$Id: README,v 1.26 2010/03/19 19:28:18 ianmacd Exp $

Introduction


Ruby/AWS is a Ruby language library that aims to make it relatively easy for the programmer to retrieve information from the popular Amazon Web site via Amazon's Associates Web Services (AWS). In addition to the original amazon.com site, the local sites amazon.co.uk, amazon.de, amazon.fr, amazon.ca and amazon.co.jp are also supported.

Development of Ruby/AWS has been quite swift since the appearance of the first alpha version, 0.0.1, in late March of 2008. Although Ruby/AWS shares almost no code with its now obsolete predecessor, Ruby/Amazon, many lessons were learnt whilst developing that library, and the experience gained has been rolled into Ruby/AWS.

As of version 0.3.0, I believe that Ruby/AWS has attained its goal of being superior to the final version of Ruby/Amazon, 0.9.2, which was released in August 2006.

History and compatibility with Ruby/Amazon


In the beginning, there was Ruby/Amazon. This library was built around version 3.x of the Amazon Web Service API and first saw the light of day in January

  1. The version of the Amazon API in use at the time was known as AWS 3.x.

Amazon later renamed AWS to ECS, or E-Commerce Service, for the launch of version 4 of their API, a complete overhaul that provided no backward compatibility with previous versions. The previous version of the API was thenceforth sometimes referred to as ECS 3.

Demonstrating the wisdom and consistency for which large companies are renowned, Amazon changed their mind once again in late 2007, reverting to the familiar name of AWS. This time, however, it was said to stand for Associates Web Service, rather than Amazon Web Service.

Since Amazon first made AWS available, the number of Amazon Web APIs has grown and AWS is now just one of many. It is therefore no longer appropriate to call this library by a name so general as Ruby/Amazon, because it provides an interface to just one of the Amazon Web APIs. Therefore, the monicker for this library is Ruby/AWS.

Unfortunately for Ruby/AWS, Amazon changed the name once again in May 2009, referring to it now as the Product Advertising API. Changing Ruby/AWS's name would create more confusion than it would mitigate, however, so I'm not about to do so. Similarly, I will continue to refer to the Amazon API in question as AWS.

Ruby/AWS is built around version 4 of the Amazon AWS API, which is fundamentally different to version 3, both in terms of how requests are made and the data returned. The underlying structure of the XML response has radically changed from previous versions.

It has therefore not been practical for Ruby/AWS to retain any level of API compatibility with Ruby/Amazon. Unfortunately, this means that any code written for Ruby/Amazon will need to be rewritten to work with Ruby/AWS. The good news is that, in most cases, this isn't as much work as it might sound.

Another bit of good news is that the /etc/amazonrc and ~/.amazonrc files used by Ruby/Amazon are compatible with Ruby/AWS. The only change required for Ruby/AWS is the addition of the 'key_id' and 'secret_key_id' parameters, which should contain your AWS Access Key ID and its secret counterpart. That fact notwithstanding, as of version 0.5.0, Ruby/AWS also supports a more flexible, locale-specific configuration syntax.

Amazon finally decomissioned v3 of the AWS API on 2008-03-31. As a result, the original Ruby/Amazon library no longer functions and is therefore obsolete.

AWS Access Key ID


You can obtain an AWS Access Key ID here:

https://aws-portal.amazon.com/gp/aws/developer/registration/index.html

You may see mention of Subscription IDs at the above location. Subscription IDs are not supported by Ruby/AWS and, in any case, are no longer supported by Amazon since the introduction of authenticated requests. Please obtain and use an AWS Access Key ID instead.

API version


Ruby/AWS currently requests the 2009-11-01 revision of the AWS API when performing its operations:

http://docs.amazonwebservices.com/AWSECommerceService/2009-11-01/DG/

However, a different version can be requested via the 'api' parameter in the user configuration file.

Status and functionality


Ruby/AWS is currently beta software. Amongst other things, this means:

  • You will encounter bugs, but hopefully not too many and none too serious. Tell me about them and I will endeavour to fix them.

  • The documentation isn't what it could be, but it's hopefully enough to get you up and running.

  • Not all features are currently implemented. Others may not yet be fully implemented. Some, I probably haven't even thought of yet. Again, if something's missing, tell me, and if it makes sense, I'll add it.

    In spite of this shortcomings, the AWS v4 API is more or less fully supported, with only small gaps in the functionality of some operations.

    Currently implemented operations are:

    BrowseNodeLookup
    CustomerContentLookup
    CustomerContentSearch
    Help
    ItemLookup
    ItemSearch
    ListLookup
    ListSearch
    SellerListingLookup
    SellerListingSearch
    SellerLookup
    SimilarityLookup
    TagLookup
    TransactionLookup
    

    Remote shopping-carts are also implemented as of version 0.3.0. The following remote shopping-cart operations are supported:

    CartCreate
    CartAdd
    CartModify
    CartClear
    

    Version 0.4.0 added the remaining shopping-cart operation, which I had first thought superfluous:

    CartGet
    

    Multiple operations are supported, but have not been heavily used, so there may be residual bugs. The implementation of multiple operations was rewritten from scratch for version 0.8.0, which appears to have resulted in code that is much more robust (not to mention easier to read and maintain).

    As of version 0.5.0, batch operations are fully supported, using the Operation#batch method. Again, the implementation was scrapped and rewritten for 0.8.0, yielding a big improvement.

    There appear to also be (undocumented) Amazon-imposed restrictions on the use of multiple operations and batch requests, so some experimentation may be required on your part to determine what works and what doesn't.

    In short, though, my investigations have demonstrated that no more than two operations may be batched together, and no more than two operations may be combined in a multiple operation. Nevertheless, by combining two batched operations in a multiple operation, one can send four base operations to Amazon in a single request. No more than two of these may be from the same class, however.

    The 2008-08-19 version of the AWS API added the following operations:

    VehiclePartLookup
    VehiclePartSearch
    VehicleSearch
    

    These are supported by Ruby/AWS from version 0.5.0 onwards.

  • Classes, methods, constants and instance variables may change name in the future. New ones may appear from nowhere and existing ones may change shape, grow, shrink or disappear without trace. Such fundamental changes will break existing code, so I will endeavour to keep them to a minimum.

In short, code written to work with this release of Ruby/AWS may stop working when you upgrade to the next. In fact, it may even stop working during this release cycle, because it's possible there are fatal conditions that I didn't encounter in my limited testing of the code. It's also possible that future (possibly unannounced) changes made by Amazon will affect Ruby/AWS in ways I can't anticipate.

That said, the Ruby/AWS API is pretty stable at this point in time. I won't break any of the method interfaces without seriously considering the merits of doing so.

Installation


Please see the INSTALL file for details of how to install Ruby/AWS. You can choose between an installation script and a RubyGems installation.

Usage


First of all, create either /etc/amazonrc or ~/.amazonrc as a plain text file. Its contents should look something like this:

# Any line that starts with a hash character is a comment.
key_id = '0Y44V8G41KCQPGF6XYZ2'
secret_key_id = 'k+kuddeoQJzUnImC0Hyy21J4xLWQc1hbvfQ+7F1G
associate = 'fuzbarorg-21'
cache = false
locale = 'uk'
encoding = 'iso-8859-15'

The ability to include your secret key was a feature added in version 0.6.0 of Ruby/AWS. If you choose to do so, your requests to AWS will be signed for authentication by Amazon's servers. If you don't, your requests will fail, because Amazon made this practice obligatory on 15th August 2009.

When Amazon checks for a valid signature, it does so by comparing its computation of the signature with the one supplied by the user. In doing this, Amazon uses the UTF-8 representation of parameter values that you supply, even if you use a different encoding.

In order to have Ruby/AWS properly reencode your strings as UTF-8, you need to tell it which encoding you are using. The 'encoding' parameter can be used for this, but you can omit it if your strings are already UTF-8, because this is the default.

As of version 0.5.0 of Ruby/AWS, the following locale-specific configuration syntax is also supported:

[global]
key_id = '0Y44V8G41KCQPGF6XYZ2'
secret_key_id = 'k+kuddeoQJzUnImC0Hyy21J4xLWQc1hbvfQ+7F1G
cache = false
locale = 'uk'
encoding = 'iso-8859-15'
# Request a specific version of the API.
# api = '2008-03-03'

[uk]
associate = 'fuzbarorg-21'

[us]
associate = 'fuzbarorg-20'

This enables the use of a different associate tag for each locale, although, oddly enough, no-one has ever requested this feature.

Because you're embedding your personal keys in the file, you should protect it (on UNIX and equivalent systems) by making it mode 0600:

$ chmod 600 ~/.amazonrc

If you define 'cache' to be 'true', you may want to also define 'cache_dir' to point to somewhere other the default cache directory, /tmp/amazon.

If you want to place .amazonrc somewhere other than $HOME, you may set the $AMAZONRCDIR environment variable, as this location is checked prior to $HOME.

If you're using Windows, $HOME is usually undefined, so a number of additional locations are checked for .amazonrc.

The exact search order is as follows:

$AMAZONRCDIR $HOME $HOMEDRIVE + $HOMEPATH $USERPROFILE

Note that only the first defined location is used, so if, for example, both $AMAZONRCDIR and $HOME are defined, but only the path specified by $HOME contains a file called .amazonrc, it will not be found.

If you want the user configuration file to be called something other than .amazonrc, you may define the $AMAZONRCFILE environment variable.

Once you have your configuration file, you can get started writing your code.

Here's a basic example, showing how to perform an ItemSearch, probably the most common type of AWS operation. Please see the ./examples subdirectory for more examples of contrived, but working code.

require 'amazon/aws/search'

# Avoid having to fully qualify our methods. # include Amazon::AWS include Amazon::AWS::Search

is = ItemSearch.new( 'Books', { 'Title' => 'Ruby' } )

# I want to receive just a small amount of data for the items found. # is.response_group = ResponseGroup.new( :Small )

req = Request.new

# Make sure I'm talking to amazon.co.uk. # req.locale = 'uk'

# Actually talk to AWS. # resp = req.search( is )

# Drill down to the meat: the array of items returned. # items = resp.item_search_response.items.item

# The following alternative shorthand would also have worked: # # items = resp.item_search_response.items.item

# Available properties for first item: # puts items.properties

items.each do |item|

attribs = item.item_attributes[0]
puts attribs.label
if attribs.list_price
  puts attribs.title, attribs.list_price[0].formatted_price, ''
end

end

Troubleshooting


HTTP 400 is the main bane of people's life since Amazon started authenticating requests to AWS. If you're trying to get Ruby/AWS to work, but are plagued by HTTP 400 responses, here are some possible causes:

  • Your ~/.amazonrc file doesn't contain a secret_key_id parameter. Add one.

  • Your computer's system clock is running more than 15 minutes slow. Amazon consider a request timestamped more than 15 minutes in the past invalid. Synchronise your system clock with a reliable time source.

    Incidentally, Amazon don't object to requests timestamped in the future, but that's something that may change at any time and therefore shouldn't be relied upon.

XML to Ruby mapping


Here, I will discuss the mapping of the XML returned from AWS to native Ruby objects and data. Note that the XML shown below was that returned at the time of writing and may look different to what you would see today if you were to execute the same request.

When this code:

resp = req.search( is )

was called in the previous section, the following URL was composed and sent to AWS as an HTTP GET operation:

ecs.amazonaws.co.uk/onca/xml?AWSAccessKeyId=01234567890123456789&AssociateTag=calibanorg-21&Operation=ItemSearch&ResponseGroup=Small&SearchIndex=Books&Service=AWSECommerceService&Title=Ruby&Version=2008-03-03

The following (abbreviated) AWS XML response was received:

<ItemSearchResponse>

<OperationRequest>
  <HTTPHeaders>
    <Header Name="UserAgent" Value="Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.13) Gecko/20080325 Fedora/2.0.0.13-1.fc7 Firefox/2.0.0.13"/>
  </HTTPHeaders>
  <RequestId>1TBGEZ48MF8KZ8TGXH65</RequestId>
  <Arguments>
    <Argument Name="SearchIndex" Value="Books"/>
    <Argument Name="Service" Value="AWSECommerceService"/>
    <Argument Name="ResponseGroup" Value="Small"/>
    <Argument Name="Operation" Value="ItemSearch"/>
    <Argument Name="Version" Value="2008-03-03"/>
    <Argument Name="AssociateTag" Value="calibanorg-21"/>
    <Argument Name="Title" Value="Ruby"/>
    <Argument Name="AWSAccessKeyId" Value="01234567890123456789"/>
  </Arguments>
  <RequestProcessingTime>0.0671439170837402</RequestProcessingTime>
</OperationRequest>
<Items>
  <Request>
    <IsValid>True</IsValid>
    <ItemSearchRequest>
      <ResponseGroup>Small</ResponseGroup>
      <SearchIndex>Books</SearchIndex>
      <Title>Ruby</Title>
    </ItemSearchRequest>
  </Request>
  <TotalResults>1804</TotalResults>
  <TotalPages>181</TotalPages>
  <Item>
    <ASIN>0439943663</ASIN>
    <DetailPageURL>

www.amazon.co.uk/gp/redirect.html%3FASIN=0439943663%26tag=calibanorg-21%26lcode=xm2%26cID=2025%26ccmID=165953%26location=/o/ASIN/0439943663%253FSubscriptionId=0Y44V8FAFNM119C6PTR2

  </DetailPageURL>
  <ItemAttributes>
    <Author>Philip Pullman</Author>
    <Manufacturer>Scholastic</Manufacturer>
    <ProductGroup>Book</ProductGroup>
    <Title>The Ruby in the Smoke (Sally Lockhart Quartet)</Title>
  </ItemAttributes>
</Item>
<Item>
  <ASIN>0596516177</ASIN>
  ...

In Ruby/AWS, each unique XML element name forms a class of the same name. All such classes are subclasses of AWSObject. For example, OperationRequest is a class, as is ItemAttributes.

As the XML tree is traversed, each element is converted to an instance of the class of the same name. Every such object has instance variables, one per unique child element name. The name of the instance variable is translated to comply with Ruby convention by adding an underscore ('_') character at word boundaries and converting the name to lower case.

For example, given the following XML:

<ItemAttributes>

<Author>Philip Pullman</Author>
<Manufacturer>Scholastic</Manufacturer>
<ProductGroup>Book</ProductGroup>
<Title>The Ruby in the Smoke (Sally Lockhart Quartet)</Title>

</ItemAttributes>

the following statements would all be true:

  • ItemAttributes, Author, Manufacturer, ProductGroup and Title would all be dynamically defined subclasses of AWSObject.

  • An instance of the ItemAttributes class would be created, with instance variables @author, @manufacturer, @product_group and @title.

  • To each of these instance variables would respectively be assigned an array of Author objects, an array of Manufacturer objects, an array of ProductGroup objects and an array of Title objects. In the above case, these would all be single element arrays, because there's only one instance of each kind of tag in the XML.

  • The Author, Manufacturer, ProductGroup and Title objects would have no instance variables of their own, because the corresponding XML elements have no children, just a value. These objects are therefore directly assigned the value in question.

So, if resp is the top level AWSObject created and returned by calling the Amazon::AWS::Search::Request#search method of the Request object, and we'd like to know the ASIN of the first item found, we can refer to this as follows:

resp.item_search_response[0].items[0].item[0].asin

Looking at each component of this chain in turn:

  • resp is an AWSObject with a single instance variable, @item_search_response. This is because the entire XML response is contained within a single <ItemSearchResponse> element, so there's nothing else at the top level.

  • resp.item_search_response is assigned an array of ItemSearchResponse objects. Because there's only a single <ItemSearchResponse> element in the whole document (containing the rest of the XML), the array contains only a single element.

  • resp.item_search_response has an instance variable, @items, which is assigned an array of Items objects. Here again, only a single element is created, because there's only one corresponding <Items> element in the XML.

  • resp.item_search_response.items has an instance variable, @item, which is an array containing the actual item(s) located by the search. It is a multi-element array, however, because more than one item was found, as represented by the multiple <Item> elements in the XML.

The creation of so many single element arrays is unfortunate. It makes user code verboser, uglier and consequently harder to read.

You might wonder why Ruby/AWS doesn't just assign the single element itself, rather than the array that contains it.

The answer is that most of these single-element arrays actually do have the potential to be multi-element, because the corresponding XML tag can appear multiple times in an AWS response. A book, for example, may have more than one <Author>. Many other types of array, however, are necessarily single element arrays. That same book, for example, is unlikely to have more than one <Title>. This is context-dependent and difficult to define programatically.

As another concrete example, an ItemSearch will probably yield many <Item> elements in the <ItemSearchResponse>, but these will invariably be nested in a single <Items> element. The @items instance variable of the ItemSearchResponse object will therefore always be a single-element array.

In other words, the following statements are both invariably true when an ItemSearch successfully locates items:

The awkwardness of using such single element arrays is alleviated in Ruby/AWS by the use of the AWSArray subclass. An instance of this class differs from a standard array by allowing element 0 of a single-element array to be dereferenced using just the array name, i.e. without a subscript.

In other words, a reference to foo.bar will actually return foo.bar when foo.size == 1. Note that this can only work because the array itself, foo, has no bar method, so the intention is unambiguous and foo can delegate the invocation of the method to foo. foo.size, on the other hand, will always invoke foo's bar method, never delegating to foo, because of the existence of the Array#size method.

This allows the ASIN of the first item returned in the above XML to be referred to using the following shorthand:

resp.item_search_response.items.item[0].asin

It's worth reiterating that it's still necessary in this example to refer to item using a subscript, because the <Items> element in the XML contains multiple <Item> elements, making item.size > 1.

Use this syntactic shorthand to your advantage, but understand when you're likely to be dealing with a single element array vs. a multiple. This will become apparent as you gain familiarity with AWS v4.

An exception will be raised if an unknown method is called on a multi-element array, as it can't be known to which array element the method invocation should be delegated. This will almost certainly stem from an incorrect assumption that an array contains only a single element when, in actual fact, it contains multiple elements.

A further important detail to note is that not all AWS operations of the same class return the same data. For example, an ItemSearch using the Books search index will return items that, amongst other things, have an ItemAttributes object containing further objects of class Author, ISBN, etc. An ItemSearch using the DVD search index, by contrast, will have no Author or ISBN, but will likely have a Director and probably one or more Actor objects.

Because of the disparity in same-class object attributes, Ruby/AWS returns nil when an attempt is made to dereference a non-existent instance variable. This approach was chosen because, more often than not, it cannot be known in advance precisely which data will be returned by a given search operation. Returning nil for non-existent attributes saves the user from having to pepper their code with exception-handling clauses.

For example:

resp.item_search_response[0].items[0].item[0].item_attributes.director

will return nil for a book, because there was no corresponding Director element in the XML returned by AWS.

Similarly:

resp.item_search_response[0].items[0].item[0].item_attributes.foo_bar

will always return nil for any item, because no kind of ItemSearch will ever yield an item with a FooBar element.

Parameter checking


There are many combinations of parameters and values that are legal for a particular type of search. For example, an ItemSearch can use a Sort parameter with a value of 'titlerank' if the SearchIndex is 'Books'. However, this value wouldn't make much sense in the 'Automotive' SearchIndex.

The very presence of a certain parameter can be illegal in certain contexts. For example, specifying the parameter 'Author' with any value would be nonsensical in the 'PetSupplies' SearchIndex.

To complicate things further, the validity of parameters and their values differs not only by search type, but also by Amazon locale (amazon.com, amazon.co.uk, amazon.de, etc.) and is prone to change with minor revisions of the Amazon AWS API.

Even worse, the operations themselves can be illegal in certain locales. TransactionLookup operations, for example, don't work in the UK locale at the time of writing, but do work in the US locale. As a rule of thumb, we can say that almost everything works in the US locale and may work in others.

Ruby/Amazon attempted to track these complex and dynamic relationships to prevent illegal or ineffective operations from being attempted. It was a time-consuming and tedious task to track the evolving API (which often changed in subtle ways without prior [or even belated] notice from Amazon), find all of the corner cases and handle undocumented quirks.

With the highly dynamic nature of the Amazon environment and its many locales, plus the sheer number of operations, parameters and their possibly legal values in the AWS v4 API, this strict approach would be completely impractical for Ruby/AWS. It therefore doesn't even try.

Instead, it's now up to you to ensure that you perform legal operations and pass sensible parameters and values for the locale in which you're working. The context is now your responsibility.

The one exception to this rule is search index checking for ItemSearch operations. Code that attempts to use an invalid SearchIndex will raise an exception. The list of allowable search indices can be found in the Amazon::AWS::Operation::ItemSearch::SEARCH_INDICES array.

Of course, even this check exposes the user to the risk that Amazon may later add new search indices, which would continue to be unrecognised and ruled invalid by Ruby/AWS until an update was issued. Whilst I have chosen to implement this very basic level of checking, it may be removed in the future if it becomes impractical to keep it current.

In short, the validity of what goes into a search operation is your own responsibility: garbage in, garbage out.

Thankfully, with the AWS Developer Guide at your side, it's largely common sense which parameters and values can be used with each type of search. It's less obvious when these differ by locale. For example, the 'Beauty' SearchIndex was valid in the 'us' locale, but not in the 'uk' locale until the 2009-01-06 revision of the AWS API.

Unfortunately, AWS abounds with such inconsistencies and they are prone to change at any time. Amazon, themselves, seem to struggle to document all of these quirks, a situation probably aggravated by the US focus of the AWS staff.

The only way to apprise yourself of such peculiarities is to read Amazon's latest developer documentation (and closely follow the release notes of each minor API revision to make sure things haven't changed). If you don't want to be exposed to such API changes, use the 'api' parameter in the user configuration file to request a particular version of the API.

The AWS Developer Connection pages may also be of use to you. In particular, the forum for discussing AWS has proved useful to me over the years:

http://developer.amazonwebservices.com/connect/forum.jspa?forumID=9

For those illegal operations that make it through to the Amazon servers, the good news is that Amazon carries out extensive run-time parameter checking in AWS v4 (much better than in v3) and will generate an error when an illegal set of parameters and/or values is given. Ruby/AWS will dynamically generate a class for the type of error reported and raise an exception of that class.

Using this approach, Ruby/AWS doesn't have to perform checks that Amazon will perform, anyway. This helps keep the code base leaner, the library faster, and reduces the chance that Ruby/AWS will disallow an operation that becomes valid following a minor revision of AWS.

Documentation


You can generate HTML documentation for the library with the following command, executed from the directory created when you unpacked the archive:

rdoc -SUx CVS lib

The documentation on how to use this library is currently incomplete, but it should be enough to get you started.

You can also use the Ruby/AWS mailing-list:

http://caliban.org/mailman/listinfo/ruby-aws

to discuss any Ruby/AWS-related subjects and issues.

Examples


The ./examples subdirectory contains working examples of code.

Licence


This software is copyright (C) 2008-2010 Ian Macdonald and distributed under the terms of the GNU GENERAL PUBLIC LICENSE, a copy of which is included.

– Ian Macdonald <ian@caliban.org>