couchdb update design doc - database

Question

I have a nodejs application where i connect to my couchdb using nano with the following script:
const { connectionString } = require('../config');
const nano = require('nano')(connectionString);
// creates database or fails silent if exists
nano.db.create('foo');
module.exports = {
foo: nano.db.use('foo')
}
This script is running on every server start, so it tries to create the database 'foo' every time the server (re)starts and just fails silently if the database already exists.
I like this idea a lot because this way I'm actually maintaining the database at the application level and don't have to create databases manually when I decide to add a new database.
Taking this approach one step further I also tried to maintain my design docs from application level.
...
nano.db.create('foo');
const foo = nano.db.use('foo');
const design = {
_id: "_design/foo",
views: {
by_name: {
map: function(doc) {
emit(doc.name, null);
}
}
}
}
foo.insert(design, (err) => {
if(err)
console.log('design insert failed');
})
module.exports = {
foo
}
Obviously this will only insert the design doc if it doesn't exist. But what if I updated my design doc and want to update it?
I tried:
foo.get("_design/foo", (err, doc) => {
if(err)
return foo.insert(design);
design._rev = doc._rev
foo.insert(design);
})
The problem now is that the design document is updated every time the server restarts (e.g it gets a new _rev on every restart).
Now... my question(s) :)
1: Is this a bad approach for bootstrapping my CouchDB with databases and designs? Should I consider some migration steps as part of my deployment process?
2: Is it a problem that my design doc gets many _revs, basically for every deployment and server restart? Even if the document itself hasn't changed? And if so, is there a way to only update the document if it changed? (I thought of manually setting the _rev to some value in my application but very unsure that would be a good idea).

Answer 1

Your approach seems quite reasonable. If the checks happen only at restarts, this won't even be a performance issue.
Too many _revs can become a problem. The history of _revs is kept as _revs_info and stored with the document itself (see the CouchDB docs for details). Depending on your setup, it might be a bad decision to create unnecessary revisions.
We had a similar challenge with some server-side scripts that required certain views. Our solution was to calculate a hash over the old and new design document and compare them. You can use any hashing function for this job, such as sha1 or md5.
Just remember to remove the _rev from the old document before hashing it, or otherwise you will get different hash values every time.

Answer 2

I tried the md5 comparison like #Bernhard Gschwantner suggested. But I ran into some difficulties because im my case I'd like to write the map/reduce functions in the design documents in pure javascript in my code.
const design = {
_id: "_design/foo",
views: {
by_name: {
map: function(doc) {
emit(doc.name, null);
}
}
}
}
while getting the design doc from CouchDb returns the map/reduce functions converted as strings:
...
"by_name": {
"map": "function (doc) {\n emit(doc.name, null);\n }"
},
...
Obviously md5 comparing does not really work here.
I ended up with the very simple solution by just putting a version number on the design doc:
const design = {
_id: "_design/foo",
version: 1,
views: {
by_name: {
map: function(doc) {
emit(doc.name, null);
}
}
}
}
When I update the design doc, I simply increment the version number and compare it with the version number in database:
const fooDesign = {...}
foo.get('_design/foo', (err, design) => {
if(err)
return foo.insert(fooDesign);
console.log('comparing foo design version', design.version, fooDesign.version);
if(design.version !== fooDisign.version) {
fooDesign._rev = design._rev;
foo.insert(fooDesign, (err) => {
if(err)
return console.log('error updating foo design', err);
console.log('foo design updated to version', fooDesign.version)
});
}
});

Answer 3

Revisiting your question again: In a recent project I used the great couchdb-push module by Johannes Schmidt. You get conditional updates for free, alongside with many other benefits inherited from its dependency couchdb-compile.
That library turned out to be a hidden gem for me. HIGHLY recommended!