CRUD operations

The core operations that a document-oriented database supports for documents are similar to other databases, and while the terminology is not perfectly standardized, most practitioners will recognize them as CRUD:

• Creation (or insertion)
• Retrieval (or query, search, read or find)
• Update (or edit)
• Deletion (or removal)

Keys

Documents are addressed in the database via a unique key that represents that document. This key is a simple identifier (or ID), typically a string, a URI, or a path. The key can be used to retrieve the document from the database. Typically the database retains an index on the key to speed up document retrieval, and in some cases the key is required to create or insert the document into the database.

Retrieval

Another defining characteristic of a document-oriented database is that, beyond the simple key-to-document lookup that can be used to retrieve a document, the database offers an API or query language that allows the user to retrieve documents based on content (or metadata). For example, you may want a query that retrieves all the documents with a certain field set to a certain value. The set of query APIs or query language features available, as well as the expected performance of the queries, varies significantly from one implementation to another. Likewise, the specific set of indexing options and configuration that are available vary greatly by implementation.

It is here that the document store varies most from the key-value store. In theory, the values in a key-value store are opaque to the store, they are essentially black boxes. They may offer search systems similar to those of a document store, but may have less understanding about the organization of the content. Document stores use the metadata in the document to classify the content, allowing them, for instance, to understand that one series of digits is a phone number, and another is a postal code. This allows them to search on those types of data, for instance, all phone numbers containing 555, which would ignore the zip code 55555.

Editing

Document databases typically provide some mechanism for updating or editing the content (or metadata) of a document, either by allowing for replacement of the entire document, or individual structural pieces of the document.

Organization

Document database implementations offer a variety of ways of organizing documents, including notions of

• Collections: groups of documents, where depending on implementation, a document may be enforced to live inside one collection, or may be allowed to live in multiple collections
• Tags and non-visible metadata: additional data outside the document content
• Directory hierarchies: groups of documents organized in a tree-like structure, typically based on path or URI

Sometimes these organizational notions vary in how much they are logical vs physical, (e.g. on disk or in memory), representations.

Relationship to key-value stores

A document-oriented database is a specialized key-value store, which itself is another NoSQL database category. In a simple key-value store, the document content is opaque. A document-oriented database provides APIs or a query/update language that exposes the ability to query or update based on the internal structure in the document. This difference may be minor for users that do not need richer query, retrieval, or editing APIs that are typically provided by document databases. Modern key-value stores often include features for working with metadata, blurring the lines between document stores.

Relationship to search engines

Some search engine (aka information retrieval) systems like Apache Solr and Elasticsearch provide enough of the core operations on documents to fit the definition of a document-oriented database.

Relationship to relational databases

In a relational database, data is first categorized into a number of predefined types, and tables are created to hold individual entries, or records, of each type. The tables define the data within each record's fields, meaning that every record in the table has the same overall form. The administrator also defines the relationships between the tables, and selects certain fields that they believe will be most commonly used for searching and defines indexes on them. A key concept in the relational design is that any data that may be repeated is normally placed in its own table, and if these instances are related to each other, a column is selected to group them together, the foreign key. This design is known as database normalization.^[3]

For example, an address book application will generally need to store the contact name, an optional image, one or more phone numbers, one or more mailing addresses, and one or more email addresses. In a canonical relational database, tables would be created for each of these rows with predefined fields for each bit of data: the CONTACT table might include FIRST_NAME, LAST_NAME and IMAGE columns, while the PHONE_NUMBER table might include COUNTRY_CODE, AREA_CODE, PHONE_NUMBER and TYPE (home, work, etc.). The PHONE_NUMBER table also contains a foreign key column, "CONTACT_ID", which holds the unique ID number assigned to the contact when it was created. In order to recreate the original contact, the database engine uses the foreign keys to look for the related items across the group of tables and reconstruct the original data.

In contrast, in a document-oriented database there may be no internal structure that maps directly onto the concept of a table, and the fields and relationships generally don't exist as predefined concepts. Instead, all of the data for an object is placed in a single document, and stored in the database as a single entry. In the address book example, the document would contain the contact's name, image, and any contact info, all in a single record. That entry is accessed through its key, which allows the database to retrieve and return the document to the application. No additional work is needed to retrieve the related data; all of this is returned in a single object.

A key difference between the document-oriented and relational models is that the data formats are not predefined in the document case. In most cases, any sort of document can be stored in any database, and those documents can change in type and form at any time. If one wishes to add a COUNTRY_FLAG to a CONTACT, this field can be added to new documents as they are inserted, this will have no effect on the database or the existing documents already stored. To aid retrieval of information from the database, document-oriented systems generally allow the administrator to provide hints to the database to look for certain types of information. These work in a similar fashion to indexes in the relational case. Most also offer the ability to add additional metadata outside of the content of the document itself, for instance, tagging entries as being part of an address book, which allows the programmer to retrieve related types of information, like "all the address book entries". This provides functionality similar to a table, but separates the concept (categories of data) from its physical implementation (tables).

In the classic normalized relational model, objects in the database are represented as separate rows of data with no inherent structure beyond that given to them as they are retrieved. This leads to problems when trying to translate programming objects to and from their associated database rows, a problem known as object-relational impedance mismatch.^[4] Document stores more closely, or in some cases directly, map programming objects into the store. These are often marketed using the term NoSQL.

XML database implementations

Most XML databases are document-oriented databases.