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Todos 06 - Building with CouchDB

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Saved by Ido Ran
on October 19, 2010 at 2:23:23 am
 

This page will show and explain how to build DataStore to work with CouchDB as backend database server.

[Show how to deploy SproutCore application into CouchDB]

 

Backend

 

What is CouchDB?

CouchDB is document-oriented database built by Apache.

CouchDB is written using Erlang language.

The best and only way to interact with CouchDB done using HTTP which makes it great fit for web applications.

You can read more and learn about CouchDB in this online book.

 

Setup the database

There are couple of ways you can setup CouchDB for the purpose of this tutorial both of them located at couch.io web site.

 

1. Run local instance

Scroll a little at couch.io until you see Download CouchOne and select the download for your favorite OS.

TODO: explain how to run locally

 

2. Register to couch.io hosting

Enter 4 bits of information at couch.io registration page and you instantly have your very own online CouchDB database.

Since CouchDB is design for web use it have administration console called Futon to let you administer your database.

 

3. Register to CloudAnt.com

Go to CloudAnt.com Oxygen registration page and in second you will have your own CouchDB database. CloudAnt have their own administration console as well as Futon.

 

Create the schema

You don't have to. That is one of the great things about CouchDB, it does not require that you will define a schema for your data. Just put your document in the database and the are stored. Because CouchDB is not relational database it does not index your data nor query your data the way SQL Server or Oracle does.

 

Create the database

Go into Futon on which ever installation you chose.

On the bottom left corner of the screen you will either see Welcome to Admin Party! Eveone is admin" or you will see Signup or Login.

For the former you don't have to do anything. Admin Party means everybody are admin and no login is require.

For the later you will need to login with your admin account.

 

At the top left corner you will see "Create database" link, click on it.

Enter "todos" as database name and click Create. That's it.

 

Play with CouchDB

Open Futon and go into your todos database.

 

Create new document

On the top left corner you will see "New Document" link, press on it.

The document edit screen will show table contain single row. The row Field will say "_id" and the value (which is now editable) will have a long UUID which compose of hex-decimal and letters. Clear the text box and write doc1 in it.

On the top left corner press the "Save Document" link.

A second row will be added to the table. This row is _rev with value of 1-XXX (XXX will be different on each machine).

You have just created a new document in your CouchDB. This is the simplest document you can create. As you see each document must have at least two fields: _id and _rev. The _id field allow you to reference this document while _rev allow CouchDB to keep track on documents as they are changing. Each time you save a document its _rev field will be change by CouchDB. _rev is built from two parts - revision number and hash code on the document content. Read about it in more detail in the CouchDB book.

 

Change a document

Click the Add Field button, it will create a new row in the table and put the focus on the text box in Field column. write description in the text box and press Tab. The focus is now on the Value text box, write Hello CouchDB and press Enter. Futon will automatically surround your text with parentesess to make it string.

Click the Save Document link. Notice the _id field stay the same but the _rev field change to 2-YYY. Also you just add a new field to your document without the need to define column in table nor type of the field.

Click Add Field again and enter isDone in Field text box and true on the Value. Notice Futon does not surround true with parentesess because it is boolean value and not a string.

Click Save Document link and see the _rev change once more.

 

See the raw document

On the right upper corner click the Source tab. Futon will change the view to show you the raw view of the document. Now you see that a document is nothing more than JSON. You can also double click on the Source content to edit the document as JSON.

Because documents are JSON they are not limited to flat name/value pairs, they can contain hierarchical structures as complex as you need.

 

Delete a document

On the middle up of the window click the Delete Document... link. A dialog will appear ask you if you sure you want to delete the document. Click Delete and the document will be delete from the database.

 

CURD with HTTP

We will now repeat the create, update and delete operation and you'll see how they are communicated to CouchDB.

Open Terminal to use curl to interact with CouchDB.

 

I will now show each command and how it change CouchDB. I am using localhost:5984 as my server, change it if you use any other address.

 

curl http://localhost:5984/todos -H "Content-Type:application/json" -d '{"description":"HTTP"}' -X POST 

POST will create new document in todos database with the content we supply, that is "description":"HTTP". CouchDB will assign _id and _rev to our new document.

As you can see CouchDB has reply to us:

{"ok":true,"id":"34ead35bfc32ab858b7b5446aa002c09","rev":"1-786e63f02916b93805d01132b917305b"}

The ok field signal everything went OK. The id field contain the id of our new document and the rev contain the revision of our new document.

 

curl http://localhost:5984/todos/34ead35bfc32ab858b7b5446aa002c09 -H "Content-Type:application/json" -d '{"isDone":false,"_rev":"1-786e63f02916b93805d01132b917305b"}' -X PUT

PUT will update exist document. First thing to notice is that we POST this request not to /todos but specifically to the address of our new document. Each document can be reached using its id field. Also we add new field isDone to our document and we also have to send the _rev field in the document with the rev return from our POST because CouchDB must know we have the latest version of this document.

Here again CouchDB reply to us:

{"ok":true,"id":"34ead35bfc32ab858b7b5446aa002c09","rev":"2-e84e08f018b870b7bdaffc0f6848e86f"} 

The ok field signal that everything is ok. the id contain the id of the document we changed and the rev contain the new revision assign the our document after the change we request has been done.

 

curl http://localhost:5984/todos/34ead35bfc32ab858b7b5446aa002c09?rev=2-e84e08f018b870b7bdaffc0f6848e86f -X DELETE

DELETE, as wired as it may sound, delete a document. Here like POST we send the request to our document but because DELETE does not have body we carry the revision information as query parameter. If you try to delete a document without including the rev query parameter you will get this reply {"error":"conflict","reason":"Document update conflict."} which means CouchDB could not verify you have the latest version and so has reject your changes due to possible conflict.

The reply we get might look strange:

{"ok":true,"id":"34ead35bfc32ab858b7b5446aa002c09","rev":"3-6347d5414facc350d74dbb26c187246d"}

We get the same reply as in POST and PUT. The ok still here and the id also here but notice rev has been incremented even though we have delete the document. You can read more about how CouchDB store the document internally but just know that when you delete a document it is not actually been delete right away. CouchDB store it (with the new revision) with field called _delete and value true. 

 

The reason it is important to play like this with CouchDB is to understand how SproutCore app will interact with it.

 

 

Design Documents

Design document are special type of document CouchDB can store. A design document is identify by have _design/<name> as its id.

I will not go into detail about design document but you can read more in this chapter of CouchDB book.

 

Views

CouchDB does not use SQL to query data. Instead CouchDB use JavaScript to define map-reduce function.

You can read the chapter about views in the CouchDB book, I will show you how to create one view here.

 

allTasks view

We need to create view which will return all of the documents in our database that represent tasks.

In Futon open todos database. On the right upper corner you will see Views drop-down list that currently say All Documents. This means Futon now show all the documents you have in your database. Open it and select Temporary view..., this will open screen for edit and test views.

As you can see view is define from two parts: map and reduce functions. For now we will only use the map function.

Enter the following text into Map Function text box:

function(doc) {
  if (!(doc.description==null || doc.isDone==null)) {
    emit(doc._id, doc);
  }
}

This function will run once for every document we have in our database. CouchDB will pass to it a single document. The function check if the document has description and isDone properties, if so it emit (select) the document. View function output is key/value pair, in our case the key is the _id field of the document and the value is the document itself.

Because we have no documents yet we cannot test the view so we will just save it for later. Click the Save As... button and enter app in the Design Document text box and enter allTasks in the View name text box. Click Save button.

 

Frontend

see: http://github.com/ido-ran/CouchTodos

DataSource

 

Settings up proxy

 

 

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