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RESTful Bag Store

Overview

This document briefly describes an approach to making BagIt style containers of content available on the Web so that content can be easily discovered, retrieved and otherwise annotated. BagIt style directories, or Bags, are a lightweight mechanism for documenting a set of files and their fixities in a manifest, and associating them with some generic metadata.

Work on an a REST API for Bags grew out of conversations started by the Educopia Institute in late 2010. Before diving into the basic design, a few use cases to describe the problem space will be described.

Use Cases

Design

This describes the public interface of an endpoint, which could be an entire service or a project-specific subdirectory on generic content storage system.

Basic Features

  • Pure HTTP
  • Authentication and access control are considered a server implementation issue using standard HTTP techniques
  • Does not require an intelligent server, with a goal of allowing simple static servers or cloud services like S3 to be used as compatible, read-only implementations

Controversial Points

  • Implementations MUST support JSON representations of resources, MAY support XML and other formats

Structure

/changes:

Feed listing new bags (should the feed list only new bags, or also e.g. deleted and modified bags?)
/bags/:

Resource listing available bags.

Responses are returned with pagination:

pagination:
offset:Offset into the list of bags
limit:Limit on the number of results
total_count:Total number of bags
next:Link to the next page of results, if available
previous:Link to the previous page of results, if available
objects:
List of bags in the following format:
href:Location of the bag
id:User-assigned bag ID

GETing /bags/ <BAG_ID> / will return a response containing the following metadata:

links:

List of links to other resources on this server (see below) using the following format, as in HTML link tags (see RFC 5988 for valid rel values).

rel:forward link types
href:URI for linked resource
type:advisory content type
info:

Parsed values from bag-info.txt returned as a list of key-value pairs
bagit:

Parsed dictionary from bagit.txt

Clients may POST to /bags/ <BAG_ID> / to perform several operations:

commit:

Complete an upload (see "Creating a bag" below)

Servers MUST not include a bag in any public listings until the bag has been committed.
validate:

Request that the server validate the bag contents against the manifest

Under /bags/ <BAG_ID> / will be several resources:

copies:

Feed listing alternate locations for this bag by URL

TODO: specify format

Mirrors can PUT their location after mirroring this bag. Servers are not required to accept these requests.

TODO: Specify rel types for instances
notes:

Feed containing comments from curators

TODO: Should this be history?
versions:

GET returns a list of version IDs:

[
    {
        "id": "<unique identifier>",
        "timestamp": "ISO 8601 representation",
        "name": "<optional symbolic name>"
    },
    …
]

Each version contains two resources:

manifest:

Resource enumerating bag contents. This is a dictionary with two keys:

tag:List of tag files as defined in the BagIt specification section 1.3 (Terminology)
payload:List of payload files as defined in the BagIt specification section 1.3 (Terminology)

Each list contains dictionaries with the following structure:

path:The file's full path relative to the bag root, i.e. data/foobar.tiff
checksum:Dictionary of encoded checksum values using the algorithm as the key. This is optional for tag files.

Example:

{
    "payload": [
        {
            "checksum": {
                "md5": "00fcbdf37a87dced7b969386efe6e132",
                "sha1": "74a272487eb513f2fb3984f2a7028871fcfb069b"
            },
            "path": "data/path/to/example.pdf"
        }
    ],
    "tag": [
        {
            "path": "bagit.txt"
        },
        {
            "path": "bag-info.txt"
        },
        {
            "path": "manifest-md5.txt"
        },
        {
            "path": "manifest-sha1.txt"
        }
    ]
}
contents:

Root for access to bag contents: for any file path in the manifest, /bags/ <BAG_ID> /contents/ <PATH> will return the raw file.
metadata:

Arbitrary additional metadata files stored in Java-style reversed domain prefixed files

GET returns a simple file list (Atom feed?), allowing clients to decide whether they wish to retrieve a file

The server promised only that the metadata files will be preserved with the same level of durability as the bag contents

Example:

[
    'gov.loc.exampleProject.backup_history.xml',
    'com.flickr.commons.userComments.json',
    'org.apache.tika.extractedMetadata.xml'
]

Versioning

The versioning semantics are designed to support the use of version control systems like Git or Mercurial as storage backends and to allow implementors to support efficient delta-based replication protocols beyond the scope of this specification.

  • All content within a version MUST be immutable but servers MAY remove old versions as desired. This allows bag copies to be compared simply by comparing the source URLs of valid bags.

    This promise of immutability applies only to to the bag contents, including the top-level tag files, and includes any file addition or deletion within the content directory. Metadata files are not versioned to avoid local additions breaking replication.

  • Arbitrary symbolic names may be provided but MUST redirect to the appropriate hash value so clients can perform consistent equality checks.

Good HTTP Citizenship

A summary of relevant points from HTTP 1.1 (RFC 2616) which are of particular value for archival and replication:

  • Servers SHOULD generate Cache-Control headers; clients MUST honor them
  • Servers MAY use HTTP redirects to direct clients to HTTP-accessible backend storage for performance reasons
  • If available, servers SHOULD return Content-MD5 or Content-SHA1 headers using the hash value from the manifest; clients SHOULD validate these values if present
  • Servers SHOULD support entity tags and If-None-Match
  • Servers SHOULD support HTTP Range to allow clients to resume transfers
  • Servers MAY provide Retry-After with HTTP 503 (Service Unavailable) to help clients, particularly when the delay is due to content being staged from slower archive storage with known latency characteristics
  • Clients MUST honor HTTP 503 Service Unavailable responses using a provided Retry-After header or using exponential back-off if Retry-After is not provided.
  • Servers SHOULD support HTTP Pipelining and Keep-Alives to avoid performance issues when transferring large numbers of small files
  • Servers SHOULD return HTTP 410 (Gone) for content which has been removed, particularly in the case of old versions for bags which are still present.

Operations

For this discussion, it is assumed that servers may return standard HTTP response code such as 401/403 to indicate that the client needs to authenticate or lacks permissions to make changes.

Creating a new bag

  1. Create the container:
    Client POSTs to /bags:
    id:unique bag identifier
    version:optional client-suggested version, which the server may choose to use

    Server returns 201 pointing to the upload location, which may be the final destination e.g. /bags/:id:/versions/:version-id:/ or a temporary location, possibly pointing to a specific back-end storage server.

    Clients MUST perform all subsequent operations using the server-provided location

    Servers MUST return 409 Conflict if the ID is already in use

  2. Client PUTs bagit.txt and bag-info.txt under contents

  3. Client PUTs one or more manifest files under /contents/

    Servers MUST return HTTP 400 if the client has not provided bagit.txt or bag-info.txt

    Clients MUST provide the manifest files before committing the bag but are free to assemble the bag in any order

  4. Client PUTs data files under contents/data/

    Servers MUST return HTTP 400 if the client has not provided at least one manifest file or attempts to PUT a file which is not listed in the manifest or fails checksum validation

    Clients MAY use DELETE or PUT to remove or replace previously uploaded files before committing the bag

  5. Client POSTs commit to the bag location

    Servers MUST return HTTP 400 if all of the files which are specified in the manifest have not been received

Deleting a bag

  1. Client DELETEs bag location

Replicating a bag

  1. Client GETs manifest
  2. Client GETs each listed content file
  3. Optionally, client performs an AtomPub POST to copies with the public URL of a copy conforming to this specification.

Requesting Server Validation

  1. Client POSTs operation=validate to /bags/ <BAG_ID>
  2. Server returns HTTP 202 Accepted and an initial status resource with the following attributes:
    uri:Unique URI which the client can GET to retrieve the current status
    status:One of In Progress, Failed, or Successful
    progress:Integer percentage or null if the server does not support partial status
    message:Human-readable summary message, which may only be available when the operation has completed
    manifest:List of bag-relative path to the manifest(s) used to perform the validation