The Compute service must know the status of each compute node to
effectively manage and use them. This can include events like a user
launching a new VM, the scheduler sending a request to a live node, or a
query to the ServiceGroup API to determine if a node is live.

When a compute worker running the nova-compute daemon starts, it
calls the join API to join the compute group. Any service (such as the
scheduler) can query the group’s membership and the status of its nodes.
Internally, the ServiceGroup client driver automatically updates the
compute worker status.

Database ServiceGroup driver

By default, Compute uses the database driver to track if a node is
live. In a compute worker, this driver periodically sends a
db update command to the database, saying “I’m OK” with a
timestamp. Compute uses a pre-defined timeout
(service_down_time) to determine if a node is dead.

The driver has limitations, which can be problematic depending on
your environment. If a lot of compute worker nodes need to be checked,
the database can be put under heavy load, which can cause the timeout to
trigger, and a live node could incorrectly be considered dead. By
default, the timeout is 60 seconds. Reducing the timeout value can help
in this situation, but you must also make the database update more
frequently, which again increases the database workload.

The database contains data that is both transient (such as whether
the node is alive) and persistent (such as entries for VM owners). With
the ServiceGroup abstraction, Compute can treat each type
separately.

Memcache ServiceGroup driver

The memcache ServiceGroup driver uses memcached, a distributed memory
object caching system that is used to increase site performance. For
more details, see memcached.org.

To use the memcache driver, you must install memcached. You might
already have it installed, as the same driver is also used for the
OpenStack Object Storage and OpenStack dashboard. To install memcached,
see the Environment -> Memcached section in the Installation Tutorials
and Guides depending on your distribution.

These values in the /etc/nova/nova.conf file are
required on every node for the memcache driver:

# Driver for the ServiceGroup service
servicegroup_driver = "mc"

# Memcached servers. Use either a list of memcached servers to use for caching (list value),
# or "<None>" for in-process caching (default).
memcached_servers = <None>

# Timeout; maximum time since last check-in for up service (integer value).
# Helps to define whether a node is dead
service_down_time = 60