PouchDB

PouchDB is a free and open-source JavaScript DBMS inspired by Apache CouchDB (document-oriented NoSQL DBMS) that is designed to run inside a web browser. It was designed with the intention of serving web developers and to run smoothly within a browser. Applications utilize a system where, while offline, data is stored locally, then, when back online, the data is synchronized with Apache CouchDB and between clients. Thus, data is synchronized regardless of login context.

Concurrency Control

Multi-version Concurrency Control (MVCC)

PouchDB's concurrency control policy is multi-version concurrency control, allowing consistency of reads.

Data Model

Document / XML

PouchDB uses an unstructured document data model, where documents are JSON serializable. It also supports attachments, which store binary data.

Indexes

B+Tree

Starting with version 2.2.0, PouchDB supports secondary indexes using B-trees. It supports an ID-based index by default.

Joins

Not Supported

PouchDB does not support joins since it uses a document-based data model. To mimic the existence of joins, the Relational Pouch plugin can be used. It enables PouchDB interaction as if it were a relational database. Regardless, the joins used here are actually created by additional requests (instead of classic join algorithms).

Logging

Not Supported

PouchDB doesn't support logging itself, however different types of logging are used by its various backends. For LevelDB-backed PouchDB (for Node.js adaption), LevelDB which uses logical logging.

Query Interface

Custom API

PouchDB utilizes a custom API (both a Javascript API and an HTTP API) for its query interface. As it is designed for web browsers, this API supports callbacks, promises, and async functions.

Storage Organization

Copy-on-Write / Shadow Paging Indexed Sequential Access Method (ISAM)

When offline, data is stored locally using WebSQL (deprecated) or IndexedDB, depending on the browser. For IndexedDB usage, the method of storage organization is ISAM. When online, data synchs with CouchDB (and other servers). CouchDB storage uses copy-on-write to update database files.

Stored Procedures

Supported

PouchDB allows for stored procedures by using view functions in JavaScript.

System Architecture

Shared-Nothing

As with other members of the Couch family, each node contains the same replica of all the data (so you don't have to worry about which node is the true source of all data). PouchDB is an AP database (referring to CAP theorem) so it guarantees availability (responses for each request) and partition tolerance (continued system operation in the event of dropped messages).

Views

Virtual Views Materialized Views

PouchDB supports both materialized (persistent) views as well as virtual (temporary) views. In PouchDB, views are synonymous with indexes (i.e. the same as CREATE INDEX in SQL-based databases). Views store information about a specified (map/reduce) query. In turn, views are stored in design documents. These views are very similar to CouchDB however PouchDB uses ASCII ordering to order view query keys. It also does not support the offset property that CouchDB does. Instead, a skip parameter is used instead of an actual offset.

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