public Builder setTombstoneTtl(com.google.protobuf.Duration value) {
      if (tombstoneTtlBuilder_ == null) {
        if (value == null) {
          throw new NullPointerException();
        }
        tombstoneTtl_ = value;
      } else {
        tombstoneTtlBuilder_.setMessage(value);
      }
      bitField0_ |= 0x00000100;
      onChanged();
      return this;
    }
<pre>
The task tombstone time to live (TTL).

After a task is deleted or executed, the task's tombstone is
retained for the length of time specified by `tombstone_ttl`.
The tombstone is used by task de-duplication; another task with the same
name can't be created until the tombstone has expired. For more information
about task de-duplication, see the documentation for
[CreateTaskRequest][google.cloud.tasks.v2beta3.CreateTaskRequest.task].

Queues created by Cloud Tasks have a default `tombstone_ttl` of 1 hour.
</pre>

<code>.google.protobuf.Duration tombstone_ttl = 9;</code>
java-tasks/proto-google-cloud-tasks-v2beta3/src/main/java/com/google/cloud/tasks/v2beta3/Queue.java