Instance Metadata

Amazon Elastic Compute Cloud: 2009-08-15

previous page next page

Instance Metadata

Amazon EC2 instances can access instance-specific metadata as well as data supplied when launching the instances. This data can be used to build more generic AMIs that can be modified by configuration files supplied at launch time.

If you run web servers for various small businesses, they can all use the same AMI and retrieve their content from the Amazon S3 bucket you specify at launch.

To add a new customer at any time, simply create a bucket for the customer, add their content, and launch your AMI.

[Note] Note

For information on categories of metadata, see Metadata Categories.

Data Retrieval

An instance retrieves the data by querying a web server using a Query API. The base URI of all requests is http://169.254.169.254/2009-08-15/ where 2009-08-15 indicates the API version.

[Note] Note

Amazon EC2 Version 1.0 is part of a legacy versioning scheme. Newer versions follow a date based versioning scheme. For more information on the versioning scheme used by Amazon EC2, go to the Amazon Elastic Compute Cloud API Reference.

The latest version of the API is always available using the URI http://169.254.169.254/latest.

Security of Launch Data

Although only your specific instance can access launch data, the data is not protected by cryptographic methods. You should take suitable precautions to protect sensitive data (such as long lived encryption keys).

[Note] Note

You are not billed for HTTP requests used to retrieve metadata and user-supplied data.

Metadata Retrieval

Requests for a specific metadata resource returns the appropriate value or a 404 HTTP error code if the resource is not available. All metadata is returned as text (content type text/plain).

Requests for a general metadata resource (i.e. an URI ending with a /) return a list of available resources or a 404 HTTP error code if there is no such resource. The list items are on separate lines terminated by line feeds (ASCII 10).

Example

The following examples list HTTP GET requests and responses. You can use a tool such as curl or wget to make these types of requests.

This example gets the available API versions.

GET http://169.254.169.254/
1.0
2007-01-19
2007-03-01
2009-08-15

This example gets the top-level metadata items.

GET http://169.254.169.254/2009-08-15/meta-data/
ami-id
ami-launch-index
ami-manifest-path
block-device-mapping/
hostname
instance-id
instance-type
local-hostname
local-ipv4
placement/
public-hostname
public-ipv4
public-keys/
reservation-id
security-groups

This example gets the value of each metadata item from the preceding example.

GET http://169.254.169.254/2009-08-15/meta-data/ami-manifest-path
my-amis/spamd-image.manifest.xml
GET http://169.254.169.254/2009-08-15/meta-data/ami-manifest-path
my-amis/spamd-image.manifest.xml
GET http://169.254.169.254/2009-08-15/meta-data/ami-id
ami-2bb65342
GET http://169.254.169.254/2009-08-15/meta-data/reservation-id
r-fea54097
GET http://169.254.169.254/2009-08-15/meta-data/hostname
ec2-67-202-51-223.compute-1.amazonaws.com

This example gets the list of available public keys.

GET http://169.254.169.254/2009-08-15/meta-data/public-keys/
0=my-public-key

This example shows the formats in which public key 0 is available. 

GET http://169.254.169.254/2009-08-15/meta-data/public-keys/0/
openssh-key

This example gets public key 0 (in the OpenSSH key format).

GET http://169.254.169.254/2009-08-15/meta-data/public-keys/0/openssh-key
ssh-rsa AAAA.....wZEf my-public-key

This example gets the product code.

GET http://169.254.169.254/2009-08-15/meta-data/product-codes
774F4FF8

User Data Retrieval

Requests for the user data returns the data as-is (content type application/x-octetstream).

[Note] Note

All user-supplied data is treated as opaque data; what you give us is what you get back. It is the responsibility of the instance to interpret this data appropriately.

Example

This shows an example of returning comma-separated user-supplied data.

GET http://169.254.169.254/2009-08-15/user-data
1234,fred,reboot,true | 4512,jimbo, | 173,,,

This shows an example of returning line-separated user-supplied data.

GET http://169.254.169.254/2009-08-15/user-data
[general]
instances: 4

[instance-0]
s3-bucket: fred

[instance-1]
reboot-on-error: yes

Use Case: AMI Launch Index Value

In this example, Alice wants to launch four instances of her favorite database AMI with the first acting as master and the remainder acting as replicas.

The master database configuration specifies various database parameters (e.g., the size of store) while the replicas' configuration specifies different parameters, such as the replication strategy. Alice decides to provide this data as an ASCII string with a pipe symbol (| delimiting the data for the various instances: 

store-size=123PB backup-every=5min | replicate-every=1min | replicate-every=2min | replicate-every=10min | replicate-every=20min

The store-size=123PB backup-every=5min defines the master database configuration, replicate-every=1min defines the first replicant's configuration, replicate-every=2min defines the second replicant's configuration, and so on.

Alice launches four instances. 

PROMPT> ec2-run-instances ami-2bb65342 -n 4 -d "store-size=123PB backup-every=5min | replicate-every=1min | replicate-every=2min | replicate-every=10min | replicate-every=20min"

RESERVATION     r-fea54097      598916040194    default
INSTANCE i-3ea74257 ami-2bb65342 pending 0 m1.small 2007-08-07T11:29:58+0000 us-east-1c
INSTANCE i-31a74258 ami-2bb65342 pending 1 m1.small 2007-08-07T11:29:58+0000 us-east-1c
INSTANCE i-31a74259 ami-2bb65342 pending 2 m1.small 2007-08-07T11:29:58+0000 us-east-1c
INSTANCE i-31a7425a ami-2bb65342 pending 3 m1.small 2007-08-07T11:29:58+0000 us-east-1c

Once launched, all instances have a copy of the user data and the common metadata shown here:

  • AMI id: ami-2bb65342

  • AMI manifest path: ec2-public-images/getting-started.manifest.xml

  • Reservation ID: r-fea54097

  • Public keys: none

  • Security group names: default

  • Instance type: m1.small

However each instance has certain unique metadata.

Instance 1

Metadata Value
instance-id i-3ea74257
ami-launch-index 0
public-hostname ec2-67-202-51-223.compute-1.amazonaws.com
public-ipv4 67.202.51.223
local-hostname ip-10-251-50-35.ec2.internal
local-ipv4 10.251.50.35

Instance 2

Metadata Value
instance-id i-31a74258
ami-launch-index 1
public-hostname ec2-67-202-51-224.compute-1.amazonaws.com
public-ipv4 67.202.51.224
local-hostname ip-10-251-50-36.ec2.internal
local-ipv4 10.251.50.36

Instance 3

Metadata Value
instance-id i-31a74259
ami-launch-index 2
public-hostname ec2-67-202-51-225.compute-1.amazonaws.com
public-ipv4 67.202.51.225
local-hostname ip-10-251-50-37.ec2.internal
local-ipv4 10.251.50.37

Instance 4

Metadata Value
instance-id i-31a7425a
ami-launch-index 3
public-hostname ec2-67-202-51-226.compute-1.amazonaws.com
public-ipv4 67.202.51.226
local-hostname ip-10-251-50-38.ec2.internal
local-ipv4 10.251.50.38

Therefore, an instance can determine its portion of the user-supplied data through the following process.

Metadata Discovery Process

1

Determine the instance in the launch group.

GET http://169.254.169.254/2009-08-15/meta-data/ami-launch-index
1

2

Retrieve the user data.

GET http://169.254.169.254/2009-08-15/user-data
          store-size=123PB backup-every=5min | replicate-every=1min | replicate-every=2min | replicate-every=10min | replicate-every=20min

3

Extract the appropriate part of the user data.

user_data.split('|')[ami_launch_index]

previous page start next page