Simple Object Access Protocol SOAP and Representational State Transfer REST are two answers to the same question how to access Web services. The choice initially may seem easy, but at times it can be surprisingly difficult. SOAP is a standards based
RoTLD REST API documentation for implementing domain registration of .ro TLD domains.
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Learn REST: A Tutorial A fa st -t ra in in g co ur se fo r REST REST - Re presentational S tate t ate T ransfer, r ansfer, a new approach to systems architecture and a lightweight alternative to web services
1. What is REST? REST stands for Re Representational presentational State Transfer. (It is sometimes spelled "ReST".) It relies on a stateless, client-server, cacheable communications protocol -- and in virtually all cases, the HTTP protocol is used.
REST is an architecture style for style for designing networked applications. The idea is that, rather than using complex mechanisms such as CORBA, RPC or SOAP to connect between machines, simple HTTP is used to make calls between machines.
In many ways, the World Wide Web itself, based on HTTP, can be viewed as a REST-based architecture.
RESTful applications use HTTP requests to post data (create and/or update), read data (e.g., make queries), and delete d ata. Thus, REST uses HTTP for all four CRUD (Create/Read/Update/Delete) operations.
REST is a lightweight alternative to mechanisms like RPC (Remote Procedure Calls) and Web Services (SOAP, WSDL, et al.). Later, we will see how much more simple REST is.
Despite being simple, REST is fully-featured; there's basically nothing you can do in Web Services that can't be done with a RESTful architecture.
REST is not a "standard". There will never be a W3C recommendataion for REST, for example. And while there are REST programming frameworks, working with REST is so simple that you can often "roll your own" with standard library features in languages like Perl, Java, or C#.
BY DR. M. ELKSTEIN
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Dr. M. Elkstein said...
Note that REST is actually used with two meanings, subtly different from each other. This can be confusing.
The pure pure meaning meaning is the architectural style -- which is completely independent of HTTP and the Web. It can can be be used with HTTP and the Web, but it doesn't have have to to be.
The real-world real-world meaning, meaning, most commonly used in the industry, is how REST is used in practice: HTTP-based, and with standard Web servers (e.g., Apache httpd) playing the part of REST servers.
Unless stated otherwise, the rest of this tutorial refers to the "real-world" usage of REST.
February 9, 2008 10:40 AM
Mangol said...
Thanks a lot for this wonderful blog. It was lot helpful. But I'm lot confused between SOAP, RPC, XMLRPC, REST, Webservices.
Can you please clear them on this blog with clear examples of each. I don't know if there are other alternatives for SOAP. If there, please compare them.
March 16, 2009 11:22 AM
Dr. M. Elkstein said...
Hello Mangol,
You might wish to take a look at pages 10 (ROA vs. SOA, REST vs. SOAP ) and 11 ( WSDL and WADL) for some answers.
The terms are often tainted, in the sense that the same term is used by different people, or in different contexts, to mean different things. Below is a very broad overview.
In general: RPC is any mechanism that allows you to execute methods remotely (although some technologies adapted that as a proper name, it's a general name for many such technologies). This includes CORBA, Java RMI, SOAP-based Web Services, and REST. Some of these systems use binary data transfer (CORBA, Java RMI), and others are text-based; some of the text-based ones are actually XML-based.
XMLRPC is a general name for any XML- based RPC system, i.e., a system where the data that passes between client and server is XML data. There are generic XMLRPC systems (Java's JAX-RPC comes to mind) but when discussing XMLRPC, the key technology is SOAP -based Web Services.
Web services is a generic name for web-based RPC. This includes SOAP-based web services, which normally go by the proper name "Web Services"; but now, REST is also considered a web-service architecture.
SOAP is an XMLRPC web service, i.e., a web-based RPC that uses XML for data transfer, and more specifically, it uses the SOAP data format (an XML schema) for the queries and their replies.
REST is a non-XML-based, web-based RPC; but it is also a design philosophy and architecture. Read this tutorial site f or more details.
2. REST as Lightweight Web Services As a programming approach, REST is a lightweight alternative to Web Services and RPC.
Much like Web Services, a REST service is:
Platform-independent (you don't care if the server is Unix, the client is a Mac, or anything else),
Language-independent (C# can talk to Java, etc.),
Standards-based (runs on top of HTTP), a nd
Can easily be used in the presence of firewalls.
Like Web Services, REST offers no built-in security features, encryption, session management, QoS guarantees, etc. But also as with Web Services, these can be added by building on top of HTTP:
For security, username/password tokens are often used.
For encryption, REST can be used on top of HTTPS (secure sockets).
... etc.
One thing that is not part of a good REST design is cookies: The "ST" in "RE ST" stands for "State Transfer", and indeed, in a good REST design operations are selfcontained, and each request carries with it (transfers) all the information (state) that the server needs in order to complete it.
BY DR. M. ELKSTEIN
3. How Simple is REST? Let's take a simple web service as an example: querying a phonebook application for the details of a given user. All we have is the user's ID.
Using Web Services and SOAP, the request would look something like this:
12345 (The details are not important; this is just an example.) The entire shebang now has to be sent (using an HTTP POST request) to the server. The result is probably an XML file, but it will be embedded, as the "payload", inside a SOAP response envelope.
And with REST? The query will probably look like this:
http://www.acme.com/phonebook/UserDetails/12345 Note that this isn't the request body -- it's just a URL. This URL is sent to the server using a simpler GET request, and the HTTP reply is the raw result data -- not embedded inside anything, just the data you need in a way you can directly use.
It's easy to see why Web Services are often used with libraries that create the SOAP/HTTP request and send it over, and then parse the SOAP r esponse. With REST, a simple network connection is all you need. You can even test the API directly, using your browser. Still, REST libraries (for simplifying things) do exist, and we will discuss some of these later.
Note how the URL's "method" part is not called "GetUserDetails", but simply "UserDetails". It is a common convention in REST design to use nouns rather than verbs to denote simpleresources.
The letter analogy A nice analogy for REST vs. SOAP is mailing a letter: with SOAP, you're using an envelope; with REST, it's a postcard. Postcards are easier to handle (by the receiver), waste less paper (i.e., consume less bandwidth), and have a short content. (Of course, REST requests aren't really limited in length, esp. if they use POST rather than GET.)
But don't carry the analogy too far: unlike letters-vs.-postcards, REST is every bit as secure as SOAP. In particular, REST can be carried over secure sockets (using the HTTPS protocol), and content can be encrypted using any mechanism you see fit. Without encryption, REST and SOAP are both insecure; with proper encryption in place, both are equally secure.
BY DR. M. ELKSTEIN
4. More Complex REST Requests The previous section included a simple example for a REST request -- with a single parameter.
REST can easily handle more complex requests, including multiple parameters. In most cases, you'll just use HTTP GET parameters in the URL.
For example:
http://www.acme.com/phonebook/UserDetails?firstName=John&lastName=Doe If you need to pass long parameters, or binary ones, you'd normally use HTTP POST requests, and include the parameters in the POST body.
As a rule, GET requests should be for read-only queries; they should not change the state of the server and its data. For creation, updating, and deleting data, use POST requests. (POST can also be used for read-only queries, as noted above, when complex parameters are required.)
In a way, this web page (like most others) can be viewed as offering services via a REST API; you use a GET request to read data, and a POST request to post a comment -- where more and longer parameters are required.
While REST services might use XML in their responses (as one way of organizing structured data), REST requests rarely use XML. As shown above, in most cases, request parameters are simple, and there is no need for the overhead of XML.
One advantage of using XML is type safety. However, in a stateless system like REST, you should always verify the validity of your input, XML or otherwise!
BY DR. M. ELKSTEIN
5. REST Server Responses A server response in REST is often an XML file; for example,
ACME Boomerang Used by Coyote in Zoom at the Top, 1962 17.32http://www.acme.com/parts/3322ACME Dehydrated Boulders Used by Coyote in Scrambled Aches, 1957
19.95 http://www.acme.com/parts/783 However, other formats can also be used; unlike SOAP services, REST is not bound to XML in any way. Possible formats include CSV (comma-separated values) and JSON (JavaScript Object Notation).
Each format has its own advantages and disadvantages. XML is easy to expand (clients should ignore unfamiliar fields) and is type-safe; CSV is more compact; and JSON is trivial to parse in JavaScript clients (and easy to parse in other languages, too).
One option is not acceptable as a REST response format, except in very specific c ases: HTML, or any other format which is meant for human consumption and is not easily processed by clients. The specific exception is, of course, when the REST s ervice is documented to return a human-readable document; and when viewing the entire WWW as a RESTful application, we find that HTML is in fact the most common REST response format...
BY DR. M. ELKSTEIN
6. Real REST Examples Here's a very partial list of service providers that use a REST API. Note that some of them also support a WSDL (Web Services) API, in addition, so you can pick which to use; but in most cases, when both alternatives are available, REST calls are easier to create, the results are easier to parse and use, and it's also less resource-heavy on your system.
So without further ado, some REST services:
Twitter has a REST API (in fact, this was their original API and, so fa r as I can tell, it's still the main API used by Twitter application developers), Flickr,
Amazon.com offer several REST services, e.g., for their S3 storage solution,
Atom is a RESTful alternative to RSS.
(This is far from an exhaustive list.)
Here's a simple example: the following URL sends a REST request to Twitter's search service: http://search.twitter.com/search.atom?q=elkstein&count=5. This specific search request searches for the string "elkstein", as set by the
q parameter (vanity searches are always fun); and limits the response to at most 5 results,
using the count parameter. There are, of course, many additional parameters, as documented on Twitter's developer pages.
The returned response is in XML. A slightly different URL (this one) can be used to obtain results in JSON format. You will note that both formats are easily machine-readable (JSON is more easily parsed in JavaScript); you will also note that, to save bandwidth and reduce renpose sizes, the results are not nicely indented. In fact, all optional whitespace is removed. It is therefore probably easier to examine these responses in an XML-aware (or JSON-aware) editor.
BY DR. M. ELKSTEIN
7. AJAX and REST AJAX is a popular web development technique that makes web pages interactive using JavaScript.
In AJAX, requests are sent to the server using
XMLHttpRequest objects. The response is used by the JavaScript code to dynamically change the current
page.
In many ways, AJAX applications follow the REST design principles. Each
XMLHttpRequest can be viewed as a REST service request, sent using GET.
And the response is often in JSON, a popular response format for REST. (See REST Server Responses, above.)
To make your AJAX application truly RESTful, follow the standard REST design principles (discussed later). You will find that most of them contribute to a good design, even if you don't think of your architecture in terms of REST.
A later section provides code samples for issuing HTTP requests in JavaScript , but if you've done any AJAX programming, you are already familiar with all that.
BY DR. M. ELKSTEIN
8. REST Architecture Components Key components of a REST architecture:
Resources , which are identified by logical URLs. Both state and functionality are represented using resources.
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The logical URLs imply that the resources are universally addressable by other parts of the system. Resources are the key element of a true RESTful design , as opposed to "methods" or "services" used in RPC and SOAP Web Services, respectively. You do not issue a " getProductName " and then a " getProductPrice " RPC calls in REST; rather, you view the product data as a resource -- and this resource should contain all the required information (or links to it).
A web of resources, meaning that a single resource should not be overwhelmingly large and contain too fine-grained details. Whenever relevant, a resource should contain links to additional information -- just as in web pages.
The system has a client-server, but of course one component's server can be another component's client.
There is no connection state; interaction is stateless (although the servers and resources can of course be stateful). Each new request should carry all the information required to complete it, and must not rely on previous interactions with the same client.
Resources should be cachable whenever possible (with an expiration date/time). The protocol must allow the server to explicitly specify which resources may be cached, and for how long.
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Since HTTP is universally used as the REST protocol, the HTTP cache-control headers are used for this purpose. Clients must respect the server's cache specification for each resource.
Proxy servers can be used as part of the architecture, to improve performance and scalability. Any standard HTTP proxy can be used.
Note that your application can use REST services (as a client) without being a REST architecture by itself; e.g., a single-machine, non-REST program can access 3rd-party REST services.
BY DR. M. ELKSTEIN
9. REST Design Guidelines Some soft guidelines for designing a REST architecture: 1.
Do not use "physical" URLs. A physical URL points at something physical -- e.g., an XML file: "http://www.acme.com/inventory/product003.xml". A logical URL does not imply a physical file: "http://www.acme.com/inventory/product/003".
o
2. 3.
4.
Sure, even with the .xml extension, the content could be dynamically generated. But it should be "humanly visible" that the URL is logical and not physical. Queries should not return an overload of data. If needed, provide a paging mechanism. For example, a "product list" GET request should return the first nproducts (e.g., the first 10), with next/prev links. Even though the REST response can be anything, make sure it's well documented, and do not change the output format lightly (since it will break existing clients).
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Remember, even if the output is human-readable, your clients aren't human users.
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Yes, this means that the output is larger. But it also means that you can easily direct clients to new URLs as needed, without requiring a change in client code.
If the output is in XML, make sure you document it with a schema or a DTD. Rather than letting clients construct URLs for additional actions, include t he actual URLs with REST responses. For example, a "product list" request could return an ID per product, and the specification says that you should use http://www.acme.com/product/ PRODUCT_ID to get additional details. That's bad design. Rather, the response should include the actual URL with each item: http://www.acme.com/product/001263, etc.
5.
GET access requests should never cause a state change. Anything that changes the server state should be a POST request (or other HTTP verbs, s uch as DELETE).
BY DR. M. ELKSTEIN
10. ROA vs. SOA, REST vs. SOAP ROA (REST Oriented Architecture) is just a fancy name for a SOA (Service Based Architecture) using REST services.
The main advantage of SOAP-based SOA over ROA is the more mature tool support; however, this could change over time. Another SOA advantages include the type-safety of XML requests (for responses, ROA can also use XML if the developers desire it).
The main advantage of ROA is ease of implementation, agility of the design, and the lightweight approach to things. In a way, SOA and SOAP i s for people in business suits; that's what you'll find used in the banking and finance industries. Conversely, somebody that needs something up-and-running quickly, with good performance and low overhead, is often better off using REST and ROA.
For example, when explaining why they chose REST over SOAP, Yahoo! people write that they " believe REST has a lower barrier to entry, is easier to use than SOAP, and is entirely sufficient for [Yahoo's] services" ( Yahoo! Developer Network FAQ, as of February 2008). This is true not only of REST vs. SOAP b ut also of ROA vs. SOA in general.
Another advantage of REST lies with performance: with better cache support, lightweight requests and responses, and easier response parsing, REST allows for nimbler clients and servers, and reduces network traffic, too.
As REST matures, expect it to become better understood and more popular even in more conservative industries.
A few words on hype vs. reality. In the comments below, I've linked to Pete Lacey's excellent criticism of SOAP, "The S Stands for Simple ". And here's another one, by Alex Bell, published in Communications of the ACM (vol. 51, no. 10, October ' 08): "DOA with SOA ".
BY DR. M. ELKSTEIN
11. Documenting REST Services: WSDL and WADL WSDL, a W3C recommendation, is the Web Services Description Language. It is commonly used to spell out in detail the services offered by a SOAP server. W hile WSDL is flexible in service binding options (for example, services can be offered via SMTP mail servers), it did not originally support HTTP operations other than GET and POST. Since REST services often use other HTTP verbs, such as PUT and DELETE, WSDL was a poor choice for documenting REST services.
With version 2.0, WSDL supports all HTTP verbs and it is now considered to be an acceptable method of documenting REST services.
The second alternative is WADL, the Web Application Description Language. WADL is championed by Sun Microsystems. Like the rest of REST, WADL is lightweight, easier to understand and easier to write than WSDL. In some respects, it is not as flexible as WSDL (no binding to SMTP servers), but it is sufficient for any REST service and much less verbose.
Here is a fragment from a WADL specification, describing Amazon's "Item Search" service:
As you can see, for format is mostly self-explanatory, and it enriches REST with such goodies as type safety using XML schema types.
The entire document is only about 10 lines longer than this fragment (including XML namespace specifications, importing schema grammers, etc.) and can be found in the WADL specification.
For another real-world WADL document, check out the W3C Unicorn project'sspecification of the CSS validator service.
Some REST advocates, however, find even the lightweight WADL to be an overkill. A nd indeed, most REST services are documented by no more than a textual description (a human-readable HTML file).
BY DR. M. ELKSTEIN
12. REST Examples in Different Languages The following sections provide brief examples for accessing REST services in different programming languages. More than anyth ing else, they present mechanisms for sending HTTP GET and/or POST requests in each language; the rest of using REST is simply a matter of architectural design.
You might wish to skip directly to your language of choice (the list is alphabetical):
C#
Java
JavaScript
Perl
PHP
Python
Ruby
Finally , the tutorial conclusion contains references to additional resources.
BY DR. M. ELKSTEIN
13. For More About REST
Purely academic: the notion of REST was created in the PhD dissertation of Roy T. Fielding .
Mostly academic: the Wikipedia article about REST.
JSR 311 is the Java Specification Request for "JAX-RS: The Java API for RESTful Web Services ".
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At the time of writing (Feb '08), still in draft review stages. Check out this presentation f or a good overview of the JSR.
Restlet is suggesting an easier way to develop REST applications in Java : restlet.org.
WADL: find the specification and tools in the Web Application Description Language's homepage.
Articles are a dime a dozen; here are a few interesting ones:
o o o o o
Second Generation Web Services by Paul Prescod. The Beauty of REST, by Jon Udell. Building Web Services the REST Way by Roger L. Costello REST vs. SOAP, by Pete Freitag. Basic SOA using REST , by Mark Hansen.
Books: At the time of writing (Feb '08), there's only one:
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RESTful Web Services by Leonard Richardson -- a concise and practical guide, with concrete examples in Ruby, Java and Python. There's also Ajax and REST Recipes: A Problem-Solution Approach, by Christian Gross, which seems to be more about AJAX than about REST.
Expect many more books to appear as REST gains increasing acceptance.
BY DR. M. ELKSTEIN
14. Questions and Answers In this section, I'll be adding common questions about REST, or real issues I've encountered while developing RESTful services.
If you have a question, feel free to post it as a comment on this page, or on any other relevant page in this tutorial.
Questions posted so far: 1. 2. 3. 4.
How do I perform atomic operations using REST? How do I delete items using a REST API? How do I handle authentication in REST? How do I send complex objects as parameters?
BY DR. M. ELKSTEIN
14.1. How do I perform atomic operations using REST? QUESTION: How do I perform atomic operations using REST? If I would like to execute multiple operations, but have them all appear as an single, atomic transaction, REST seems inappropriate.
ANSWER: REST faces the exact same problem as SOAP-based web services with regards to atomic transactions. There is no stateful connection, and every operation is immediately committed; performing a series of operations means other clients can see interim states.
Unless, of course, you take care of this by design. First, ask yourself: do I have a standard set of atomic operations? This is commonly the case. For example, for a banking operation, removing a sum from one account and adding the same sum to a different account is often a required atomic operation. But rather than
exporting just the primitive building blocks, the REST API should provide a single "transfer" operation, which encapsulates the entire process. This provides the desired atomicity, while also making client code much simpler. This appracoh is known as low granularity services, or high-level batch operations.
If there is no simple, pre-defined set of desired atomic operation sequences, the problem is more severe. A common solution is the batch command pattern. Define one REST method to demarcate the beginning of a transaction, and another to demarcate its end (a 'c ommit' request). Anything sent between these sets of operations is queued by the server but not committed, until the commit request is sent.
This pattern complicates the server significantly -- it must maintain a state per client. Normally, the first operation ('begin transaction') returns a transaction ID (TID), and all subsequent operations, up to and including the commit, must include this TID as a parameter.
It is a good idea to enforce a timeout on transactions: if too much time has passed since the initial 'begin transaction' request, or since the last step, the server has the right to abort the transaction. This prevents a potential DoS attack that causes the server to waste resources by keeping too many transactions open. The client design must keep in mind that each operation must be checked for a timeout response.
It is also a good idea to allow the client to abort a transaction, by providing a 'rollback' API.
The usual care required in designing code that uses multiple concurrent transactions applies as usual in this complex design scenario. If at all possible, try to limit the use of transactions, and support high-level batch operations instead.
BY DR. M. ELKSTEIN
14.2. How do I delete items using a REST API? QUESTION: How do I provide a REST API for deleting resources in the system (e.g., a record from a database)? I was told that using a simple GET API, or even POST, is inappropriate, but I don't know why.
ANSWER: There's a well-known story about a company that had its entire database wiped out by Google. Why? Because they had a "delete" link on every web page, and Googlebot crawled all links it could find...
The lesson was quickly learned by the entire industry: do not expose deletion link, or more generally, state-changing links, as simple web links. And since a REST API is often described as a simple web URL, the convention was carried over.
In fact, if you never expose your REST action URLs as we b links, you could use standard GET operations for deletion. However, it's not a good idea. On a conceptual level, a good REST design takes advantage of the various HTTP verbs, to prevent errors, confusion, or mistaken operations. And there's a very specific HTTP verb for deletion, namely DELETE.
See also item #5 in the page about REST Design Guidelines in this tutorial.
BY DR. M. ELKSTEIN
14.3. How do I handle authentication in REST?
QUESTION: Most services require user to login before exploring further. If cookies are not recommended in REST, does it mean I have to go thru authentication on every single request (e.g., HTTP AUTH)?
ANSWER: There are two basic approaches here. The first is, use HTTP AUTH. The (human) user will be prompted for the credential just once (or they will be loaded from file, depending on the system at hand). The client software will compute the Base64 encoding of the credentials and will include them in each future HTTP request to the server (using the "Authorization" HTTP header).
The second alternative is to create a dedicated login service, that accepts credentials and returns a token. This token should then be included, as a URL argument, to each following request (e.g., by addding "&authtoken= XYZ " to the URL). Of course, the API (including the argument's name) should be clearly defined.
Each approach has its benefits, and its limitations. The first approach is easy to test using browsers. Just connect to the s ervice with your browser, and on the first visit, you'll get a the username/password prompt. The browser than caches your credentials and automatically sends them on all following requests (just like the REST client software would have done!). The main advantage of the second approach is that tokens can be created with an expiration date (managed on the server, unlike cookie expiration). For some services, this is important.
And what about cookies? As I said elsewhere, these violate the "state transfer" notion of REST, where the complete state should be passed with e ach request. While cookies can be limited to function just like the Authorization header, why do that when you have a dedicated, special-purpose header?
BY DR. M. ELKSTEIN
14.4. How do I send complex objects as parameters? QUESTION: The REST client is sending complex objects as a list of standalone parameters: for example, firstName, lastName, birthDate, etc. The backend, which I'm developing, has to turn these into high-level-language objects, such as User. How do I convert these arguments into objects?
ANSWER: The most straightforward way to do this would be a constructor to the complex object class (here, User) which accepts a map of key/value pairs; these would be the fine-grained parameters passed by the client. For example, in Java, class
User could have a constructor with a single argument, of
type Map .
Now, how do we convert the map into a new object? In languages with good reflection mechanisms, such as Java or C#, it might be tempting to use reflection here: go over the map keys, for each map key try to find a data member in the class, and set that data member to the value mapped to this key. For example, if the first key in the map is
firstName, a call to getField on the Class object representing this would provide you with a field object that can be used
to set the actual field. A simple loop for setting all fields, and you're done!
I strongly advise against this approach. It could present a security risk (for example, you end up setting a private field. Do you really want to re-implement the language's security mechanisms?). It also circumvents any setters that might be defined for these fields, and which are used to enforce any class invariants. It is also probably not very efficient, though this is less of a configuration.
Instead, write simple, possibly "boring" code: for each field you'd like to set, find if a matching key exists in the map (note that the map key and the field name don't have to be identical now; the field could be
firstName while the REST parameter was first_name ). If found, pass the matching value from the
map to the field's setter method.
Avoid validity checks in this constructor. If your code insists that first-name should be at least two letters long, this should be verified in the setFirstName method (or property setter, in C#). The most validity-checks that should appear in this constructor is in converting string values to non-string ones, for example for setting integer fields.
