Clustering

LXD can be run in clustering mode, where any number of LXD servers share the same distributed database and can be managed uniformly using the lxc client or the REST API.

Note that this feature was introduced as part of the API extension "clustering".

Forming a cluster

First you need to choose a bootstrap LXD node. It can be an existing LXD server or a brand new one. Then you need to initialize the bootstrap node and join further nodes to the cluster. This can be done interactively or with a preseed file.

Note that all further nodes joining the cluster must have identical configuration to the bootstrap node, in terms of storage pools and networks. The only configuration that can be node-specific are the source and size keys for storage pools and the bridge.external_interfaces key for networks.

It is strongly recommended that the number of nodes in the cluster be at least three, so the cluster can survive the loss of at least one node and still be able to establish quorum for its distributed state (which is kept in a SQLite database replicated using the Raft algorithm). If the number of nodes is less than three, then only one node in the cluster will store the SQLite database. When the third node joins the cluster, both the second and third nodes will receive a replica of the database.

Interactively

Run lxd init and answer yes to the very first question ("Would you like to use LXD clustering?"). Then choose a name for identifying the node, and an IP or DNS address that other nodes can use to connect to it, and answer no to the question about whether you're joining an existing cluster. Finally, optionally create a storage pool and a network bridge. At this point your first cluster node should be up and available on your network.

You can now join further nodes to the cluster. Note however that these nodes should be brand new LXD servers, or alternatively you should clear their contents before joining, since any existing data on them will be lost.

To add an additional node, run lxd init and answer yes to the question about whether to use clustering. Choose a node name that is different from the one chosen for the bootstrap node or any other nodes you have joined so far. Then pick an IP or DNS address for the node and answer yes to the question about whether you're joining an existing cluster. Pick an address of an existing node in the cluster and check the fingerprint that gets printed.

Preseed

Create a preseed file for the bootstrap node with the configuration you want, for example:

config:
  core.trust_password: sekret
  core.https_address: 10.55.60.171:8443
  images.auto_update_interval: 15
storage_pools:
- name: default
  driver: dir
networks:
- name: lxdbr0
  type: bridge
  config:
    ipv4.address: 192.168.100.14/24
    ipv6.address: none
profiles:
- name: default
  devices:
    root:
      path: /
      pool: default
      type: disk
    eth0:
      name: eth0
      nictype: bridged
      parent: lxdbr0
      type: nic
cluster:
  server_name: node1
  enabled: true

Then run cat <preseed-file> | lxd init --preseed and your first node should be bootstrapped.

Now create a bootstrap file for another node. You only need to fill in the cluster section with data and config values that are specific to the joining node.

Be sure to include the address and certificate of the target bootstrap node. To create a YAML-compatible entry for the cluster_certificate key you can use a command like sed ':a;N;$!ba;s/\n/\n\n/g' /var/lib/lxd/server.crt, which you have to run on the bootstrap node.

For example:

cluster:
  enabled: true
  server_name: node2
  server_address: 10.55.60.155:8443
  cluster_address: 10.55.60.171:8443
  cluster_certificate: "-----BEGIN CERTIFICATE-----

opyQ1VRpAg2sV2C4W8irbNqeUsTeZZxhLqp4vNOXXBBrSqUCdPu1JXADV0kavg1l

2sXYoMobyV3K+RaJgsr1OiHjacGiGCQT3YyNGGY/n5zgT/8xI0Dquvja0bNkaf6f

...

-----END CERTIFICATE-----
"
  cluster_password: sekret
  member_config:
  - entity: storage-pool
    name: default
    key: source
    value: ""

Managing a cluster

Once your cluster is formed you can see a list of its nodes and their status by running lxc cluster list. More detailed information about an individual node is available with lxc cluster show <node name>.

Voting and stand-by members

The cluster uses a distributed database to store its state. All nodes in the cluster need to access such distributed database in order to serve user requests.

If the cluster has many nodes, only some of them will be picked to replicate database data. Each node that is picked can replicate data either as "voter" or as "stand-by". The database (and hence the cluster) will remain available as long as a majority of voters is online. A stand-by node will automatically be promoted to voter when another voter is shutdown gracefully or when its detected to be offline.

The default number of voting nodes is 3 and the default number of stand-by nodes is 2. This means that your cluster will remain operation as long as you switch off at most one voting node at a time.

You can change the desired number of voting and stand-by nodes with:

lxc config set cluster.max_voters <n>

and

lxc config set cluster.max_standby <n>

with the constraint that the maximum number of voters must be odd and must be least 3, while the maximum number of stand-by nodes must be between 0 and 5.

Deleting nodes

To cleanly delete a node from the cluster use lxc cluster remove <node name>.

Offline nodes and fault tolerance

At each time there will be an elected cluster leader that will monitor the health of the other nodes. If a node is down for more than 20 seconds, its status will be marked as OFFLINE and no operation will be possible on it, as well as operations that require a state change across all nodes.

If the node that goes offline is the leader itself, the other nodes will elect a new leader.

As soon as the offline node comes back online, operations will be available again.

If you can't or don't want to bring the node back online, you can delete it from the cluster using lxc cluster remove --force <node name>.

You can tweak the amount of seconds after which a non-responding node will be considered offline by running:

lxc config set cluster.offline_threshold <n seconds>

The minimum value is 10 seconds.

Upgrading nodes

To upgrade a cluster you need to upgrade all of its nodes, making sure that they all upgrade to the same version of LXD.

To upgrade a single node, simply upgrade the lxd/lxc binaries on the host (via snap or other packaging systems) and restart the lxd daemon.

If the new version of the daemon has database schema or API changes, the restarted node might transition into a Blocked state. That happens if there are still nodes in the cluster that have not been upgraded and that are running an older version. When a node is in the Blocked state it will not serve any LXD API requests (in particular, lxc commands on that node will not work, although any running instance will continue to run).

You can see if some nodes are blocked by running lxc cluster list on a node which is not blocked.

As you proceed upgrading the rest of the nodes, they will all transition to the Blocked state, until you upgrade the very last one. At that point the blocked nodes will notice that there is no out-of-date node left and will become operational again.

Failure domains

Failure domains can be used to indicate which nodes should be given preference when trying to assign roles to a cluster member that has been shutdown or has crashed. For example, if a cluster member that currently has the database role gets shutdown, LXD will try to assign its database role to another cluster member in the same failure domain, if one is available.

To change the failure domain of a cluster member you can use the lxc cluster edit <member> command line tool, or the PUT /1.0/cluster/<member> REST API.

Recover from quorum loss

Every LXD cluster has up to 3 members that serve as database nodes. If you permanently lose a majority of the cluster members that are serving as database nodes (for example you have a 3-member cluster and you lose 2 members), the cluster will become unavailable. However, if at least one database node has survived, you will be able to recover the cluster.

In order to check which cluster members are configured as database nodes, log on any survived member of your cluster and run the command:

lxd cluster list-database

This will work even if the LXD daemon is not running.

Among the listed members, pick the one that has survived and log into it (if it differs from the one you have run the command on).

Now make sure the LXD daemon is not running and then issue the command:

lxd cluster recover-from-quorum-loss

At this point you can restart the LXD daemon and the database should be back online.

Note that no information has been deleted from the database, in particular all information about the cluster members that you have lost is still there, including the metadata about their instances. This can help you with further recovery steps in case you need to re-create the lost instances.

In order to permanently delete the cluster members that you have lost, you can run the command:

lxc cluster remove <name> --force

Note that this time you have to use the regular lxc command line tool, not lxd.

Instances

You can launch an instance on any node in the cluster from any node in the cluster. For example, from node1:

lxc launch --target node2 ubuntu:18.04 bionic

will launch an Ubuntu 18.04 container on node2.

When you launch an instance without defining a target, the instance will be launched on the server which has the lowest number of instances. If all the servers have the same amount of instances, it will choose one at random.

You can list all instances in the cluster with:

lxc list

The NODE column will indicate on which node they are running.

After an instance is launched, you can operate it from any node. For example, from node1:

lxc exec bionic ls /
lxc stop bionic
lxc delete bionic
lxc pull file bionic/etc/hosts .

Manually altering Raft membership

There might be situations in which you need to manually alter the Raft membership configuration of the cluster because some unexpected behavior occurred.

For example if you have a cluster member that was removed uncleanly it might not show up in lxc cluster list but still be part of the Raft configuration (you can see that with `lxd sql local "SELECT * FROM raft_nodes").

In that case you can run:

lxd cluster remove-raft-node <address>

to remove the leftover node.

Images

By default, LXD will replicate images on as many cluster members as you have database members. This typically means up to 3 copies within the cluster.

That number can be increased to improve fault tolerance and likelihood of the image being locally available.

The special value of "-1" may be used to have the image copied on all nodes.

You can disable the image replication in the cluster by setting the count down to 1:

lxc config set cluster.images_minimal_replica 1

Storage pools

As mentioned above, all nodes must have identical storage pools. The only difference between pools on different nodes might be their source, size or zfs.pool\_name configuration keys.

To create a new storage pool, you first have to define it across all nodes, for example:

lxc storage create --target node1 data zfs source=/dev/vdb1
lxc storage create --target node2 data zfs source=/dev/vdc1

Note that when defining a new storage pool on a node the only valid configuration keys you can pass are the node-specific ones mentioned above.

At this point the pool hasn't been actually created yet, but just defined (it's state is marked as Pending if you run lxc storage list).

Now run:

lxc storage create data zfs

and the storage will be instantiated on all nodes. If you didn't define it on a particular node, or a node is down, an error will be returned.

You can pass to this final storage create command any configuration key which is not node-specific (see above).

Storage volumes

Each volume lives on a specific node. The lxc storage volume list includes a NODE column to indicate on which node a certain volume resides.

Different volumes can have the same name as long as they live on different nodes (for example image volumes). You can manage storage volumes in the same way you do in non-clustered deployments, except that you'll have to pass a --target <node name> parameter to volume commands if more than one node has a volume with the given name.

For example:

# Create a volume on the node this client is pointing at
lxc storage volume create default web

# Create a volume with the same node on another node
lxc storage volume create default web --target node2

# Show the two volumes defined
lxc storage volume show default web --target node1
lxc storage volume show default web --target node2

Networks

As mentioned above, all nodes must have identical networks defined.

The only difference between networks on different nodes might be their optional configuration keys. When defining a new network on a specific clustered node the only valid optional configuration keys you can pass are bridge.external_interfaces and parent, as these can be different on each node (see documentation about network configuration for a definition of each).

To create a new network, you first have to define it across all nodes, for example:

lxc network create --target node1 my-network
lxc network create --target node2 my-network

At this point the network hasn't been actually created yet, but just defined (it's state is marked as Pending if you run lxc network list).

Now run:

lxc network create my-network

The network will be instantiated on all nodes. If you didn't define it on a particular node, or a node is down, an error will be returned.

You can pass to this final network create command any configuration key which is not node-specific (see above).

Separate REST API and clustering networks

You can configure different networks for the REST API endpoint of your clients and for internal traffic between the nodes of your cluster (for example in order to use a virtual address for your REST API, with DNS round robin).

To do that, you need to bootstrap the first node of the cluster using the cluster.https_address config key. For example, when using preseed:

config:
  core.trust_password: sekret
  core.https_address: my.lxd.cluster:8443
  cluster.https_address: 10.55.60.171:8443
...

(the rest of the preseed YAML is the same as above).

To join a new node, first set its REST API address, for instance using the lxc client:

lxc config set core.https_address my.lxd.cluster:8443

and then use the PUT /1.0/cluster API endpoint as usual, specifying the address of the joining node with the server_address field. If you use preseed, the YAML payload would be exactly like the one above.