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<binding...>
   <wireFormat.dynamic>
</binding...>

Cache control using ETags, Last-Modified and other HTTP Headers

The HTTP specification provides a set of methods for HTTP clients and servers to interact. These methods form the foundation of the World Wide Web. Tuscany implements many of these methods a binding interface to a collection. The main methods are:

• GET - retrieves an item from a collection
• POST - creates or adds an item to a collection
• PUT - updates or replaces an item in a collection
• DELETE - removes an item in a a collection

The HTTP specification (HTTP 1.1 Chapter 13 - Caching) also provides a mechanism by which these methods may be executed conditionally. To perform conditional methods, an HTTP client puts 3 items in the HTTP request header:

• ETag - entity tag, a unique identifier to an item in a collection. Normally created and returned by the server when creating (POST) a new item.
• LastModified - an updated field. Normally a string containing a date and time of the last modification of the item.
• Predicate - a logical test (e.g. IfModified, IfUnmodified) to use with the ETag and LastModified to determine whether to act.

The complete list of predicates is given in the HTTP specification.

The most common use of conditional methods is to prevent two requests to the server instead of one conditional request. For example, a common scenario is to check if an item has been modified, if not changed update it with a new version, if changed do not update it. With a conditional PUT method (using the IfUnmodifed predicate and a LastModified date), this can be done in one action. Another common use is to prevent multiple GETs of an item to ensure we have a valid copy. Rather than doing a second request of a large item, one can do a conditional GET request (using an IfModified predicate and a LastModified date), and avoid the second request if our object is still valid. The server responds with either a normal response body, or status code 304 (Not Modified), or status code 412 (precondition failed).

Default cache control is done by using generated ETags based on response content checksum. To avoid data to be overwriten during concurrent updates, include an HTTP If-Match header that contains the original content ETag value. If you want to force an update regardless of whether someone else has updated it since you retrieved it, then use If-Match: * and don't include the ETag.

Store scenarios goes REST - Catalog Services using binding.rest

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@Scope("COMPOSITE")
public class FruitsCatalogImpl implements Catalog {
    
    @Property
    public String currencyCode = "USD";
    
    @Reference
    public CurrencyConverter currencyConverter;
    
    private Map<String, Item> catalog = new HashMap<String, Item>();

    @Init
    public void init() {
        String currencySymbol = currencyConverter.getCurrencySymbol(currencyCode);
        catalog.put("Apple", new Item("Apple",  currencySymbol + currencyConverter.getConversion("USD", currencyCode, 2.99)));
        catalog.put("Orange", new Item("Orange", currencySymbol + currencyConverter.getConversion("USD", currencyCode, 3.55)));
        catalog.put("Pear", new Item("Pear", currencySymbol + currencyConverter.getConversion("USD", currencyCode, 1.55)));
    }

    public Item[] getAll() {
        Item[] catalogArray = new Item[catalog.size()];
        catalog.values().toArray(catalogArray);
        return catalogArray;
    }
    
    public Item getItemById(String itemId) {
        return catalog.get(itemId);
    }
    
    public void addItem(Item item) {
        catalog.put(item.getName(),item);
    }
    
    public void updateItem(Item item) {
        if(catalog.get(item.getName()) != null) {
            catalog.put(item.getName(), item);
        }
    }
    
    public void deleteItem(String itemId) {
        if(catalog.get(itemId) != null) {
            catalog.remove(itemId);
        }        
    }
}

Advanced Features of the Tuscany HTTP Binding

HTTP Conditional Actions and Caching using ETags and Last-Modified

The HTTP specification provides a set of methods for HTTP clients and servers to interact. These methods form the foundation of the World Wide Web. Tuscany implements many of these methods a binding interface to a collection. The main methods are:

• GET - retrieves an item from a collection
• POST - creates or adds an item to a collection
• PUT - updates or replaces an item in a collection
• DELETE - removes an item in a a collection

The HTTP specification (HTTP 1.1 Chapter 13 - Caching) also provides a mechanism by which these methods may be executed conditionally. To perform conditional methods, an HTTP client puts 3 items in the HTTP request header:

• ETag - entity tag, a unique identifier to an item in a collection. Normally created and returned by the server when creating (POST) a new item.
• LastModified - an updated field. Normally a string containing a date and time of the last modification of the item.
• Predicate - a logical test (e.g. IfModified, IfUnmodified) to use with the ETag and LastModified to determine whether to act.

The complete list of predicates is given in the HTTP specification.

The most common use of conditional methods is to prevent two requests to the server instead of one conditional request. For example, a common scenario is to check if an item has been modified, if not changed update it with a new version, if changed do not update it. With a conditional PUT method (using the IfUnmodifed predicate and a LastModified date), this can be done in one action. Another common use is to prevent multiple GETs of an item to ensure we have a valid copy. Rather than doing a second request of a large item, one can do a conditional GET request (using an IfModified predicate and a LastModified date), and avoid the second request if our object is still valid. The server responds with either a normal response body, or status code 304 (Not Modified), or status code 412 (precondition failed).

Tuscany implements the logic to move these caching items to and from the HTTP request and response headers, and deliver them to the collection implementation. The items are delivered to a user implementation via a serializable object called CacheContext. This object contains the value of the ETag, the LastModified value, and any predicates. It is up to the implementer of a collection to provide the correct server logic to act on these predicates. The CacheContext class is found in found in the Tuscany module package org.apache.tuscany.sca.binding.http.

To implement conditional actions on your service, the developer or implementer implements any of the condional HTTP actions: conditionalPost, conditionalGet, conditionalPut, or conditionalDelete. Tuscany automatically routes a request with proper request header fields (ETag, LastModified, and predicates) to the proper collection method.

For example, the TestBindingCacheImpl class in package org.apache.tuscany.sca.binding.http implements a server collection which pays attention to conditional methods. Notice that this collection implements conditionalGet, conditionalPut, conditionalPost, and conditionalDelete methods as well as get, put, post, delete. The server collection can look at the CacheContext obect to determine whether an item or a status code should be returned. In this example code, the conditionalGet checked the If-Modified predicate and determines whether the item is not modified and if so, throws a NotModifiedException.

	public InputStream conditionalGet(String id, HTTPCacheContext cacheContext)
			throws NotModifiedException, PreconditionFailedException {

		if (cacheContext != null) {
			if (cacheContext.ifModifiedSince) {
				if ((id.equals("1"))
						&& (0 > cacheContext.lastModifiedDate
								.compareTo(new Date())))
					throw new NotModifiedException("item 1 was modified on "
							+ new Date());
			}
                ...
        ...

For a full example of all conditional methods and many combinations of predicates, one can look to the module http-binding unit tests to understand how these conditional request work. The HTTPBindingCacheTestCase contains 36 tests of all 4 HTTP conditonal methods. Various predicates are tested with various ETags and LastModified fields. The fields are tested in both the positive and negative cases.

Here is a complete list of the tests:

• testGet() - tests normal GET method of collection, expects item
• testConditionalGetIfModifiedNegative() - tests not modified GET, expects item
• testConditionalGetIfModifiedPositive() - tests modified GET, expect code 304
• testConditionalGetIfUnmodifiedNegative() - tests unmodifed GET, expects item
• testConditionalGetIfUnmodifiedPositive() - tests modified GET, expects code 412
• testConditionalGetIfMatchNegative() - tests matching GET, expects code 412
• testConditionalGetIfMatchPositive() - tests matching GET, expects item
• testConditionalGetIfNoneMatchNegative - tests unmatching GET, expects item
• testConditionalGetIfNoneMatchPositive() - tests unmatching GET, expects code 412

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