Zope Object Database: object database and persistence
The Zope Object Database provides an object-oriented database for Python that provides a high-degree of transparency. Applications can take advantage of object database features with few, if any, changes to application logic. ZODB includes features such as a plugable storage interface, rich transaction support, and undo.
- Change History
The ZODB package provides a set of tools for using the Zope Object Database (ZODB). The components you get with the ZODB release are as follows:
- Core ZODB, including the persistence machinery
- Standard storages such as FileStorage
- The persistent BTrees modules
- ZEO, for scalability needs
- documentation (needs a lot more work)
Our primary development platforms are Linux, Mac OS X, and Windows XP. The test suite should pass without error on all of these platforms, although it can take a long time on Windows – longer if you use ZoneAlarm. Many particularly slow tests are skipped unless you pass –all as an argument to test.py.
ZODB 3.10 requires Python 2.5 or later.
- Note –
- When using ZEO and upgrading from Python 2.4, you need to upgrade clients and servers at the same time, or upgrade clients first and then servers. Clients running Python 2.5 or 2.6 will work with servers running Python 2.4. Clients running Python 2.4 won’t work properly with servers running Python 2.5 or later due to changes in the way Python implements exceptions.
ZODB ZEO clients from ZODB 3.2 on can talk to ZODB 3.10 servers. ZODB ZEO 3.10 Clients can talk to ZODB 3.8, 3.9, and 3.10 ZEO servers.
- Note –
- ZEO 3.10 servers don’t support undo for older clients.
You must have Python installed. If you’re using a system Python install, make sure development support is installed too.
You also need the transaction, zc.lockfile, ZConfig, zdaemon, zope.event, zope.interface, zope.proxy and zope.testing packages. If you don’t have them and you can connect to the Python Package Index, then these will be installed for you if you don’t have them.
To install by hand, first install the dependencies, ZConfig, zdaemon, zope.interface, zope.proxy and zope.testing. These can be found in the Python Package Index.
To run the tests, use the test setup command:
python setup.py test
It will download dependencies if needed. If this happens, ou may get an import error when the test command gets to looking for tests. Try running the test command a second time and you should see the tests run.
python setup.py test
To install, use the install command:
python setup.py install
The ZODB checkouts are buildouts. When working from a ZODB checkout, first run the bootstrap.py script to initialize the buildout:
% python bootstrap.py
and then use the buildout script to build ZODB and gather the dependencies:
This creates a test script:
% bin/test -v
This command will run all the tests, printing a single dot for each test. When it finishes, it will print a test summary. The exact number of tests can vary depending on platform and available third-party libraries.:
Ran 1182 tests in 241.269s OK
The test script has many more options. Use the -h or --help options to see a file list of options. The default test suite omits several tests that depend on third-party software or that take a long time to run. To run all the available tests use the --all option. Running all the tests takes much longer.:
Ran 1561 tests in 1461.557s OK
Several scripts are provided with the ZODB and can help for analyzing, debugging, checking for consistency, summarizing content, reporting space used by objects, doing backups, artificial load testing, etc. Look at the ZODB/script directory for more informations.
The historical version numbering schemes for ZODB and ZEO are complicated. Starting with ZODB 3.4, the ZODB and ZEO version numbers are the same.
In the ZODB 3.1 through 3.3 lines, the ZEO version number was “one smaller” than the ZODB version number; e.g., ZODB 3.2.7 included ZEO 2.2.7. ZODB and ZEO were distinct releases prior to ZODB 3.1, and had independent version numbers.
Historically, ZODB was distributed as a part of the Zope application server. Jim Fulton’s paper at the Python conference in 2000 described a version of ZODB he called ZODB 3, based on an earlier persistent object system called BoboPOS. The earliest versions of ZODB 3 were released with Zope 2.0.
Andrew Kuchling extracted ZODB from Zope 2.4.1 and packaged it for use by standalone Python programs. He called this version “StandaloneZODB”. Andrew’s guide to using ZODB is included in the Doc directory. This version of ZODB was hosted at http://sf.net/projects/zodb. It supported Python 1.5.2, and might still be of interest to users of this very old Python version.
Zope Corporation released a version of ZODB called “StandaloneZODB 1.0” in Feb. 2002. This release was based on Andrew’s packaging, but built from the same CVS repository as Zope. It is roughly equivalent to the ZODB in Zope 2.5.
Why not call the current release StandaloneZODB? The name StandaloneZODB is a bit of a mouthful. The standalone part of the name suggests that the Zope version is the real version and that this is an afterthought, which isn’t the case. So we’re calling this release “ZODB”. We also worked on a ZODB4 package for a while and made a couple of alpha releases. We’ve now abandoned that effort, because we didn’t have the resources to pursue ot while also maintaining ZODB(3).
ZODB is distributed under the Zope Public License, an OSI-approved open source license. Please see the LICENSE.txt file for terms and conditions.
The ZODB/ZEO Programming Guide included in the documentation is a modified version of Andrew Kuchling’s original guide, provided under the terms of the GNU Free Documentation License.
We maintain a Wiki page about all things ZODB, including status on future directions for ZODB. Please see
and feel free to contribute your comments. There is a Mailman mailing list in place to discuss all issues related to ZODB. You can send questions to
or subscribe at
and view its archives at
Note that Zope Corp mailing lists have a subscriber-only posting policy.
Andrew’s ZODB Programmers Guide is made available in several forms, including DVI and HTML. To view it online, point your browser at the file Doc/guide/zodb/index.html
Added a ‘–kill-old-on-full’ argument to the backup options: if passed, remove any older full or incremental backup files from the repository after doing a full backup. (https://bugs.launchpad.net/zope2/+bug/143158)
When transactions are aborted, new object ids allocated during the transaction are saved and used in subsequent transactions. This can help in situations where object ids are used as BTree keys and the sequential allocation of object ids leads to conflict errors.
ZEO servers now support a server_status method for for getting information the number of clients, lock requests and general statistics.
ZEO clients now support a client_label constructor argument and client-label configuration-file option to specify a label for a client in server logs. This makes it easier to identify specific clients corresponding to server log entries, especially when there are multiple clients originating from the same machine.
The mkzeoinst script has been moved to a separate project:
and is no-longer included with ZODB.
Improved ZEO server commit lock logging. Now, locking activity is logged at the debug level until the number of wating lock requests gets above 3. Log at the critical level when the number of waiting lock requests gets above 9.
ZEO servers no longer log their pids in every log message. It’s just not interesting. :)
Fixed bug in cPickleCache’s byte size estimation logic. (https://bugs.launchpad.net/zodb/+bug/533015)
Fixed a serious bug that caused cache failures when run with Python optimization turned on.
When using using a ClientStorage in a Storage server, there was a threading bug that caused clients to get disconnected.
Fixed a bug that caused savepoint rollback to not properly set object state when objects implemented _p_invalidate methods that reloaded ther state (unghostifiable objects).
cross-database wekrefs weren’t handled correctly.
Fixed some problems in ZEO server commit lock management.
On Mac OS X, clients that connected and disconnected quickly could cause a ZEO server to stop accepting connections, due to a failure to catch errors in the initial part of the connection process.
The failure to properly handle exceptions while accepting connections is potentially problematic on other platforms.
- There are a number of performance enhancements for ZEO storage servers.
- FileStorage indexes use a new format. They are saved and loaded much faster and take less space. Old indexes can still be read, but new indexes won’t be readable by older versions of ZODB.
- The API for undoing multiple transactions has changed. To undo multiple transactions in a single transaction, pass a list of transaction identifiers to a database’s undoMultiple method. Calling a database’s undo method multiple times in the same transaction now raises an exception.
- The ZEO protocol for undo has changed. The only user-visible consequence of this is that when ZODB 3.10 ZEO servers won’t support undo for older clients.
- The storage API (IStorage) has been tightened. Now, storages should raise a StorageTransactionError when invalid transactions are passed to tpc_begin, tpc_vote, or tpc_finish.
- ZEO clients (ClientStorage instances) now work in forked processes, including those created via multiprocessing.Process instances.
- Broken objects now provide the IBroken interface.
- As a convenience, you can now pass an integer port as an address to the ZEO ClientStorage constructor.
- As a convenience, there’s a new client function in the ZEO package for constructing a ClientStorage instance. It takes the same arguments as the ClientStorage constructor.
- A ZEO threading bug could cause transactions to read inconsistent data. (This sometimes caused an AssertionError in Connection._setstate_noncurrent.)
- DemoStorage.loadBefore sometimes returned invalid data which would trigger AssertionErrors in ZODB.Connection.
- History support was broken when using stprages that work with ZODB 3.8 and 3.9.
- zope.testing was an unnecessary non-testing dependency.
- Internal ZEO errors were logged at the INFO level, rather than at the error level.
- The FileStorage backup and restore script, repozo, gave a deprecation warning under Python 2.6.
- C Header files weren’t installed correctly.
- The undo implementation was incorrect in ways that could cause subtle missbehaviors.
- 2 BTree bugs, introduced by a bug fix in 3.9.0c2, sometimes caused deletion of keys to be improperly handled, resulting in data being available via iteraation but not item access.
- ZEO manages a separate thread for client network IO. It created this thread on import, which caused problems for applications that implemented daemon behavior by forking. Now, the client thread isn’t created until needed.
- File-storage pack clean-up tasks that can take a long time unnecessarily blocked other activity.
- In certain rare situations, ZEO client connections would hang during the initial connection setup.
- Conflict errors committing blobs caused ZEO servers to stop committing transactions.
The Database class now has an xrefs keyword argument and a corresponding allow-implicit-cross-references configuration option. which default to true. When set to false, cross-database references are disallowed.
Added support for RelStorage.
As a convenience, the connection root method for returning the root object can now also be used as an object with attributes mapped to the root-object keys.
Databases have a new method, transaction, that can be used with the Python (2.5 and later) with statement:
db = ZODB.DB(...) with db.transaction() as conn: # ... do stuff with conn
This uses a private transaction manager for the connection. If control exits the block without an error, the transaction is committed, otherwise, it is aborted.
Convenience functions ZODB.connection and ZEO.connection provide a convenient way to open a connection to a database. They open a database and return a connection to it. When the connection is closed, the database is closed as well.
The ZODB.config databaseFrom… methods now support multi-databases. If multiple zodb sections are used to define multiple databases, the databases are connected in a multi-database arrangement and the first of the defined databases is returned.
The zeopack script has gotten a number of improvements:
- Simplified command-line interface. (The old interface is still supported, except that support for ZEO version 1 servers has been dropped.)
- Multiple storages can be packed in sequence.
- This simplifies pack scheduling on servers serving multiple databases.
- All storages are packed to the same time.
- You can now specify a time of day to pack to.
- The script will now time out if it can’t connect to s storage in 60 seconds.
The connection now estimates the object size based on its pickle size and informs the cache about size changes.
The database got additional configurations options (cache-size-bytes and historical-cache-size-bytes) to limit the cache size based on the estimated total size of cached objects. The default values are 0 which has the interpretation “do not limit based on the total estimated size”. There are corresponding methods to read and set the new configuration parameters.
Connections now have a public opened attribute that is true when the connection is open, and false otherwise. When true, it is the seconds since the epoch (time.time()) when the connection was opened. This is a renaming of the previous _opened private variable.
FileStorage now supports blobs directly.
You can now control whether FileStorages keep .old files when packing.
POSKeyErrors are no longer logged by ZEO servers, because they are really client errors.
A new storage interface, IExternalGC, to support external garbage collection, http://wiki.zope.org/ZODB/ExternalGC, has been defined and implemented for FileStorage and ClientStorage.
As a small convenience (mainly for tests), you can now specify initial data as a string argument to the Blob constructor.
ZEO Servers now provide an option, invalidation-age, that allows quick verification of ZEO clients have been disconnected for less than a given time even if the number of transactions the client hasn’t seen exceeds the invalidation queue size. This is only recommended if the storage being served supports efficient iteration from a point near the end of the transaction history.
The FileStorage iterator now handles large files better. When iterating from a starting transaction near the end of the file, the iterator will scan backward from the end of the file to find the starting point. This enhancement makes it practical to take advantage of the new storage server invalidation-age option.
Previously, database connections were managed as a stack. This tended to cause the same connection(s) to be used over and over. For example, the most used connection would typically be the only connection used. In some rare situations, extra connections could be opened and end up on the top of the stack, causing extreme memory wastage. Now, when connections are placed on the stack, they sink below existing connections that have more active objects.
There is a new pool-timeout database configuration option to specify that connections unused after the given time interval should be garbage collection. This will provide a means of dealing with extra connections that are created in rare circumstances and that would consume an unreasonable amount of memory.
The Blob open method now supports a new mode, ‘c’, to open committed data for reading as an ordinary file, rather than as a blob file. The ordinary file may be used outside the current transaction and even after the blob’s database connection has been closed.
ClientStorage now provides blob cache management. When using non-shared blob directories, you can set a target cache size and the cache will periodically be reduced try to keep it below the target size.
The client blob directory layout has changed. If you have existing non-shared blob directories, you will have to remove them.
ZODB 3.9 ZEO clients can connect to ZODB 3.8 servers. ZODB ZEO clients from ZODB 3.2 on can connect to ZODB 3.9 servers.
When a ZEO cache is stale and would need verification, a ZEO.interfaces.StaleCache event is published (to zope.event). Applications may handle this event and take action such as exiting the application without verifying the cache or starting cold.
There’s a new convenience function, ZEO.DB, for creating databases using ZEO Client Storages. Just call ZEO.DB with the same arguments you would otherwise pass to ZEO.ClientStorage.ClientStorage:
import ZEO db = ZEO.DB(('some_host', 8200))
Object saves are a little faster
When configuring storages in a storage server, the storage name now defaults to “1”. In the overwhelmingly common case that a single storage, the name can now be omitted.
FileStorage now provides optional garbage collection. A ‘gc’ keyword option can be passed to the pack method. A false value prevents garbage collection.
The FileStorage constructor now provides a boolean pack_gc option, which defaults to True, to control whether garbage collection is performed when packing by default. This can be overridden with the gc option to the pack method.
The ZConfig configuration for FileStorage now includes a pack-gc option, corresponding to the pack_gc constructor argument.
The FileStorage constructor now has a packer keyword argument that allows an alternative packer to be supplied.
The ZConfig configuration for FileStorage now includes a packer option, corresponding to the packer constructor argument.
MappingStorage now supports multi-version concurrency control and iteration and provides a better storage implementation example.
DemoStorage has a number of new features:
- The ability to use a separate storage, such as a file storage to store changes
- Blob support
- Multi-version concurrency control and iteration
- Explicit support for demo-storage stacking via push and pop methods.
Wen calling ZODB.DB to create a database, you can now pass a file name, rather than a storage to use a file storage.
Added support for copying and recovery of blob storages:
Added a helper function, ZODB.blob.is_blob_record for testing whether a data record is for a blob. This can be used when iterating over a storage to detect blob records so that blob data can be copied.
In the future, we may want to build this into a blob-aware iteration interface, so that records get blob file attributes automatically.
Added the IBlobStorageRestoreable interfaces for blob storages that support recovery via a restoreBlob method.
Updated ZODB.blob.BlobStorage to implement IBlobStorageRestoreable and to have a copyTransactionsFrom method that also copies blob data.
New ClientStorage configuration option drop_cache_rather_verify. If this option is true then the ZEO client cache is dropped instead of the long (unoptimized) verification. For large caches, setting this option can avoid effective down times in the order of hours when the connection to the ZEO server was interrupted for a longer time.
Cleaned-up the storage iteration API and provided an iterator implementation for ZEO.
Versions are no-longer supported.
Document conflict resolution (see ZODB/ConflictResolution.txt).
Support multi-database references in conflict resolution.
Make it possible to examine oid and (in some situations) database name of persistent object references during conflict resolution.
Moved the ‘transaction’ module out of ZODB. ZODB depends upon this module, but it must be installed separately.
ZODB installation now requires setuptools.
Added offset information to output of fstail script. Added test harness for this script.
Added support for read-only, historical connections based on datetimes or serials (TIDs). See src/ZODB/historical_connections.txt.
Removed the ThreadedAsync module.
Now depend on zc.lockfile
CVE-2009-2701: Fixed a vulnerability in ZEO storage servers when blobs are available. Someone with write access to a ZEO server configured to support blobs could read any file on the system readable by the server process and remove any file removable by the server process.
BTrees (and TreeSets) kept references to internal keys. https://bugs.launchpad.net/zope3/+bug/294788
BTree Sets and TreeSets don’t support the standard set add method. (Now either add or the original insert method can be used to add an object to a BTree-based set.)
The runzeo script didn’t work without a configuration file. (https://bugs.launchpad.net/zodb/+bug/410571)
Officially deprecated PersistentDict (https://bugs.launchpad.net/zodb/+bug/400775)
Calling __setstate__ on a persistent object could under certain uncommon cause the process to crash.
When committing transactions involving blobs to ClientStorages with non-shared blob directories, a failure could occur in tpc_finish if there was insufficient disk space to copy the blob file or if the file wasn’t available. https://bugs.launchpad.net/zodb/+bug/224169
Savepoint blob data wasn’t properly isolated. If multiple simultaneous savepoints in separate transactions modified the same blob, data from one savepoint would overwrite data for another.
Savepoint blob data wasn’t cleaned up after a transaction abort. https://bugs.launchpad.net/zodb/+bug/323067
Opening a blob with modes ‘r+’ or ‘a’ would fail when the blob had no committed changes.
PersistentList’s sort method did not allow passing of keyword parameters. Changed its sort parameter list to match that of its (Python 2.4+) UserList base class.
Certain ZEO server errors could cause a client to get into a state where it couldn’t commit transactions. https://bugs.launchpad.net/zodb/+bug/374737
Fixed vulnerabilities in the ZEO network protocol that allow:
- CVE-2009-0668 Arbitrary Python code execution in ZODB ZEO storage servers
- CVE-2009-0669 Authentication bypass in ZODB ZEO storage servers
The vulnerabilities only apply if you are using ZEO to share a database among multiple applications or application instances and if untrusted clients are able to connect to your ZEO servers.
Fixed the setup test command. It previously depended on private functions in zope.testing.testrunner that don’t exist any more.
ZEO client threads were unnamed, making it hard to debug thread management.
ZEO protocol 2 support was broken. This caused very old clients to be unable to use new servers.
zeopack was less flexible than it was before. -h should default to local host.
The “lawn” layout was being selected by default if the root of the blob directory happened to contain a hidden file or directory such as “.svn”. Now hidden files and directories are ignored when choosing the default layout.
BlobStorage was not compatible with MVCC storages because the wrappers were being removed by each database connection. Fixed.
Saving indexes for large file storages failed (with the error: RuntimeError: maximum recursion depth exceeded). This can cause a FileStorage to fail to start because it gets an error trying to save its index.
Sizes of new objects weren’t added to the object cache size estimation, causing the object-cache size limiting feature to let the cache grow too large when many objects were added.
Deleted records weren’t removed when packing file storages.
Fixed analyze.py and added test.
fixed Python 2.6 compatibility issue with ZEO/zeoserverlog.py
using hashlib.sha1 if available in order to avoid DeprecationWarning under Python 2.6
made runzeo -h work
The monitor server didn’t correctly report the actual number of clients.
Packing could return spurious errors due to errors notifying disconnected clients of new database size statistics.
Undo sometimes failed for FileStorages configured to support blobs.
Starting ClientStorages sometimes failed with non-new but empty cache files.
The history method on ZEO clients failed.
Fix for bug #251037: Make packing of blob storages non-blocking.
Fix for bug #220856: Completed implementation of ZEO authentication.
Fix for bug #184057: Make initialisation of small ZEO client file cache sizes not fail.
Fix for bug #184054: MappingStorage used to raise a KeyError during load instead of a POSKeyError.
Fixed bug in Connection.TmpStore: load() would not defer to the backend storage for loading blobs.
Fix for bug #181712: Make ClientStorage update lastTransaction directly after connecting to a server, even when no cache verification is necessary.
Fixed bug in blob filesystem helper: the isSecure check was inverted.
Fixed bug in transaction buffer: a tuple was unpacked incorrectly in clear.
Bugfix the situation in which comparing persistent objects (for instance, as members in BTree set or keys of BTree) might cause data inconsistency during conflict resolution.
Fixed bug 153316: persistent and BTrees were using int for memory sizes which caused errors on x86_64 Intel Xeon machines (using 64-bit Linux).
Fixed small bug that the Connection.isReadOnly method didn’t work after a savepoint.
Bug #98275: Made ZEO cache more tolerant when invalidating current versions of objects.
Fixed a serious bug that could cause client I/O to stop (hang). This was accompanied by a critical log message along the lines of: “RuntimeError: dictionary changed size during iteration”.
Fixed bug #127182: Blobs were subclassable which was not desired.
Fixed bug #126007: tpc_abort had untested code path that was broken.
Fixed bug #129921: getSize() function in BlobStorage could not deal with garbage files
Fixed bug in which MVCC would not work for blobs.
Fixed bug in ClientCache that occurred with objects larger than the total cache size.
When an error occured attempting to lock a file and logging of said error was enabled.
FileStorages previously saved indexes after a certain number of writes. This was done during the last phase of two-phase commit, which made this critical phase more subject to errors than it should have been. Also, for large databases, saves were done so infrequently as to be useless. The feature was removed to reduce the chance for errors during the last phase of two-phase commit.
File storages previously kept an internal object id to transaction id mapping as an optimization. This mapping caused excessive memory usage and failures during the last phase of two-phase commit. This optimization has been removed.
Refactored handling of invalidations on ZEO clients to fix a possible ordering problem for invalidation messages.
On many systems, it was impossible to create more than 32K blobs. Added a new blob-directory layout to work around this limitation.
Fixed bug that could lead to memory errors due to the use of a Python dictionary for a mapping that can grow large.
Fixed bug #251037: Made packing of blob storages non-blocking.
Fixed a bug that could cause InvalidObjectReference errors for objects that were explicitly added to a database if the object was modified after a savepoint that added the object.
Fixed several bugs that caused ZEO cache corruption when connecting to servers. These bugs affected both persistent and non-persistent caches.
Improved the the ZEO client shutdown support to try to avoid spurious errors on exit, especially for scripts, such as zeopack.
Packing failed for databases containing cross-database references.
Cross-database references to databases with empty names weren’t constructed properly.
The zeo client cache used an excessive amount of memory, causing applications with large caches to exhaust available memory.
Fixed a number of bugs in the handling of persistent ZEO caches:
- Cache records are written in several steps. If a process exits after writing begins and before it is finishes, the cache will be corrupt on restart. The way records are written was changed to make cache record updates atomic.
- There was no lock file to prevent opening a cache multiple times at once, which would lead to corruption. Persistent caches now use lock files, in the same way that file storages do.
- A bug in the cache-opening logic led to cache failure in the unlikely event that a cache has no free blocks.
When using ZEO Client Storages, Errors occured when trying to store objects too big to fit in the ZEO cache file.
Fixed bug in blob filesystem helper: the isSecure check was inverted.
Fixed bug in transaction buffer: a tuple was unpacked incorrectly in clear.
Fixed bug in Connection.TmpStore: load() would not defer to the back-end storage for loading blobs.
Fixed bug #190884: Wrong reference to POSKeyError caused NameError.
Completed implementation of ZEO authentication. This fixes issue 220856.
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