
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
   [RegionInfo:us-east-1,
    RegionInfo:ap-northeast-1,
    RegionInfo:us-west-1,
    RegionInfo:us-west-2,
    RegionInfo:ap-southeast-1,
    RegionInfo:eu-west-1]
   >>> elb = regions[-1].connect()

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

   [Boto]
   elb_region_name = eu-west-1
   elb_region_endpoint = elasticloadbalancing.eu-west-1.amazonaws.com


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'])
[LoadBalancer:load-balancer-prod]

"get_all_load_balancers" returns a "boto.resultset.ResultSet" that
contains instances of "boto.ec2.elb.loadbalancer.LoadBalancer", each
of which abstracts access to a load balancer. "ResultSet" works very
much like a list.

>>> balancers = conn.get_all_load_balancers()
>>> balancers[0]
LoadBalancer:load-balancer-prod


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:

        TCP:PORT_TO_CHECK

     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:

        HTTP:PORT_TO_CHECK/RESOURCE

     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(
        interval=20,
        healthy_threshold=3,
        unhealthy_threshold=5,
        target='HTTP:8080/health'
    )


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
my_elb-123456789.us-east-1.elb.amazonaws.com

You can then CNAME map a better name, i.e. www.MYWEBSITE.com 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
      *Autoscale tutorial* 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()
