274 lines
12 KiB
ReStructuredText
274 lines
12 KiB
ReStructuredText
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=============================================
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Using Swift as Backing Store for Service Data
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=============================================
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----------
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Background
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----------
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This section provides guidance to OpenStack Service developers for how to
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store your users' data in Swift. An example of this is that a user requests
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that Nova save a snapshot of a VM. Nova passes the request to Glance,
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Glance writes the image to a Swift container as a set of objects.
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Throughout this section, the following terminology and concepts are used:
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* User or end-user. This is a person making a request that will result in
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an OpenStack Service making a request to Swift.
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* Project (also known as Tenant). This is the unit of resource ownership.
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While data such as snapshot images or block volume backups may be
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stored as a result of an end-user's request, the reality is that these
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are project data.
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* Service. This is a program or system used by end-users. Specifically, it
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is any program or system that is capable of receiving end-user's tokens and
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validating the token with the Keystone Service and has a need to store
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data in Swift. Glance and Cinder are examples of such Services.
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* Service User. This is a Keystone user that has been assigned to a Service.
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This allows the Service to generate and use its own tokens so that it
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can interact with other Services as itself.
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* Service Project. This is a project (tenant) that is associated with a
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Service. There may be a single project shared by many Services or there
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may be a project dedicated to each Service. In this document, the
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main purpose of the Service Project is to allow the system operator
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to configure specific roles for each Service User.
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-------------------------------
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Alternate Backing Store Schemes
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-------------------------------
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There are three schemes described here:
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* Dedicated Service Account (Single Tenant)
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Your Service has a dedicated Service Project (hence a single dedicated
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Swift account). Data for all users and projects are stored in this
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account. Your Service must have a user assigned to it (the Service User).
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When you have data to store on behalf of one of your users, you use the
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Service User credentials to get a token for the Service Project and
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request Swift to store the data in the Service Project.
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With this scheme, data for all users is stored in a single account. This
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is transparent to your users and since the credentials for the Service User
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are typically not shared with anyone, your users' cannot access their
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data by making a request directly to Swift. However, since data belonging
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to all users is stored in one account, it presents a single point of
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vulnerably to accidental deletion or a leak of the service-user
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credentials.
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* Multi Project (Multi Tenant)
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Data belonging to a project is stored in the Swift account
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associated with the project. Users make requests to your Service using
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a token scoped to a project in the normal way. You can then use this
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same token to store the user data in the project's Swift account.
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The effect is that data is stored in multiple projects (aka tenants).
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Hence this scheme has been known as the "multi tenant" scheme.
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With this scheme, access is controlled by Keystone. The users must
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have a role that allows them to perform the request to your Service. In
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addition, they must have a role that also allows them to store data in
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the Swift account. By default, the admin or swiftoperator roles are
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used for this purpose (specific systems may use other role names). If the
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user does not have the appropriate roles, when your Service attempts
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to access Swift, the operation will fail.
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Since you are using the user's token to access the data, it follows that
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the user can use the same token to access Swift directly -- bypassing your
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Service. When end-users are browsing containers, they will also see
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your Service's containers and objects -- and may potentially delete
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the data. Conversely, there is no single account where all data so leakage
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of credentials will only affect a single project/tenant.
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* Service Prefix Account
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Data belonging to a project is stored in a Swift account associated
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with the project. This is similar to the Multi Project scheme described
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above. However, the Swift account is different than the account that
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users access. Specifically, it has a different account prefix. For example,
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for the project 1234, the user account is named AUTH_1234. Your Service uses
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a different account, for example, SERVICE_1234.
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To access the SERVICE_1234 account, you must present two tokens: the user's
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token is put in the X-Auth-Token header. You present your Service's token
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in the X-Service-Token header. Swift is configured such that only when both
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tokens are presented will it allow access. Specifically, the user cannot
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bypass your Service because they only have their own token. Conversely, your
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Service can only access the data while it has a copy of the user's token --
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the Service's token by itself will not grant access.
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The data stored in the Service Prefix Account cannot be seen by end-users.
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So they cannot delete this data -- they can only access the data if they
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make a request through your Service. The data is also more secure. To make
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an unauthorized access, someone would need to compromise both an end-user's
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and your Service User credentials. Even then, this would only expose one
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project -- not other projects.
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The Service Prefix Account scheme combines features of the Dedicated Service
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Account and Multi Project schemes. It has the private, dedicated,
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characteristics of the Dedicated Service Account scheme but does not present
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a single point of attack. Using the Service Prefix Account scheme is a little
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more involved than the other schemes, so the rest of this document describes
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it more detail.
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-------------------------------
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Service Prefix Account Overview
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-------------------------------
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The following diagram shows the flow through the system from the end-user,
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to your Service and then onto Swift::
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client
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\
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\ <request>: <path-specific-to-the-service>
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\ x-auth-token: <user-token>
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\
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SERVICE
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\
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\ PUT: /v1/SERVICE_1234/<container>/<object>
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\ x-auth-token: <user-token>
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\ x-service-token: <service-token>
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\
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Swift
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The sequence of events and actions are as follows:
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* Request arrives at your Service
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* The <user-token> is validated by the keystonemiddleware.auth_token
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middleware. The user's role(s) are used to determine if the user
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can perform the request. See :doc:`overview_auth` for technical
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information on the authentication system.
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* As part of this request, your Service needs to access Swift (either to
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write or read a container or object). In this example, you want to perform
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a PUT on <container>/<object>.
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* In the wsgi environment, the auth_token module will have populated the
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HTTP_X_SERVICE_CATALOG item. This lists the Swift endpoint and account.
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This is something such as https://<netloc>/v1/AUTH_1234 where ``AUTH_``
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is a prefix and ``1234`` is the project id.
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* The ``AUTH_`` prefix is the default value. However, your system may use a
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different prefix. To determine the actual prefix, search for the first
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underscore ('_') character in the account name. If there is no underscore
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character in the account name, this means there is no prefix.
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* Your Service should have a configuration parameter that provides the
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appropriate prefix to use for storing data in Swift. There is more
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discussion of this below, but for now assume the prefix is ``SERVICE_``.
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* Replace the prefix (``AUTH_`` in above examples) in the path with
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``SERVICE_``, so the full URL to access the object becomes
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https://<netloc>/v1/SERVICE_1234/<container>/<object>.
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* Make the request to Swift, using this URL. In the X-Auth-Token header place
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a copy of the <user-token>. In the X-Service-Token header, place your
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Service's token. If you use python-swiftclient you can achieve this
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by:
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* Putting the URL in the ``preauthurl`` parameter
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* Putting the <user-token> in ``preauthtoken`` paramater
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* Adding the X-Service-Token to the ``headers`` parameter
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Using the HTTP_X_SERVICE_CATALOG to get Swift Account Name
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----------------------------------------------------------
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The auth_token middleware populates the wsgi environment with information when
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it validates the user's token. The HTTP_X_SERVICE_CATALOG item is a JSON
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string containing details of the OpenStack endpoints. For Swift, this also
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contains the project's Swift account name. Here is an example of a catalog
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entry for Swift::
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"serviceCatalog": [
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...
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{
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....
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"type": "object-store",
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"endpoints": [
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...
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{
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...
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"publicURL": "https://<netloc>/v1/AUTH_1234",
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"region": "<region-name>"
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...
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}
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...
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...
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}
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}
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To get the End-user's account:
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* Look for an entry with ``type`` of ``object-store``
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* If there are several regions, there will be several endpoints. Use the
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appropriate region name and select the ``publicURL`` item.
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* The Swift account name is the final item in the path ("AUTH_1234" in this
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example).
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Getting a Service Token
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-----------------------
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A Service Token is no different than any other token and is requested
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from Keystone using user credentials and project in the usual way. The core
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requirement is that your Service User has the appropriate role. In practice:
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* Your Service must have a user assigned to it (the Service User).
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* Your Service has a project assigned to it (the Service Project).
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* The Service User must have a role on the Service Project. This role is
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distinct from any of the normal end-user roles.
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* The role used must the role configured in the /etc/swift/proxy-server.conf.
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This is the ``<prefix>_service_roles`` option. In this example, the role
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is the ``service`` role::
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[keystoneauth]
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reseller_prefix = AUTH_, SERVICE_
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SERVICE_service_role = service
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The ``service`` role should only be granted to OpenStack Services. It should
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not be granted to users.
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Single or multiple Service Prefixes?
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------------------------------------
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Most of the examples used in this document used a single prefix. The
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prefix, ``SERVICE`` was used. By using a single prefix, an operator is
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allowing all OpenStack Services to share the same account for data
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associated with a given project. For test systems or deployments well protected
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on private firewalled networks, this is appropriate.
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However, if one Service is compromised, that Service can access
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data created by another Service. To prevent this, multiple Service Prefixes may
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be used. This also requires that the operator configure multiple service
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roles. For example, in a system that has Glance and Cinder, the following
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Swift configuration could be used::
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[keystoneauth]
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reseller_prefix = AUTH_, IMAGE_, BLOCK_
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IMAGE_service_roles = image_service
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BLOCK_service_roles = block_service
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The Service User for Glance would be granted the ``image_service`` role on its
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Service Project and the Cinder Service user is granted the ``block_service``
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role on its project. In this scheme, if the Cinder Service was compromised,
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it would not be able to access any Glance data.
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Container Naming
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----------------
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Since a single Service Prefix is possible, container names should be prefixed
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with a unique string to prevent name clashes. We suggest you use the service
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type field (as used in the service catalog). For example, The Glance Service
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would use "image" as a prefix.
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