openstack-manuals/doc/admin-guide/source/compute-service-groups.rst

Configure Compute service groups

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.

ZooKeeper ServiceGroup driver

The ZooKeeper ServiceGroup driver works by using ZooKeeper ephemeral nodes. ZooKeeper, unlike databases, is a distributed system, with its load divided among several servers. On a compute worker node, the driver can establish a ZooKeeper session, then create an ephemeral znode in the group directory. Ephemeral znodes have the same lifespan as the session. If the worker node or the nova-compute daemon crashes, or a network partition is in place between the worker and the ZooKeeper server quorums, the ephemeral znodes are removed automatically. The driver can be given group membership by running the ls command in the group directory.

The ZooKeeper driver requires the ZooKeeper servers and client libraries. Setting up ZooKeeper servers is outside the scope of this guide (for more information, see Apache Zookeeper). These client-side Python libraries must be installed on every compute node:

python-zookeeper

The official Zookeeper Python binding

evzookeeper

This library makes the binding work with the eventlet threading model.

This example assumes the ZooKeeper server addresses and ports are 192.168.2.1:2181, 192.168.2.2:2181, and 192.168.2.3:2181.

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

# Driver for the ServiceGroup service
servicegroup_driver="zk"

[zookeeper]
address="192.168.2.1:2181,192.168.2.2:2181,192.168.2.3:2181"

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. If you need to install memcached, see the instructions in the OpenStack Installation Guide.

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