondingly to delegate loading of original objects back to underlying Connection.
DB.close is modified - similarly to ZODB5 - to release DB's Connections carefully with preventing connections from DB poll from implicitly starting new transactions via afterCompletion hook.
ZODB.nxd_patches is introduced to indicate to client software that this particular patch is present and can be relied upon.
Tests are updated correspondingly. In 227953b9 Jim talks about converting many tests - because
"Lots of tests didn't clean up databases and connections properly"
and because new MVCC approach
"makes database and connection hygiene a bit more important, especially for tests, because a connection will continue to interact with storages if it isn't properly closed, which can lead to errors if the storage is closed."
but finally implementing automatic cleanup at transaction boundaries because there are too many tests to fix. We backport only automatic cleanup + necessary explicit test fixes to keep the diff minimal.
All tests pass. This includes tests for ZODB itself, ZEO and NEO test over hereby modified ZODB(*), my test programs from
and ERP5 tests. Upcoming wendelin.core 2 also work with this change.
(*) ZEO, NEO and ERP5 tests fail sometimes, but there is no regression here because ZEO, NEO and ERP5 tests are failing regularly, and in the same way, even with unmodified ZODB.
This appendix provides motivation for the backport:
For wendelin.core v2 we need a way to know at which particular database state application-level ZODB connection is viewing the database. Knowing that state, WCFS client library interacts with WCFS filesystem server and, in simple terms, requests the server to provide data as of that particular database state. Let us call the function that for a client ZODB connection returns database state corresponding to its database view zconn_at.
Now here is the problem: zconn_at is relatively easy to implement for ZODB5 - see e.g. here:
however, for ZODB4, since its operational models is not directly MVCC, it is not that straightforward. Still, even for older ZODB4, for every client connection, there is such at that corresponds to that connection view of the database.
We need ZODB4 support, because ZODB4 is currently the version that Nexedi uses, and my understanding is that it will stay like this for not a small time. I have the feeling that ZODB5 was reworked in better direction, but without caring enough about quality which resulted in concurrency bugs with data corruption effects like
Even though the first one is now fixed (but it broke other parts and so both ZODB had to be fixed again and NEO had to be fixed for that ZODB fix to work currently), I feel that upgrading into ZODB5 for Nexedi will require some non-negligible amount of QA work, and thus it is better if we move step-by-step - even if we eventually upgrade to ZODB5 - it is better we first migrate wendelin.core 1 -> wendelin.core 2 with keeping current version of ZODB.
Now please note what would happen if zconn_at gives, even a bit, wrong answer: wcfs client will ask wcfs server to provide array data as of different database state compared to current on-client ZODB connection. This will result in that data accessed via ZBigArray will not correspond to all other data accessed via regular ZODB mechanism. It is, in other words, a data corruptions. In some scenarios it can be harmless, but generally it is the worst that can happen to a database.
It is good to keep in mind ZODB issue290 when imagining corner cases that zconn_at has to deal with. Even though that issue is ZODB5 only, it shows what kind of bugs it can be in zconn_at implementation for ZODB4.
Just for the reference: in Wendelin-based systems there is usually constant stream of database ingestions coming from many places simultaneously. Plus many activities crunching on the DB at the same time as well. And the more clients a system handles, the more there will be level-of-concurrency increase. This means that the problem of correctly handling concurrency issues in zconn_at is not purely theoretical, but has direct relation to our systems.
With this backport, zconn_at for ZODB4 becomes trivial and robust to implement:
I would like to thank Joshua Wölfel whose internship helped this topic to shape up: