Sample code showing how to use the MongoDB Java drivers to make updates to documents in an environment where there is potential for simultaneous updates by multiple clients to overwrite one another.
We update the documents using updateMany in three phases:
Firstly we try to set a lock on the documents to be updated by adding our process ID to the "lockStatus" field on each document to be updated, but only if the current value of the "lockStatus" field is set to "unlocked". As part of this first stage we also add a timestamp to show when the lock was set.
Next, we look at the result object returned by the initial "lock" update and verify the number of documents that were updated matches what we were expecting. If it is less than what we were expecting, it would indicate that another process has one or more documents locked, and we should try again, possibly with an appropriate wait period first.
If the number of documents marked as locked matches what we expected, we can then go ahead and make our updates. Depending on the nature of the updates and how you calculate the updated values, you might need to retrieve the documents first to ensure you have the latest version of any fields used to calculate the update values.
Finally, we do a final update to unlock the documents. We do this in the 'finally' clause of a try block to make sure the unlock happens even in the event of an exception occurring.
This process is reliant on the fact that an update to a document in MongoDB is always ACIDic. Note also that if an exception is thrown by the UpdateMany command, none of the matching documents are updated.
Using the processID of the locking process and adding a lockTimestamp will allow you to add a cleanup process to handle a client process dying before removing its locks. The cleanup process should periodically look for documents locked by a process that no longer exists, or has been locked for longer than a reasonable timeout value.
For this example, we assume we're working with a collection called UpdateExamples containing the following documents:
{"_id": 1, "lockStatus": "unlocked", "lockTS": null, "dataPayload": "" },
{"_id": 2, "lockStatus": "unlocked", "lockTS": null, "dataPayload": "" },
{"_id": 3, "lockStatus": "unlocked", "lockTS": null, "dataPayload": "" },
{"_id": 4, "lockStatus": "unlocked", "lockTS": null, "dataPayload": "" },
{"_id": 5, "lockStatus": "unlocked", "lockTS": null, "dataPayload": "" }