# Blobstore Go API Overview - Google App Engine - Google Code

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# Blobstore Go API Overview

The Blobstore API allows your application to serve data objects, called blobs, that are much larger than the size allowed for objects in the Datastore service. Blobs are useful for serving large files, such as video or image files, and for allowing users to upload large data files. Blobs are created by uploading a file through an HTTP request. Typically, your applications will do this by presenting a form with a file upload field to the user. When the form is submitted, the Blobstore creates a blob from the file's contents and returns an opaque reference to the blob, called a blob key, which you can later use to serve the blob. The application can serve the complete blob value in response to a user request, or it can read the value directly using a streaming file-like interface.

- Introducing the Blobstore
- Using the Blobstore
- Uploading a Blob
- Serving a Blob
- Complete Sample Application
- Writing Files to the BlobstoreExperimental!
- Limits

## Introducing the Blobstore

Google App Engine includes the Blobstore service, which allows applications to serve data objects limited only by the amount of data that can be uploaded or downloaded over a single HTTP connection. These objects are called Blobstore values, or blobs. Blobstore values are served as responses from request handlers and are created as uploads via web forms. Applications do not create blob data directly; instead, blobs are created indirectly, by a submitted web form or other HTTP `POST` request. Blobstore values can be served to the user, or accessed by the application in a file-like stream, using the Blobstore API.

Note: Blobs as defined by the Blobstore service are not related to blob property values used by the datastore.

To prompt a user to upload a Blobstore value, your application presents a web form with a file upload field. The application generates the form's action URL by calling the Blobstore API. The user's browser uploads the file directly to the Blobstore via the generated URL. Blobstore then stores the blob, rewrites the request to contain the blob key, and passes it to a path in your application. A request handler at that path in your application can perform additional form processing.

To serve a blob, your application sets a header on the outgoing response, and App Engine replaces the response with the blob value.

Blobs can't be modified after they're created, though they can be deleted. Each blob has a corresponding blob info record, stored in the datastore, that provides details about the blob, such as its creation time and content type. You can use the blob key to fetch blob info records and query their properties.

## Using the Blobstore

Applications can use the Blobstore to accept large files as uploads from users and to serve those files. Files are called blobs once they're uploaded. Applications don't access blobs directly by filename. Instead, applications refer to blobs through the `appengine.BlobKey` type.

The user creates a blob by submitting an HTML form that includes one or more file input fields. Your application sets `blobstore.UploadURL` as the destination (action) of this form, passing the function a URL path of a handler in your application. When the user submits the form, the user's browser uploads the specified files directly to the Blobstore. The Blobstore rewrites the user's request and stores the uploaded file data, replacing the uploaded file data with one or more corresponding blob keys, then passes the rewritten request to the handler at the URL path you provided to `blobstore.UploadURL`. This handler can do additional processing based on the blob key. Finally, the handler must return a headers-only, redirect response (301, 302, or 303), typically a browser redirect to another page indicating the status of the blob upload.

The application can read portions of a Blobstore value using a `blobstore.Reader`, and can also programatically generate blobs using a `blobstore.Writer`.

## Uploading a Blob

To create and upload a blob, follow this procedure:

- Call `blobstore.UploadUrl` to create an upload URL for the form that the user will fill out, passing the application path to load when the `POST` of the form is completed.

      c := appengine.NewContext(r)
      uploadURL, err := blobstore.UploadURL(c, "/upload", nil)
      if err != nil {
      	serveError(c, w, err)
      	return
      }
      w.Header().Set("Content-Type", "text/html")
      err = rootTemplate.Execute(w, uploadURL)
      if err != nil {
      	c.Errorf("%v", err)
      }

- The form must include a file upload field, and the form's `enctype` must be set to `multipart/form-data`. When the user submits the form, the `POST` is handled by the Blobstore API, which creates the blob. The API also creates an info record for the blob and stores the record in the datastore, and passes the rewritten request to your application on the given path as a blob key:

      var rootTemplate = template.MustParse(rootTemplateHTML, nil)

      const rootTemplateHTML = `
      <html><body>
      <form action="{@}" method="POST" enctype="multipart/form-data">
      Upload File: <input type="file" name="file"><br>
      <input type="submit" name="submit" value="Submit">
      </form></body></html>
      `

- In this handler, you can store the blob key with the rest of your application's data model. The blob key itself remains accessible from the blob info entity in the datastore. Note that after the user submits the form and your handler is called, the blob has already been saved and the blob info added to the datastore. If your application doesn't want to keep the blob, you should delete the blob immediately to prevent it from becoming orphaned:

      c := appengine.NewContext(r)
      blobs, _, err := blobstore.ParseUpload(r)
      if err != nil {
      	serveError(c, w, err)
      	return
      }
      file := blobs["file"]
      if len(file) == 0 {
      	c.Errorf("no file uploaded")
      	http.Redirect(w, r, "/", http.StatusFound)
      	return
      }
      http.Redirect(w, r, "/serve/?blobKey="+string(file[0].BlobKey), http.StatusFound)

When the Blobstore rewrites the user's request, the MIME parts of the uploaded files have their bodies emptied, and the blob key is added as a MIME part header. All other form fields and parts are preserved and passed to the upload handler. If you don't specify a content type, the Blobstore will try to infer it from the file extension. If no content type can be determined, the newly created blob is assigned content type `application/octet-stream`.

## Serving a Blob

To serve blobs, you must include a blob download handler as a path in your application. This handler should pass the blob key for the desired blob to `blobstore.Send`. In this example, the blob key is passed to the download handler as the URL argument `r.FormValue("blobKey")`. In practice, the download handler can get the blob key by any means you choose, such as through another method or user action.

    blobstore.Send(w, appengine.BlobKey(r.FormValue("blobKey")))

Blobs can be served from any application URL. To serve a blob in your application, you put a special header in the response containing the blob key. App Engine replaces the body of the response with the content of the blob.

### Blob Byte Ranges

The Blobstore supports serving part of a large value instead of the full value in response to a request. To serve a partial value, include the `X-AppEngine-BlobRange` header in the outgoing response. Its value is a standard HTTP byte range. The byte numbering is zero-based. A blank `X-AppEngine-BlobRange` instructs the API to ignore the range header and serve the full blob. Example ranges include:

- `0-499` serves the first 500 bytes of the value (bytes 0 through 499, inclusive).
- `500-999` serves 500 bytes starting with the 501st byte.
- `500-` serves all bytes starting with the 501st byte to the end of the value.
- `-500` serves the last 500 bytes of the value.

If the byte range is valid for the Blobstore value, the Blobstore sends a `206` `Partial` `Content` status code and the requested byte range to the client. If the range is not valid for the value, the Blobstore sends `416` `Requested` `Range` `Not` `Satisfiable`.

The Blobstore does not support multiple byte ranges in a single request (for example, `100-199,200-299`), whether or not they overlap.

## Complete Sample Application

In the following sample application, the application's main URL loads the form that asks the user for a file to upload, and the upload handler immediately calls the download handler to serve the data. This is to simplify the sample application. In practice, you would probably not use the main URL to request upload data, nor would you immediately serve a blob you had just uploaded.

    package blobstore_example

    import (
    	"http"
    	"io"
    	"os"
    	"template"

    	"appengine"
    	"appengine/blobstore"
    )

    func serveError(c appengine.Context, w http.ResponseWriter, err os.Error) {
    	w.WriteHeader(http.StatusInternalServerError)
    	w.Header().Set("Content-Type", "text/plain")
    	io.WriteString(w, "Internal Server Error")
    	c.Errorf("%v", err)
    }

    var rootTemplate = template.MustParse(rootTemplateHTML, nil)

    const rootTemplateHTML = `
    <html><body>
    <form action="{@}" method="POST" enctype="multipart/form-data">
    Upload File: <input type="file" name="file"><br>
    <input type="submit" name="submit" value="Submit">
    </form></body></html>
    `

    func handleRoot(w http.ResponseWriter, r *http.Request) {
    	c := appengine.NewContext(r)
    	uploadURL, err := blobstore.UploadURL(c, "/upload", nil)
    	if err != nil {
    		serveError(c, w, err)
    		return
    	}
    	w.Header().Set("Content-Type", "text/html")
    	err = rootTemplate.Execute(w, uploadURL)
    	if err != nil {
    		c.Errorf("%v", err)
    	}
    }

    func handleServe(w http.ResponseWriter, r *http.Request) {
    	blobstore.Send(w, appengine.BlobKey(r.FormValue("blobKey")))
    }

    func handleUpload(w http.ResponseWriter, r *http.Request) {
    	c := appengine.NewContext(r)
    	blobs, _, err := blobstore.ParseUpload(r)
    	if err != nil {
    		serveError(c, w, err)
    		return
    	}
    	file := blobs[[]"file"]
    	if len(file) == 0 {
    		c.Errorf("no file uploaded")
    		http.Redirect(w, r, "/", http.StatusFound)
    		return
    	}
    	http.Redirect(w, r, "/serve/?blobKey="+string(file[[]0].BlobKey), http.StatusFound)
    }

    func init() {
    	http.HandleFunc("/", handleRoot)
    	http.HandleFunc("/serve/", handleServe)
    	http.HandleFunc("/upload", handleUpload)
    }

## Writing Files to the Blobstore (Experimental)

App Engine allows you to create Blobstore blobs programmatically, providing a file-like API that you can use to read and write to blobs. Some common uses of this functionality include exporting data and generating reports—or any function that involves generating large binary data objects.

You begin by creating a new (empty) Blobstore file using the `Create` function, which creates an `io.WriteCloser`.

This is a low-level API.

The following sample shows how to create a new Blobstore file and get its blob key:

    var k appengine.BlobKey
    w, err := blobstore.Create(c, "application/octet-stream")
    if err != nil {
    	return k, err
    }
    _, err = w.Write([]byte("... some data ..."))
    if err != nil {
    	return k, err
    }
    err = w.Close()
    if err != nil {
    	return k, err
    }
    return w.Key()

## Quotas

Space used for Blobstore values contributes to the Stored Data (billable) quota. Blob info entities in the datastore count towards datastore-related limits.

For more information on systemwide safety quotas, see Quotas, and the Quota Details section of the Administration Console.

In addition to systemwide safety quotas, a limit applies specifically to the use of the Blobstore: the maximum size of Blobstore data that can be read by the application with one API call is 32 megabytes.