This is Durus, a persistent object system for applications written in the Python programming language.
Durus was written by the MEMS Exchange software development team at the Corporation for National Research Initiatives (CNRI). Durus is designed to be the storage component for the Python-powered web sites operated by the MEMS Exchange, and it provides the features that we need for this purpose, and no more. If you are looking for a language-independent, long term persistent storage system for digital objects of all kinds, designed to provide secure, managed access to extensible server-side information services, you should read more about CNRI's Digital Object Store® at http://www.cnri.reston.va.us/digital_object_store.html.
Durus offers an easy way to use and maintain a consistent collection of object instances used by one or more processes. Access and change of a persistent instances is managed through a cached Connection instance which includes commit() and abort() methods so that changes are transactional.
- Quick Demo:
Run "durus -s" in one window. This starts a durus storage server using a temporary file and listening for clients on localhost port 2972. Run "durus -c" in another window. This connects to the storage server on the port 2972 on the localhost. When you start, you have access to only one dictionary-like persistent object, "root". If you make changes to items of root and run "connection.commit()", the changes are written to the (in this case, temporary) file. If you make changes to attributes of root, and then run "connection.abort()", the attributes revert back to the values they had at the last commit.
Run another "durus -c" in a third window, and you can see how committed changes to root in the first client are available in the second client when it starts. Subsequent changes committed in any client are visible in any other client that synchronizes by calling either "connection.abort()" or "connection.commit()".
You can stop the server by Control-C or by running "durus -s --stop". You can stop the clients by Control-D or by your usual method of terminating a python interaction.
This demonstrates simple transactional behavior, but not persistence, since the temporary file is removed as soon as the durus server is stopped.
To see how persistence works, follow the same procedure again, except add "--file test.durus" to the command that starts the server. Make changes to attributes of root, run "connection.commit()", and "durus -s --stop", and the changes to root will be stored in test.durus, so that you"ll see the changes again if you restart again with the "--file test.durus" option.
Finally, note that you can run "durus -c --file test.durus" (after stopping the durus server) to use the file storage directly and exclusively. Everything works the same way as before, except that no server is involved.
Both the "durus -s" and "durus -c" commands accept "--help" command line options that explain more about their usage.
- Using Durus in a Program:
To use Durus, a Python program needs to make a Storage instance and a Connection instance. For the Storage instance, you have two choices: FileStorage or ClientStorage. If your program is to be one of several processes accessing a shared collection of objects, then you want ClientStorage. If your program has no competition, then choose FileStorage. There is only one Connection class, and the constructor takes a storage instance as an argument.
Example using FileStorage to open a Connection to a file:
from durus.file_storage import FileStorage from durus.connection import Connection connection = Connection(FileStorage("test.durus"))
Example using ClientStorage to open a Connection to a Durus server:
from durus.client_storage import ClientStorage from durus.connection import Connection connection = Connection(ClientStorage())
Note that the ClientStorage constructor supports the "address" keyword that you can use to specify the address to use. The value must be either a (host, port) tuple or a string giving a path to use for a unix domain socket. If you provide the address you should be sure to start the storage server the same way. The "durus" command line tool also supports options to specify the address.
The connection instance has a get_root() method that you can use to obtain the root object.
In your program, you can make changes to the root object attributes, and call connection.commit() or connection.abort() to lock in or revert changes made since the last commit. The root object is actually an instance of durus.persistent_dict.PersistentDict, which means that it can be used like a regular dict, except that changes will be managed by the Connection. There is a similar class, durus.persistent_list.PersistentList that provides list-like behavior, except managed by the Connection.
PersistentList and PersistentDict both inherit from durus.persistent.Persistent, and this is the key to making your own classes participate in the Durus persistence system. Just add Persistent class A"s list of bases, and your instances will know how to manage changes to their attributes through a Connection. To actually store an instance x of A in the storage, though, you need to commit a reference to x in some object that is already stored in the database. The root object is always there, for example, so you can do something like this:
# Assume mymodule defines A as a subclass of Persistent. from mymodule import A x = A() root = connection.get_root() # connection set as shown above. root["sample"] = x # root is dict-like connection.commit() # Now x is stored.
Subsequent changes to x, or to new A instances put on attributes of X, and so on, will all be managed by the Connection just as for the root object. This management of the Persistent instance continues as long as the instance is in the storage. Sometimes, though, we wish to remove "garbage" Persistent instances from the storage so that the file can be smaller. This garbage collection can be done manually by calling the Connection's pack() method. If you are using a storage server to share a Storage, you can use the gcinterval argument to tell it to take care of garbage collection automatically.
- Non-Persistent Containers.
When you change an attribute of a Persistent instance, the fact that the instance has been changed is noted with the Connection, so that the Connection knows what instances need to be stored on the next commit(). The same change-tracking occurs automatically when you make dict-like changes to PersistentDict instances or list-like changes to PersistentList instances. If, however, you make changes to a non-persistent container, even if it is the value of an attribute of a Persistent instance, the changes are not automatically noted with the Connection. To make sure that your changes do get saved, you must call the _p_note_change() method of the Persistent instance that refers to the changed non-persistent container. You can see an example of this by looking at the source code of PersistentDict and PersistentList, both of which maintain a non-persistent container on a "data" attribute, shadow the methods of the underlying container, and add calls to self._p_note_change() in every method that makes changes.
Copyright (c) Corporation for National Research Initiatives 2009. All Rights Reserved.
- Source of Support:
This work was supported by DARPA/MTO under Contract MDA972-03-1-0022.