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title = "Docker Registry HTTP API V2"
description = "This is a specification for the API."
keywords = ["registry, service, driver, images, storage,  api"]
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parent="smn_registry_ref"
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# Docker Registry HTTP API V2

## Introduction

The _Docker Registry HTTP API_ is the protocol to facilitate distribution of
images to the docker engine. It interacts with instances of the docker
registry, which is a service to manage information about docker images and
enable their distribution. The specification covers the operation of version 2
of this API, known as _Docker Registry HTTP API V2_.

While the V1 registry protocol is usable, there are several problems with the
architecture that have led to this new version. The main driver of this
specification these changes to the docker the image format, covered in
docker/docker#8093. The new, self-contained image manifest simplifies image
definition and improves security. This specification will build on that work,
leveraging new properties of the manifest format to improve performance,
reduce bandwidth usage and decrease the likelihood of backend corruption.

For relevant details and history leading up to this specification, please see
the following issues:

- [docker/docker#8093](https://github.com/docker/docker/issues/8093)
- [docker/docker#9015](https://github.com/docker/docker/issues/9015)
- [docker/docker-registry#612](https://github.com/docker/docker-registry/issues/612)

### Scope

This specification covers the URL layout and protocols of the interaction
between docker registry and docker core. This will affect the docker core
registry API and the rewrite of docker-registry. Docker registry
implementations may implement other API endpoints, but they are not covered by
this specification.

This includes the following features:

- Namespace-oriented URI Layout
- PUSH/PULL registry server for V2 image manifest format
- Resumable layer PUSH support
- V2 Client library implementation

While authentication and authorization support will influence this
specification, details of the protocol will be left to a future specification.
Relevant header definitions and error codes are present to provide an
indication of what a client may encounter.

#### Future

There are features that have been discussed during the process of cutting this
specification. The following is an incomplete list:

- Immutable image references
- Multiple architecture support
- Migration from v2compatibility representation

These may represent features that are either out of the scope of this
specification, the purview of another specification or have been deferred to a
future version.

### Use Cases

For the most part, the use cases of the former registry API apply to the new
version. Differentiating use cases are covered below.

#### Image Verification

A docker engine instance would like to run verified image named
"library/ubuntu", with the tag "latest". The engine contacts the registry,
requesting the manifest for "library/ubuntu:latest". An untrusted registry
returns a manifest. Before proceeding to download the individual layers, the
engine verifies the manifest's signature, ensuring that the content was
produced from a trusted source and no tampering has occured. After each layer
is downloaded, the engine verifies the digest of the layer, ensuring that the
content matches that specified by the manifest.

#### Resumable Push

Company X's build servers lose connectivity to docker registry before
completing an image layer transfer. After connectivity returns, the build
server attempts to re-upload the image. The registry notifies the build server
that the upload has already been partially attempted. The build server
responds by only sending the remaining data to complete the image file.

#### Resumable Pull

Company X is having more connectivity problems but this time in their
deployment datacenter. When downloading an image, the connection is
interrupted before completion. The client keeps the partial data and uses http
`Range` requests to avoid downloading repeated data.

#### Layer Upload De-duplication

Company Y's build system creates two identical docker layers from build
processes A and B. Build process A completes uploading the layer before B.
When process B attempts to upload the layer, the registry indicates that its
not necessary because the layer is already known.

If process A and B upload the same layer at the same time, both operations
will proceed and the first to complete will be stored in the registry (Note:
we may modify this to prevent dogpile with some locking mechanism).

### Changes

The V2 specification has been written to work as a living document, specifying
only what is certain and leaving what is not specified open or to future
changes. Only non-conflicting additions should be made to the API and accepted
changes should avoid preventing future changes from happening.

This section should be updated when changes are made to the specification,
indicating what is different. Optionally, we may start marking parts of the
specification to correspond with the versions enumerated here.

<dl>

  <dt>2.0.4</dt>>
  <dd>
    <ul>
      <li>Added support for listing registry contents.</li>
      <li>Added pagination to tags API.</li>
      <li>Added common approach to support pagination.</li>
    </ul>
  </dd>

  <dt>2.0.3</dt>
  <dd>
    <li>Allow repository name components to be one character.</li>
    <li>Clarified that single component names are allowed.</li>
  </dd>

  <dt>2.0.2</dt>
  <dd>
    <li>Added section covering digest format.</li>
    <li>Added more clarification that manifest cannot be deleted by tag.</li>
  </dd>
	<dt>2.0.1</dt>
	<dd>
		<ul>
			<li>Added capability of doing streaming upload to PATCH blob upload.</li>
			<li>Updated PUT blob upload to no longer take final chunk, now requires entire data or no data.</li>
			<li>Removed `416 Requested Range Not Satisfiable` response status from PUT blob upload.</li>
		</ul>
	</dd>

	<dt>2.0.0</dt>
	<dd>
		<ul>
			<li>Added support for immutable manifest references in manifest endpoints.</li>
			<li>Deleting a manifest by tag has been deprecated.</li>
			<li>Specified `Docker-Content-Digest` header for appropriate entities.</li>
			<li>Added error code for unsupported operations.</li>
		</ul>
	</dd>

	<dt>2.0</dt>
	<dd>
		This is the baseline specification.
	</dd>
</dl>

## Overview

This section covers client flows and details of the API endpoints. The URI
layout of the new API is structured to support a rich authentication and
authorization model by leveraging namespaces. All endpoints will be prefixed
by the API version and the repository name:

    /v2/<name>/

For example, an API endpoint that will work with the `library/ubuntu`
repository, the URI prefix will be:

    /v2/library/ubuntu/

This scheme provides rich access control over various operations and methods
using the URI prefix and http methods that can be controlled in variety of
ways.

Classically, repository names have always been two path components where each
path component is less than 30 characters. The V2 registry API does not
enforce this. The rules for a repository name are as follows:

1. A repository name is broken up into _path components_. A component of a
   repository name must be at least one lowercase, alpha-numeric characters,
   optionally separated by periods, dashes or underscores. More strictly, it
   must match the regular expression `[a-z0-9]+(?:[._-][a-z0-9]+)*`.
2. If a repository  name has two or more path components, they must be
   separated by a forward slash ("/").
3. The total length of a repository name, including slashes, must be less the
   256 characters.

These name requirements _only_ apply to the registry API and should accept a
superset of what is supported by other docker ecosystem components.

All endpoints should support aggressive http caching, compression and range
headers, where appropriate. The new API attempts to leverage HTTP semantics
where possible but may break from standards to implement targeted features.

For detail on individual endpoints, please see the [_Detail_](#detail)
section.

### Errors

Actionable failure conditions, covered in detail in their relevant sections,
are reported as part of 4xx responses, in a json response body. One or more
errors will be returned in the following format:

    {
        "errors:" [{
                "code": <error identifier>,
                "message": <message describing condition>,
                "detail": <unstructured>
            },
            ...
        ]
    }

The `code` field will be a unique identifier, all caps with underscores by
convention. The `message` field will be a human readable string. The optional
`detail` field may contain arbitrary json data providing information the
client can use to resolve the issue.

While the client can take action on certain error codes, the registry may add
new error codes over time. All client implementations should treat unknown
error codes as `UNKNOWN`, allowing future error codes to be added without
breaking API compatibility. For the purposes of the specification error codes
will only be added and never removed.

For a complete account of all error codes, please see the _Detail_ section.

### API Version Check

A minimal endpoint, mounted at `/v2/` will provide version support information
based on its response statuses. The request format is as follows:

    GET /v2/

If a `200 OK` response is returned, the registry implements the V2(.1)
registry API and the client may proceed safely with other V2 operations.
Optionally, the response may contain information about the supported paths in
the response body. The client should be prepared to ignore this data.

If a `401 Unauthorized` response is returned, the client should take action
based on the contents of the "WWW-Authenticate" header and try the endpoint
again. Depending on access control setup, the client may still have to
authenticate against different resources, even if this check succeeds.

If `404 Not Found` response status, or other unexpected status, is returned,
the client should proceed with the assumption that the registry does not
implement V2 of the API.

When a `200 OK` or `401 Unauthorized` response is returned, the
"Docker-Distribution-API-Version" header should be set to "registry/2.0".
Clients may require this header value to determine if the endpoint serves this
API. When this header is omitted, clients may fallback to an older API version.

### Content Digests

This API design is driven heavily by [content addressability](http://en.wikipedia.org/wiki/Content-addressable_storage).
The core of this design is the concept of a content addressable identifier. It
uniquely identifies content by taking a collision-resistent hash of the bytes.
Such an identifier can be independently calculated and verified by selection
of a common _algorithm_. If such an identifier can be communicated in a secure
manner, one can retrieve the content from an insecure source, calculate it
independently and be certain that the correct content was obtained. Put simply,
the identifier is a property of the content.

To disambiguate from other concepts, we call this identifier a _digest_. A
_digest_ is a serialized hash result, consisting of a _algorithm_ and _hex_
portion. The _algorithm_ identifies the methodology used to calculate the
digest. The _hex_ portion is the hex-encoded result of the hash.

We define a _digest_ string to match the following grammar:

  digest      := algorithm ":" hex
  algorithm   := /[A-Fa-f0-9_+.-]+/
  hex         := /[A-Fa-f0-9]+/

Some examples of _digests_ include the following:

digest                                                                            | description                                   |
----------------------------------------------------------------------------------|------------------------------------------------
sha256:6c3c624b58dbbcd3c0dd82b4c53f04194d1247c6eebdaab7c610cf7d66709b3b           | Common sha256 based digest                    |
tarsum.v1+sha256:6c3c624b58dbbcd3c0dd82b4c53f04194d1247c6eebdaab7c610cf7d66709b3b | Tarsum digest, used for legacy layer digests. |

> __NOTE:__ While we show an example of using a `tarsum` digest, the security
> of tarsum has not been verified. It is recommended that most implementations
> use sha256 for interoperability.

While the _algorithm_ does allow one to implement a wide variety of
algorithms, compliant implementations should use sha256. Heavy processing of
input before calculating a hash is discouraged to avoid degrading the
uniqueness of the _digest_ but some canonicalization may be performed to
ensure consistent identifiers.

Let's use a simple example in pseudo-code to demonstrate a digest calculation:

```
let C = 'a small string'
let B = sha256(C)
let D = 'sha256:' + EncodeHex(B)
let ID(C) = D
```

Above, we have bytestring _C_ passed into a function, _SHA256_, that returns a
bytestring B, which is the hash of _C_. _D_ gets the algorithm concatenated
with the hex encoding of _B_. We then define the identifier of _C_ to _ID(C)_
as equal to _D_. A digest can be verified by independently calculating _D_ and
comparing it with identifier _ID(C)_

#### Digest Header

To provide verification of http content, any response may include a `Docker-
Content-Digest` header. This will include the digest of the target entity
returned in the response. For blobs, this is the entire blob content. For
manifests, this is the manifest body without the signature content, also known
as the JWS payload. Note that the commonly used canonicalization for digest
calculation may be dependent on the mediatype of the content, such as with
manifests.

The client may choose to ignore the header or may verify it to ensure content
integrity and transport security. This is most important when fetching by a
digest. To ensure security, the content should be verified against the digest
used to fetch the content. At times, the returned digest may differ from that
used to initiate a request. Such digests are considered to be from different
_domains_, meaning they have different values for _algorithm_. In such a case,
the client may choose to verify the digests in both domains or ignore the
server's digest. To maintain security, the client _must_ always verify the
content against the _digest_ used to fetch the content.

> __IMPORTANT:__ If a _digest_ is used to fetch content, the client should use
> the same digest used to fetch the content to verify it. The header `Docker-
> Content-Digest` should not be trusted over the "local" digest.

### Pulling An Image

An "image" is a combination of a JSON manifest and individual layer files. The
process of pulling an image centers around retrieving these two components.

The first step in pulling an image is to retrieve the manifest. For reference,
the relevant manifest fields for the registry are the following:

 field    | description                                    |
----------|------------------------------------------------|
name      | The name of the image.                         |
tag       | The tag for this version of the image.         |
fsLayers  | A list of layer descriptors (including tarsum) |
signature | A JWS used to verify the manifest content      |

For more information about the manifest format, please see
[docker/docker#8093](https://github.com/docker/docker/issues/8093).

When the manifest is in hand, the client must verify the signature to ensure
the names and layers are valid. Once confirmed, the client will then use the
tarsums to download the individual layers. Layers are stored in as blobs in
the V2 registry API, keyed by their tarsum digest.

#### Pulling an Image Manifest

The image manifest can be fetched with the following url:

```
GET /v2/<name>/manifests/<reference>
```

The `name` and `reference` parameter identify the image and are required. The
reference may include a tag or digest.

A `404 Not Found` response will be returned if the image is unknown to the
registry. If the image exists and the response is successful, the image
manifest will be returned, with the following format (see docker/docker#8093
for details):

    {
       "name": <name>,
       "tag": <tag>,
       "fsLayers": [
          {
             "blobSum": <tarsum>
          },
          ...
        ]
       ],
       "history": <v1 images>,
       "signature": <JWS>
    }

The client should verify the returned manifest signature for authenticity
before fetching layers.

#### Pulling a Layer

Layers are stored in the blob portion of the registry, keyed by tarsum digest.
Pulling a layer is carried out by a standard http request. The URL is as
follows:

    GET /v2/<name>/blobs/<tarsum>

Access to a layer will be gated by the `name` of the repository but is
identified uniquely in the registry by `tarsum`. The `tarsum` parameter is an
opaque field, to be interpreted by the tarsum library.

This endpoint may issue a 307 (302 for <HTTP 1.1) redirect to another service
for downloading the layer and clients should be prepared to handle redirects.

This endpoint should support aggressive HTTP caching for image layers. Support
for Etags, modification dates and other cache control headers should be
included. To allow for incremental downloads, `Range` requests should be
supported, as well.

### Pushing An Image

Pushing an image works in the opposite order as a pull. After assembling the
image manifest, the client must first push the individual layers. When the
layers are fully pushed into the registry, the client should upload the signed
manifest.

The details of each step of the process are covered in the following sections.

#### Pushing a Layer

All layer uploads use two steps to manage the upload process. The first step
starts the upload in the registry service, returning a url to carry out the
second step. The second step uses the upload url to transfer the actual data.
Uploads are started with a POST request which returns a url that can be used
to push data and check upload status.

The `Location` header will be used to communicate the upload location after
each request. While it won't change in the this specification, clients should
use the most recent value returned by the API.

##### Starting An Upload

To begin the process, a POST request should be issued in the following format:

```
POST /v2/<name>/blobs/uploads/
```

The parameters of this request are the image namespace under which the layer
will be linked. Responses to this request are covered below.

##### Existing Layers

The existence of a layer can be checked via a `HEAD` request to the blob store
API. The request should be formatted as follows:

```
HEAD /v2/<name>/blobs/<digest>
```

If the layer with the tarsum specified in `digest` is available, a 200 OK
response will be received, with no actual body content (this is according to
http specification). The response will look as follows:

```
200 OK
Content-Length: <length of blob>
Docker-Content-Digest: <digest>
```

When this response is received, the client can assume that the layer is
already available in the registry under the given name and should take no
further action to upload the layer. Note that the binary digests may differ
for the existing registry layer, but the tarsums will be guaranteed to match.

##### Uploading the Layer

If the POST request is successful, a `202 Accepted` response will be returned
with the upload URL in the `Location` header:

```
202 Accepted
Location: /v2/<name>/blobs/uploads/<uuid>
Range: bytes=0-<offset>
Content-Length: 0
Docker-Upload-UUID: <uuid>
```

The rest of the upload process can be carried out with the returned url,
called the "Upload URL" from the `Location` header. All responses to the
upload url, whether sending data or getting status, will be in this format.
Though the URI format (`/v2/<name>/blobs/uploads/<uuid>`) for the `Location`
header is specified, clients should treat it as an opaque url and should never
try to assemble the it. While the `uuid` parameter may be an actual UUID, this
proposal imposes no constraints on the format and clients should never impose
any.

If clients need to correlate local upload state with remote upload state, the
contents of the `Docker-Upload-UUID` header should be used. Such an id can be
used to key the last used location header when implementing resumable uploads.

##### Upload Progress

The progress and chunk coordination of the upload process will be coordinated
through the `Range` header. While this is a non-standard use of the `Range`
header, there are examples of [similar approaches](https://developers.google.com/youtube/v3/guides/using_resumable_upload_protocol) in APIs with heavy use.
For an upload that just started, for an example with a 1000 byte layer file,
the `Range` header would be as follows:

```
Range: bytes=0-0
```

To get the status of an upload, issue a GET request to the upload URL:

```
GET /v2/<name>/blobs/uploads/<uuid>
Host: <registry host>
```

The response will be similar to the above, except will return 204 status:

```
204 No Content
Location: /v2/<name>/blobs/uploads/<uuid>
Range: bytes=0-<offset>
Docker-Upload-UUID: <uuid>
```

Note that the HTTP `Range` header byte ranges are inclusive and that will be
honored, even in non-standard use cases.

##### Monolithic Upload

A monolithic upload is simply a chunked upload with a single chunk and may be
favored by clients that would like to avoided the complexity of chunking. To
carry out a "monolithic" upload, one can simply put the entire content blob to
the provided URL:

```
PUT /v2/<name>/blobs/uploads/<uuid>?digest=<tarsum>[&digest=sha256:<hex digest>]
Content-Length: <size of layer>
Content-Type: application/octet-stream

<Layer Binary Data>
```

The "digest" parameter must be included with the PUT request. Please see the
_Completed Upload_ section for details on the parameters and expected
responses.

Additionally, the upload can be completed with a single `POST` request to
the uploads endpoint, including the "size" and "digest" parameters:

```
POST /v2/<name>/blobs/uploads/?digest=<tarsum>[&digest=sha256:<hex digest>]
Content-Length: <size of layer>
Content-Type: application/octet-stream
  
<Layer Binary Data>
```

On the registry service, this should allocate a download, accept and verify
the data and return the same  response as the final chunk of an upload. If the
POST request fails collecting the data in any way, the registry should attempt
to return an error response to the client with the `Location` header providing
a place to continue the download.

The single `POST` method is provided for convenience and most clients should
implement `POST` + `PUT` to support reliable resume of uploads.
  
##### Chunked Upload

To carry out an upload of a chunk, the client can specify a range header and
only include that part of the layer file:

```
PATCH /v2/<name>/blobs/uploads/<uuid>
Content-Length: <size of chunk>
Content-Range: <start of range>-<end of range>
Content-Type: application/octet-stream

<Layer Chunk Binary Data>
```

There is no enforcement on layer chunk splits other than that the server must
receive them in order. The server may enforce a minimum chunk size. If the
server cannot accept the chunk, a `416 Requested Range Not Satisfiable`
response will be returned and will include a `Range` header indicating the
current status:

```
416 Requested Range Not Satisfiable
Location: /v2/<name>/blobs/uploads/<uuid>
Range: 0-<last valid range>
Content-Length: 0
Docker-Upload-UUID: <uuid>
```

If this response is received, the client should resume from the "last valid
range" and upload the subsequent chunk. A 416 will be returned under the
following conditions:

- Invalid Content-Range header format
- Out of order chunk: the range of the next chunk must start immediately after
  the "last valid range" from the previous response.

When a chunk is accepted as part of the upload, a `202 Accepted` response will
be returned, including a `Range` header with the current upload status:

```
202 Accepted
Location: /v2/<name>/blobs/uploads/<uuid>
Range: bytes=0-<offset>
Content-Length: 0
Docker-Upload-UUID: <uuid>
```

##### Completed Upload

For an upload to be considered complete, the client must submit a `PUT`
request on the upload endpoint with a digest parameter. If it is not provided,
the upload will not be considered complete. The format for the final chunk
will be as follows:

```
PUT /v2/<name>/blob/uploads/<uuid>?digest=<tarsum>[&digest=sha256:<hex digest>]
Content-Length: <size of chunk>
Content-Range: <start of range>-<end of range>
Content-Type: application/octet-stream

<Last Layer Chunk Binary Data>
```

Optionally, if all chunks have already been uploaded, a `PUT` request with a
`digest` parameter and zero-length body may be sent to complete and validated
the upload. Multiple "digest" parameters may be provided with different
digests. The server may verify none or all of them but _must_ notify the
client if the content is rejected.

When the last chunk is received and the layer has been validated, the client
will receive a `201 Created` response:

```
201 Created
Location: /v2/<name>/blobs/<tarsum>
Content-Length: 0
Docker-Content-Digest: <digest>
```

The `Location` header will contain the registry URL to access the accepted
layer file. The `Docker-Content-Digest` header returns the canonical digest of
the uploaded blob which may differ from the provided digest. Most clients may
ignore the value but if it is used, the client should verify the value against
the uploaded blob data.

###### Digest Parameter

The "digest" parameter is designed as an opaque parameter to support
verification of a successful transfer. The initial version of the registry API
will support a tarsum digest, in the standard tarsum format. For example, a
HTTP URI parameter might be as follows:

```
tarsum.v1+sha256:6c3c624b58dbbcd3c0dd82b4c53f04194d1247c6eebdaab7c610cf7d66709b3b
```

Given this parameter, the registry will verify that the provided content does
result in this tarsum. Optionally, the registry can support other other digest
parameters for non-tarfile content stored as a layer. A regular hash digest
might be specified as follows:

```
sha256:6c3c624b58dbbcd3c0dd82b4c53f04194d1247c6eebdaab7c610cf7d66709b3b
```

Such a parameter would be used to verify that the binary content (as opposed
to the tar content) would be verified at the end of the upload process.

For the initial version, registry servers are only required to support the
tarsum format.

##### Canceling an Upload

An upload can be cancelled by issuing a DELETE request to the upload endpoint.
The format will be as follows:

```
DELETE /v2/<name>/blobs/uploads/<uuid>
```

After this request is issued, the upload uuid will no longer be valid and the
registry server will dump all intermediate data. While uploads will time out
if not completed, clients should issue this request if they encounter a fatal
error but still have the ability to issue an http request.

##### Errors

If an 502, 503 or 504 error is received, the client should assume that the
download can proceed due to a temporary condition, honoring the appropriate
retry mechanism. Other 5xx errors should be treated as terminal.

If there is a problem with the upload, a 4xx error will be returned indicating
the problem. After receiving a 4xx response (except 416, as called out above),
the upload will be considered failed and the client should take appropriate
action.

Note that the upload url will not be available forever. If the upload uuid is
unknown to the registry, a `404 Not Found` response will be returned and the
client must restart the upload process.

#### Pushing an Image Manifest

Once all of the layers for an image are uploaded, the client can upload the
image manifest. An image can be pushed using the following request format:

    PUT /v2/<name>/manifests/<reference>

    {
       "name": <name>,
       "tag": <tag>,
       "fsLayers": [
          {
             "blobSum": <tarsum>
          },
          ...
        ]
       ],
       "history": <v1 images>,
       "signature": <JWS>,
       ...
    }

The `name` and `reference` fields of the response body must match those specified in
the URL. The `reference` field may be a "tag" or a "digest".

If there is a problem with pushing the manifest, a relevant 4xx response will
be returned with a JSON error message. Please see the _PUT Manifest section
for details on possible error codes that may be returned.

If one or more layers are unknown to the registry, `BLOB_UNKNOWN` errors are
returned. The `detail` field of the error response will have a `digest` field
identifying the missing blob, which will be a tarsum. An error is returned for
each unknown blob. The response format is as follows:

    {
        "errors:" [{
                "code": "BLOB_UNKNOWN",
                "message": "blob unknown to registry",
                "detail": {
                    "digest": <tarsum>
                }
            },
            ...
        ]
    }



### Listing Image Tags

It may be necessary to list all of the tags under a given repository. The tags
for an image repository can be retrieved with the following request:

    GET /v2/<name>/tags/list

The response will be in the following format:

    200 OK
    Content-Type: application/json

    {
        "name": <name>,
        "tags": [
            <tag>,
            ...
        ]
    }

For repositories with a large number of tags, this response may be quite
large. If such a response is expected, one should use the pagination.

#### Pagination

Paginated tag results can be retrieved by adding the appropriate pagination
parameters. Starting a paginated flow may begin as follows:

```
GET /v2/<name>/tags/list?n=<integer>
```

The above specifies that a tags response should be returned, from the start of
the result set, ordered lexically, limiting the number of results to `n`. The
response to such a request would look as follows:

```
200 OK
Content-Type: application/json

{
  "name": <name>,
  "tags": [
    <tag>,
    ...
  ]
  "next": <url>?n=<n from the request>&last=<last tag value from previous response>
}
```

> __TODO(stevvooe):__ Consider using a Header here, rather than a body parameter. A
header would allow one to issue the next request before parsing the response
body.

To get the next result set, a client would issue the request as follows, using
the value of "next" from the response body:

```
GET /v2/<name>/tags/list?n=<n from the request>&last=<last tag value from previous response>
```

The above process should then be repeated until the `next` parameter is no
longer set in the response.

The behavior of `last` is quite simple and can be demonstrated with an
example. Let's say the repository has the following tags:

```
a
b
c
d
```

If the value of `n` is 2, _a_ and _b_ will be returned on the first response.
The `next` url within the respone will have `n` set to 2 and last set to _b_:

```
"next": <url>?n=2&last=b
```

The client can then issue the response, receiving the values _c_ and _d_. Note
that n may change on second to last response or be omitted fully, if the
server may so choose.

### Listing Repositories

Images are stored in collections, known as a _repository_, which is keyed by a
`name`, as seen throughout the API specification. A registry instance may
contain several repositories. The list of available repositories, or image
names, is made available through the _catalog_.

The catalog for a given registry can be retrived with the following request:

```
GET /v2/_catalog
```

The response will be in the following format:

```
200 OK
Content-Type: application/json

{
  "repositories": [
    <name>,
    ...
  ]
}
```

For registries with a large number of repositories, this response may be quite
large. If such a response is expected, one should use the pagination.

#### Pagination

Paginated repository results can be retrieved by adding the appropriate
pagination parameters, which are similar to those available in the tag API.
Starting a paginated flow may begin as follows:

```
GET /v2/_catalog?n=<integer>
```

The above specifies that a catalog response should be returned, from the start of
the result set, ordered lexically, limiting the number of results to `n`. The
response to such a request would look as follows:

```
200 OK
Content-Type: application/json

{
  "repositories": [
    <name>,
    ...
  ]
  "next": <url>?n=<n from the request>&last=<last repository value from previous response>
}
```

> __TODO(stevvooe):__ Consider using a Header here, rather than a body parameter. A
header would allow one to issue the next request before parsing the response
body.

To get the next result set, a client would issue the request as follows, using
the value of "next" from the response body:

```
GET /v2/_catalog?n=<n from the request>&last=<last repostory value from previous response>
```

The above process should then be repeated until the `next` parameter is no
longer set in the response.

The result set of repository names is represented abstractly as a lexically
sorted list, where the position in that list can be specified by the query
term `last`. The entries in the response start _after_ the term specified by
`last`, up to `n` entries.

The behavior of `last` is quite simple when demonstrated with an example.
Let's say the registry has the following repositories:

```
a
b
c
d
```

If the value of `n` is 2, _a_ and _b_ will be returned on the first response.
The `next` url within the respone will have `n` set to 2 and last set to _b_:

```
"next": <url>?n=2&last=b
```

The client can then issue the request with above value of `next`, receiving
the values _c_ and _d_. Note that n may change on second to last response or
be omitted fully, if the server may so choose.

### Deleting an Image

An image may be deleted from the registry via its `name` and `reference`. A
delete may be issued with the following request format:

    DELETE /v2/<name>/manifests/<reference>

For deletes, `reference` *must* be a digest or the delete will fail. If the
image exists and has been successfully deleted, the following response will be
issued:

    202 Accepted
    Content-Length: None

If the image had already been deleted or did not exist, a `404 Not Found`
response will be issued instead.

## Detail

> **Note**: This section is still under construction. For the purposes of
> implementation, if any details below differ from the described request flows
> above, the section below should be corrected. When they match, this note
> should be removed.

The behavior of the endpoints are covered in detail in this section, organized
by route and entity. All aspects of the request and responses are covered,
including headers, parameters and body formats. Examples of requests and their
corresponding responses, with success and failure, are enumerated.

> **Note**: The sections on endpoint detail are arranged with an example
> request, a description of the request, followed by information about that
> request.

A list of methods and URIs are covered in the table below:

|Method|Path|Entity|Description|
|------|----|------|-----------|
{{range $route := .RouteDescriptors}}{{range $method := .Methods}}| {{$method.Method}} | `{{$route.Path|prettygorilla}}` | {{$route.Entity}} | {{$method.Description}} |
{{end}}{{end}}

The detail for each endpoint is covered in the following sections.

### Errors

The error codes encountered via the API are enumerated in the following table:

|Code|Message|Description|
|----|-------|-----------|
{{range $err := .ErrorDescriptors}} `{{$err.Value}}` | {{$err.Message}} | {{$err.Description|removenewlines}}
{{end}}

{{range $route := .RouteDescriptors}}
### {{.Entity}}

{{.Description}}

{{range $method := $route.Methods}}

#### {{.Method}} {{$route.Entity}}

{{.Description}}

{{if .Requests}}{{range .Requests}}{{if .Name}}
##### {{.Name}}{{end}}

```
{{$method.Method}} {{$route.Path|prettygorilla}}{{if .QueryParameters}}?{{range .QueryParameters}}{{.Name}}={{.Format}}{{end}}{{end}}{{range .Headers}}
{{.Name}}: {{.Format}}{{end}}{{if .Body.ContentType}}
Content-Type: {{.Body.ContentType}}{{end}}{{if .Body.Format}}

{{.Body.Format}}{{end}}
```

{{.Description}}

{{if or .Headers .PathParameters .QueryParameters}}
The following parameters should be specified on the request:

|Name|Kind|Description|
|----|----|-----------|
{{range .Headers}}|`{{.Name}}`|header|{{.Description}}|
{{end}}{{range .PathParameters}}|`{{.Name}}`|path|{{.Description}}|
{{end}}{{range .QueryParameters}}|`{{.Name}}`|query|{{.Description}}|
{{end}}{{end}}

{{if .Successes}}
{{range .Successes}}
###### On Success: {{if .Name}}{{.Name}}{{else}}{{.StatusCode | statustext}}{{end}}

```
{{.StatusCode}} {{.StatusCode | statustext}}{{range .Headers}}
{{.Name}}: {{.Format}}{{end}}{{if .Body.ContentType}}
Content-Type: {{.Body.ContentType}}{{end}}{{if .Body.Format}}

{{.Body.Format}}{{end}}
```

{{.Description}}
{{if .Fields}}The following fields may be returned in the response body:

|Name|Description|
|----|-----------|
{{range .Fields}}|`{{.Name}}`|{{.Description}}|
{{end}}{{end}}{{if .Headers}}
The following headers will be returned with the response:

|Name|Description|
|----|-----------|
{{range .Headers}}|`{{.Name}}`|{{.Description}}|
{{end}}{{end}}{{end}}{{end}}

{{if .Failures}}
{{range .Failures}}
###### On Failure: {{if .Name}}{{.Name}}{{else}}{{.StatusCode | statustext}}{{end}}

```
{{.StatusCode}} {{.StatusCode | statustext}}{{range .Headers}}
{{.Name}}: {{.Format}}{{end}}{{if .Body.ContentType}}
Content-Type: {{.Body.ContentType}}{{end}}{{if .Body.Format}}

{{.Body.Format}}{{end}}
```

{{.Description}}
{{if .Headers}}
The following headers will be returned on the response:

|Name|Description|
|----|-----------|
{{range .Headers}}|`{{.Name}}`|{{.Description}}|
{{end}}{{end}}

{{if .ErrorCodes}}
The error codes that may be included in the response body are enumerated below:

|Code|Message|Description|
|----|-------|-----------|
{{range $err := .ErrorCodes}}| `{{$err}}` | {{$err.Descriptor.Message}} | {{$err.Descriptor.Description|removenewlines}} |
{{end}}

{{end}}{{end}}{{end}}{{end}}{{end}}{{end}}

{{end}}