[Checkins] [zopefoundation/ZEO] ab86bd: Include both modified and just created objects int...

[Kirill Smelkov]

  Branch: refs/heads/master
  Home:   https://github.com/zopefoundation/ZEO
  Commit: ab86bd725e538b4f36a9b9d5395dd883d0204af8
      https://github.com/zopefoundation/ZEO/commit/ab86bd725e538b4f36a9b9d5395dd883d0204af8
  Author: Kirill Smelkov <kirr at nexedi.com>
  Date:   2020-07-29 (Wed, 29 Jul 2020)

  Changed paths:
    M src/ZEO/StorageServer.py
    M src/ZEO/tests/testZEO.py

  Log Message:
  -----------
  Include both modified and just created objects into invalidations

Starting from 1999 (b3805a2f "just getting started") only modified - not
just created - objects were included into ZEO invalidation messages:

https://github.com/zopefoundation/ZEO/commit/b3805a2f#diff-52fb76aaf08a1643cdb8fdaf69e37802R126-R127

In 2000 this behaviour was further changed to not send invalidation
message at all if the only objects a transaction has were the created ones:

https://github.com/zopefoundation/ZEO/commit/230ffbe8#diff-52fb76aaf08a1643cdb8fdaf69e37802L163-R163

In 2016 the latter was reconsidered as bug and fixed in ZEO5 because
ZODB5 relies more heavily on MVCC semantic and needs to be notified
about every transaction committed to storage to be able to properly
update ZODB.Connection view:

https://github.com/zopefoundation/ZEO/commit/02943acd#diff-52fb76aaf08a1643cdb8fdaf69e37802L889-R834
https://github.com/zopefoundation/ZEO/commit/9613f09b

In 2020, with this patch, I'm proposing to reconsider initial "send only
modified, not created objects" as bug, and include both modified and
just created objects into invalidation messages at least for the
following reasons:

- a ZODB client (not necessarily native ZODB/py client) can maintain
  raw cache for the storage. If such client tries to load an oid at
  database view when that object did not existed yet, gets "no object"
  reply and stores that information into raw cache, to properly invalidate
  the cache it needs an invalidation message from ZODB server that
  _includes_ created object.

- tools like `zodb watch` [1,2,3] don't work properly (give incorrect output)
  if not all objects modified/created by a transaction are included into
  invalidation messages.

- similarly to `zodb watch`, a monitoring tool, that would want to be
  notified of all created/modified objects, won't see full
  database-change picture, and so won't work properly without knowing
  which objects were created.

- wendelin.core 2 - which builds data from ZODB BTrees and data objects
  into virtual filesystem - needs to get invalidation messages with both
  modified and created objects to properly implement its own lazy
  invalidation and isolation protocol for file blocks in OS cache: when
  a block of file is accessed, all clients, that have this block mmaped,
  need to be notified and asked to remmap that block into particular
  revision of the file depending on a client's view of the filesystem and
  database [4,5].

  To compute to where a client needs to remmap the block, WCFS server
  (that in turn acts as ZODB client wrt ZEO/NEO server), needs to be able
  to see whether client's view of the filesystem is before object creation
  (and then ask that client to pin that block to hole), or after creation
  (and then ask the client to pin that block to corresponding revision).

  This computation needs ZODB server to send invalidation messages in
  full: with both modified and just created objects.

The patch is simple - it removes `if serial != b"\0\0\0\0\0\0\0\0"`
before queuing oid into ZEOStorage.invalidated, and adjusts the tests
correspondingly. From my point of view and experience, in practice, this
patch should not cause any compatibility break nor performance regressions.

Thanks beforehand,
Kirill

/cc @jimfulton

[1] https://lab.nexedi.com/kirr/neo/blob/ea53a795/go/zodb/zodbtools/watch.go
[2] https://lab.nexedi.com/kirr/neo/commit/e0d59f5d
[3] https://lab.nexedi.com/kirr/neo/commit/c41c2907

[4] https://lab.nexedi.com/kirr/wendelin.core/blob/1efb5876/wcfs/wcfs.go#L94-182
[5] https://lab.nexedi.com/kirr/wendelin.core/blob/1efb5876/wcfs/client/wcfs.h#L20-71