==========================
DemoStorage demo (doctest)
==========================

DemoStorages provide a way to provide incremental updates to an
existing, base, storage without updating the storage.

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 = db.open()
    >>> 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 ZODB.DemoStorage import DemoStorage
    >>> storage = DemoStorage(base=base, changes=changes)

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

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

Let's add some data:

    >>> db = DB(storage)
    >>> conn = db.open()
    >>> 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')
    True

But we have modified the changes database:

    >>> len(changes)
    2

Our lastTransaction reflects the lastTransaction of the changes:

    >>> storage.lastTransaction() > base.lastTransaction()
    True

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

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

    >>> from ZODB.utils import p64, u64
    >>> storage.load(p64(0), '') == changes.load(p64(0), '')
    True
    >>> storage.load(p64(0), '') == base.load(p64(0), '')
    False
    >>> storage.load(p64(1), '') == base.load(p64(1), '')
    True

    >>> serial = base.getTid(p64(0))
    >>> storage.loadSerial(p64(0), serial) == base.loadSerial(p64(0), serial)
    True

    >>> serial = changes.getTid(p64(0))
    >>> storage.loadSerial(p64(0), serial) == changes.loadSerial(p64(0),
    ...                                                          serial)
    True

The object id of the new object is quite random, and typically large:

    >>> print u64(conn.root()['2']._p_oid)
    7106521602475165646

Let's look at some other methods:

    >>> storage.getName()
    "DemoStorage('base.fs', 'changes.fs')"

    >>> storage.sortKey() == changes.sortKey()
    True

    >>> storage.getSize() == changes.getSize()
    True
    
    >>> len(storage) == len(changes)
    True

    
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]

    >>> db.close()

Storage Stacking
================

A common use case is to stack demo storages.  DemoStorage provides
some helper functions to help with this.  The push method, just
creates a new demo storage who's base is the original demo storage:

    >>> demo = DemoStorage()
    >>> demo2 = demo.push()
    >>> demo2.base is demo
    True

We can also supply an explicit changes storage, if we wish:

    >>> from ZODB.MappingStorage import MappingStorage
    >>> changes = MappingStorage()
    >>> demo3 = demo2.push(changes)
    >>> demo3.changes is changes, demo3.base is demo2
    (True, True)

The pop method closes the changes storage and returns the base
*without* closing it:

    >>> demo3.pop() is demo2
    True

    >>> changes.opened()
    False

Special backward compatibility support
--------------------------------------

Normally, when a demo storage is closed, it's base and changes
storage are closed:

    >>> demo = DemoStorage(base=MappingStorage(), changes=MappingStorage())
    >>> demo.close()
    >>> demo.base.opened(), demo.changes.opened()
    (False, False)

Older versions of DemoStorage didn't have a separate changes storage
and didn't close or discard their changes when they were closed.  When
a stack was built solely of demo storages, the close method
effectively did nothing.  To maintain backward compatibility, when no
base or changes storage is supplied in the constructor, the underlying
storage created by the demo storage isn't closed by the demo storage.
This backward-compatibility is deprecated.

    >>> demo = DemoStorage()
    >>> demo.close()
    >>> demo.changes.opened(), demo.base.opened()
    (True, True)

    >>> demo = DemoStorage(base=MappingStorage())
    >>> demo2 = demo.push()
    >>> demo2.close()
    >>> demo2.changes.opened(), demo2.base.base.opened()
    (True, False)

Blob Support
============

DemoStorage supports Blobs if the changes database supports blobs.

    >>> import ZODB.blob
    >>> base = ZODB.blob.BlobStorage('base', FileStorage('base.fs'))
    >>> db = DB(base)
    >>> conn = db.open()
    >>> conn.root()['blob'] = ZODB.blob.Blob()
    >>> conn.root()['blob'].open('w').write('state 1')
    >>> transaction.commit()
    >>> db.close()

    >>> base = ZODB.blob.BlobStorage('base',
    ...                               FileStorage('base.fs', read_only=True))
    >>> changes = ZODB.blob.BlobStorage('changes',
    ...                                 FileStorage('changes.fs', create=True))
    >>> storage = DemoStorage(base=base, changes=changes)

    >>> db = DB(storage)
    >>> conn = db.open()
    >>> conn.root()['blob'].open().read()
    'state 1'
    >>> _ = transaction.begin()
    >>> conn.root()['blob'].open('w').write('state 2')
    >>> transaction.commit()
    >>> conn.root()['blob'].open().read()
    'state 2'
    
    >>> storage.temporaryDirectory() == changes.temporaryDirectory()
    True

    >>> db.close()

It isn't necessary for the base database to support blobs.

    >>> base = FileStorage('base.fs', read_only=True)
    >>> changes = ZODB.blob.BlobStorage('changes', FileStorage('changes.fs'))
    >>> storage = DemoStorage(base=base, changes=changes)
    >>> db = DB(storage)
    >>> conn = db.open()
    >>> conn.root()['blob'].open().read()
    'state 2'

    >>> _ = transaction.begin()
    >>> conn.root()['blob2'] = ZODB.blob.Blob()
    >>> conn.root()['blob2'].open('w').write('state 1')
    >>> conn.root()['blob2'].open().read()
    'state 1'

    >>> db.close()

If the changes database is created implicitly, it will get a blob
storage wrapped around it when necessary:

    >>> base = ZODB.blob.BlobStorage('base',
    ...                               FileStorage('base.fs', read_only=True))
    >>> storage = DemoStorage(base=base)

    >>> type(storage.changes).__name__
    'MappingStorage'

    >>> db = DB(storage)
    >>> conn = db.open()
    >>> conn.root()['blob'].open().read()
    'state 1'

    >>> type(storage.changes).__name__
    'BlobStorage'

    >>> _ = transaction.begin()
    >>> conn.root()['blob'].open('w').write('state 2')
    >>> transaction.commit()
    >>> conn.root()['blob'].open().read()
    'state 2'
    
    >>> storage.temporaryDirectory() == storage.changes.temporaryDirectory()
    True

    >>> db.close()

.. Check that the temporary directory is gone

   For now, it won't go until the storage does.

   >>> transaction.abort()
   >>> conn.close()
   >>> blobdir = storage.temporaryDirectory()
   >>> del db, conn, storage, _
   
   >>> import gc
   >>> _ = gc.collect()

   >>> import os
   >>> os.path.exists(blobdir)
   False

ZConfig support
===============

You can configure demo storages using ZConfig, using name, changes,
and base options:

    >>> import ZODB.config
    >>> storage = ZODB.config.storageFromString("""
    ... <demostorage>
    ... </demostorage>
    ... """)
    >>> storage.getName()
    "DemoStorage('MappingStorage', 'MappingStorage')"

    >>> storage = ZODB.config.storageFromString("""
    ... <demostorage>
    ...    <filestorage base>
    ...       path base.fs
    ...    </filestorage>
    ...
    ...    <filestorage changes>
    ...       path changes.fs
    ...    </filestorage>
    ... </demostorage>
    ... """)
    >>> storage.getName()
    "DemoStorage('base.fs', 'changes.fs')"

    >>> storage.close()

    >>> storage = ZODB.config.storageFromString("""
    ... <demostorage>
    ...    name bob
    ...    <filestorage>
    ...       path base.fs
    ...    </filestorage>
    ...
    ...    <filestorage changes>
    ...       path changes.fs
    ...    </filestorage>
    ... </demostorage>
    ... """)
    >>> storage.getName()
    'bob'
    >>> storage.base.getName()
    'base.fs'

    >>> storage.close()


Generating OIDs
===============

When asked for a new OID DemoStorage randomly chooses a value and then verifies
that neither the base or changes storages already contain that OID.

Under rare circumstances an OID can be chosen that has already been handed out,
but which hasn't yet been comitted.  Lets verify that if the same OID is chosen
twice during a transaction that everything will still work.

First we'll get a single OID.

    >>> import random
    >>> storage = DemoStorage.push(storage)
    >>> random.seed(47)
    >>> storage.new_oid()
    '\x10\x01\xa6bZ\x12\x98\xa2'

Then we'll force the random number generator to use the same seed for the
subsequent call to "new_oid" and show that we get a different OID.

    >>> random.seed(47)
    >>> oid = storage.new_oid()
    >>> oid
    'A\xe6\xcb\x06W\xed\xa2\x15'

DemoStorage keeps up with the issued OIDs to know when not to reissue them...

    >>> oid in storage._issued_oids
    True

...but once data is stored with a given OID...

    >>> t = transaction.begin()
    >>> storage.tpc_begin(t)
    >>> tid = storage.store(oid, 0, 'data', '', t)

...there's no need to remember it any longer:

    >>> oid in storage._issued_oids
    False

We're done with the storage, so "unwrap" the underlying storage.

    >>> storage = storage.pop()
