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Updating Instance Metadata

You can add and update custom metadata for a compute instance using the Command Line Interface (CLI) or REST API documentation.

When you create an instance using the LaunchInstance operation, you can specify custom metadata for the instance in the LaunchInstanceDetails datatype's metadata or extendedMetadata attributes.

To update an instance's metadata, use the UpdateInstance operation, specifying the custom metadata in the UpdateInstanceDetails datatype's metadata or extendedMetadata attributes.

The metadata attribute supports key/value string pairs. The extendedMetadata attribute supports nested JSON objects. The combined size of these two attributes can be a maximum of 32,000 bytes.

It might take up to a minute for the changes to be reflected in the instance metadata service.

For permissions, see Required IAM Policy for Working with Instances.

Using the API

When you use the UpdateInstance operation, the instance's metadata will be the combination of the values specified in the UpdateInstanceDetails datatype's metadata or extendedMetadata attributes. Any set of key/value pairs specified for these attributes in the UpdateInstance operation will replace the existing values for these attributes, so you need to include all the metadata values for the instance in each call, not just the ones you want to add. If you leave the attribute empty when calling UpdateInstance, the existing metadata values in that attribute will be used. You cannot specify a value for the same metadata key twice, as this will cause the UpdateInstance operation to fail due to there being duplicate keys.

To understand this, consider the example scenario where you created an instance using the LaunchInstance operation and specified the following key/value pair for the metadata attribute:

"myCustomMetadataKey" : "myCustomMetadataValue"

If you then call the UpdateInstance operation, and add new metadata by specifying additional key/value pairs in the extendedMetadata attribute, but you leave the metadata attribute empty, do not include the myCustomMetadataKey key/value in the extendedMetadata attribute, as this will cause the operation to fail since that key already exists. If you do specify values for the metadata attribute, you need to include the myCustomMetadataKey key/value to maintain it in the instance's metadata. In this case, you can specify it in either of the attributes.

There are two reserved keys, user_data and ssh_authorized_keys, that can only be set for an instance at launch time, they cannot be updated later. If you use the metadata attribute to add or update metadata for an instance, you need to ensure that you include the values specified at launch time for both these keys, otherwise the UpdateInstance operation will fail.

Best Practices for Updating an Instance's Metadata

When using the UpdateInstance operation, we recommend the following:

  • Use the GetInstance operation to retrieve the existing custom metadata for the instance to ensure that you include the values you want to maintain in the appropriate attributes when you call UpdateInstance. The metadata values are returned in the metadata and extendedMetadata attributes for the Instance . For a code example demonstrating this, see the UpdateInstanceExample in the SDK for Java.

  • Unless you are updating custom metadata that was added using the metadata attribute, use the extendedMetadata attribute to add custom metadata. Otherwise you need to include the launch time values for the user_data and ssh_authorized_keys reserved keys. If you use the metadata attribute to add values and you leave out the values for these reserved keys or specify different values for them, the UpdateInstance call will fail.

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