Overview

This guide covers various automation-oriented cluster formation and peer discovery features. For a general overview of RabbitMQ clustering, please refer to the Clustering Guide.

This guide assumes general familiarity with RabbitMQ clustering and focuses on the peer discovery subsystem. For example, it will not cover what ports must be open for inter-node communication, how nodes authenticate to each other, and so on. Besides discovery mechanisms and their configuration, this guide also covers a closely related topic of rejoining nodes, the problem of initial cluster formation with nodes booting in parallel as well as additional health checks offered by some discovery implementations.

The guide also covers the basics of peer discovery troubleshooting.

What is Peer Discovery?

To form a cluster, new ("blank") nodes need to be able to discover their peers. This can be done using a variety of mechanisms (backends). Some mechanisms assume all cluster members are known ahead of time (for example, listed in the config file), others are dynamic (nodes can come and go).

All peer discovery mechanisms assume that newly joining nodes will be able to contact their peers in the cluster and authenticate with them successfully. The mechanisms that rely on an external service (e.g. DNS or Consul) or API (e.g. AWS or Kubernetes) require the service(s) or API(s) to be available and reachable on their standard ports. Inability to reach the services will lead to node's inability to join the cluster.

Available Discovery Mechanisms

The following mechanisms are built into the core and always available:

Additional peer discovery mechanisms are available via plugins. The following peer discovery plugins ship with supported RabbitMQ versions:

The above plugins do not need to be installed but like all plugins they must be enabled or preconfigured before they can be used.

For peer discovery plugins, which must be available on node boot, this means they must be enabled before first node boot. The example below uses rabbitmq-plugins' --offline mode:

rabbitmq-plugins --offline enable <plugin name>

A more specific example:

rabbitmq-plugins --offline enable rabbitmq_peer_discovery_k8s

A node with configuration settings that belong a non-enabled peer discovery plugin will fail to start and report those settings as unknown.

Specifying the Peer Discovery Mechanism

The discovery mechanism to use is specified in the config file, as are various mechanism-specific settings, for example, discovery service hostnames, credentials, and so on. cluster_formation.peer_discovery_backend is the key that controls what discovery module (implementation) is used:

cluster_formation.peer_discovery_backend = classic_config

# The backend can also be specified using its module name. Note that
# module names do not necessarily match plugin names exactly.
# cluster_formation.peer_discovery_backend = rabbit_peer_discovery_classic_config

The module has to implement the rabbit_peer_discovery_backend behaviour. Plugins therefore can introduce their own discovery mechanisms.

How Peer Discovery Works

When a node starts and detects it doesn't have a previously initialised database, it will check if there's a peer discovery mechanism configured. If that's the case, it will then perform the discovery and attempt to contact each discovered peer in order. Finally, it will attempt to join the cluster of the first reachable peer.

Depending on the backend (mechanism) used, the process of peer discovery may involve contacting external services, for example, an AWS API endpoint, a Consul node or performing a DNS query. Some backends require nodes to register (tell the backend that the node is up and should be counted as a cluster member): for example, Consul and etcd both support registration. With other backends the list of nodes is configured ahead of time (e.g. config file). Those backends are said to not support node registration.

In some cases node registration is implicit or managed by an external service. AWS autoscaling groups is a good example: AWS keeps track of group membership, so nodes don't have to (or cannot) explicitly register. However, the list of cluster members is not predefined. Such backends usually include a no-op registration step and apply one of the race condition mitigation mechanisms described below.

When the configured backend supports registration, nodes unregister when they stop.

If peer discovery isn't configured, or it repeatedly fails, or no peers are reachable, a node that wasn't a cluster member in the past will initialise from scratch and proceed as a standalone node. Peer discovery progress and outcomes will be logged by the node.

If a node previously was a cluster member, it will try to contact and rejoin its "last seen" peer for a period of time. In this case, no peer discovery will be performed. This is true for all backends.

Nodes Rejoining Their Existing Cluster

A new node joining a cluster is just one possible case. Another common scenario is when an existing cluster member temporarily leaves and then rejoins the cluster. While the peer discovery subsystem does not affect the behavior described in this section, it's important to understand how nodes behave when they rejoin their cluster after a restart or failure.

Existing cluster members will not perform peer discovery. Instead they will try to contact their previously known peers.

If a node previously was a cluster member, when it boots it will try to contact its "last seen" peer for a period of time. If the peer is not booted (e.g. when a full cluster restart or upgrade is performed) or cannot be reached, the node will retry the operation a number of times.

Default values are 10 retries and 30 seconds per attempt, respectively, or 5 minutes total. In environments where nodes can take a long and/or uneven time to start it is recommended that the number of retries is increased.

If a node is reset since losing contact with the cluster, it will behave like a blank node. Note that other cluster members might still consider it to be a cluster member, in which case the two sides will disagree and the node will fail to join. Such reset nodes must also be removed from the cluster using rabbitmqctl forget_cluster_node executed against an existing cluster member.

If a node was explicitly removed from the cluster by the operator and then reset, it will be able to join the cluster as a new member. In this case it will behave exactly like a blank node would.

A node rejoining after a node name or host name change can start as a blank node if its data directory path changes as a result. Such nodes will fail to rejoin the cluster. While the node is offline, its peers can be reset or started with a blank data directory. In that case the recovering node will fail to rejoin its peer as well since internal data store cluster identity would no longer match.

Consider the following scenario:

  1. A cluster of 3 nodes, A, B and C is formed
  2. Node A is shut down
  3. Node B is reset
  4. Node A is started
  5. Node A tries to rejoin B but B's cluster identity has changed
  6. Node B doesn't recognise A as a known cluster member because it's been reset

in this case node B will reject the clustering attempt from A with an appropriate error message in the log:

Node 'rabbit@node1.local' thinks it's clustered with node 'rabbit@node2.local', but 'rabbit@node2.local' disagrees

In this case B can be reset again and then will be able to join A, or A can be reset and will successfully join B.

How to Configure Peer Discovery

Peer discovery plugins are configured just like the core server and other plugins: using a config file.

cluster_formation.peer_discovery_backend is the key that controls what peer discovery backend will be used. Each backend will also have a number of configuration settings specific to it. The rest of the guide will cover configurable settings specific to a particular mechanism as well as provide examples for each one.

Config File Peer Discovery Backend

Config File Peer Discovery Overview

The most basic way for a node to discover its cluster peers is to read a list of nodes from the config file. The set of cluster members is assumed to be known at deployment time.

Configuration

The peer nodes are listed using the cluster_formation.classic_config.nodes config setting:

cluster_formation.peer_discovery_backend = classic_config

# the backend can also be specified using its module name
# cluster_formation.peer_discovery_backend = rabbit_peer_discovery_classic_config

cluster_formation.classic_config.nodes.1 = rabbit@hostname1.eng.example.local
cluster_formation.classic_config.nodes.2 = rabbit@hostname2.eng.example.local

DNS Peer Discovery Backend

DNS Peer Discovery Overview

Another built-in peer discovery mechanism as of RabbitMQ 3.7.0 is DNS-based. It relies on a pre-configured hostname ("seed hostname") with DNS A (or AAAA) records and reverse DNS lookups to perform peer discovery. More specifically, this mechanism will perform the following steps:

  1. Query DNS A records of the seed hostname.
  2. For each returned DNS record's IP address, perform a reverse DNS lookup.
  3. Append current node's prefix (e.g. rabbit in rabbit@hostname1.example.local) to each hostname and return the result.

For example, let's consider a seed hostname of discovery.eng.example.local. It has 2 DNS A records that return two IP addresses: 192.168.100.1 and 192.168.100.2. Reverse DNS lookups for those IP addresses return node1.eng.example.local and node2.eng.example.local, respectively. Current node's name is not set and defaults to rabbit@$(hostname). The final list of nodes discovered will contain two nodes: rabbit@node1.eng.example.local and rabbit@node2.eng.example.local.

Configuration

The seed hostname is set using the cluster_formation.dns.hostname config setting:

cluster_formation.peer_discovery_backend = dns

# the backend can also be specified using its module name
# cluster_formation.peer_discovery_backend = rabbit_peer_discovery_dns

cluster_formation.dns.hostname = discovery.eng.example.local

Peer Discovery on AWS (EC2)

AWS Peer Discovery Overview

An AWS (EC2)-specific discovery mechanism is available via a plugin.

As with any plugin, it must be enabled before it can be used. For peer discovery plugins it means they must be enabled or preconfigured before first node boot:

rabbitmq-plugins --offline enable rabbitmq_peer_discovery_aws

The plugin provides two ways for a node to discover its peers:

  • Using EC2 instance tags
  • Using AWS autoscaling group membership

Both methods rely on AWS-specific APIs (endpoints) and features and thus cannot work in other IaaS environments. Once a list of cluster member instances is retrieved, final node names are computed using instance hostnames or IP addresses.

Configuration and Credentials

Before a node can perform any operations on AWS, it needs to have a set of AWS account credentials configured. This can be done in a couple of ways:

  1. Via config file
  2. Using environment variables AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY

EC2 Instance Metadata service for the region will also be consulted.

The following example snippet configures RabbitMQ to use the AWS peer discovery backend and provides information about AWS region as well as a set of credentials:

cluster_formation.peer_discovery_backend = aws

# the backend can also be specified using its module name
# cluster_formation.peer_discovery_backend = rabbit_peer_discovery_aws

cluster_formation.aws.region = us-east-1
cluster_formation.aws.access_key_id = ANIDEXAMPLE
cluster_formation.aws.secret_key = WjalrxuTnFEMI/K7MDENG+bPxRfiCYEXAMPLEKEY

If region is left unconfigured, us-east-1 will be used by default. Sensitive values in configuration file can optionally be encrypted.

If an IAM role is assigned to EC2 instances running RabbitMQ nodes, a policy has to be used to allow said instances use EC2 Instance Metadata Service. When the plugin is configured to use Autoscaling group members, a policy has to grant access to describe autoscaling group members (instances). Below is an example of a policy that covers both use cases:

{
"Version": "2012-10-17",
"Statement": [
              {
              "Effect": "Allow",
              "Action": [
                         "autoscaling:DescribeAutoScalingInstances",
                         "ec2:DescribeInstances"
                         ],
              "Resource": [
                           "*"
                           ]
              }
              ]
}

Using Autoscaling Group Membership

When autoscaling-based peer discovery is used, current node's EC2 instance autoscaling group members will be listed and used to produce the list of discovered peers.

To use autoscaling group membership, set the cluster_formation.aws.use_autoscaling_group key to true:

cluster_formation.peer_discovery_backend = aws

cluster_formation.aws.region = us-east-1
cluster_formation.aws.access_key_id = ANIDEXAMPLE
cluster_formation.aws.secret_key = WjalrxuTnFEMI/K7MDENG+bPxRfiCYEXAMPLEKEY

cluster_formation.aws.use_autoscaling_group = true

Using EC2 Instance Tags

When tags-based peer discovery is used, the plugin will list EC2 instances using EC2 API and filter them by configured instance tags. Resulting instance set will be used to produce the list of discovered peers.

Tags are configured using the cluster_formation.aws.instance_tags key. The example below uses three tags: region, service, and environment.

cluster_formation.peer_discovery_backend = aws

cluster_formation.aws.region = us-east-1
cluster_formation.aws.access_key_id = ANIDEXAMPLE
cluster_formation.aws.secret_key = WjalrxuTnFEMI/K7MDENG+bPxRfiCYEXAMPLEKEY

cluster_formation.aws.instance_tags.region = us-east-1
cluster_formation.aws.instance_tags.service = rabbitmq
cluster_formation.aws.instance_tags.environment = staging

Using Private EC2 Instance IPs

By default peer discovery will use private DNS hostnames to compute node names. This option is most convenient and is highly recommended.

However, it is possible to opt into using private IPs instead by setting the cluster_formation.aws.use_private_ip key to true. For this setup to work, RABBITMQ_NODENAME must be set to the private IP address at node deployment time.

RABBITMQ_USE_LOGNAME also has to be set to true or an IP address won't be considered a valid part of node name.

cluster_formation.peer_discovery_backend = aws

cluster_formation.aws.region = us-east-1
cluster_formation.aws.access_key_id = ANIDEXAMPLE
cluster_formation.aws.secret_key = WjalrxuTnFEMI/K7MDENG+bPxRfiCYEXAMPLEKEY

cluster_formation.aws.use_autoscaling_group = true
cluster_formation.aws.use_private_ip = true

Peer Discovery on Kubernetes

Kubernetes Peer Discovery Overview

A Kubernetes-based discovery mechanism is available via a plugin.

As with any plugin, it must be enabled before it can be used. For peer discovery plugins it means they must be enabled or preconfigured before first node boot:

rabbitmq-plugins --offline enable rabbitmq_peer_discovery_k8s

Important: Prerequisites and Deployment Considerations

With this mechanism, nodes fetch a list of their peers from a Kubernetes API endpoint using a set of configured values: a URI scheme, host, port, as well as the token and certificate paths.

There are several prerequisites and deployment choices that must be taken into account when deploying RabbitMQ to Kubernetes, with this peer discovery mechanism and in general.

Use a Stateful Set

A RabbitMQ cluster deployed to Kubernetes will use a set of pods. The set must be a stateful set. A headless service must be used to control network identity of the pods (their hostnames), which in turn affect RabbitMQ node names. On the headless service spec, field publishNotReadyAddresses must be set to true to propagate SRV DNS records for its Pods for the purpose of peer discovery.

If a stateless set is used recreated nodes will not have their persisted data and will start as blank nodes. This can lead to data loss and higher network traffic volume due to more frequent data synchronisation of both quorum queues and classic queue mirrors on newly joining nodes.

Use Persistent Volumes

How storage is configured is generally orthogonal to peer discovery. However, it does not make sense to run a stateful data service such as RabbitMQ with node data directory stored on a transient volume. Use of transient volumes can lead nodes to not have their persisted data after a restart. This has the same consequences as with stateless sets covered above.

Make Sure /etc/rabbitmq is Mounted as Writeable

RabbitMQ nodes and images may need to update a file under /etc/rabbitmq, the default configuration file location on Linux. This may involve configuration file generation performed by the image used, enabled plugins file updates, and so on.

It is therefore highly recommended that /etc/rabbitmq is mounted as writeable and owned by RabbitMQ's effective user (typically rabbitmq).

Use Most Basic Health Checks for RabbitMQ Pod Readiness Probes

A readiness probe that expects the node to be fully booted and have rejoined its cluster peers can prevent a deployment that restarts all RabbitMQ pods and relies on the OrderedReady pod management policy. Deployments that use the Parallel pod management policy will not be affected.

One health check that does not expect a node to be fully booted and have schema tables synced is

# a very basic check that will succeed for the nodes that are currently waiting for
# a peer to sync schema from
rabbitmq-diagnostics ping

This basic check would allow the deployment to proceed and the nodes to eventually rejoin each other, assuming they are compatible.

See Schema Syncing from Online Peers in the Clustering guide.

Examples

A minimalistic runnable example of Kubernetes peer discovery mechanism can be found on GitHub.

The example can be run using either MiniKube or Kind.

Configuration

To use Kubernetes for peer discovery, set the cluster_formation.peer_discovery_backend to k8s or kubernetes or its module name, rabbit_peer_discovery_k8s (note: the name of the module is slightly different from plugin name):

cluster_formation.peer_discovery_backend = k8s

# the backend can also be specified using its module name
# cluster_formation.peer_discovery_backend = rabbit_peer_discovery_k8s

# Kubernetes API hostname (or IP address). Default value is kubernetes.default.svc.cluster.local
cluster_formation.k8s.host = kubernetes.default.example.local

It is possible to configure Kubernetes API port and URI scheme:

cluster_formation.peer_discovery_backend = k8s

cluster_formation.k8s.host = kubernetes.default.example.local
# 443 is used by default
cluster_formation.k8s.port = 443
# https is used by default
cluster_formation.k8s.scheme = https

Kubernetes token file path is configurable via cluster_formation.k8s.token_path:

cluster_formation.peer_discovery_backend = k8s

cluster_formation.k8s.host = kubernetes.default.example.local
# default value is /var/run/secrets/kubernetes.io/serviceaccount/token
cluster_formation.k8s.token_path = /var/run/secrets/kubernetes.io/serviceaccount/token

It must point to a local file that exists and is readable by RabbitMQ.

Certificate and namespace paths use cluster_formation.k8s.cert_path and cluster_formation.k8s.namespace_path, respectively:

cluster_formation.peer_discovery_backend = k8s

cluster_formation.k8s.host = kubernetes.default.example.local
# default value is /var/run/secrets/kubernetes.io/serviceaccount/token
cluster_formation.k8s.token_path = /var/run/secrets/kubernetes.io/serviceaccount/token

# default value is /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
cluster_formation.k8s.cert_path = /var/run/secrets/kubernetes.io/serviceaccount/ca.crt

# default value is /var/run/secrets/kubernetes.io/serviceaccount/namespace
cluster_formation.k8s.namespace_path = /var/run/secrets/kubernetes.io/serviceaccount/namespace

Just like with the token path key both must point to a local file that exists and is readable by RabbitMQ.

When a list of peer nodes is computed from a list of pod containers returned by Kubernetes, either hostnames or IP addresses can be used. This is configurable using the cluster_formation.k8s.address_type key:

cluster_formation.peer_discovery_backend = k8s

cluster_formation.k8s.host = kubernetes.default.example.local

cluster_formation.k8s.token_path = /var/run/secrets/kubernetes.io/serviceaccount/token
cluster_formation.k8s.cert_path = /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
cluster_formation.k8s.namespace_path = /var/run/secrets/kubernetes.io/serviceaccount/namespace

# should result set use hostnames or IP addresses
# of Kubernetes API-reported containers?
# supported values are "hostname" and "ip"
cluster_formation.k8s.address_type = hostname

Supported values are ip or hostname. hostname is the recommended option but has limitations: it can only be used with stateful sets (also highly recommended) and headless services. ip is used by default for better compatibility.

It is possible to append a suffix to peer hostnames returned by Kubernetes using cluster_formation.k8s.hostname_suffix:

cluster_formation.peer_discovery_backend = k8s

cluster_formation.k8s.host = kubernetes.default.example.local

cluster_formation.k8s.token_path = /var/run/secrets/kubernetes.io/serviceaccount/token
cluster_formation.k8s.cert_path = /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
cluster_formation.k8s.namespace_path = /var/run/secrets/kubernetes.io/serviceaccount/namespace

# no suffix is appended by default
cluster_formation.k8s.hostname_suffix = rmq.eng.example.local

Service name is rabbitmq by default but can be overridden using the cluster_formation.k8s.service_name key if needed:

cluster_formation.peer_discovery_backend = k8s

cluster_formation.k8s.host = kubernetes.default.example.local

cluster_formation.k8s.token_path = /var/run/secrets/kubernetes.io/serviceaccount/token
cluster_formation.k8s.cert_path = /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
cluster_formation.k8s.namespace_path = /var/run/secrets/kubernetes.io/serviceaccount/namespace

# overrides Kubernetes service name. Default value is "rabbitmq".
cluster_formation.k8s.service_name = rmq-qa

Peer Discovery Using Consul

Consul Peer Discovery Overview

A Consul-based discovery mechanism is available via a plugin.

As with any plugin, it must be enabled before it can be used. For peer discovery plugins it means they must be enabled or preconfigured before first node boot:

rabbitmq-plugins --offline enable rabbitmq_peer_discovery_consul

The plugin supports Consul 0.8.0 and later versions.

Nodes register with Consul on boot and unregister when they leave. Prior to registration, nodes will attempt to acquire a lock in Consul to reduce the probability of a race condition during initial cluster formation. When a node registers with Consul, it will set up a periodic health check for itself (more on this below).

Configuration

To use Consul for peer discovery, set the cluster_formation.peer_discovery_backend to consul or its module name, rabbit_peer_discovery_consul (note: the name of the module is slightly different from plugin name):

cluster_formation.peer_discovery_backend = consul

# the backend can also be specified using its module name
# cluster_formation.peer_discovery_backend = rabbit_peer_discovery_consul

# Consul host (hostname or IP address). Default value is localhost
cluster_formation.consul.host = consul.eng.example.local

Consul Endpoint

It is possible to configure Consul port and URI scheme:

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local
# 8500 is used by default
cluster_formation.consul.port = 8500
# http is used by default
cluster_formation.consul.scheme = http

Consul ACL Token

To configure Consul ACL token, use cluster_formation.consul.acl_token:

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local
cluster_formation.consul.acl_token = acl-token-value

Service name (as registered in Consul) defaults to "rabbitmq" but can be overridden:

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local
# rabbitmq is used by default
cluster_formation.consul.svc = rabbitmq

Service Address

Service hostname (address) as registered in Consul will be fetched by peers and therefore must resolve on all nodes. The hostname can be computed by the plugin or specified by the user. When computed automatically, a number of nodes and OS properties can be used:

  • Hostname (as returned by gethostname(2))
  • Node name (without the rabbit@ prefix)
  • IP address of an NIC (network controller interface)

When cluster_formation.consul.svc_addr_auto is set to false, service name will be taken as is from cluster_formation.consul.svc_addr. When it is set to true, other options explained below come into play.

In the following example, the service address reported to Consul is hardcoded to hostname1.rmq.eng.example.local instead of being computed automatically from the environment:

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local

cluster_formation.consul.svc = rabbitmq
# do not compute service address, it will be specified below
cluster_formation.consul.svc_addr_auto = false
# service address, will be communicated to other nodes
cluster_formation.consul.svc_addr = hostname1.rmq.eng.example.local
# use long RabbitMQ node names?
cluster_formation.consul.use_longname = true

In this example, the service address reported to Consul is parsed from node name (the rabbit@ prefix will be dropped):

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local

cluster_formation.consul.svc = rabbitmq
# do compute service address
cluster_formation.consul.svc_addr_auto = true
# compute service address using node name
cluster_formation.consul.svc_addr_use_nodename = true
# use long RabbitMQ node names?
cluster_formation.consul.use_longname = true

cluster_formation.consul.svc_addr_use_nodename is a boolean field that instructs Consul peer discovery backend to compute service address using RabbitMQ node name.

In the next example, the service address is computed using hostname as reported by the OS instead of node name:

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local

cluster_formation.consul.svc = rabbitmq
# do compute service address
cluster_formation.consul.svc_addr_auto = true
# compute service address using host name and not node name
cluster_formation.consul.svc_addr_use_nodename = false
# use long RabbitMQ node names?
cluster_formation.consul.use_longname = true

In the example below, the service address is computed by taking the IP address of a provided NIC, en0:

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local

cluster_formation.consul.svc = rabbitmq
# do compute service address
cluster_formation.consul.svc_addr_auto = true
# compute service address using the IP address of a NIC, en0
cluster_formation.consul.svc_addr_nic = en0
cluster_formation.consul.svc_addr_use_nodename = false
# use long RabbitMQ node names?
cluster_formation.consul.use_longname = true

Service Port

Service port as registered in Consul can be overridden. This is only necessary if RabbitMQ uses a non-standard port for client (technically AMQP 0-9-1 and AMQP 1.0) connections since default value is 5672.

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local
# 5672 is used by default
cluster_formation.consul.svc_port = 6674

Service Tags and Metadata

It is possible to provide Consul service tags:

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local
# Define tags for the RabbitMQ service: "qa" and "3.8"
cluster_formation.consul.svc_tags.1 = qa
cluster_formation.consul.svc_tags.2 = 3.8

It is possible to configure Consul service metadata, which is a map of string keys to string values with certain restrictions (see Consul documentation to learn more):

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local

# Define metadata for the RabbitMQ service. Both keys and values have a
# maximum length limit enforced by Consul. This can be used to provide additional
# context about the service (RabbitMQ cluster) for operators or other tools.
cluster_formation.consul.svc_meta.owner = team-xyz
cluster_formation.consul.svc_meta.service = service-one
cluster_formation.consul.svc_meta.stats_url = https://service-one.eng.megacorp.local/stats/

Service Health Checks

When a node registers with Consul, it will set up a periodic health check for itself. Online nodes will periodically send a health check update to Consul to indicate the service is available. This interval can be configured:

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local
# health check interval (node TTL) in seconds
# default: 30
cluster_formation.consul.svc_ttl = 40

A node that failed its health check is considered to be in the warning state by Consul. Such nodes can be automatically unregistered by Consul after a period of time (note: this is a separate interval value from the TTL above). The period cannot be less than 60 seconds.

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local
# health check interval (node TTL) in seconds
cluster_formation.consul.svc_ttl = 30
# how soon should nodes that fail their health checks be unregistered by Consul?
# this value is in seconds and must not be lower than 60 (a Consul requirement)
cluster_formation.consul.deregister_after = 90

Please see a section on automatic cleanup of nodes below.

Nodes in the warning state are excluded from peer discovery results by default. It is possible to opt into including them by setting cluster_formation.consul.include_nodes_with_warnings to true:

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local
# health check interval (node TTL) in seconds
cluster_formation.consul.svc_ttl = 30
# include node in the warning state into discovery result set
cluster_formation.consul.include_nodes_with_warnings = true

Node Name Suffixes

If node name is computed and long node names are used, it is possible to append a suffix to node names retrieved from Consul. The format is .node.. This can be useful in environments with DNS conventions, e.g. when all service nodes are organised in a separate subdomain. Here's an example:

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local

cluster_formation.consul.svc = rabbitmq
# do compute service address
cluster_formation.consul.svc_addr_auto = true
# compute service address using node name
cluster_formation.consul.svc_addr_use_nodename = true
# use long RabbitMQ node names?
cluster_formation.consul.use_longname = true
# append a suffix (node.rabbitmq.example.local) to node names retrieved from Consul
cluster_formation.consul.domain_suffix = example.local

With this setup node names will be computed to rabbit@192.168.100.1.node.example.local instead of rabbit@192.168.100.1.

Distributed Lock Acquisition

When a node tries to acquire a lock on boot and the lock is already taken, it will wait for the lock to become available for a limited amount of time. Default value is 300 seconds but it can be configured:

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local
# lock acquisition timeout in seconds
# default: 300
# cluster_formation.consul.lock_wait_time is an alias
cluster_formation.consul.lock_timeout = 60

Lock key prefix is rabbitmq by default. It can also be overridden:

cluster_formation.peer_discovery_backend = consul

cluster_formation.consul.host = consul.eng.example.local
cluster_formation.consul.lock_timeout = 60
# should the Consul key used for locking be prefixed with something
# other than "rabbitmq"?
cluster_formation.consul.lock_prefix = environments-qa

Peer Discovery Using Etcd

Etcd Peer Discovery Overview

An etcd-based discovery mechanism is available via a plugin.

As of RabbitMQ 3.8.4, the plugin uses a v3 API, gRPC-based etcd client and requires etcd 3.4 or a later version.

As with any plugin, it must be enabled before it can be used. For peer discovery plugins it means they must be enabled or preconfigured before first node boot:

rabbitmq-plugins --offline enable rabbitmq_peer_discovery_etcd

Nodes register with etcd on boot by creating a key in a conventionally named directory. The keys have a short (say, a minute) expiration period. The keys are deleted when nodes stop cleanly.

Prior to registration, nodes will attempt to acquire a lock in etcd to reduce the probability of a race condition during initial cluster formation.

Every node's key has an associated lease with a configurable TTL. Nodes keep their key's leases alive. If a node loses connectivity and cannot update its lease, its key will be cleaned up by etcd after TTL expires. Such nodes won't be discovered by newly joining nodes. If configured, such nodes can be forcefully removed from the cluster.

Configuration

etcd Endpoints and Authentication

To use etcd for peer discovery, set the cluster_formation.peer_discovery_backend to etcd or its module name, rabbit_peer_discovery_etcd (note: the name of the module is slightly different from plugin name).

The plugin requires a configured etcd endpoint for the plugin to connect to:

cluster_formation.peer_discovery_backend = etcd

# the backend can also be specified using its module name
# cluster_formation.peer_discovery_backend = rabbit_peer_discovery_etcd

# etcd endpoints. This property is required or peer discovery won't be performed.
cluster_formation.etcd.endpoints.1 = one.etcd.eng.example.local:2379

It is possible to configure multiple etcd endpoints. The first randomly chosen one that the plugin can successfully connect to will be used.

cluster_formation.peer_discovery_backend = etcd

cluster_formation.etcd.endpoints.1 = one.etcd.eng.example.local:2379
cluster_formation.etcd.endpoints.2 = two.etcd.eng.example.local:2479
cluster_formation.etcd.endpoints.3 = three.etcd.eng.example.local:2579

If authentication is enabled for etcd, the plugin can be configured to use a pair of credentials:

cluster_formation.peer_discovery_backend = etcd

cluster_formation.etcd.endpoints.1 = one.etcd.eng.example.local:2379
cluster_formation.etcd.endpoints.2 = two.etcd.eng.example.local:2479
cluster_formation.etcd.endpoints.3 = three.etcd.eng.example.local:2579

cluster_formation.etcd.username = rabbitmq
cluster_formation.etcd.password = s3kR37

It is possible to use advanced.config file to encrypt the password value listed in the config. In this case all plugin settings must be moved to the advanced config:

%% advanced.config file
[
 {rabbit,
     [{cluster_formation,
          [{peer_discovery_etcd, [
                {endpoints, [
                  "one.etcd.eng.example.local:2379",
                  "two.etcd.eng.example.local:2479",
                  "three.etcd.eng.example.local:2579"
                ]},

                {etcd_prefix,   "rabbitmq"},
                {cluster_name,  "default"},

                {etcd_username, "etcd user"},
                {etcd_password, {encrypted, <<"cPAymwqmMnbPXXRVqVzpxJdrS8mHEKuo2V+3vt1u/fymexD9oztQ2G/oJ4PAaSb2c5N/hRJ2aqP/X0VAfx8xOQ==">>}
                }]
           }]
      }]
 },

  {config_entry_decoder, [
             {passphrase, <<"decryption key passphrase">>}
         ]}
].

Key Naming

Directories and keys used by the peer discovery mechanism follow a naming scheme. Since in etcd v3 API the key space is flat, a hardcoded prefix is used. It allows the plugin to predictably perform key range queries using a well-known prefix:

# for node presence keys
/rabbitmq/discovery/{prefix}/clusters/{cluster name}/nodes/{node name}
# for registration lock keys
/rabbitmq/locks/{prefix}/clusters/{cluster name}/registration

Here's an example of a key that would be used by node rabbit@hostname1 with default user-provided key prefix and cluster name:

/rabbitmq/discovery/rabbitmq/clusters/default/nodes/rabbit@hostname1

Default key prefix is simply "rabbitmq". It rarely needs overriding but that's supported:

cluster_formation.peer_discovery_backend = etcd

cluster_formation.etcd.endpoints.1 = one.etcd.eng.example.local:2379
cluster_formation.etcd.endpoints.2 = two.etcd.eng.example.local:2479
cluster_formation.etcd.endpoints.3 = three.etcd.eng.example.local:2579

# rabbitmq is used by default
cluster_formation.etcd.key_prefix = rabbitmq_discovery

If multiple RabbitMQ clusters share an etcd installation, each cluster must use a unique name:

cluster_formation.peer_discovery_backend = etcd

cluster_formation.etcd.endpoints.1 = one.etcd.eng.example.local:2379
cluster_formation.etcd.endpoints.2 = two.etcd.eng.example.local:2479
cluster_formation.etcd.endpoints.3 = three.etcd.eng.example.local:2579

# default name: "default"
cluster_formation.etcd.cluster_name = staging

Key Leases and TTL

Key used for node registration will have a lease with a TTL associated with them. Online nodes will periodically keep the leases alive (refresh). The TTL value can be configured:

cluster_formation.peer_discovery_backend = etcd

cluster_formation.etcd.endpoints.1 = one.etcd.eng.example.local:2379
cluster_formation.etcd.endpoints.2 = two.etcd.eng.example.local:2479
cluster_formation.etcd.endpoints.3 = three.etcd.eng.example.local:2579

# node TTL in seconds
# default: 30
cluster_formation.etcd.node_ttl = 40

Key leases are updated periodically while the node is running and the plugin remains enabled.

It is possible to forcefully remove the nodes that fail to refresh their keys from the cluster. This is covered later in this guide.

Locks

When a node tries to acquire a lock on boot and the lock is already taken, it will wait for the lock to become available for a limited amount of time. Default value is 300 seconds but it can be configured:

cluster_formation.peer_discovery_backend = etcd

cluster_formation.etcd.endpoints.1 = one.etcd.eng.example.local:2379
cluster_formation.etcd.endpoints.2 = two.etcd.eng.example.local:2479

# lock acquisition timeout in seconds
# default: 300
# cluster_formation.consul.lock_wait_time is an alias
cluster_formation.etcd.lock_timeout = 60

Inspecting Keys

In order to list all keys used by the etcd-based peer discovery mechanism, use etcdctl get like so:

etcdctl get --prefix=true "/rabbitmq"

TLS

It is possible to configure the plugin to use TLS when connecting to etcd. TLS will be enabled if any of the TLS options listed below are configured, otherwise connections will use "plain TCP" without TLS.

The plugin acts as a TLS client. A trusted CA certificate file must be provided as well as a client certificate and private key pair:

cluster_formation.peer_discovery_backend = etcd

cluster_formation.etcd.endpoints.1 = one.etcd.eng.example.local:2379
cluster_formation.etcd.endpoints.2 = two.etcd.eng.example.local:2479

# trusted CA certificate file path
cluster_formation.etcd.ssl_options.cacertfile = /path/to/ca_certificate.pem
# client certificate (public key) file path
cluster_formation.etcd.ssl_options.certfile   = /path/to/client_certificate.pem
# client private key file path
cluster_formation.etcd.ssl_options.keyfile    = /path/to/client_key.pem

# use TLSv1.2 for connections
cluster_formation.etcd.ssl_options.versions.1 = tlsv1.2

# enables peer verification (the plugin will verify the certificate chain of the server)
cluster_formation.etcd.ssl_options.verify               = verify_peer
cluster_formation.etcd.ssl_options.fail_if_no_peer_cert = true

More TLS options are supported such as cipher suites and client-side session renegotiation options:

cluster_formation.peer_discovery_backend = etcd

cluster_formation.etcd.endpoints.1 = one.etcd.eng.example.local:2379
cluster_formation.etcd.endpoints.2 = two.etcd.eng.example.local:2479

# trusted CA certificate file path
cluster_formation.etcd.ssl_options.cacertfile = /path/to/ca_certificate.pem
# client certificate (public key) file path
cluster_formation.etcd.ssl_options.certfile   = /path/to/client_certificate.pem
# client private key file path
cluster_formation.etcd.ssl_options.keyfile    = /path/to/client_key.pem

# use TLSv1.2 for connections
cluster_formation.etcd.ssl_options.versions.1 = tlsv1.2

# enables peer verification (the plugin will verify the certificate chain of the server)
cluster_formation.etcd.ssl_options.verify               = verify_peer
cluster_formation.etcd.ssl_options.fail_if_no_peer_cert = true

# use secure session renegotiation
cluster_formation.etcd.ssl_options.secure_renegotiate   = true

# Explicitly list enabled cipher suites. This can break connectivity
# and is not necessary most of the time.
cluster_formation.etcd.ssl_options.ciphers.1  = ECDHE-ECDSA-AES256-GCM-SHA384
cluster_formation.etcd.ssl_options.ciphers.2  = ECDHE-RSA-AES256-GCM-SHA384
cluster_formation.etcd.ssl_options.ciphers.3  = ECDH-ECDSA-AES256-GCM-SHA384
cluster_formation.etcd.ssl_options.ciphers.4  = ECDH-RSA-AES256-GCM-SHA384
cluster_formation.etcd.ssl_options.ciphers.5  = DHE-RSA-AES256-GCM-SHA384
cluster_formation.etcd.ssl_options.ciphers.6  = DHE-DSS-AES256-GCM-SHA384
cluster_formation.etcd.ssl_options.ciphers.7  = ECDHE-ECDSA-AES128-GCM-SHA256
cluster_formation.etcd.ssl_options.ciphers.8  = ECDHE-RSA-AES128-GCM-SHA256
cluster_formation.etcd.ssl_options.ciphers.9  = ECDH-ECDSA-AES128-GCM-SHA256
cluster_formation.etcd.ssl_options.ciphers.10 = ECDH-RSA-AES128-GCM-SHA256
cluster_formation.etcd.ssl_options.ciphers.11 = DHE-RSA-AES128-GCM-SHA256
cluster_formation.etcd.ssl_options.ciphers.12 = DHE-DSS-AES128-GCM-SHA256

Race Conditions During Initial Cluster Formation

For successful cluster formation, only one node should form the cluster initially, that is, start as a standalone node and initializing its database. If this was not the case, multiple clusters would be formed instead of just one, violating operator expectations.

Consider a deployment where the entire cluster is provisioned at once and all nodes start in parallel. In this case, a natural race condition occurs between the starting nodes. To prevent multiple nodes forming separate clusters, peer discovery backends try to acquire a lock when either forming the cluster (seeding) or joining a peer. What locks are used varies from backend to backend:

  • Classic config file, K8s, and AWS backends use a built-in locking library provided by the runtime
  • The Consul peer discovery backend sets a lock in Consul
  • The etcd peer discovery backend sets a lock in etcd

Node Health Checks and Forced Removal

Nodes in clusters formed using peer discovery can fail, become unavailable or be permanently removed (decomissioned). Some operators may want such nodes to be automatically removed from the cluster after a period of time. Such automated forced removal also can produce unforeseen side effects, so RabbitMQ does not enforce this behavior. It should be used with great care and only if the side effects are fully understood and considered.

For example, consider a cluster that uses the AWS backend configured to use autoscaling group membership. If an EC2 instance in that group fails and is later re-created as a new node, its original "incarnation" will be considered a separate, now permanently unavailable node in the same cluster.

With peer discovery backends that offer dynamic node management (as opposed to, say, a fixed list of nodes in the configuration file), such unknown nodes can be logged or forcefully removed from the cluster.

They are

Forced node removal can be dangerous and should be carefully considered. For example, a node that's temporarily unavailable but will be rejoining (or recreated with its persistent storage re-attached from its previous incarnation) can be kicked out of the cluster permanently by automatic cleanup, thus failing to rejoin.

Before enabling the configuration keys covered below make sure that a compatible peer discovery plugin is enabled. If that's not the case the node will report the settings to be unknown and will fail to start.

To log warnings for the unknown nodes, cluster_formation.node_cleanup.only_log_warning should be set to true:

# Don't remove cluster members unknown to the peer discovery backend but log
# warnings.
#
# This setting can only be used if a compatible peer discovery plugin is enabled.
cluster_formation.node_cleanup.only_log_warning = true

This is the default behavior.

To forcefully delete the unknown nodes from the cluster, cluster_formation.node_cleanup.only_log_warning should be set to false.

# Forcefully remove cluster members unknown to the peer discovery backend. Once removed,
# the nodes won't be able to rejoin. Use this mode with great care!
#
# This setting can only be used if a compatible peer discovery plugin is enabled.
cluster_formation.node_cleanup.only_log_warning = false

Note that this option should be used with care, in particular with discovery backends other than AWS.

The cleanup checks are performed periodically. The interval is 60 seconds by default and can be overridden:

# perform the check every 90 seconds
cluster_formation.node_cleanup.interval = 90

Some backends (Consul, etcd) support node health checks or TTL. These checks should not to be confused with monitoring health checks. They allow peer discovery services (such as etcd or Consul) keep track of what nodes are still around (have checked in recently).

With service discovery health checks, nodes set a TTL on their keys and/or periodically notify their respective discovery service that they are still present. If no notifications from a node come in after a period of time, the node's key will eventually expire (with Consul, such nodes will be considered to be in a warning state).

With etcd, such nodes will no longer show up in discovery results. With Consul, they can either be removed (deregistered) or their warning state can be reported. Please see documentation for those backends to learn more.

Automatic cleanup of absent nodes makes most sense in environments where failed/discontinued nodes will be replaced with brand new ones (including cases when persistent storage won't be re-attached).

When automatic node cleanup is disabled (switched to the warning mode), operators have to explicitly remove absent cluster nodes using rabbitmqctl forget_cluster_node.

Negative Side Effects of Automatic Removal

Automatic node removal has a number of negative side effects operators should be aware of. A node that's temporarily unreachable, for example, because it's lost connectivity to the rest of the network or its VM was temporarily suspended, will be removed and will then come back. Such node won't be able to rejoin its cluster and will log a similar message:

Node 'rabbit@node1.local' thinks it's clustered with node 'rabbit@node2.local', but 'rabbit@node2.local' disagrees

In addition, such nodes can begin to fail their monitoring health checks, as they would be in a permanent "partitioned off" state. Even though such nodes might have been replaced with a new one and the cluster would be operating as expected, such automatically removed and replaced nodes can produce monitoring false positives.

The list of side effects is not limited to those two scenarios but they all have the same root cause: an automatically removed node can come back without realising that it's been kicked out of its cluster. Monitoring systems and operators won't be immediately aware of that event either.

Peer Discovery Failures and Retries

In latest releases if a peer discovery attempt fail, it will be retried up to a certain number of times with a delay between each attempt. This is similar to the peer sync retries nodes perform when they come online after a restart.

For example, with the Kubernetes peer discovery mechanism this means that Kubernetes API requests that list pods will be retried should they fail. With the AWS mechanism, EC2 API requests are retried, and so on.

Such retries by no means handle every possible failure scenario but they improve the resilience of peer discovery and thus cluster and node deployments in practice. However, if clustered nodes fail to authenticate with each other, retries will simply merely the inevitable failure of cluster formation.

Nodes that fail to perform peer discovery will log their remaining recovery attempts:

2020-06-27 06:35:36.426 [error] <0.277.0> Trying to join discovered peers failed. Will retry after a delay of 500 ms, 4 retries left...
2020-06-27 06:35:36.928 [warning] <0.277.0> Could not auto-cluster with node rabbit@hostname2: {badrpc,nodedown}
2020-06-27 06:35:36.930 [warning] <0.277.0> Could not auto-cluster with node rabbit@hostname3: {badrpc,nodedown}
2020-06-27 06:35:36.930 [error] <0.277.0> Trying to join discovered peers failed. Will retry after a delay of 500 ms, 3 retries left...
2020-06-27 06:35:37.432 [warning] <0.277.0> Could not auto-cluster with node rabbit@hostname2: {badrpc,nodedown}
2020-06-27 06:35:37.434 [warning] <0.277.0> Could not auto-cluster with node rabbit@hostname3: {badrpc,nodedown}

If a node fails to perform peer discovery and exhausts all retries, enable debug logging is highly recommended for troubleshooting.

The number of retries and the delay can be configured:

# These are the default values

# Retry peer discovery operations up to ten times
cluster_formation.discovery_retry_limit = 10

# 500 milliseconds
cluster_formation.discovery_retry_interval = 500

The defaults cover five seconds of unavailability of services, API endpoints or nodes involved in peer discovery. These values are sufficient to cover sporadic failures. They will require increasing in environments where dependent services (DNS, etcd, Consul, etc) may be provisioned concurrently with RabbitMQ cluster deployment and thus can become available only after a period of time.

HTTP Proxy Settings

Peer discovery mechanisms that use HTTP to interact with its dependencies (e.g. AWS, Consul and etcd ones) can proxy their requests using an HTTP proxy.

There are separate proxy settings for HTTP and HTTPS:

# example HTTP and HTTPS proxy servers, values in your environment
# will vary
cluster_formation.proxy.http_proxy = 192.168.0.98
cluster_formation.proxy.https_proxy = 192.168.0.98

Some hosts can be excluded from proxying, e.g. the link-local AWS instance metadata IP address:

# example HTTP and HTTPS proxy servers, values in your environment
# will vary
cluster_formation.proxy.http_proxy = 192.168.0.98
cluster_formation.proxy.https_proxy = 192.168.0.98

# requests to these hosts won't go via proxy
cluster_formation.proxy.proxy_exclusions.1 = 169.254.169.254
cluster_formation.proxy.proxy_exclusions.2 = excluded.example.local

Troubleshooting

The peer discovery subsystem and individual mechanism implementations log important discovery procedure steps at the info log level. More extensive logging is available at the debug level. Mechanisms that depend on external services accessible over HTTP will log all outgoing HTTP requests and response codes at debug level. See the logging guide for more information about logging configuration.

If the log does not contain any entries that demonstrate peer discovery progress, for example, the list of nodes retrieved by the mechanism or clustering attempts, it may mean that the node already has an initialised data directory or is already a member of the cluster. In those cases peer discovery won't be performed.

Peer discovery relies on inter-node network connectivity and successful authentication via a shared secret. Verifying that nodes can communicate with one another and use the expected Erlang cookie value (that's also identical across all cluster nodes). See the main Clustering guide for more information.

A methodology for network connectivity troubleshooting as well as commonly used tools are covered in the Troubleshooting Network Connectivity guide.

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