ICM Server Cache 
Purpose
The ICM server cache or Internet server cache saves HTTP(S) objects before they are sent to the client. The next time an object is requested, then, the application gets the content directly from the cache before sending it to the client.
The HTTP
request handler uses the ICM Server Cache when, for example, response pages
need to be re-used, such as the entry page of an online shop
application.The ICM
server cache saves the pages before they are sent to the client. When the
page is next called, then the application gets the page directly from the ICM
and sends it to the client (provided that the expiry period has not run out)
(see 
icm/HTTP/server_cache_<xx>/expiration),
and the work process does not have to be opened (see Interaction
Model).
Objects in the MIME repository are saved to the cache by default. Method calls are used to activate the ICM server cache. This is described later.
Implementation Considerations
Using the ICM server cache can increase performance considerably.
Benchmarking tests for cache hits in the main memory have resulted in latent response times of less than one millisecond per request, and a total run of under 3,000 requests per second on a 4 CPU hardware.
These results are based on a strong parallel and multithreaded architecture that supports simultaneous read and write accesses with versioning. Furthermore, a patented indexing algorithm is used to access the cache directory quickly. This algorithm is particularly suitable for long Web URLs as cache keys.
Integration
The ICM server cache is part of the Internet Communication Manager (ICM).
Features
The following graphic displays the architecture of the ICM server cache.
The ICM server cache contains the following:
● Two-level cache hierarchy: memory cache and disk cache
● Dynamic caching technology
● Active caching
● UFO caching (unfound objects)
● Browser-dependent caching
Two-level Cache Hierarchy
The ICM server cache consists of a two-level hierarchy: a very quick main memory (memory cache) and a drive-based disk cache. This has the advantage that both the speed of the main memory access and the large storage capacity of the hard disk repository can be used.
Dynamic caching technology
Conventional Web-caching scenarios are based on HTTP proxies and usually only support caching of static contents, such as GIF pictures. The Internet server cache, on the other hand, can store dynamic Web content as well as static content, such as JSP and BSPs. This is especially important as Web applications are becoming increasingly dynamic.
Active caching
An
additional difference between the existing standard Web caching solutions and
the Internet server cache consists of the cache control technology, which is
integrated in the Internet Communication
Framework and in the BSP programming
model.
Active caching means that the application has full control over how current the objects are that are in the cache. This happens by using asynchronous content invalidation, which is triggered by application-dependent events. Even this is contrary to the standard HTTP caching, with which the application only has limited control over how current the objects in the cache actually are. With the standard HTTP caching technology, expiry time heuristics are usually used.
UFO Caching
The Internet server cache supports caching invalid requests. These are requests that lead to errors on the application server or on the database backend, such as “not found” errors. As a result, the backend is protected from overload situations in which the system is flooded with invalid Web requests.
Note that this function is only used as general protection and cannot protect the system from more polished attacks.
Browser-dependent caching for BSPs
A Business Server Page may appear differently in different browsers, since the HTML code is not always interpreted in the same way.
This is why the ICM server cache can store BSPs according to the browser type.If the same page is requested by a different browser, the request cannot be retrieved from the cache.To inform the runtime environment if the content is browser-dependent, you can set the relevant flag in the BSP properties. By default the flag is not set.
See Caching
BSPs.
How Does the ICM Server Cache Work?
The following documentation describes how the cache identifies objects, stores and invalidates them.
Invalidating Objects in the Cache
Storing Data Objects
in the ICM Server Cache
Search Sequence in the ICM Server Cache
Profile Parameters for Configuring the ICM Server Cache
The following parameters are used to parameterize the ICM server cache:
icm/HTTP/server_cache_<xx>/size_MB
icm/HTTP/server_cache_<xx>/memory_size_MB
icm/HTTP/server_cache_<x>/max_entries
icm/HTTP/server_cache_<xx>/clear
icm/HTTP/server_cache_<xx>/expiration
icm/HTTP/server_cache_<xx>/max_name_len
icm/HTTP/server_cache_<xx>/max_ufo_entries
icm/HTTP/server_cache_<xx>/ufo_expiration
You can
monitor the ICM server cache from the system using the ICM
monitor.This is
described in Monitoring and
Administrating the ICM Server Cache.