Question

I’m writing some Java code using MongoDB with Java API and I’m unsure of some part of the Javadoc.

In a multi-thread context I use DBCollection.html#update(com.mongodb.DBObject, com.mongodb.DBObject) to update a unique document, but I saw that two threads could try to write concurrently. In this context, I observed that only one write was done, as Mongodb seems to use optimistic write lock, but I wanted to find out programmatically in which thread the write was the one who wrote, and which one was not. As a “no update” behavior was silent (I mean no exception or something), I searched into the API some way to answer my issue and after some tests found out this method: WriteResult#getN()

public int getN()
    Gets the "n" field
    Returns:

The description is, hum… not really exhaustive. My tests showed that the thread that win the write has a getN() that return 1, and the other 0.

So my question is: Could someone confirm this ?

Answer 1

From the GetLastError() documentation

The return value from the command is an object with various fields. The common fields are listed below; there may also be other fields.

• ok – true indicates the getLastError command completed successfully. This does NOT indicate there wasn’t a last error.
• err – if non-null, indicates an error occurred. Value is a textual description of the error.
• code – if set, indicates the error code which occurred. connectionId – the id of the connection
• lastOp – the op-id from the last operation

For updates:

• n – if an update was done, this is the number of documents updated.

So in this context, ‘get “n” field’ means get n which is the number of documents updated. Without “multi” being set to true it can only be either 0 or 1.