Devoxx11: Building Web Applications with MongoDB

Brendan McAdams

mongoDB is a nosql DB, ie. a "non-relational" DB

• no joins
• no referential integrity
• no complex transactions --> not suitable for higly transactional apps
• but: focus on horizontal scalability
  • sharding
  • replica's
• a scheme-less DB
  • unique ObjectID is automatically generated (based on time, machine, pid & counter)
  • flexible structures: adding/omitting elements is easy
• native JSON support
  • hierarchical 'documents'
  • arrays
  • internally, an optimized format is used: BSON:http://bsonspec.org
• support for indexes (+ 'multi-indexes' on arrays)

technical specifics:

• based on memory mapped files:
  • OS maps the files in the RAM.
  • pagefaults are expensive --> keep working set in RAM
    • provide sufficient RAM
    • prewarm the DB
  • only use 64bit OS & binaries
• Intel-only (little endian)
• filesystem
  • Ext4 & Xfs: only filesystems with support for posix_fallocate()
  • set noatime, noadirtime in /etc/fstab
• 'documents' (i.e. objects) have a max. size of 16mb
• accessible over tcp/ip: Mongo Wire Protocol
• UTF-8
• the data is stored in fysical files / 'extents'

modeling for mongoDB:

• think of 'documents' instead of 'rows'
• favor embedding over referencing -- mongoDB offers no referential integrity (aka foreign keys)
• think of the access patterns of your data

other specifics:

• support for MapReduce
• geospatial indexing
• indexes (internally btrees)
• query optimization
• advanced queries ($lt, $all, $in...)
• gridfs:
  • large 'chunked' files
  • distributed (sharded)
  • accessible through a regular java.io.File