Use Mongosniff to clarify what your MongoDB hears and says

Preamble

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Writing an application on top of the framework on top of the driver on top of the database is a bit like a game on the phone: you say “insert foo”, and the database says “dishwasher with purple monkeys”.
Mongosniff allows you to see exactly what the database hears and says.
It comes with a binary distribution, so if you have a mongod, you should have mongosniff .

To try this, first, run the mongod instance as usual:

$ ./mongod

When you run mongosniff you must tell it to listen to the loopback interface (localhost). This interface is usually called “lo”, but my Mac calls it “lo0”, so run ifconfig to make sure you give it the right name. Now run it:

$ sudo ./mongosniff --source NET lo
sniffing... 27017

Note “sudo”: this never worked for me from my user account, probably because of some stupid network permissions.

Now run the Mongo shell and try to insert something:

db.foo.insert({x:1})

If you look at the mongosniff output, you will see:

127.0.0.1:57856 -->> 127.0.0.1:27017 test.foo 62 bytes id:430131ca 1124151754
insert: { _id: ObjectId('4c7fb007b5d697849addc650'), x: 1.0 }
127.0.0.1:57856 -->> 127.0.0.1:27017 test.$cmd 76 bytes id:430131cb 1124151755
query: { getlasterror: 1.0, w: 1.0 } ntoreturn: -1 ntoskip: 0
127.0.0.1:27017 <<-- 127.0.0.1:57856 76 bytes id:474447bf 1195657151 - 1124151755
reply n:1 cursorId: 0
{ err: null, n: 0, wtime: 0, ok: 1.0 }

There are three requests, all for one miserable insert. By cutting the first request, we can find out:

source – >>
place of destination

Our client,
In this case, mongo , works through port 57856 and sent a message to the database (127.0.0.1:27017).
dB 
collection

This query is for the “foo” collection of the “test” database.
Byte length

The request length is 62 bytes. This may be convenient if your requests are close to the maximum request length (16 MB).

id:
hexadecimal
identifier

This is the request ID in hexadecimal and decimal form (in case you do not have a computer, obviously). Each query to the database has a unique identifier associated with it for tax purposes.
op :
content .

This is the actual content of the request: we insert this document. Note that it inserts a floating point value of 1.0 although we have entered 1 in the shell. This is because JavaScript has only one number type, so each number entered in the shell is converted to a double.
The next request in the output of mongosniff is the database command: it checks that the insertion is successful (the shell always performs safe inserts).

The last message heard is slightly different: it goes from database to shell. This is the database response to the getlasterror command. It shows that only one document was returned (answer n: 1) and that there are no more results waiting in the database (cursorId: 0). If this was a “real” query and there was another package of results to send from the database, cursorId would be non-zero.

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