Launch the MongoDB client and connect to MongoDB server:

$ mongo

Switch to a database: (We call it test)

> use test

List the existing collections (there are none):

> show collections
system.indexes  0.000MB / 0.000MB

Alternatively, list all the entries in the counter collection: (there are none):

> db.counter.find()
Fetched 0 record(s) in 0ms

Insert a counter

Insert a document where we decide the _id, this is going to be the key of the counter, and it has aanother field called 'val' that is set to the initial value of 1.

> db.counter.insert({ '_id' : 'a', 'val' : 1 })
Inserted 1 record(s) in 9ms
WriteResult({
  "nInserted": 1
})

Retreive a document

We can retreive the counter by specifying its name:

> db.counter.find( { '_id' : 'a' })
{
  "_id": "a",
  "val": 1
}
Fetched 1 record(s) in 1ms

Increment the counter

> db.counter.update({ '_id' : 'a'}, {'$inc' : { 'val' : 1 } })
Updated 1 existing record(s) in 3ms
WriteResult({
  "nMatched": 1,
  "nUpserted": 0,
  "nModified": 1
})

and retreive the new value:

> db.counter.find( { '_id' : 'a' })
{
  "_id": "a",
  "val": 2
}

Insert another counter

> db.counter.insert({ '_id' : 'b', 'val' : 1 })

Increment both of them separately:

> db.counter.update({ '_id' : 'a'}, {'$inc' : { 'val' : 1 } })
> db.counter.update({ '_id' : 'a'}, {'$inc' : { 'val' : 1 } })
> db.counter.update({ '_id' : 'a'}, {'$inc' : { 'val' : 1 } })
> db.counter.update({ '_id' : 'b'}, {'$inc' : { 'val' : 1 } })

Retreive the individual counters

> db.counter.find( { '_id' : 'a' })
{
  "_id": "a",
  "val": 5
}
Fetched 1 record(s) in 1ms

> db.counter.find( { '_id' : 'b' })
{
  "_id": "b",
  "val": 2
}
Fetched 1 record(s) in 0ms

Retreive all the counters

> db.counter.find()
{
  "_id": "a",
  "val": 5
}
{
  "_id": "b",
  "val": 2
}
Fetched 2 record(s) in 1ms

findAndModify and upsert

An even better solution is to use the findAndModify function call, and even include an upsert field to make sure the docuemnt is created if it does not exist.

> db.counter.findAndModify({ query: { '_id' : 'y' }, update : { '$inc' : { val : 1 } }, new: true, upsert: true })
{
  "_id": "y",
  "val": 1
}