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Pharo CouchDB Client

CouchDB

This provides a lightweight Pharo client for CouchDB NoSQL Document Database with support for basic JSON documents as well objects serialization/deserialization in JSON format.

The source is based on an old version for VisualWorks, but this one uses Zinc HTTP client (ZnClient), URL objects (ZnUrl) instead of plain URL strings, has the Couch prefix for the class names (due to the lack of namespaces), and has other refactorings that I considered relevant (although there are many more to do). This version also supports document attachment operations using Base64 encoding for download.

There are several pending features to be added to this client, if you have any issues please create an issue and/or send a pull request. Or if you need a full-featured, including a native View Server and App Contexts you can use the SCouchDB Client.

Client Installation

Metacello new
	baseline: 'CouchDB';
	repository: 'github://eMaringolo/pharo-couchdb/src';
	load.

Server Installation

Download CouchDB and install it following the instructions in CouchDB website.

If you install it using defaults, it will a create a default server running at http://127.0.0.1:5984/ without authentication. All the examples will assume the server is running at that address and port.

How to use

The current version provides the class CouchInterface that will provide control over the server databases.

You can instantiate an interface by evaluating:

client := CouchInterface url: 'http://127.0.0.1:5984/'.

But that above will not attempt any connection to the server. So you can assert whether the server exists you can evaluate:

client exists

Then you can create a database by asking the interface to create a new database.

database := client create: 'pharodb`

If the database was already created you can also instantiate it by evaluating:

database := CouchDatabase url: 'http://localhost:5984/pharodb'

And then, you're ready to store your first document. You can store anything that can be serialized as JSON.

By default this CouchDB client sends the #couchJson message to objects to convert them to a String with JSON syntax that will be stored as the document.

document := database create: { 'foo' -> true. 'baz' -> (1 to: 5) } asDictionary

This will return a CouchDocument that will contain a dynamically assigned _id and other information such as the _rev number. And more importantly, you can can ask for the URL of the recently created document.

document url "http://127.0.0.1:5984/pharodb/36557e332476cf324dd0bcc009005801"

Then you can copy that URL and paste it in your web browser of preference, and retrieve the contents of the document.

Note: Even when all documents are stored as JSON, when served by CouchDB they are returned as text/plain.

A CouchDocument works a wrapper for an object, which by default, as in our above example, is a Dictionary.

So in our above sample document, we can do:

document object at: 'foo' put: false.
document save

This will save the created document and also increment the revision number, which you can query by sending #revision to the document.

Document attachments

All documents can have attachments that will get versioned along the document itself. You can add any file as attachment, you can do it directly by passing the file name, the MIME type and the contents, or by instantiating a CouchAttachment object. The contents are expected to be bytes, so encode and decode strings accordingly.

self 
  saveAttachmentNamed: 'foo.txt'
  mimeType: ZnMimeType textPlain
  contents: 'This is a sample attachment' utf8encoded

You can save an attachment with the same name as many times as you want, each time you save it it will increment the revision number of the document.

Every time you query for a CouchDocument it will fetch the attached file, but will present them only as stubs without the content.

So, following our example we could do:

document := CouchDocument url: 'http://127.0.0.1:5984/pharodb/36557e332476cf324dd0bcc009005801'.
document attachments

That will return a Dictionary whose keys will be the attachment names and instances of CouchAttachmentStub its values. You can send #download to an attachment stub to retrieve its full contents.

Mapping objects

Although it is convenient to serialize/deserialize simple objects such as Dictionaries and Arrays into a JSON representation, the original implementation of the client devised a way to "map" objects to/from JSON using a few extra properties.

If you have the CouchDB-Examples package loaded, you'll find the CouchSofa and CouchPillow example classes, these classes have nothing special, but serve as a reference of how to save/restore an object, with dependencies, from the database.

database create: CouchSofa example1

This will create a regular JSON document for the body, but will add a +class property with the name of the Smalltalk class of the mapped object.

How mapping works

The basic classes such as SequenceableCollection, Dictionary, String, etc. know how to write themselves as JSON, but other classes must define what attributes must be mapped. This is achieved by means of the couchDocumentProperties method.

CouchSofa>>#couchDocumentProperties 

	^Dictionary new
		at: 'weight' put: #weight;
		at: 'pillows' put: #pillows;
		yourself

This Dictionary defines a mapping between a key (that will be used as the property name when serialized as JSON) and a symbol that will work as a getter (sending asMutator to be used as setter). This of course could use some more advanced mapping technique or at least better accessors such as those in Magritte, but for the most part if works without issues.

The pillows accessor returns a collection of CouchPillow instances, CouchPillow also defines couchDocumentProperties, otherwise the instances would be serialized as empty JSON objects and deserialized as empty Dictionaries.

Remarks

Although this guide is small, it is suggested to use the GT Inspector to "dive" into each message send, because CouchDB objects works as containers in the following sequence "Server" -> "Database" -> "Document" -> "Attachment".