5. Liveliness

The Liveliness QoS can be used to ensure whether specific entities are alive or not. There are three values to specify the liveliness’ kind: AUTOMATIC, MANUAL_BY_PARTICIPANT or MANUAL_BY_TOPIC liveliness. If any of the first two is selected, a value for the lease duration and announcement period can be set. However, if MANUAL_BY_TOPIC is selected, only the lease duration can be configured, as the announcement period is not used with this configuration. With the AUTOMATIC liveliness kind, the service takes the responsibility for renewing the timer associated to the lease duration, and as long as the remote participant keeps running and remains connected, all the entities within that participant will be considered alive. The other two kinds (MANUAL_BY_PARTICIPANT and MANUAL_BY_TOPIC) need a periodic assertion to consider the remote participants as alive. Please refer to Fast DDS LivelinessQosPolicy Documentation for more information on Liveliness QoS.

In this test, a publisher and subscriber using AUTOMATIC liveliness will be created, and a lease duration value higher than the write rate of the publisher will be set.

5.1. Step-by-step example implementation

First, launch two instances and create a publisher and a subscriber:

1. Create a red square publisher:

   • Start eProsima Shapes Demo. (We will refer to this instance as Instance1)

   • Click on Publish.

   • Select SQUARE option for Shape and RED for Color.

   • Select AUTOMATIC for liveliness kind.

   • Set Lease Duration to 150. (The default write rate is 75 ms)

2. Create a square subscriber:

   • Start eProsima Shapes Demo. (We will refer to this instance as Instance2)

   • Click on Subscribe.

   • Select SQUARE option for Shape.

   • Select AUTOMATIC for liveliness kind.

   • Set a value for the Lease Duration higher or equal to the one stated for the publisher. (If the value of subscriber lease duration is lower the entities do not match)

The Output Tab of Instance2 shows that the subscriber has recovered the liveliness once it matches with the publisher.

Then, kill the process corresponding to the publisher (Instance1). As a result, the subscriber reported that liveliness was lost, as the publisher did not terminate cleanly.