Amazon EC2 for SAP Sailing Analytics

Table of Contents

Quickstart

Servers

  • Web Server: ec2-54-229-94-254.eu-west-1.compute.amazonaws.com
  • Database and Queue Server: 172.31.25.253

Starting an instance

  • Which instance type to choose:
    • Archive: m2.2xlarge
    • Live: c1.xlarge

You may need to select "All generations" instead of "Current generation" to see these instance configurations. Of course, you may choose variations of those as you feel is appropriate for your use case.

  • Using a release, set the following in the instance's user data, replacing myspecificevent by a unique name of the event or series you'll be running on that instance, such as kielerwoche2014 or similar.
INSTALL_FROM_RELEASE=`name-of-release`
USE_ENVIRONMENT=live-server
MONGODB_NAME=myspecificevent
REPLICATION_CHANNEL=myspecificevent
SERVER_NAME=MYSPECIFICEVENT
BUILD_COMPLETE_NOTIFY=simon.marcel.pamies@sap.com
SERVER_STARTUP_NOTIFY=simon.marcel.pamies@sap.com

Note that when you select to install an environment using the USE_ENVIRONMENT variable, any other variable that you specify in the user data, such as the MONGODB_NAME or REPLICATION_CHANNEL properties in the example above, these additional user data properties will override whatever comes from the environment specified by the USE_ENVIRONMENT parameter.

  • To build from git, install and start, set the following in the instance's user data, adjusting the branch name (BUILD_FROM), the myspecificevent naming and memory settings according to your needs:
BUILD_BEFORE_START=True
BUILD_FROM=master
RUN_TESTS=False
COMPILE_GWT=True
BUILD_COMPLETE_NOTIFY=you@email.com
SERVER_STARTUP_NOTIFY=
SERVER_NAME=MYSPECIFICEVENT
MEMORY=2048m
REPLICATION_HOST=172.31.25.253
REPLICATION_CHANNEL=myspecificevent
MONGODB_HOST=172.31.25.253
MONGODB_PORT=10202
MONGODB_NAME=myspecificevent

Setting up a new image (AMI) from scratch (more or less)

See here

Receiving wind from Expedition

  • To receive and forward wind with an Expedition connector, log into webserver as user trac and switch to $HOME/servers/udpmirror. Start the mirror and forward it to the instance you want. In order to receive wind through the Igtimi connector, this step is not required as the wind data is received directly from the Igtimi server.

Setting up Master and Replica

  • Fire up a master with the following configuration. There is a preconfigured master environment at http://releases.sapsailing.com/environments/live-master-server that you should use.
INSTALL_FROM_RELEASE=(name-of-release)
USE_ENVIRONMENT=live-master-server
SERVER_NAME=MYSPECIFICEVENT
REPLICATION_CHANNEL=myspecificevent
MONGODB_NAME=myspecificevent
SERVER_STARTUP_NOTIFY=you@email.com
  • After your master server is ready, note the internal IP and configure your replica instances. Make sure to use the preconfigured environment from http://releases.sapsailing.com/environments/live-replica-server. Then absolutely make sure to add the line "REPLICATE_MASTER_SERVLET_HOST" to the user-data and adjust the myspecificevent master exchange name to the REPLICATION_CHANNEL setting you used for the master configuration.
INSTALL_FROM_RELEASE=(name-of-release)
USE_ENVIRONMENT=live-replica-server
REPLICATE_MASTER_SERVLET_HOST=(IP of your master server)
REPLICATE_MASTER_EXCHANGE_NAME=myspecificevent
REPLICATE_ON_START=com.sap.sailing.server.impl.RacingEventServiceImpl,com.sap.sse.security.impl.SecurityServiceImpl,com.sap.sse.filestorage.impl.FileStorageManagementServiceImpl,com.sap.sse.mail.impl.MailServiceImpl,com.sap.sailing.polars.impl.PolarDataServiceImpl
SERVER_NAME=MYSPECIFICEVENT
MONGODB_NAME=myspecificevent-replica
EVENT_ID=<some-uuid-of-an-event-you-want-to-feature>
SERVER_STARTUP_NOTIFY=you@email.com

Costs per month

To give you a feeling about the costs you can refer to the following table. To get all details go to http://www.awsnow.info/

Server Type Cost per Month Cost per Month (Reserved instance for 12 months)
m2.2xlarge (Archive) $800 $400
c1.xlarge (Build and Live) $500 $350

General Information and Security

Since XXX 2013 this project is using EC2 as the server provider. Amazon Elastic Compute Cloud (EC2) is a central part of Amazon.com's cloud computing platform, Amazon Web Services (AWS). EC2 allows users to rent virtual computers on which to run their own computer applications. EC2 allows scalable deployment of applications by providing a Web service through which a user can boot an Amazon Machine Image to create a virtual machine, which Amazon calls an "instance", containing any software desired. A user can create, launch, and terminate server instances as needed, paying by the hour for active servers, hence the term "elastic".

This project is associated with an SAP Sailing Analytics account that, for billing purposes, is a subsidiary of a main SAP billing account. The Analytics account number is "0173-6397-0217 (simon.marcel.pamies@sap.com)" and connected to "SAP CMC Production (hagen.stanek@sap.com)". It has "Dr. Axel Uhl (axel.uhl@sap.com)" configured as operations officer that can be contacted by Amazon in case of problems with the instances.

The main entry point for the account is https://console.aws.amazon.com/. There you can only log in using the root account. You will then have access to not only the EC2 Console but also to the main account details (including billing details).

Associated to the root account are n users that can be configured using the IAM (User Management, https://console.aws.amazon.com/iam/home). Each of these users can belong to different groups that have different rights associated. Currently two groups exist:

  • Administrators: Users belonging to this group have access to all EC2 services (including IAM). They do not have the right to manage main account information (like billing).

  • Seniors: Everyone belonging to this group can not access IAM but everything else.

Users configured in the IAM and at least belonging to the group Seniors can log in using the following url https://017363970217.signin.aws.amazon.com/console. All users that belong to one of these groups absolutely need to have MFA activated. MFA (Multi-Factor-Authentication) can be compared to the RSA token that needs to be input every time one wants to access the SAP network. After activation users need to synchronize their device using a barcode that is displayed in IAM. The device can be a software (Google Authenticator for iOS and Android) or a physical device.

In addition to having a password and MFA set for one user one can activate "Access Keys". These keys are a combination of hashed username ("ID") and a password ("Key"). These are needed in case of API related access (e.g. S3 uploader scripts). One user should not have more than 1 access key active because of security concerns and never distribute them over insecure channels.

EC2 Server Architecture for Sailing Analytics

The architecture is divided into logical tiers. These are represented by firewall configurations (Security Groups) that can be associated to Instances. Each tier can contain one or more instances. The following image depicts the parts of the architecture.

Tiers

  • Webserver: Holds one or more webserver instances that represent the public facing part of the architecture. Only instances running in this tier should have an Elastic IP assigned. In the image you can see one configured instance that delivers content for sapsailing.com. It has some services running on it like an Apache, the GIT repository and the UDP mirror. The Apache is configured to proxy HTTP(S) connections to an Archive or Live server.
  • Balancer: Features an Elastic Load Balancer. Such balancers can be configured to distribute traffic among many other running instances. Internally an ELB consists of multiple balancing instances on which load is distributed by a DNS round robin so that bandwidth is not a limiting factor.
  • Database: Instances handling all operations related to persistence. Must be reachable by the "Instance" and "Balancer+Group" tier. In the standard setup this tier only contains one database server that handles connections to MongoDB, MySQL and RabbitMQ.
  • Instances: Space where all instances, that are not logically grouped, live. In the image one can see three running instances. One serving archived data, one serving a live event and one for build and test purposes.
  • Balancer+Group: Analytics instances grouped and managed by an Elastic Load Balancer. A group is just a term describing multiple instances replicating from one master instance. The word "group" does in this context not refer to the so called "Placement Groups".

Instances

Name Access Key(s) Security Group Services Description
Webserver (Elastic IP: 54.229.94.254) Administrator Webserver Apache, GIT, Piwik, Bugzilla, Wiki This tier holds one instance that has one public Elastic IP associated. This instance manages all domains and subdomains associated with this project. It also contains the public GIT repository.
DB & Messaging Administrator Database and Messaging MongoDB, MySQL, RabbitMQ All databases needed by either the Analytics applications or tools like Piwik and Bugzilla are managed by this instance.
Archive Administrator, Sailing User Sailing Analytics App Java App Instance handling the access to all historical races.
Build and Test Administrator, Sailing User Sailing Analytics App X11,Firefox,Hudson Instance that can be used to run tests

HowTo

Create a new Analytics application instance ready for production

Create a new Analytics instance as described in detail here wiki/amazon-ec2-create-new-app-instance. You should use a configuration like the following. You have two possibilities of making sure that the server uses code from a specific branch.

  • First you can use a release file. These files can be usually found at http://releases.sapsailing.com/ and represent a certain point in time. These files can be built by using the buildAndUpdateProduct.sh with the parameter release. In addition to the release file you can specify an environment configuration. These usually can be found here http://releases.sapsailing.com/environments. A configuration then could look like this:
INSTALL_FROM_RELEASE=master-201311062138
USE_ENVIRONMENT=live-server
BUILD_COMPLETE_NOTIFY=simon.marcel.pamies@sap.com
SERVER_STARTUP_NOTIFY=simon.marcel.pamies@sap.com
  • The second option is to let the instance build itself from a specified branch. It is currently not supported to then specify an environment file. Attention: You can not start the building process on t1.micro instances having less than 1.5 GB of RAM! The configuration then looks like this:
BUILD_BEFORE_START=True
BUILD_FROM=master
RUN_TESTS=False
COMPILE_GWT=True
BUILD_COMPLETE_NOTIFY=simon.marcel.pamies@sap.com
SERVER_STARTUP_NOTIFY=
SERVER_NAME=LIVE1
MEMORY=2048m
REPLICATION_HOST=172.31.25.253
REPLICATION_CHANNEL=sapsailinganalytics-live
TELNET_PORT=14888
SERVER_PORT=8888
MONGODB_HOST=172.31.25.253
MONGODB_PORT=10202
EXPEDITION_PORT=2010
REPLICATE_ON_START=False
REPLICATE_MASTER_SERVLET_HOST=
REPLICATE_MASTER_SERVLET_PORT=
REPLICATE_MASTER_QUEUE_HOST=
REPLICATE_MASTER_QUEUE_PORT=
INSTALL_FROM_RELEASE=
USE_ENVIRONMENT=

After your instance has been started (and build and tests are through) it will be publicly reachable if you chose a port between 8090 and 8099. If you filled the BUILD_COMPLETE_NOTIFY field then you will get an email once the server has been built. You can also add your email address to the field SERVER_STARTUP_NOTIFY to get an email whenever the server has been started.

You can now access this instance by either using the Administrator key (for root User) or the Sailing User key (for user sailing):

ssh -i .ssh/Administrator.pem root@ec2-54-246-247-194.eu-west-1.compute.amazonaws.com

or

ssh -i .ssh/SailingUser.pem sailing@ec2-54-246-247-194.eu-west-1.compute.amazonaws.com

If you want to connect your instance to a subdomain then log onto the main webserver with the Administrator key as root, open the file /etc/httpd/conf.d/001-events.conf and put something like this there. As you can see you have to specify the IP address and the port the java server is running on. Make sure to always use the internal IP.

Use Event idm.sapsailing.com "<uuid-of-event-object>" 172.31.22.12 8888

Testing code on a server

Starting a test is as easy as starting up a new instance. Just make sure that you fill the field RUN_TESTS and set it to True. Also set the field BUILD_FROM to a gitspec that matches the code branch that you want to test. After tests has been run and the server has been started you will get an email giving you all the details. You can then access your instance or simply shut it down.

Build, deploy, start and stop an instance

Log on to the instance using ssh as user sailing. Change to the ~/code directory and fetch the latest git branch, e.g., using git fetch origin; git merge origin/master. Make sure you have a MongoDB instance running on the default port 27017 (see here). You can then trigger a build. It may be a good idea to do this in a tmux session because this will allow you to log off or get disconnected while the build is running. To start a tmux session, simply enter the command tmux at the console and you will get a new tmux session. Check the man page of tmux for more details and note that the usual Ctrl-B shortcut has been redefined in our instances to Ctrl-A to mimic the behavior of the old screen tool we used before we switched to tmux. Suffice it to say that you can detach from the tmux session by pressing Ctrl-A d which will keep the session running. To reconnect, enter tmux attach which works as long as you only have one tmux session running. In other cases, refer to the tmux man page again.

To launch the build, enter configuration/buildAndUpdateProduct.sh build as usual. Used without options, the build script will tell the options available. After the build has completed, use configuration/buildAndUpdateProduct.sh -s server install to install the product to ~/servers/server.

To launch the instance, change to the ~/servers/server directory and enter ./start. The instance will by default launch on port 8888. See ~/servers/server/env.sh for the instance's settings. To stop the instance again, from that same directory enter ./stop.

To access your instance externally, make sure to use the external Amazon DNS name, such as ec2-54-72-6-31.eu-west-1.compute.amazonaws.com. This is required for our Google Maps API key to recognize the domain. The map won't work when you simply use the IP number as the URL. A good URL to start with would therefore look something like http://ec2-54-72-6-31.eu-west-1.compute.amazonaws.com:8888/gwt/AdminConsole.html.

Setup replicated instances with ELB

The main concept behind ELB is that there is one instance that you configure in the "Load Balancers" tab that serves as the main entry point for all requests going to your application. This instance can be told to pass through requests from one port to another. In order to make this ELB instance aware of the Analytics EC2 Instances it should balance over you need to add all instances that should be part of the setup to the ELB instance.

A closer look reveals that an ELB instance consists itself of many other invisible instances. These are behind a DNS round robin configuration that redirects each incoming request to one of these instances. These invisible instances then decide upon the rules you've created how and where to distribute this request to one of the associated instances.

In a live event scenario, the SAP Sailing Analytics are largely bandwidth bound. Adding more users that watch races live doesn't add much CPU load, but it adds traffic linearly. Therefore, as the number of concurrent users grows, a single instance can quickly max out its bandwidth which for usual instances peaks at around 100Mbit/s. It is then essential that an ELB can offload the traffic to multiple instances which are replicas of a common master in our case.

To still get the usual logging and URL re-writing features, replicas need to run their local Apache server with a bit of configuration. Luckily, most of the grunt work is done for you automatically. You simply need to tell the replicas in their instance details to start replicating automatically, provide an EVENT_ID and set the SERVER_NAME variable properly. The Apache configuration on the replica will then automatically be adjusted such that the lower-case version of $SERVER_NAME.sapsailing.com will re-direct users to the event page for the event with ID $EVENT_ID.

Here are the steps to create a load balanced setup:

  • Create a master instance holding all data (see http://wiki.sapsailing.com/wiki/amazon-ec2#Setting-up-Master-and-Replica)
  • When using the Race Committee App (RCApp), try to make sure the app is configured to send its data to the master instance and not the ELB (otherwise, write requests may end up at replicas which then have to reverse-replicate these to the master which is as of this writing (2014-12-18) an EXPERIMENTAL feature). You may want to configure a separate URL for the master server for this purpose, so you don't have to re-configure the RCApp devices when switching to a different master server.
  • Create n instances that are configured to connect to the master server, automatically launching replication by using one of the *...-replica-...* environment from http://releases.sapsailing.com/environments.
  • Create a load balancer that redirects everything from HTTP port 8888 to HTTP port 8888 and leave the other switches and checkboxes on their default value
  • As "Ping Port" enter HTTP port 8888 and use /index.html as the "Ping Path." Leave the other values on their defaults again.
  • Put the ELB into the "Sailing Analytics App" security group as it will appear in the landscape as a regular sailing analytics application server.
  • Associate all your instances
  • Connect your domain with the IP of the load balancer. It could be a good idea to use an Elastic IP that always stays the same for the domain and associate it with your load balancer. That way you can also easily switch between a load balancer and a single instance setup. Again, remember not to let the RCApp devices point to the ELB domain as their updates could hit a replica which wouldn't know how to handle!

It is important to understand that it wouldn't help to let all traffic run through our central Apache httpd server which usually acts as a reverse proxy with comprehensive URL rewriting rules and macros. This would make the Apache server the bandwidth bottleneck. Instead, the event traffic needs to go straight to the ELB which requires the event DNS domain name to be mapped to the ELB's host name. You need to set this up in the "Route 53" DNS server which you find in the Amazon Services drop-down.

Go to the "Hosted Zones" entry

and select the sapsailing.com. row,

then click on "Go to Record Sets." You will then see the record sets for the sapsailing.com. domain:

Click on "Create Record Set" and fill in the subdomain name (myspecificevent in the example shown below) and as the value use the host name (A-record) of the ELB that you find in the ELB configuration.

Amazon ELB is designed to handle unlimited concurrent requests per second with “gradually increasing” load pattern (although it's initial capacity is described to reach 20k requests/secs). It is not designed to handle heavy sudden spike of load or flash traffic because of its internal structure where it needs to fire up more instances when load increases. ELB's can be pre-warmed though by writing to the AWS Support Team.

With this set-up, please keep in mind that administration of the sailing server instance always needs to happen through the master instance. A fat, red warning is displayed in the administration console of the replica instances that shall keep you from making administrative changes there. Change them on the master, and the changes will be replicated to the replicas.

You can monitor the central RabbitMQ message queueing system at http://54.246.250.138:15672/#/exchanges. Use guest/guest for username and password. You should find the exchange name you configured for you master there and will be able to see the queues bound to the exchange as well as the traffic running through the exchange.

Using Latency-Based DNS across Regions

ELBs don't work across regions (such as eu-west-1 and ap-southeast-1) but only across the availability zones within one region. Therefore, if you want location and latency-based routing for your event, you have to set up so-called latency-based routing policies using alias record sets that point to your ELBs, one per region. The alias records are all for the same domain name and are each an alias for one ELB in one region. The DNS service will then find out for a requesting client which of the ELBs for the same name will provide the shortest network latency and return that record. Additionally, the DNS service can be configured to evaluate the ELBs health checking status. If the ELB has bad health, DNS requests will subsequently be answered using other alias records, therefore pointing to ELBs in other regions.

The above image shows what you need to do: After you've set up an ELB in each of the regions as described above, for each of them add an alias record set. Enter the domain name and make sure it's the same for all ELBs. Change the "Alias" radio button to "Yes." The alias target can usually be selected from a drop-down, but in case you aren't offered the ELB you want to add, you can as well copy the ELB DNS name shown in the configuration page of that ELB.

Change the routing policy to "Latency" and select the region in which your ELB is located. A meaninful ID string may be helpful later to identify the record set from a list of record sets.

Select the "Yes" radio button for "Evaluate Target Health" in order to make sure the DNS server checks the ELB's health status and fails over to any of your other latency-based record sets if the ELB has bad health.

Should you be switching from an Apache reverse proxy set-up with a record in /etc/httpd/conf.d/001-events.conf for your sub-domain, remember to comment or remove this record and to reload the httpd service on the central web server using service httpd reload. Its effects may otherwise interfere with the effects of the DNS entries for that same sub-domain. Conversely, before removing the DNS record sets for the sub-domain, if migrating back to a non-ELB, non-DNS scenario, remember to first re-activate the 001-events.conf reverse proxy entry before removing the DNS record sets.

Access MongoDB database

To launch a local MongoDB instance on the default port, enter a command such as mkdir /home/sailing/mongodb; /opt/mongodb-linux-x86_64-1.8.1/bin/mongod --dbpath /home/sailing/mongodb. To make sure the process survives log-off, you may want to launch it in a tmux session. See above for how to create a tmux session. In an existing tmux session, a new window can be created using Ctrl-a c.

Upgrade the Sailing Analytics App AMI image

There are a number of use cases that suggest an upgrade of the AMI that we use to start new SAP Sailing Analytics instances. One can be that we want to add or modify the set of SSH keys authorized for access to the root and sailing accounts. Another can be that we would like to upgrade the git contents under ~sailing/code for an update to the /etc/init.d/sailing or the /home/sailing/code/java/target/refreshInstance.sh script that is central to the automated launch process of a new instance. Another use case can be applying an upgrade to the underlying operating system (currently CentOS).

Follow these steps to upgrade the AMI:

  • Launch a new instance based on the existing AMI
  • Log in as user root
  • Run yum update to update the operating system
  • Remove any obsolete logs from /home/sailing/servers/server/logs
  • Update the git contents (essential for up-to-date versions of /etc/init.d/sailing which links to the git, and the refreshInstance.sh script used during automatic instance launch), and clean any build artifacts by doing <pre> su - sailing cd code mvn clean git fetch git merge origin/master</pre>
  • Then, still as user sailing, edit ~sailing/servers/server/env.sh and remove everything after the line # **** Overwritten environment variables **** as this will then be the place where any downloaded environment and the EC2 user data variables will be appended later during automatic installation upon reboot.
  • Check the sizes of the mounted partitions by doing df; swapon -s. These will come in handy after creating the new AMI in order to tag the new volume snapshots accordingly
  • Update any keys in /root/.ssh/authorized_keys and /home/sailing/.ssh/authorized_keys
  • In the EC2 administration console go to the "Instances" tab, select your running instance and from the "Actions" drop-down select "Create Image". Give the image the name "SAP Sailing Analytics App x.y" where "x.y" is the updated version number of the image. Just make sure it's greater than the previous one. If you feel like it, you may provide a short description telling the most important features of the image.
  • Once the image creation has completed, go to the Snapshots list in the "Elastic Block Store" category and name the new snapshots appropriately. Now the information about the device sizes obtained earlier from the df and swapon commands will help you to identify which snapshot is which. Usually, the three snapshots would be something like AMI Analytics Home x.y, AMI Analytics System x.y and AMI Analytics Swap x.y with "x.y" being the version number matching that of your image.
  • Now you can remove any earlier Sailing Server AMI version and the corresponding snapshots.

Migration Checklist

Before switching sapsailing.com to the EC2 webserver

  • fire up archive server and load it (DONE)
  • configure 001-events.conf starting with a copy from old sapsailing.com, using test URLs (rombalur.de) (DONE)
  • clone entire MongoDB content (DONE)
  • migrate MySQL for Bugzilla
  • ensure that all users have access; either solicit their public keys and enter to ~trac/.ssh/authorized_keys or migrate /etc/passwd and /etc/group settings for access to trac group (DONE)
  • run test build and deploy (DONE)
  • fire up a live server and test it (DONE)
  • fire up a replica and check that it works correctly (ERROR!)
  • check that UDP mirror is working (DONE)
  • check that SwissTiming StoreAndForward is working
  • check that we can fire up a live2 / archive2 server and switch transparently

Just before the migration on Sunday evening

  • check that sapsailing.com is entered everywhere a hostname / domain name is required, particularly in /etc/httpd/conf.d/001-events.conf and /opt/piwik-scripts and all of /etc - also have a look at piwik and bugzilla configuration (DONE)
  • disable bugzilla on old.sapsailing.com because Nameserver switch can take up to 48 hours for everyone (DONE)
  • copy /home/trac/releases to webserver (DONE)
  • import bugzilla to mysql (DONE)
  • git fetch –all on webserver (DONE)
  • tell SAP hostmaster to point old.sapsailing.com to 195.227.10.246

Immediately after switching the sapsailing.com domain to the EC2 webserver on Sunday evening

  • check that old.sapsailing.com points to 195.227.10.246
  • check that EC2 web server is responding to sapsailing.com now
  • fetch all git branches from what is now old.sapsailing.com; also sync gollum wiki git
  • ask people (including internal Git team) to update their known_hosts files according to the new web server's key
  • check if build server can access new sapsailing.com
  • check why swisstiminglistener doesn't receive connections and fix

Glossary

Term Description
Instance Virtual machine that runs on a Xen host. Such an instance runs forever until it is stopped. It will be billed by hours it ran. Each start will be billed by a full hour.
Spot Instance Instances that run whenever there are free resources. It is not possible to control when or where these instances run. These instances are much cheaper than normal instances.
Amazon Machine Image (AMI) Amazon Machine Image: Image file that contains a filesystem and a preinstalled operating system. One can create AMIs very easily from a stopped Instance by first creating a snapshot and then converting it to an AMI.
Volume An active harddisk that can be associated to one Instance.
IOPS Input/Output operations per second. Metric used to denote the performance of a volume. The higher the IOPS value the better the speed. Be aware of the fact that IOPS is metered by IOPS/h and is very expensive. Use with care!
Snapshot Snapshot of a Volume
Elastic IP IP address that can be associated to an instance. Any Elastic-IP not associated to a running Instance costs some amount of money per hour.
Security Group Firewall configuration that can be associated to an instance. There is no need of configuring iptables or such. One can associate many instances the the same Security Group.
Elastic Load Balancer (ELB) Service that makes it possible to balance over services running on different instances.
Network Interfaces Virtual network interfaces that are mapped to physical network interfaces on instances.
Placement Groups Enables applications to get the full-bisection bandwidth and low-latency network performance required for tightly coupled, node-to-node communication. Placement Groups can only contain HVM instance and have other limitations described here: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using_cluster_computing.html