How to use update function to upload attachment in CouchDB - function

Question

I would like to know what can I do to upload attachments in CouchDB using the update function.
here you will find an example of my update function to add documents:
function(doc, req){
if (!doc) {
if (!req.form._id) {
req.form._id = req.uuid;
}
req.form['|edited_by'] = req.userCtx.name
req.form['|edited_on'] = new Date();
return [req.form, JSON.stringify(req.form)];
}
else {
return [null, "Use POST to add a document."]
}
}
example for remove documents:
function(doc, req){
if (doc) {
for (var i in req.form) {
doc[i] = req.form[i];
}
doc['|edited_by'] = req.userCtx.name
doc['|edited_on'] = new Date();
doc._deleted = true;
return [doc, JSON.stringify(doc)];
}
else {
return [null, "Document does not exist."]
}
}
thanks for your help,

Answer 1

It is possible to add attachments to a document using an update function by modifying the document's _attachments property. Here's an example of an update function which will add an attachment to an existing document:
function (doc, req) {
// skipping the create document case for simplicity
if (!doc) {
return [null, "update only"];
}
// ensure that the required form parameters are present
if (!req.form || !req.form.name || !req.form.data) {
return [null, "missing required post fields"];
}
// if there isn't an _attachments property on the doc already, create one
if (!doc._attachments) {
doc._attachments = {};
}
// create the attachment using the form data POSTed by the client
doc._attachments[req.form.name] = {
content_type: req.form.content_type || 'application/octet-stream',
data: req.form.data
};
return [doc, "saved attachment"];
}
For each attachment, you need a name, a content type, and body data encoded as base64. The example function above requires that the client sends an HTTP POST in application/x-www-form-urlencoded format with at least two parameters: name and data (a content_type parameter will be used if provided):
name=logo.png&content_type=image/png&data=iVBORw0KGgoA...
To test the update function:
Find a small image and base64 encode it:
$ base64 logo.png | sed 's/+/%2b/g' > post.txt
The sed script encodes + characters so they don't get converted to spaces.
Edit post.txt and add name=logo.png&content_type=image/png&data= to the top of the document.
Create a new document in CouchDB using Futon.
Use curl to call the update function with the post.txt file as the body, substituting in the ID of the document you just created.
curl -X POST -d #post.txt http://127.0.0.1:5984/mydb/_design/myddoc/_update/upload/193ecff8618678f96d83770cea002910
This was tested on CouchDB 1.6.1 running on OSX.
Update: #janl was kind enough to provide some details on why this answer can lead to performance and scaling issues. Uploading attachments via an upload handler has two main problems:
The upload handlers are written in JavaScript, so the CouchDB server may have to fork() a couchjs process to handle the upload. Even if a couchjs process is already running, the server has to stream the entire HTTP request to the external process over stdin. For large attachments, the transfer of the request can take significant time and system resources. For each concurrent request to an update function like this, CouchDB will have to fork a new couchjs process. Since the process runtime will be rather long because of what is explained next, you can easily run out of RAM, CPU or the ability to handle more concurrent requests.
After the _attachments property is populated by the upload handler and streamed back to the CouchDB server (!), the server must parse the response JSON, decode the base64-encoded attachment body, and write the binary body to disk. The standard method of adding an attachment to a document -- PUT /db/docid/attachmentname -- streams the binary request body directly to disk and does not require the two processing steps.
The function above will work, but there are non-trivial issues to consider before using it in a highly-scalable system.