PNCounter

class PNCounter(client, service_name, name)

Bases: hazelcast.proxy.base.Proxy

PN (Positive-Negative) CRDT counter.

The counter supports adding and subtracting values as well as retrieving the current counter value. Each replica of this counter can perform operations locally without coordination with the other replicas, thus increasing availability. The counter guarantees that whenever two nodes have received the same set of updates, possibly in a different order, their state is identical, and any conflicting updates are merged automatically. If no new updates are made to the shared state, all nodes that can communicate will eventually have the same data.

When invoking updates from the client, the invocation is remote. This may lead to indeterminate state - the update may be applied but the response has not been received. In this case, the caller will be notified with a TargetDisconnectedError.

The read and write methods provide monotonic read and RYW (read-your-write) guarantees. These guarantees are session guarantees which means that if no replica with the previously observed state is reachable, the session guarantees are lost and the method invocation will throw a ConsistencyLostError. This does not mean that an update is lost. All of the updates are part of some replica and will be eventually reflected in the state of all other replicas. This exception just means that you cannot observe your own writes because all replicas that contain your updates are currently unreachable. After you have received a ConsistencyLostError, you can either wait for a sufficiently up-to-date replica to become reachable in which case the session can be continued or you can reset the session by calling the reset() method. If you have called the reset() method, a new session is started with the next invocation to a CRDT replica.

NOTE: The CRDT state is kept entirely on non-lite (data) members. If there aren’t any and the methods here are invoked on a lite member, they will fail with an NoDataMemberInClusterError.

Requires Hazelcast IMDG 3.10+.

get()

Returns the current value of the counter.

Raises:
Returns:

(int), the current value of the counter.
get_and_add(delta)

Adds the given value to the current value and returns the previous value.

Raises:
Parameters:

delta – (int), the value to add.
Returns:

(int), the previous value.
add_and_get(delta)

Adds the given value to the current value and returns the updated value.

Raises:
Parameters:

delta – (int), the value to add.
Returns:

(int), the updated value.
get_and_subtract(delta)

Subtracts the given value from the current value and returns the previous value.

Raises:
Parameters:

delta – (int), the value to subtract.
Returns:

(int), the previous value.
subtract_and_get(delta)

Subtracts the given value from the current value and returns the updated value.

Raises:
Parameters:

delta – (int), the value to subtract.
Returns:

(int), the updated value.
get_and_decrement()

Decrements the counter value by one and returns the previous value.

Raises:
Returns:

(int), the previous value.
decrement_and_get()

Decrements the counter value by one and returns the updated value.

Raises:
Returns:

(int), the updated value.
get_and_increment()

Increments the counter value by one and returns the previous value.

Raises:
Returns:

(int), the previous value.
increment_and_get()

Increments the counter value by one and returns the updated value.

Raises:
Returns:

(int), the updated value.
reset()

Resets the observed state by this PN counter. This method may be used after a method invocation has thrown a ConsistencyLostError to reset the proxy and to be able to start a new session.