This is probably the simplest (functional) storage possible.

src/ZODB/MappingStorage.py

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+##############################################################################
+"""Very Simple Mapping ZODB storage
+
+The Mapping storage provides an extremely simple storage
+implementation that doesn't provide undo or version support.
+
+It is meant to illustrate the simplest possible storage.
+
+The Mapping storage uses a single data structure to map
+object ids to data.
+
+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 strorage can have a "base" storage that is used in a
+read-only fashion. The base storage must not not to 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 4-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, serial, pre, vdata, p, 
+
+where:
+
+     oid -- object id
+
+     serial -- object serial number
+
+     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
+
+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.  Foe example, in Zope, you can create an external
+method::
+
+  import Zope
+
+  def info(RESPONSE):
+      RESPONSE['Content-type']= 'text/plain'
+
+      return Zope.DB._storage._splat()
+
+and call it to minotor the storage.
+
+"""
+__version__='$Revision: 1.1 $'[11:-2]
+
+import base64, POSException, BTree, BaseStorage, time, string, utils
+from TimeStamp import TimeStamp
+
+class DemoStorageError: pass
+
+class MappingStorage(BaseStorage.BaseStorage):
+
+    def __init__(self, name='Mapping Storage'):
+
+        BaseStorage.BaseStorage.__init__(self, name)
+
+        # We use a BTree because the items are sorted!
+        self._index={}
+        self._tindex=[]
+
+    def __len__(self):
+        return len(self._index)
+        
+    def getSize(self):
+        s=32
+        for oid, p in self._index.items():
+            s=s+56+len(p)
+            
+        return s
+
+    def load(self, oid, version):
+        self._lock_acquire()
+        try:
+            p=self._index[oid]
+            return p[8:], p[:8] # pickle, serial
+        finally: self._lock_release()
+
+    def store(self, oid, serial, data, version, transaction):
+        if transaction is not self._transaction:
+            raise POSException.StorageTransactionError(self, transaction)
+
+        if version:
+            raise POSException.Unsupported, "Versions aren't supported"
+
+        self._lock_acquire()
+        try:
+            old=self._index.get(oid, None)
+            if old is not None:
+                oserial=old[:8]
+                if serial != oserial: raise POSException.ConflictError
+                
+            serial=self._serial
+            self._tindex.append((oid,serial+data))
+        finally: self._lock_release()
+
+        return serial
+
+    def _clear_temp(self):
+        self._tindex=[]
+
+    def _finish(self, tid, user, desc, ext):
+
+        index=self._index
+        for oid, p in self._tindex: index[oid]=p
+
+    def pack(self, t, referencesf):
+        
+        self._lock_acquire()
+        try:    
+            # Build an index of *only* those objects reachable
+            # from the root.
+            index=self._index
+            rootl=['\0\0\0\0\0\0\0\0']
+            pop=rootl.pop
+            pindex={}
+            referenced=pindex.has_key
+            while rootl:
+                oid=pop()
+                if referenced(oid): continue
+    
+                # Scan non-version pickle for references
+                r=index[oid]
+                pindex[oid]=r
+                p=r[8:]
+                referencesf(p, rootl)
+
+            # Now delete any unreferenced entries:
+            for oid in index.keys():
+                if not referenced(oid): del index[oid]
+    
+        finally: self._lock_release()
+
+    def _splat(self):
+        """Spit out a string showing state.
+        """
+        o=[]
+        o.append('Index:')
+        items=self._index.items()
+        items.sort()
+        for oid, r in items:
+            o.append('  %s: %s, %s' %
+                     (utils.u64(oid),TimeStamp(r[:8]),`r[8:]`))
+            
+        return string.join(o,'\n')