Storm in a Clustered Mode - Real-time Analytics with Storm and Cassandra - page 80

Here, we are specifying that for 5 executors dedicated to the spout, the total value
of numtasks is 10 , so two tasks each will be spawned on each of the executors.
What we see on the UI is a total of all numtasks values across all topology
components.
Section 2
This section holds the various actions that can be performed on the topology:
• Activate : The UI provides a facility to revive or reactivate a topology that has
been suspended earlier. Once activated, it can again start consuming the messages
from the spout and process them.
• Deactivate : When this action is executed, the topology immediately turns off the
spout, that is, no new messages are read from the spout and pushed downstream
to the DAG. Existing messages that are already being processed in various bolts
are processed completely.
• Rebalance : This action is executed when the worker allocation to a live topology
is altered.
• Kill : As the name suggests, this is used to send a termination signal for the topo-
logy to the Storm framework. It's always advisable to provide a reasonable kill
time so that the topology drains completely and is able to clean the pipelined
events before it terminates.
Section 3
This section displays a capture of the number of messages processed on the timeline. It
has the following key sections:
• Window : This field specifies the time duration in the following segments: last for
10 minutes, last 3 hours, the past day, or all the time. The topology's progress is
captured against these time sections.
• Emitted : This captures the number of tuples emitted by the spout at various time
segments.
• Transferred : This specifies the number of tuples sent to other components in the
topology. Please note that the number of emitted tuples may or may not be equal
to the number of transferred tuples as the former is the exact number of times the
emit method of the spout is executed, while the latter is the number transferred
based on the grouping used; for example, if we have bound a spout to a bolt that
has the parallelism of two tuples using all grouping, then for every x tuples emit-
ted by the spout, 2x tuples will be transferred.