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Load Balancing GBrowse2 with AWS Spot Instances

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As of GBrowse version 2.54, you may offload some of the work involved in rendering tracks to a farm of render slaves running under Amazon Web Services. The render slaves use Amazon's Spot Instance facility, which provides servers for just pennies per hour, and the number of slaves is automatically adjusted up and down in response to load.


Synchronize the data on the master with the slave image:

% -c /etc/gbrowse2/aws_balancer.conf -mysql /var/lib/mysql

Launch the balancer in the foreground

% --conf         /etc/gbrowse2/aws_balancer.conf \
                          --access_key   XYZZY \
                          --secret_key   Plugh

Launch the balancer in the background as a daemon:

% --background \
                          --conf         /etc/gbrowse2/aws_balancer.conf \
                          --access_key   XYZZY \
                          --secret_key   Plugh \
                          --logfile      /var/log/gbrowse/aws_balancer.log \
                          --pidfile      /var/run/ \
                          --user         nobody

Kill a running balancer daemon:

% --kill \
                          --conf         /etc/gbrowse2/aws_balancer.conf \
                          --access_key   XYZZY \
                          --secret_key   Plugh \
                          --logfile      /var/log/gbrowse/aws_balancer.log \
                          --pidfile      /var/run/ \
                          --user         nobody

Use the init script:

% sudo /etc/init.d/gbrowse-aws-balancer start
% sudo /etc/init.d/gbrowse-aws-balancer restart
% sudo /etc/init.d/gbrowse-aws-balancer stop
% sudo /etc/init.d/gbrowse-aws-balancer status


The AWS balancer scripts help you synchronize the contents of your local GBrowse databases with a snapshot (or series of snapshots) on Amazon Web Services, and to launch track rendering slaves as Amazon Spot Instances in response to high load levels on the master server.

Load balancing is most convenient to run in conjunction with a GBrowse instance running within the Amazon Web Service EC2 cloud (see GBrowse2_VMs#Getting_Started_with_the_EC2_VM), but you may also use this to supplement a server running on local hardware. The sections below describe the configuration needed for these two scenarios.

Note that you must have an Amazon Web Services account to use this facility, and to install a small number of additional perl modules on the master machine.

Initial Setup

1. You must have the Perl modules VM::EC2 (v1.22 or later), and Parse::Apache::Server::Status installed on the machine you intend to run the balancer on. The balancer must run on the same machine that GBrowse is running on. To install these modules, run:

perl -MCPAN -e 'install VM::EC2; install Parse::Apache::Server::Status'

2. You must have an account on Amazon Web Services and must be familiar with using the AWS Console to launch and terminate EC2 instances. If you run GBrowse on local hardware, then you will need to provide the script with your access key and secret access key when launching it. It may be safer to create and use an IAM user (Identity and Access Management) who has more limited privileges. See CONFIGURATION below for some suggestions.

3. GBrowse must be running under Apache.

4. Apache must be configured to enable the mod_status module and to allow password-less requests to this module from localhost ( This is the recommended configuration:

<IfModule mod_status.c>
   ExtendedStatus on 
   <Location /server-status>
           SetHandler server-status
           Order deny,allow
           Deny from all
           Allow from ::1

5. If you are running GBrowse on local hardware, the local hardware must be connected to the Internet or have a Virtual Private Cloud (VPC) connection to Amazon.

The aws_balancer.conf Configuration File

The balancer requires a configuration file, ordinarily named aws_balancer.conf and located in the GBrowse configuration directory (e.g. /etc/gbrowse2/aws_balancer.conf). On the GBrowse2 Cloud VM, this file is located at /opt/gbrowse/etc/aws_balancer.conf.

The distribution config file looks like this:

#req/s  min max
0.1     0   1
0.5     0   2
1.0     1   3
2.0     2   4
5.0     3   6
6.0     4   6
7.0     5   7
10.0    6   8
15.0    8   8

external_ip       =                # optional; will figure it out if needed
poll_interval     = 0.5            # minutes between polling steps
server_status_url = http://localhost/server-status    # URL on master that invokes Apache server-status

instance_type     = m1.large       # pretty nice performance, might also try medium
spot_bid          = 0.10           # the pricing will cost no more than $0.10/hr
ports             = 8101           # can be several space-delimited port numbers
region            = us-west-2      # needed only when run from a non-AWS computer
image_id          = GBrowse-2.54   # AMI of the slave
data_snapshots    =                # EBS snapshot(s) to attach to the slave
availability_zone =                # optional
subnet            =                # optional
security_group    =                # optional; will manage own security group if needed

The configuration file has three sections:


This section describes the number of slave instances to launch for different load levels. It consists of a three-column space-delimited table with the following columns:

<requests/sec>    <min instances>    <max instances>

For example, the first few rows of the default table reads:

0.1     0   1
0.5     0   2
1.0     1   3
2.0     2   4

This is read as meaning that when the number of requests per second on the GBrowse server is greater than 0.1 but less than 0.5, run at least 0 slave servers but no more than 1 slave server. When the number of requests is between 0.5 and 1.0, run between 0 and 2 slave instances. When the rate is between 1.0 and 2.0, run at least 1 slave instance, but no more than 3. Load levels below the lowest value on the table (0.1 in this case) will run no slave servers, while levels above the highest value on the table (2.0) will launch the minimum and maximum number of slaves for that load value (between 2 and 4 in this case).

The reason for having a range of instance counts for each load range is to avoid unecessarily launching and killing slaves repeatedly when the load fluctuates around the boundary. You may wish to tune the values in this table to maximize the performance of your GBrowse installation.

Note that the server load includes both GBrowse requests and all other requests on the web server. If this is a problem, you may wish to run GBrowse on a separate Apache port or virtual host.


The options in this sections configure the master GBrowse instance. Three options are recognized:

external_ip (optional)
This controls the externally-visible IP address of the GBrowse master, which is needed by the firewall rule for master/slave communications. This option can usually be left blank: when the master is running on EC2, then the IP address is known; when the master is running on a local machine, the externally-visible IP address is looked up using a web service. It is only in the rare case that this lookup is incorrect that you will need to configure this option yourself. The external IP that the balancer script finds can be seen in a log message when verbosity is 2 or higher.
poll_interval (required)
This is the interval, in minutes, that the balancer script will periodically check the Apache load and adjust the number of slave instances. The suggested value is 0.5 (30s intervals).
server_status_url (required)
This is the URL to call to fetch the server load from Apache's server_status module.


The options in this section apply to the render slaves launched by the balancer.

instance_type (required)
This is the EC2 instance type. Faster instances give better performance. High-IO instances give the best performance, but cost more.
spot_bid (required)
This is the maximum, in US dollars, that you are willing to pay per hour to run a slave spot instance. Typically you will pay less than the bid price. If the spot price increases beyond the maximum bid, then the spot instances will be terminated and the balancer will wait until the spot price decreases below the maximum bid before launching additional slaves.
ports (required)
This is a space-delimited list of TCP port numbers on which the render slaves should listen for incoming render requests from the master. Generally it is only necessary to listen on a single port; multiple ports were supported for performance reasons in earlier single-threaded versions of the slave.
region (required for local masters)
The Amazon region in which to launch slaves. When the master is running in EC2, this is automatically chosen to be the same as the master's region and can be left blank.
image_id (required for local masters)
This is the name or ID of the AMI that will be used to launch slaves. When the master is running on an EC2 instance, then the image ID used to launch the master will be used. Otherwise, this option should contain the AMI ID of a GBrowse image in the desired region, or a partial name that will be used to find the correct AMI.
Before launching the slave, attach EBS volumes created from one or more volume snapshots listed in this option. :Multiple snapshots can be attached by providing a space-delimited list:
data_snapshots = snap-12345 snap-abcdef
Ordinarily, this option is updated by the script and should not be manually edited!
availability_zone (optional)
This option will force the slave into the named availability zone. If not specified, an availability zone in the current region will be chosen at random.
subnet (optional)
If you are in a VPC (virtual private cloud) environment, then this option will force the slave into the named subnet. Ordinarily the balancer script will launch slaves into non-VPC instances if the master is running on local hardware or a non-VPC EC2 instance. The balancer will launch slaves into the same VPC subnet as the master if the master is running on a VPC instance.
security_group (optional)
This specifies the security group to assign the slaves to. If not specified, a properly-configured security group will be created as needed and destroyed when the balancer script exits. If you choose to manage the security group manually, be sure to configure the firewall ingress rule to allow access to the slave port(s) (see the "ports" option) from the master's group or IP address.

Configuring AWS Credentials

To work, the balancer and synchronization scripts must be able to make spot instance requests and to monitor and terminate instances. To perform these operations the script must have access to the appropriate AWS credentials (access key and secret key) on the command line or as environment variables.

While the script does its best to shield the credentials from prying eyes, there is still a chance that the credentials can be intercepted by another party with login access to the machine that the master runs on and use the credentials to run up your AWS bill. For this reason you may prefer to create an EC2 account or role with limited access to AWS resources.

  1. Your personal EC2 credentials -- You may provide the balancer script with --access_key and --secret_key command line arguments using your personal EC2 credentials or set the environment variables EC2_ACCESS_KEY and EC2_SECRET_KEY. If not provided, the script will interactively prompt for one or both of these values. This is the simplest method, but has the risk that if the credentials are intercepted by a malicious third party, he or she gains access to all your EC2 resources.
  2. The credentials of a restricted IAM account -- You may use the Amazon AWS console to create an IAM (Identity Access and Management) user with restricted permissions, and provide that user's credentials to the script on the command line or with environment variables. The following IAM permission policy is the minimum needed for the balancer script to work properly:
            "Statement": [
                "Sid": "BalancerPolicy",
                "Action": [
                "Effect": "Allow",
                "Resource": [

Note that even with these restrictions, an unauthorized user with access to the credentials could still launch a large number of spot instances or terminate bona fide instances. This is just a fundamental limitation of the granularity of EC2's permissions system.

IAM Roles

Alternatively, if the master is running on an EC2 instance, then the most convenient way to pass credentials is by assigning the instance an IAM role. The balancer script can then obtain temporary credentials by making internal EC2 calls. The credentials do not need to be provided on the command line or in environment variables, and are only valid for short periods of time, limiting the effect of theft.

  1. First, create an IAM role using the Amazon Console. Select IAM->Roles->Create New Role, and give the role the name "GBrowseMaster" (or whatever you prefer).
  2. Next, when prompted for the role type, select AWS Service Roles->Amazon EC2.
  3. On the Select Role Permissions screen, choose "Custom Policy". Give the policy a name like "GBrowseBalancer" and cut and paste into the Policy Document text field the permission policy listed above in the instructions for creating a restriced IAM account. Be sure to remove the whitespace before the beginning of the first curly brace, or the console will complain about an invalid policy.
  4. You only need to do this once. After this, whenever you launch an instance that will run the GBrowse master (typically from a GBrowse AMI), specify the "GBrowseMaster" IAM role name. This can be done from the AWS console's instance launch wizard, or by passing the -p option to the ec2-run-instances command-line tool. When run on the master machine, the balancer scripts can now be run without passing any credentials.


This script launches a process that monitors the load on the local GBrowse instance. If the load exceeds certain predefined levels, then it uses Amazon web services to launch one or more GBrowse slave instances. The work of rendering tracks is then handed off to these instances, reducing the load on the local instance.


Options can be abbreviated. For example, you can use -a for --access_key:

     --access_key   EC2 access key
     --secret_key   EC2 secret key
     --conf         Path to balancer configuration file
     --pidfile      Path to file that holds daemon process ID
     --logfile      Path to file that records log messages
     --user         User to run daemon under (script must be
                        started as root)
     --verbosity    Logging verbosity. 0=least, 3=most.
     --background   Go into the background and run as daemon.
     --kill         Kill a previously-launched daemon. Must provide
                        the same --pidfile argument as used when
                        the daemon was started.
     --ssh_key      Enable ssh login on the slave(s) using the specified
                        AWS ssh keypair. Login will only be available
                        from the host this script is run on.


The gbrowse-aws-balancer init script can be used on Ubuntu and Debian-based systems to simplify launching the balancer at boot time. It can be found in /etc/init.d by default, and is called in the following manner:

start the service

% sudo /etc/init.d/gbrowse-aws-balancer start

stop the service

% sudo /etc/init.d/gbrowse-aws-balancer stop

stop and restart the service

% sudo /etc/init.d/gbrowse-aws-balancer restart

show the status of the service (running, stopped)

% sudo /etc/init.d/gbrowse-aws-balancer status

The various script options are all set in a single configuration file named /etc/default/gbrowse-aws-balancer. The distribution contents of this file looks like this:


The variables in this file set the location of the balancer script, the location of its configuration file, the verbosity to run with, and where to write the script's process ID and log information. In addition, you can place your (or another authorized user's) EC2 access and secret key in this file. Please make sure that this file is only readable by root.

The script should be run on the master each time you add a new database to an existing data source, or if you add a whole new data source. What it does is to prepare a new Amazon EBS snapshot containing a copy of all the data needed for the GBrowse slave to run. This snapshot is then attached to new slave instances.

After running, it updates the conf file with the current versions of the slave AMI and the data snapshot.

% sudo --conf     /etc/gbrowse2/aws_balancer.conf \
                                  --mysql    /var/lib/mysql \
                                  --postgres /var/lib/postgresql

The --conf argument is required. The script will create a snapshot of the appropriate size, mount it on a temporary staging instance, and rsync a copy of your gbrowse databases directory (e.g. /var/lib/gbrowse2/databases) to the snapshot. If you have created mysql or postgres databases, you must also give the paths to their database file directories, as shown in the example.

Note that ALL your mysql and postgres data files located on the server will be copied; not just those used for track display.

Environment Variables

The following environment variables are used by and if the corresponding command line options are not present:

EC2_ACCESS_KEY     AWS EC2 access key
EC2_SECRET_KEY     AWS EC2 secret key


If slaves are returning track renderinge errors, then there is likely an issue with data synchronization. This typically happens when the data on the master differs from the data on the slave, or path names are different on the two systems.

To debug this, launch with the -ssh_key option:

 % --conf         /etc/gbrowse2/aws_balancer.conf \
                          --access_key   XYZZY \
                          --secret_key   Plugh \
                          --ssh_key      John_Doe_default

You may then ssh into the slave using the specified ssh key and the username "admin". A useful thing to do is to tail the slave log file:

ssh -i .ssh/John_Doe_default admin@ tail -f /var/log/gbrowse/gbrowse_slave

Replace the IP number with the correct IP number of one of the running slaves, which you can find in /etc/gbrowse2/renderfarm.conf.


Lincoln Stein,

Copyright (c) 2012 Ontario Institute for Cancer Research

This package and its accompanying libraries is free software; you can redistribute it and/or modify it under the terms of the GPL (either version 1, or at your option, any later version) or the Artistic License 2.0. Refer to LICENSE for the full license text. In addition, please see DISCLAIMER.txt for disclaimers of warranty.