Atomic operation examples

A key goal of MongoDB is to handle a good breadth of use cases, and to handle in a way that is easy for the developer. We found that a good number of use cases require atomic operations / strong consistency; thus that is a feature of the product. Below are some examples (in mongo shell syntax).

// register a new user (atomically)
db.users.insert( { _id : 'joe123' } )
if( db.getLastErrorObj().err ) 
  print("name is use try another")
else 
  print("you are registered")


// decrement y if y > 0 (atomically)
db.stats.update(
  { _id : ‘myid’, y : { $gt : 0 } },
  { $inc : { y : -1 } } 
)


// assure everyone’s email address is unique
db.users.ensureIndex( {email:1} , {unique:true} )


// if joe hasn’t already voted, let him vote (without races)
db.posts.update( 
  { _id : ‘some_post_of_interest’ , voters : { $ne : ‘joe’ } },
  { votes : { $inc : 1 } }
)

Atomic operation examples

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