Durus

This is Durus, a persistent object system for applications written in the Python programming language. It could be called a noSQL database. However, it does provide "ACID" properties (Atomicity, Consistency, Isolation, Durability).

The implementation of Durus is not multi-threaded but does provide concurrency via a client/server model. It is optimized for read heavy work loads and aggressively caches persistent objects in memory. For many applications, this design enables good performance with minimal effort from application programmers.

Origin

Durus was originally written by the MEMS Exchange software development team at the Corporation for National Research Initiatives (CNRI). Durus was designed to be the storage component for the Python-powered web sites operated by the MEMS Exchange. See doc/README_CNRI.txt for more details.

Overview

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.

Storage back-ends

This version of Durus includes a number of back-end storage implementations that may be used. The default is FileStorage, an append-only journal that includes an on-disk index of object record offsets. This module has the advantage of fast startup time with slightly slower read performance (two disk seeks per object load).

Also available is FileStorage2, an older version of the FileStorage format. It uses an in-memory index for object offsets and so it has slower startup time (reading the index into memory takes time, especially on large databases) but faster read performance (one seek per object load).

Finally, there is an experimental Sqlite storage module, SqliteStorage. The module uses a SQLite3 database to persist object data. One disadvantage of this module compared to the others is that online backups are more difficult (for the other two it is safe to just copy the file while the server is running). You also lose the ability to do point-in-time recovery (which the other two storage implementations provide, assuming you did not yet pack the DB).

License

Durus is released under an open-source license. See LICENSE.txt for details.

License

nascheme/durus

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