Document
MongoDB can be thought of as a document-oriented database. By 'document', we mean structured documents, not freeform text documents. These documents canbe thought of as objectsbut only the data of an object, not the code, methods or class hierarchy. Additionally, there is much less linking between documents in MongoDB data models than there is between objects in a program written in an object-oriented programming language.
In MongoDB the documents are conceptually JSON. More specifically the documents are represented in a format calledBSON(standing for Binary JSON).
Documents are stored inCollections.
Maximum Document Size
MongoDB limits the data size of individual BSON objects/documents. At the time of this writing the limit is 16MB.
This limit is designed as a sanity-check; it is not a technical limit on document sizes. The thinking is that if documents are larger than this size, it is likely the schema is not ideal. Further it allows drivers to make some assumptions on the max size of documents.
The concept is that the maximum document size is a limit that ensures each document does not require an excessive amount of RAM from the machine, or require too much network bandwidth to fetch. For example, fetching a full 100MB document would take over 1 second to fetch over a gigabit ethernet connection. In this situation one would be limited to 1 request per second.
Over time, as computers grow in capacity, the limit will be adjusted upward.
Collection
MongoDB collections are essentially named groupings of documents. You can think of them as roughly equivalent to relational database tables.
A MongoDB collection is a collection ofBSONdocuments. These documents usually have the same structure, but this is not a requirement since MongoDB is a schema-free (or more accurately, "dynamic schema") database. You may store a heterogeneous set of documents within a collection, as you do not need predefine the collection's "columns" or fields.
A collection is created when the first document is inserted.
Collection names should begin with letters or an underscore and may include numbers; $ is reserved. Collections can be organized in namespaces; these are named groups of collections defined using a dot notation. For example, you could define collections blog.posts and blog.authors, both reside under "blog". Note that this is simply an organizational mechanism for the user -- the collection namespace is flat from the database's perspective.
The maximum size of a collection name is 128 characters (including the name of the db and indexes). It is probably best to keep it under 80/90 chars.
Namespace
MongoDB stores BSON objects in collections. The concatenation of the database name and the collection name (with a period in between) is called a namespace.
For example, acme.users is a namespace, where acme is the database name, and users is the collection name. Note that periods can occur in collection names, so a name such as acme.blog.posts is legal too (in that case blog.posts is the collection name.