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FAQ: MongoDB Fundamentals

This document addresses basic high level questions about MongoDB and its use.

If you don’t find the answer you’re looking for, check the complete list of FAQs or post your question to the MongoDB User Mailing List.

What kind of database is MongoDB?

MongoDB is a document-oriented DBMS. Think of MySQL but with JSON-like objects comprising the data model, rather than RDBMS tables. Significantly, MongoDB supports neither joins nor transactions. However, it features secondary indexes, an expressive query language, atomic writes on a per-document level, and fully-consistent reads.

Operationally, MongoDB features master-slave replication with automated failover and built-in horizontal scaling via automated range-based partitioning.

Note

MongoDB uses BSON, a binary object format similar to, but more expressive than JSON.

Do MongoDB databases have tables?

Instead of tables, a MongoDB database stores its data in collections, which are the rough equivalent of RDBMS tables. A collection holds one or more documents, which corresponds to a record or a row in a relational database table, and each document has one or more fields, which corresponds to a column in a relational database table.

Collections have important differences from RDBMS tables. Documents in a single collection may have a unique combination and set of fields. Documents need not have identical fields. You can add a field to some documents in a collection without adding that field to all documents in the collection.

Do MongoDB databases have schemas?

MongoDB uses dynamic schemas. You can create collections without defining the structure, i.e. the fields or the types of their values, of the documents in the collection. You can change the structure of documents simply by adding new fields or deleting existing ones. Documents in a collection need not have an identical set of fields.

In practice, it is common for the documents in a collection to have a largely homogeneous structure; however, this is not a requirement. MongoDB’s flexible schemas mean that schema migration and augmentation are very easy in practice, and you will rarely, if ever, need to write scripts that perform “alter table” type operations, which simplifies and facilitates iterative software development with MongoDB.

What languages can I use to work with MongoDB?

MongoDB client drivers exist for all of the most popular programming languages, and many other ones. See the latest list of drivers for details.

Does MongoDB support SQL?

No.

However, MongoDB does support a rich, ad-hoc query language of its own.

See also

Operators

What are typical uses for MongoDB?

MongoDB has a general-purpose design, making it appropriate for a large number of use cases. Examples include content management systems, mobile applications, gaming, e-commerce, analytics, archiving, and logging.

Do not use MongoDB for systems that require SQL, joins, and multi-object transactions.

Does MongoDB support ACID transactions?

MongoDB does not support multi-document transactions.

However, MongoDB does provide atomic operations on a single document. Often these document-level atomic operations are sufficient to solve problems that would require ACID transactions in a relational database.

For example, in MongoDB, you can embed related data in nested arrays or nested documents within a single document and update the entire document in a single atomic operation. Relational databases might represent the same kind of data with multiple tables and rows, which would require transaction support to update the data atomically.

MongoDB allows clients to read documents inserted or modified before it commits these modifications to disk, regardless of write concern level or journaling configuration. As a result, applications may observe two classes of behaviors:

  • For systems with multiple concurrent readers and writers, MongoDB will allow clients to read the results of a write operation before the write operation returns.
  • If the mongod terminates before the journal commits, even if a write returns successfully, queries may have read data that will not exist after the mongod restarts.

Other database systems refer to these isolation semantics as read uncommitted. For all inserts and updates, MongoDB modifies each document in isolation: clients never see documents in intermediate states. For multi-document operations, MongoDB does not provide any multi-document transactions or isolation.

When mongod returns a successful journaled write concern, the data is fully committed to disk and will be available after mongod restarts.

For replica sets, write operations are durable only after a write replicates and commits to the journal of a majority of the members of the set. MongoDB regularly commits data to the journal regardless of journaled write concern: use the commitIntervalMs to control how often a mongod commits the journal.

Does MongoDB require a lot of RAM?

Not necessarily. It’s certainly possible to run MongoDB on a machine with a small amount of free RAM.

MongoDB automatically uses all free memory on the machine as its cache. System resource monitors show that MongoDB uses a lot of memory, but its usage is dynamic. If another process suddenly needs half the server’s RAM, MongoDB will yield cached memory to the other process.

Technically, the operating system’s virtual memory subsystem manages MongoDB’s memory. This means that MongoDB will use as much free memory as it can, swapping to disk as needed. Deployments with enough memory to fit the application’s working data set in RAM will achieve the best performance.

See also

FAQ: MongoDB Diagnostics for answers to additional questions about MongoDB and Memory use.

How do I configure the cache size?

MongoDB has no configurable cache. MongoDB uses all free memory on the system automatically by way of memory-mapped files. Operating systems use the same approach with their file system caches.

Does MongoDB require a separate caching layer for application-level caching?

No. In MongoDB, a document’s representation in the database is similar to its representation in application memory. This means the database already stores the usable form of data, making the data usable in both the persistent store and in the application cache. This eliminates the need for a separate caching layer in the application.

This differs from relational databases, where caching data is more expensive. Relational databases must transform data into object representations that applications can read and must store the transformed data in a separate cache: if these transformation from data to application objects require joins, this process increases the overhead related to using the database which increases the importance of the caching layer.

Does MongoDB handle caching?

Yes. MongoDB keeps all of the most recently used data in RAM. If you have created indexes for your queries and your working data set fits in RAM, MongoDB serves all queries from memory.

MongoDB does not implement a query cache: MongoDB serves all queries directly from the indexes and/or data files.

Are writes written to disk immediately, or lazily?

Writes are physically written to the journal within 100 milliseconds, by default. At that point, the write is “durable” in the sense that after a pull-plug-from-wall event, the data will still be recoverable after a hard restart. See commitIntervalMs for more information on the journal commit window.

While the journal commit is nearly instant, MongoDB writes to the data files lazily. MongoDB may wait to write data to the data files for as much as one minute by default. This does not affect durability, as the journal has enough information to ensure crash recovery. To change the interval for writing to the data files, see syncPeriodSecs.

What language is MongoDB written in?

MongoDB is implemented in C++. Drivers and client libraries are typically written in their respective languages, although some drivers use C extensions for better performance.

What are the limitations of 32-bit versions of MongoDB?

MongoDB uses memory-mapped files. When running a 32-bit build of MongoDB, the total storage size for the server, including data and indexes, is 2 gigabytes. For this reason, do not deploy MongoDB to production on 32-bit machines.

If you’re running a 64-bit build of MongoDB, there’s virtually no limit to storage size. For production deployments, 64-bit builds and operating systems are strongly recommended.

Note

32-bit builds disable journaling by default because journaling further limits the maximum amount of data that the database can store.