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s¾ Demo ZODB storage
The Demo storage serves two purposes:
- Provide an example implementation of a full storage without
distracting storage details,
- Provide a volatile storage that is useful for giving demonstrations.
The demo storage can have a "base" storage that is used in a
read-only fashion. The base storage must not contain version
data.
There are three main data structures:
_data -- Transaction logging information necessary for undo
This is a mapping from transaction id to transaction, where
a transaction is simply a 5-tuple:
packed, user, description, extension_data, records
where extension_data is a dictionary or None and records are the
actual records in chronological order. Packed is a flag
indicating whethe the transaction has been packed or not
_index -- A mapping from oid to record
_vindex -- A mapping from version name to version data
where version data is a mapping from oid to record
A record is a tuple:
oid, pre, vdata, p, tid
where:
oid -- object id
pre -- The previous record for this object (or None)
vdata -- version data
None if not a version, ortherwise:
version, non-version-record
p -- the pickle data or None
tid -- the transaction id that wrote the record
The pickle data will be None for a record for an object created in
an aborted version.
It is instructive to watch what happens to the internal data structures
as changes are made. For example, in Zope, you can create an external
method::
import Zope2
def info(RESPONSE):
RESPONSE['Content-type']= 'text/plain'
return Zope2.DB._storage._splat()
and call it to monitor the storage.
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| i j o t i d ƒ ‚ n Wd | i! ƒ X| i S( NR@ i i R# t serialst dataiH i i i s™ Quota Exceeded
The maximum quota for this demonstration storage
has been exceeded.
Have a nice day.(" R) R R* R R+ R. R R/ R# R t oldR
RB R R RA R3 R$ R% R2 t VersionLockErrort serialt
ConflictErrorRI R6 R1 R5 R4 t _tsizeR R R
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,, & c C s d S( Ni ( ( R ( ( R t supportsVersions* s c C s g | _ | i d | _ d S( Ni ( R R5 R RN ( R ( ( R R - s c C s | i S( N( R R ( R ( ( R t lastTransaction1 s c C s2 | i d t | ƒ t | ƒ t | ƒ | _ d S( Nix ( R R R R R R! RN ( R R R R R! ( ( R t _begin4 s c C s3 | i | _ d | | | t | i ƒ f | i |