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# Glossary
This page contains definitions for distribution related terms.
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< dt id = "blob" > < h4 > Blob< / h4 > < / dt >
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< blockquote > A blob is any kind of content that is stored by a Registry under a content-addressable identifier (a "digest").< / blockquote >
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< a href = " #layer " > Layers</ a > are a good example of "blobs".
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< / p >
< / dd >
< dt id = "image" > < h4 > Image< / h4 > < / dt >
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< blockquote > An image is a named set of immutable data from which a container can be created.< / blockquote >
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< p >
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An image is represented by a json file called a < a href = " #manifest " > manifest</ a > , and is conceptually a set of < a href = " #layer " > layers</ a > .
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Image names indicate the location where they can be pulled from and pushed to, as they usually start with a < a href = " #registry " > registry</ a > domain name and port.
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< / p >
< / dd >
< dt id = "layer" > < h4 > Layer< / h4 > < / dt >
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< blockquote > A layer is a tar archive bundling partial content from a filesystem.< / blockquote >
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Layers from an < a href = " #image " > image</ a > are usually extracted in order on top of each other to make up a root filesystem from which containers run out.
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< / p >
< / dd >
< dt id = "manifest" > < h4 > Manifest< / h4 > < / dt >
< dd > < blockquote > A manifest is the JSON representation of an image.< / blockquote > < / dd >
< dt id = "namespace" > < h4 > Namespace< / h4 > < / dt >
< dd > < blockquote > A namespace is a collection of repositories with a common name prefix.< / blockquote >
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The namespace with an empty prefix is considered the Global Namespace.
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< / dd >
< dt id = "registry" > < h4 > Registry< / h4 > < / dt >
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< dd >< blockquote > A registry is a service that let you store and deliver < a href = " #images " > images</ a > and other content.</ blockquote >
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< / dd >
< dt id = "registry" > < h4 > Repository< / h4 > < / dt >
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< blockquote > A repository is a set of data containing all versions of a given image.< / blockquote >
< / dd >
< dt id = "scope" > < h4 > Scope< / h4 > < / dt >
< dd > < blockquote > A scope is the portion of a namespace onto which a given authorization token is granted.< / blockquote > < / dd >
< dt id = "tag" > < h4 > Tag< / h4 > < / dt >
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< dd >< blockquote > A tag is conceptually a "version" of a < a href = " #image " > named image</ a > .</ blockquote >
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< p >
Example: `docker pull myimage:latest` instructs docker to pull the image "myimage" in version "latest".
< / p >
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< / dd >
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< / dl >