Microsoft's Approach to Big Data - Microsoft Big Data Solutions - page 26

Once one node has been defined as the coordinator, the other nodes act
as workhorses performing delegated tasks on data subsets as defined by
the coordinator. Each workhorse operates over data and provides interim
results back to the coordinator, who will be responsible for the final result
set.
The Impala daemons are in constant contact with the Statestore daemon to
see which nodes in the cluster are healthy and are accepting tasks.
Impala Statestore
TheStatestore isanother daemon known asstatestored. Itsjobistomonitor
all the Impala daemons, confirming their availability to perform tasks and
informing them of the health of other Impala daemons in the cluster. It
therefore helps to make sure that tasks are not assigned to a node that is
currently unreachable. This is important because Impala sacrifices runtime
resilience for speed. Unlike MapReduce, queries that experience a node
failure are canceled; so, the sooner the cluster knows about an issue, the
better.
Note that only one Statestore daemon is deployed on the cluster. However,
this is not an availability issue per se. This process is not critical to the
operation of Impala. The cluster does become susceptible to runtime
stability for query operation, but does not go offline.
Impala Catalog Service
The Catalog Service is the third daemon and is named catalogd. Its job is
to distribute metadata changes to all nodes in the cluster. Again, only one
Catalog Service daemon is in operation on the cluster, and it is commonly
deployed on the same node as the Statestore owing to the fact that it uses
the Statestore as the vehicle for transmitting its messages to the Impala
daemons.
The catalog service removes the need to issue REFRESH and INVALIDATE
METADATA statements, which would otherwise be required when using Data
Definition Language (DDL) or Data Modification Language (DML) in
Impala. By distributing metadata changes it ensures that any Impala
daemon can act as a coordinator without any additional actions on the part
of the calling application.