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    How to Enable Optimistic Concurrency

    • By default, optimistic concurrency checks are disabled. Changes made outside of the session object will be overwritten. Concurrent changes to the same document will use the Last Write Wins strategy so a lost update anomaly is possible with the default configuration of the session.

    • Optimistic concurrency can be enabled for:

      • A specific document
      • A specific session (enable on a per-session basis)
      • All sessions (enable globally, at the document store level)

    • With optimistic concurrency enabled, RavenDB will generate a concurrency exception (and abort all modifications in the current transaction) when trying to save a document that has been modified on the server side after the client loaded and modified it.

    • The ConcurrencyException that might be thrown upon the save_changes call needs to be handled by the caller. The operation can be retried (the document needs to be reloaded since it got changed meanwhile) or handle the error in a way that is suitable in a given scenario.

    • In this page:

    • Note that the use_optimistic_concurrency setting only applies to documents that have been modified by the current session. E.g., if you load documents users/1-A and users/2-A in a session, make modifications only to users/1-A, and then call save_changes, the operation will succeed regardless of the optimistic concurrency setting, even if users/2-A has been changed by another process in the meantime.

    • However, if you modify both documents and attempt to save changes with optimistic concurrency enabled, an exception will be raised if users/2-A has been modified externally.
      In this case, the updates to both users/1-A and users/2-A will be cancelled.

    A detailed description of transactions and concurrency control in RavenDB is available here: Transaction support in RavenDB

    Enable for specific session 

    with store.open_session() as session:
    # Enable optimistic concurrency for this session
    session.advanced.use_optimistic_concurrency = True

    # Save a document in this session
    product = Product(name="Some name")
    session.store(product, "products/999")
    session.save_changes()

    # Modify the document 'externally' by another session
    with store.open_session() as other_session:
        other_product = other_session.load("products/999")
        other_product.name = "Other name"
        other_session.save_changes()

    # Trying to modify the document without reloading it first will throw
    product.name = "Better Name"
    session.save_changes()  # This will throw a ConcurrencyException

gion optimistic_concurrency_2
able for all sessions that will be opened within this document store
e.conventions.use_optimistic_concurrency = True
 store.open_session() as session:
is_session_using_optimistic_concurrency = session.advanced.use_optimistic_concurrency  # will return True

    • Enabling optimistic concurrency in a session will ensure that changes made to a document will only be persisted if the version of the document sent in the save_changes call matches its version from the time it was initially read (loaded from the server).

    • Note that it's necessary to enable optimistic concurrency for ALL sessions that modify the documents for which you want to guarantee that no writes will be silently discarded. If optimistic concurrency is enabled in some sessions but not in others, and they modify the same documents, the risk of the lost update anomaly still exists.

    Enable globally 

    # Enable for all sessions that will be opened within this document store
store.conventions.use_optimistic_concurrency = True
with store.open_session() as session:
    is_session_using_optimistic_concurrency = session.advanced.use_optimistic_concurrency  # will return True

    Disable for specific document (when enabled on session)

    • Optimistic concurrency can be disabled when storing a specific document,
      even when it is enabled for an entire session (or globally).

    • This is done by passing None as a change vector value to the store method.

    with store.open_session() as session:
    # Store document 'products/999'
    session.store(Product(name="Some name", Id="products/999"))
    session.save_changes()

with store.open_session() as session:
    # Enable optimistic concurrency for the session
    session.advanced.use_optimistic_concurrency = True

    # Store the same document
    # Pass 'null' as the change_vector to turn OFF optimistic concurrency for this document
    session.store(Product(name="Some Other Name"), change_vector=None, key="products/999")

    # This will NOT throw a ConcurrencyException, and the document will be saved
    session.save_changes()

    Enable for specific document (when disabled on session)

    • Optimistic concurrency can be enabled when storing a specific document,
      even when it is disabled for an entire session (or globally).

    • This is done by passing an empty str as the change vector value to the store method.
      Setting the change vector to an empty string will cause RavenDB to ensure that this document is a new one and doesn't already exist. A ConcurrencyException will be thrown if the document already exists.

    • If you do not provide a change vector or if the change vector is None, optimistic concurrency will be disabled.

    • Setting optimistic concurrency for a specific document overrides the use_optimistic_concurrency property from the advanced session operations.

    with store.open_session() as session:
    # Store document 'products/999'
    session.store(Product(name="Some name", Id="products/999"))
    session.save_changes()

with store.open_session() as session:
    # Disable optimistic concurrency for the session
    session.advanced.use_optimistic_concurrency = False

    # Store the same document
    # Pass empty str as the change_vector to turn ON optimistic concurrency for this document
    session.store(Product(name="Some Other Name"), key="products/999", change_vector="")

    # This will throw a ConcurrencyException, and the document will NOT be saved
    session.save_changes()

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