Retrieving information from back end or remote systems might be slow and consume a lot of resources. You can use advisory locks on records in order to control access to these resources. Instead of letting any client access Couchbase Server and potentially overwhelm the server with high concurrent client requests, you could create an advisory lock to allow only one client at a time access the information.

You can create a lock in Couchbase by setting an expiration on specific item and by using the add() and delete() methods to access that named item. The add() and delete() methods are atomic, so you can be assured that only one client will become the advisory lock owner.

The first client that tries to add a named lock item with an expiration timeout will succeed. Other clients will see error responses to an add() command on that named lock item; they will know that some other client owns the named lock item. When the current lock owner is finished owning the lock, it can send an explicit delete() command on the named lock item to free the lock.

If a client that owns a lock crashes, the lock will automatically become available to the next client that requests for lock via add() after the expiration timeout.

As a convenience, several Couchbase SDKs provide get-and-lock as a single operation and single server request. This will accomplish the functional equivalent of adding a lock and deleting it. The following is an example from the Python SDK:

import uuid

key, value = str(uuid.uuid4()), str(uuid.uuid4())

client.set(key, 0, 0, value)
client.getl((key)[2], value)
client.set(key, 0, 0, value)

After we set the key and values to unique strings, we lock the key. The subsequent set() request will fail and return an error.