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ZODB storage that stores changes relative to a base storage

Project description

The zc.demostorage2 module provides a storage implementation that wraps two storages, a base storage and a storage to hold changes. The base storage is never written to. All new records are written to the changes storage. Both storages are expected to:

  • Use packed 64-bit unsigned integers as object ids,
  • Allocate object ids sequentially, starting from 0, and
  • in the case of the changes storage, accept object ids assigned externally.

In addition, it is assumed that less than 2**63 object ids have been allocated in the first storage.

Note that DemoStorage also assumes that it’s base storage uses 64-bit unsigned integer object ids allocated sequentially.

Change History

0.1.1 (2008-02-07)

Fixed a packaging bug that caused some files to be omitted.

0.1 (2008-02-04)

Initial release.


The section below shows how to create zc.demostorage2 storages from Python. If you’re using ZConfig, you need to:

  • import zc.demostroage2
  • include a demostroage2 section

Here’s an example that shows how to configure demo storage and how to use the configuration from python:

>>> import ZODB.config
>>> storage = ZODB.config.storageFromString("""
... %import zc.demostorage2
... <demostorage2>
...    <filestorage base>
...       path base.fs
...    </filestorage>
...    <filestorage changes>
...       path changes.fs
...    </filestorage>
... </demostorage2>
... """)

This creates a demo storage that gets base data from a file storage named base.fs and stores changes in a file storage named changes.fs.

>>> storage
<DemoStorage2: DemoStorage2(base.fs, changes.fs)>
>>> storage.close()

Demo (doctest)

Note that most people will configure the storage through ZConfig. If you are one of those people, you may want to stop here. :) The examples below show you how to use the storage from Python, but they also exercise lots of details you might not be interested in.

To see how this works, we’ll start by creating a base storage and puting an object (in addition to the root object) in it:

>>> from ZODB.FileStorage import FileStorage
>>> base = FileStorage('base.fs')
>>> from ZODB.DB import DB
>>> db = DB(base)
>>> from persistent.mapping import PersistentMapping
>>> conn =
>>> conn.root()['1'] = PersistentMapping({'a': 1, 'b':2})
>>> import transaction
>>> transaction.commit()
>>> db.close()
>>> import os
>>> original_size = os.path.getsize('base.fs')

Now, lets reopen the base storage in read-only mode:

>>> base = FileStorage('base.fs', read_only=True)

And open a new storage to store changes:

>>> changes = FileStorage('changes.fs')

and combine the 2 in a demofilestorage:

>>> from zc.demostorage2 import DemoStorage2
>>> storage = DemoStorage2(base, changes)

If there are no transactions, the storage reports the lastTransaction of the base database:

>>> storage.lastTransaction() == base.lastTransaction()

Let’s add some data:

>>> db = DB(storage)
>>> conn =
>>> items = conn.root()['1'].items()
>>> items.sort()
>>> items
[('a', 1), ('b', 2)]
>>> conn.root()['2'] = PersistentMapping({'a': 3, 'b':4})
>>> transaction.commit()
>>> conn.root()['2']['c'] = 5
>>> transaction.commit()

Here we can see that we haven’t modified the base storage:

>>> original_size == os.path.getsize('base.fs')

But we have modified the changes database:

>>> len(changes)

Our lastTransaction reflects the lastTransaction of the changes:

>>> storage.lastTransaction() > base.lastTransaction()
>>> storage.lastTransaction() == changes.lastTransaction()

Let’s walk over some of the methods so ewe can see how we delegate to the new oderlying storages:

>>> from ZODB.utils import p64, u64
>>> storage.load(p64(0), '') == changes.load(p64(0), '')
>>> storage.load(p64(0), '') == base.load(p64(0), '')
>>> storage.load(p64(1), '') == base.load(p64(1), '')
>>> serial = base.getTid(p64(0))
>>> storage.loadSerial(p64(0), serial) == base.loadSerial(p64(0), serial)
>>> serial = changes.getTid(p64(0))
>>> storage.loadSerial(p64(0), serial) == changes.loadSerial(p64(0),
...                                                          serial)

The object id of the new object is quite large:

>>> u64(conn.root()['2']._p_oid)

Let’s look at some other methods:

>>> storage.getName()
'DemoStorage2(base.fs, changes.fs)'
>>> storage.sortKey() == changes.sortKey()
>>> storage.getSize() == changes.getSize()
>>> len(storage) == len(changes)

Undo methods are simply copied from the changes storage:

>>> [getattr(storage, name) == getattr(changes, name)
...  for name in ('supportsUndo', 'undo', 'undoLog', 'undoInfo')
...  ]
[True, True, True, True]

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