Scripts to download files from google storage based on sha1 sums

third_party/gsutil/boto/docs/source/elb_tut.rst

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+.. _elb_tut:
+
+==========================================================
+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: http://aws.amazon.com/elasticloadbalancing/
+
+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.
+
+.. _Elastic Compute Cloud: http://aws.amazon.com/ec2/
+
+Creating a Connection
+---------------------
+
+The first step in accessing ELB is to create a connection to the service.
+
+>>> import boto
+>>> conn = boto.connect_elb(
+        aws_access_key_id='YOUR-KEY-ID-HERE',
+        aws_secret_access_key='YOUR-SECRET-HERE'
+    )
+
+
+A Note About Regions and Endpoints
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+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, instantiate the
+ELBConnection object with that region's information.
+
+>>> from boto.regioninfo import RegionInfo
+>>> reg = RegionInfo(
+        name='eu-west-1',
+        endpoint='elasticloadbalancing.eu-west-1.amazonaws.com'
+    )
+>>> conn = boto.connect_elb(
+        aws_access_key_id='YOUR-KEY-ID-HERE',
+        aws_secret_access_key='YOUR-SECRET-HERE',
+        region=reg
+    )
+
+Another way to connect to an alternative region is like this:
+
+>>> import boto.ec2.elb
+>>> elb = boto.ec2.elb.connect_to_region('eu-west-1')
+
+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: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]
+
+: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]
+[LoadBalancer:load-balancer-prod]
+
+Creating a Load Balancer
+------------------------
+To create a load balancer you need the following:
+ #. The specific **ports and protocols** you want to load balancer over, and what port
+    you want to connect to all instances.
+ #. 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.
+ #. 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.
+
+.. _elb-configuring-a-health-check:
+
+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*:
+
+>>> regions = ['us-east-1a', 'us-east-1b']
+>>> ports = [(80, 8080, 'http'), (443, 8443, 'tcp')]
+>>> lb = conn.create_load_balancer('my-lb', regions, 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:
+
+ #. Manually, adding each instance in turn.
+ #. 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()
+
+