Deploying the Aurora Scheduler

When setting up your cluster, you will install the scheduler on a small number (usually 3 or 5) of machines. This guide helps you get the scheduler set up and troubleshoot some common hurdles.

Installing Aurora

The Aurora scheduler is a standalone Java server. As part of the build process it creates a bundle of all its dependencies, with the notable exceptions of the JVM and libmesos. Each target server should have a JVM (Java 7 or higher) and libmesos (0.20.1) installed.

Creating the Distribution .zip File (Optional)

To create a distribution for installation you will need build tools installed. On Ubuntu this can be done with sudo apt-get install build-essential default-jdk.

git clone http://git-wip-us.apache.org/repos/asf/incubator-aurora.git
cd incubator-aurora
./gradlew distZip

Copy the generated dist/distributions/aurora-scheduler-*.zip to each node that will run a scheduler.

Installing Aurora

Extract the aurora-scheduler zip file. The example configurations assume it is extracted to /usr/local/aurora-scheduler.

sudo unzip dist/distributions/aurora-scheduler-*.zip -d /usr/local
sudo ln -nfs "$(ls -dt /usr/local/aurora-scheduler-* | head -1)" /usr/local/aurora-scheduler

Configuring Aurora

A Note on Configuration

Like Mesos, Aurora uses command-line flags for runtime configuration. As such the Aurora “configuration file” is typically a scheduler.sh shell script of the form.

#!/bin/bash
AURORA_HOME=/usr/local/aurora-scheduler

# Flags controlling the JVM.
JAVA_OPTS=(
  -Xmx2g
  -Xms2g
  # GC tuning, etc.
)

# Flags controlling the scheduler.
AURORA_FLAGS=(
  -http_port=8081
  # Log configuration, etc.
)

# Environment variables controlling libmesos
export JAVA_HOME=...
export GLOG_v=1
export LIBPROCESS_PORT=8083

JAVA_OPTS="${JAVA_OPTS[*]}" exec "$AURORA_HOME/bin/aurora-scheduler" "${AURORA_FLAGS[@]}"

That way Aurora’s current flags are visible in ps and in the /vars admin endpoint.

Examples are available under examples/scheduler/. For a list of available Aurora flags and their documentation run

/usr/local/aurora-scheduler/bin/aurora-scheduler -help

Replicated Log Configuration

All Aurora state is persisted to a replicated log. This includes all jobs Aurora is running including where in the cluster they are being run and the configuration for running them, as well as other information such as metadata needed to reconnect to the Mesos master, resource quotas, and any other locks in place.

Aurora schedulers use ZooKeeper to discover log replicas and elect a leader. Only one scheduler is leader at a given time - the other schedulers follow log writes and prepare to take over as leader but do not communicate with the Mesos master. Either 3 or 5 schedulers are recommended in a production deployment depending on failure tolerance and they must have persistent storage.

In a cluster with N schedulers, the flag -native_log_quorum_size should be set to floor(N/2) + 1. So in a cluster with 1 scheduler it should be set to 1, in a cluster with 3 it should be set to 2, and in a cluster of 5 it should be set to 3.

Number of schedulers (N) -native_log_quorum_size setting (floor(N/2) + 1)
1 1
3 2
5 3
7 4

Incorrectly setting this flag will cause data corruption to occur!

See this document for more replicated log and storage configuration options.

Initializing the Replicated Log

Before you start Aurora you will also need to initialize the log on the first master.

mesos-log initialize --path="$AURORA_HOME/scheduler/db"

Failing to do this will result the following message when you try to start the scheduler.

Replica in EMPTY status received a broadcasted recover request

Storage Performance Considerations

See this document for scheduler storage performance considerations.

Network considerations

The Aurora scheduler listens on 2 ports - an HTTP port used for client RPCs and a web UI, and a libprocess (HTTP+Protobuf) port used to communicate with the Mesos master and for the log replication protocol. These can be left unconfigured (the scheduler publishes all selected ports to ZooKeeper) or explicitly set in the startup script as follows:

# ...
AURORA_FLAGS=(
  # ...
  -http_port=8081
  # ...
)
# ...
export LIBPROCESS_PORT=8083
# ...

Running Aurora

Configure a supervisor like Monit or supervisord to run the created scheduler.sh file and restart it whenever it fails. Aurora expects to be restarted by an external process when it fails. Aurora supports an active health checking protocol on its admin HTTP interface - if a GET /health times out or returns anything other than 200 OK the scheduler process is unhealthy and should be restarted.

For example, monit can be configured with

if failed port 8081 send "GET /health HTTP/1.0\r\n" expect "OK\n" with timeout 2 seconds for 10 cycles then restart

assuming you set -http_port=8081.

Maintaining an Aurora Installation

Monitoring

Please see our dedicated monitoring guide for in-depth discussion on monitoring.

Running stateful services

Aurora is best suited to run stateless applications, but it also accommodates for stateful services like databases, or services that otherwise need to always run on the same machines.

Dedicated attribute

The Mesos slave has the --attributes command line argument which can be used to mark a slave with static attributes (not to be confused with --resources, which are dynamic and accounted).

Aurora makes these attributes available for matching with scheduling constraints. Most of these constraints are arbitrary and available for custom use. There is one exception, though: the dedicated attribute. Aurora treats this specially, and only allows matching jobs to run on these machines, and will only schedule matching jobs on these machines.

Syntax

The dedicated attribute has semantic meaning. The format is $role(/.*)?. When a job is created, the scheduler requires that the $role component matches the role field in the job configuration, and will reject the job creation otherwise. The remainder of the attribute is free-form. We’ve developed the idiom of formatting this attribute as $role/$job, but do not enforce this.

Example

Consider the following slave command line:

mesos-slave --attributes="host:$HOST;rack:$RACK;dedicated:db_team/redis" ...

And this job configuration:

Service(
  name = 'redis',
  role = 'db_team',
  constraints = {
    'dedicated': 'db_team/redis'
  }
  ...
)

The job configuration is indicating that it should only be scheduled on slaves with the attribute dedicated:dba_team/redis. Additionally, Aurora will prevent any tasks that do not have that constraint from running on those slaves.

Common problems

So you’ve started your first cluster and are running into some issues? We’ve collected some common stumbling blocks and solutions here to help get you moving.

Replicated log not initialized

Symptoms

  • Scheduler RPCs and web interface claim Storage is not READY
  • Scheduler log repeatedly prints messages like
  I1016 16:12:27.234133 26081 replica.cpp:638] Replica in EMPTY status
  received a broadcasted recover request
  I1016 16:12:27.234256 26084 recover.cpp:188] Received a recover response
  from a replica in EMPTY status

Solution

When you create a new cluster, you need to inform a quorum of schedulers that they are safe to consider their database to be empty by initializing the replicated log. This is done to prevent the scheduler from modifying the cluster state in the event of multiple simultaneous disk failures or, more likely, misconfiguration of the replicated log path.

Scheduler not registered

Symptoms

Scheduler log contains

Framework has not been registered within the tolerated delay.

Solution

Double-check that the scheduler is configured correctly to reach the master. If you are registering the master in ZooKeeper, make sure command line argument to the master:

--zk=zk://$ZK_HOST:2181/mesos/master

is the same as the one on the scheduler:

-mesos_master_address=zk://$ZK_HOST:2181/mesos/master

Tasks are stuck in PENDING forever

Symptoms

The scheduler is registered, and (receiving offers](docs/monitoring.md#schedulerresourceoffers), but tasks are perpetually shown as PENDING - Constraint not satisfied: host.

Solution

Check that your slaves are configured with host and rack attributes. Aurora requires that slaves are tagged with these two common failure domains to ensure that it can safely place tasks such that jobs are resilient to failure.

See our vagrant example for details.