System Architecture

Shared-Disk

It depends on which pluggable storage layer is used. The default storage layer ZEO is a single-server model, so there is no sharing. There are many distributed-server models written for ZODB, such as NEO and ZRS. NEO and ZRS store data on multiple machines across the Internet, and support data duplication, load-balancing read and fault tolerance.

Joins

Not Supported

The relationships between objects are automatically maintained by Python object reference, which is a key internal mechanism of Python.

Query Interface

Custom API

ZODB only provides Python APIs. Querying for an object in ZODB is similar to querying for a value in a Python dictionary.

Concurrency Control

Multi-version Concurrency Control (MVCC)

ZODB implements MVCC as an additional layer for those pluggable storage layers without MVCC, such as ZEO and NEO. ZODB does not implement MVCC layer for relStorage, which already has MVCC.

Foreign Keys

Not Supported

Views

Not Supported

Indexes

B+Tree

ZODB does not support building indexes on a set of stored objects. However, there is a variable type named Btree functioning as an index in ZODB. If objects are inserted into a Btree, they are maintained in order, and range queries are supported. Btree has variants: IOBTrees, OIBTrees, and IIBTrees, which are optimized for integer keys and integer values.

Logging

Not Supported

The default storage layer FileStorage is itself a log-structured file. So there is no need to implement logging in ZODB. For other alternative storage layers, whether they have logging or not is determined by the implementation of their backend.

Storage Architecture

Disk-oriented In-Memory

ZODB has pluggable storage layers, including ZEO, NEO, relstorage, Directory Storage, Demo Storage, and so on. DemoStorage is used for development and testing, so it is a in-memory storage for convenient. Others are disk-oriented.

Stored Procedures

Supported

ZODB is used as a Python library. Therefore, Python functions can be viewed as stored procedures for ZODB.

Query Compilation

Not Supported

ZODB does not support complex queries, so there is no need for query compilation.

Checkpoints

Not Supported

ZODB does not provide ckeckpoint API.

Data Model

Object-Oriented

ZODB stores data as Python objects.

Compression

Naïve (Page-Level)

The default storage FileStorage does not compress the data. It simply appends objects to the end of a file. However, there is an optional layer named zlibstorage which uses gzip to compress the file in the FileStorage. Moreover, since ZODB has a pluggable storage layer, it can support compression by using different storage layers such as NEO and relStorage. NEO implements client-side compression, and RelStorage supports compression using MySQL, PostgreSQL, and Oracle backend.

Isolation Levels

Snapshot Isolation

ZODB supports snapshot isolation with MVCC.

Storage Organization

Log-structured Copy-on-Write / Shadow Paging

It depends on which pluggable storage layer is used. DemoStorage is a copy-on-write storage layer. FileStorage is a log-structured storage layer, which has only one large file for all data.