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An Introduction to boto's Elastic Load Balancing interface

This tutorial focuses on the boto interface for Elastic Load Balancing from Amazon Web Services. This tutorial assumes that you have already downloaded and installed boto, and are familiar with the boto ec2 interface.

Elastic Load Balancing Concepts

Elastic Load Balancing (ELB) is intimately connected with Amazon's Elastic Compute Cloud (EC2) service. Using the ELB service allows you to create a load balancer - a DNS endpoint and set of ports that distributes incoming requests to a set of EC2 instances. The advantages of using a load balancer is that it allows you to truly scale up or down a set of backend instances without disrupting service. Before the ELB service, you had to do this manually by launching an EC2 instance and installing load balancer software on it (nginx, haproxy, perlbal, etc.) to distribute traffic to other EC2 instances.

Recall that the EC2 service is split into Regions, which are further divided into Availability Zones (AZ). For example, the US-East region is divided into us-east-1a, us-east-1b, us-east-1c, us-east-1d, and us-east-1e. You can think of AZs as data centers - each runs off a different set of ISP backbones and power providers. ELB load balancers can span multiple AZs but cannot span multiple regions. That means that if you'd like to create a set of instances spanning both the US and Europe Regions you'd have to create two load balancers and have some sort of other means of distributing requests between the two load balancers. An example of this could be using GeoIP techniques to choose the correct load balancer, or perhaps DNS round robin. Keep in mind also that traffic is distributed equally over all AZs the ELB balancer spans. This means you should have an equal number of instances in each AZ if you want to equally distribute load amongst all your instances.

Creating a Connection

The first step in accessing ELB is to create a connection to the service.

Like EC2, the ELB service has a different endpoint for each region. By default the US East endpoint is used. To choose a specific region, use the connect_to_region function:

>>> import boto.ec2.elb
>>> elb = boto.ec2.elb.connect_to_region('us-west-2')

Here's yet another way to discover what regions are available and then connect to one:

>>> import boto.ec2.elb
>>> regions = boto.ec2.elb.regions()
>>> regions
>>> elb = regions[-1].connect()

Alternatively, edit your boto.cfg with the default ELB endpoint to use:

elb_region_name = eu-west-1
elb_region_endpoint =

Getting Existing Load Balancers

To retrieve any exiting load balancers:

>>> conn.get_all_load_balancers()
[LoadBalancer:load-balancer-prod, LoadBalancer:load-balancer-staging]

You can also filter by name

>>> conn.get_all_load_balancers(load_balancer_names=['load-balancer-prod'])

:py:meth:`get_all_load_balancers <boto.ec2.elb.ELBConnection.get_all_load_balancers>` returns a :py:class:`boto.resultset.ResultSet` that contains instances of :class:`boto.ec2.elb.loadbalancer.LoadBalancer`, each of which abstracts access to a load balancer. :py:class:`ResultSet <boto.resultset.ResultSet>` works very much like a list.

>>> balancers = conn.get_all_load_balancers()
>>> balancers[0]

Creating a Load Balancer

To create a load balancer you need the following:
  1. The specific ports and protocols you want to load balancer over, and what port you want to connect to all instances.
  2. A health check - the ELB concept of a heart beat or ping. ELB will use this health check to see whether your instances are up or down. If they go down, the load balancer will no longer send requests to them.
  3. A list of Availability Zones you'd like to create your load balancer over.

Ports and Protocols

An incoming connection to your load balancer will come on one or more ports - for example 80 (HTTP) and 443 (HTTPS). Each can be using a protocol - currently, the supported protocols are TCP and HTTP. We also need to tell the load balancer which port to route connects to on each instance. For example, to create a load balancer for a website that accepts connections on 80 and 443, and that routes connections to port 8080 and 8443 on each instance, you would specify that the load balancer ports and protocols are:

  • 80, 8080, HTTP
  • 443, 8443, TCP

This says that the load balancer will listen on two ports - 80 and 443. Connections on 80 will use an HTTP load balancer to forward connections to port 8080 on instances. Likewise, the load balancer will listen on 443 to forward connections to 8443 on each instance using the TCP balancer. We need to use TCP for the HTTPS port because it is encrypted at the application layer. Of course, we could specify the load balancer use TCP for port 80, however specifying HTTP allows you to let ELB handle some work for you - for example HTTP header parsing.

Configuring a Health Check

A health check allows ELB to determine which instances are alive and able to respond to requests. A health check is essentially a tuple consisting of:

  • Target: What to check on an instance. For a TCP check this is comprised of:


    Which attempts to open a connection on PORT_TO_CHECK. If the connection opens successfully, that specific instance is deemed healthy, otherwise it is marked temporarily as unhealthy. For HTTP, the situation is slightly different:


    This means that the health check will connect to the resource /RESOURCE on PORT_TO_CHECK. If an HTTP 200 status is returned the instance is deemed healthy.

  • Interval: How often the check is made. This is given in seconds and defaults to 30. The valid range of intervals goes from 5 seconds to 600 seconds.

  • Timeout: The number of seconds the load balancer will wait for a check to return a result.

  • Unhealthy threshold: The number of consecutive failed checks to deem the instance as being dead. The default is 5, and the range of valid values lies from 2 to 10.

The following example creates a health check called instance_health that simply checks instances every 20 seconds on port 80 over HTTP at the resource /health for 200 successes.

>>> from boto.ec2.elb import HealthCheck
>>> hc = HealthCheck(

Putting It All Together

Finally, let's create a load balancer in the US region that listens on ports 80 and 443 and distributes requests to instances on 8080 and 8443 over HTTP and TCP. We want the load balancer to span the availability zones us-east-1a and us-east-1b:

>>> zones = ['us-east-1a', 'us-east-1b']
>>> ports = [(80, 8080, 'http'), (443, 8443, 'tcp')]
>>> lb = conn.create_load_balancer('my-lb', zones, ports)
>>> # This is from the previous section.
>>> lb.configure_health_check(hc)

The load balancer has been created. To see where you can actually connect to it, do:

>>> print lb.dns_name

You can then CNAME map a better name, i.e. to the above address.

Adding Instances To a Load Balancer

Now that the load balancer has been created, there are two ways to add instances to it:

  1. Manually, adding each instance in turn.
  2. Mapping an autoscale group to the load balancer. Please see the :doc:`Autoscale tutorial <autoscale_tut>` for information on how to do this.

Manually Adding and Removing Instances

Assuming you have a list of instance ids, you can add them to the load balancer

>>> instance_ids = ['i-4f8cf126', 'i-0bb7ca62']
>>> lb.register_instances(instance_ids)

Keep in mind that these instances should be in Security Groups that match the internal ports of the load balancer you just created (for this example, they should allow incoming connections on 8080 and 8443).

To remove instances:

>>> lb.deregister_instances(instance_ids)

Modifying Availability Zones for a Load Balancer

If you wanted to disable one or more zones from an existing load balancer:

>>> lb.disable_zones(['us-east-1a'])

You can then terminate each instance in the disabled zone and then deregister then from your load balancer.

To enable zones:

>>> lb.enable_zones(['us-east-1c'])

Deleting a Load Balancer

>>> lb.delete()
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