Stanchion

Stanchion is an application used by Riak CS to manage the serialization of requests, which enables Riak CS to manage globally unique entities like users and bucket names. Serialization in this context means that the entire cluster agrees upon a single value for any globally unique entity at any given time; when that value is changed, the new value must be recognized throughout the entire cluster.

The Role of Stanchion in a Riak CS Cluster

Unlike Riak and Riak CS, which both run on multiple nodes in your cluster, there should be only one running Stanchion instance in your Riak CS cluster at any time. Correspondingly, your Stanchion installation must be managed and configured separately. For more information, see the following documents:

• Configuring Stanchion
• Installing Stanchion
• The Stantion Command-line Interface

For a more in-depth discussion of implementation details, see the project’s README on GitHub.

Globally Unique Entities

There are two types of entities that must be globally unique within a Riak CS system:

1. User identifiers — Riak CS mandates that each user create an account using an email address as an identifier. Stanchion takes steps to ensure that an email address has not already been used before accepting a user creation request.
2. Bucket names — Bucket names must be unique within a Riak CS system (just as they must be unique in S3 and other systems) and any attempt to create a bucket with a name that is already in use are rejected.

The uniqueness of these entities is enforced by serializing any creation or modification requests that involve them. This process is handled by Stanchion. What happens under the hood is essentially that Stanchion mandates that all vnodes in the underlying Riak cluster that are responsible for the user or bucket being created must be available at creation time.

One result of this enforcement is that user creation requests and bucket creation or modification, i.e. deletion, requests are not highly available like other Riak CS system operations. If the Stanchion application is unavailable or cannot be reached for whatever reason, you will not be able to carry out user- and bucket-related operations. In addition, instability in the Riak cluster may lead to user and bucket requests being disallowed. If this happens, you will see something like this in the Stanchion console or error logs:

2013-01-03 05:24:24.028 [warning] <0.110.0>@stanchion_utils:bucket_available:501 Error occurred trying to check if the bucket <<"mybucket">> exists. Reason: <<"{pr_val_unsatisfied,3,2}">>

Because of this, user- and bucket-related operations should be used only as preparation for a workflow and not included as part of a highly available workflow.