Java access to MongoDb Lock/Unlock Concurrency

[Han Chen]

I read some articles and they tell me MongoDb has Lock/Unlock Concurrency feature itself. So for example I have 2 Java instances access to one MongoDb to read & update data, each of them does the same thing:

    1. Query all the qualified data from MongoDb  [read]

       (using DBCursor cursor = collection.find(query))

    2. Do some calculation and update/write back to MongoDb (maybe all of them or 

       one of them) [write]

I want to make sure all the qualified data which query from the MongoDb should be "locked" because they might be updated/changed later; if the other Java instance accesses to the same data at the same time, it will return the wrong result. So all of them must be accessed by only one instance at the same time until the update/write process is done (Step 2). 

How does MongoDb handle this situation above? Does it have some features to achieve it itself automatically or I have to do it manually (using other application such as zookeeper)? 

[Justin Lee]

You will have to manage that concurrency yourself.

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[Han Chen]

OK. Do you have advise about how to manage this concurrency?

[Karl Yang]

Maybe you can set all your operations going through only one object, and give this object a lock.

[Asya Kamsky]

One possible way is to set a field on a document when you are editing it, and then make the save conditional on that field not changing *or* don't allow another thread to read this object for editing if that field is set (but be careful of processes that start editing the object but never finish).

Asya