[jboss-cvs] JBossCache/docs/faq-pojo/en ...
Ben Wang
bwang at jboss.com
Sat Sep 23 08:44:37 EDT 2006
User: bwang
Date: 06/09/23 08:44:37
Added: docs/faq-pojo/en master.xml
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1.1 date: 2006/09/23 12:44:37; author: bwang; state: Exp;JBossCache/docs/faq-pojo/en/master.xml
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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"../../../../docbook-support/support/docbook-dtd/docbookx.dtd"
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<article class="faq" lang="en">
<articleinfo>
<title>Frequently Asked Questions about PojoCache</title>
<releaseinfo>Release 2.0</releaseinfo>
<pubdate>September 2006</pubdate>
<author>
<firstname>Ben</firstname>
<surname>Wang</surname>
<email>ben.wang at jboss.com</email>
</author>
</articleinfo>
<para>These are frequently asked questions regarding Pojocache.</para>
<qandaset defaultlabel="qanda">
<title>General Information</title>
<qandaentry>
<question id="a49">
<para>What is PojoCache?</para>
</question>
<answer>
<para>PojoCache is a fine-grained field-level replicated and
transactional POJO (plain old Java object) cache. By POJO, we mean
that the cache: 1) automatically manages object mapping and
relationship for a client under both local and replicated cache
mode, 2) provides support for inheritance relationship between
"aspectized" POJOs. By leveraging the dynamic AOP in JBossAop, it is
able to map a complex object into the cache store, preserve and
manage the object relationship behind the scene. During replication
mode, it performs fine-granularity (i.e., on a per-field basis)
update, and thus has the potential to boost cache performance and
minimize network traffic.</para>
<para>From a user perspective, once your POJO is managed by the
cache, all cache operations are transparent. Therefore, all the
usual in-VM POJO method semantics are still preserved, providing
ease of use. For example, if a POJO has been put in PojoCache (by
calling <literal>attach</literal>, for example), then any POJO get/set
method will be
intercepted by PojoCache to provide the data from the
cache.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a1">
<para>What is the relationship between Cache and PojoCache?</para>
</question>
<answer>
<para>The core JBoss Cache library <literal>Cache</literal> is a traditional generic distributed cache system.
PojoCache uses Cache as the underlying distributed state system to achieve POJO caching. It uses Cache as a
delegate. As a result, all the replication aspects are configured with the Cache configuration XML.
Additionally, PojoCache also has API to expose the Cache interface (via <literal>getCache()</literal> API).</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a52">
<para>What is the difference between Cache and
PojoCache?</para>
</question>
<answer>
<para>Think of PojoCache as a Cache on steroids. :-)
Seriously, both are cache stores-- one is a generic cache and the other other one POJO Cache.
However, while Cache only
provides pure object reference storage (e.g., <literal>put(FQN fqn,
Object key, Object value)</literal>), PojoCache goes beyond that
and performs fine-grained field level replication object mapping and
relationship management for a user behind the scenes. As a result,
if you have complex object systems that you would like to cache, you
can have PojoCache manage it for you. You simply treat your
object systems as they are residing in-memory, e.g., use your
regular POJO methods without worrying about cache management.
Furthermore, this is true in replication mode as well.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a521">
<para>How does PojoCache work then?</para>
</question>
<answer>
<para>PojoCache uses the so-called AOP technology (aspect oriented programming) to do field level
interception. Currently, it uses <literal>JBoss Aop</literal> library to do it.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a522">
<para>What's changed between 1.x and 2.x release then?</para>
</question>
<answer>
<para>Starting in 2.0 release, we have a separate library for PojoCache, <literal>pojocache.jar</literal> that
is extra to the core <literal>jboss-cache.jar</literal>. Since we uses Cache as a delegate, user
will need to have a regular xml to configure the core Cache functionality (e.g., replication and locking
aspect). In addition, there is also the <literal>jboss-aop.xml</literal> that specifies the PojoCache
interceptor stack (that can be left as default).</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a53">
<para>What are the steps to use the PojoCache feature?</para>
</question>
<answer>
<para>In order to use PojoCache, you will need to:</para>
<itemizedlist>
<listitem><para>prepare POJO. You can do either via xml declaration or JDK50 annotation.
This is the step to declare your POJO such that it will be instrumented by
<literal>JBoss Aop</literal>.</para></listitem>
<listitem><para>instrumentation. You will need to instrument your POJO either at compile- or load-time.
If you do it during compile-time, you use so-called an aop pre-compiler (aopc) to do bytecode manipulation.
If you do it via load-time, however, you need either a special system class loader or, in JDK50, you can
use the javaagent option. Either way, <literal>JBoss Aop</literal> will byte code manipulate your POJO
class such that all field access can be intercepted.</para></listitem>
</itemizedlist>
<para>So if you use JDK50, for example, with annotation and load-time instrumentation, then you won't need
any pre-processing step to use PojoCache. For a full example, please refer to the distro examples directory.
There are numerous PojoCache examples that uses different options.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a541">
<para>What is the JDK version required to run PojoCache 2.x?</para>
</question>
<answer>
<para>PojoCache 2.x requires JDK5.0 since it uses the annotation extensively.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a542">
<para>Can I run PojoCache as a standalone mode?</para>
</question>
<answer>
<para>Yes, same as the core Cache library, you can run PojoCache either as a standalone or
inside an application server.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a543">
<para>What is the JBoss AS recommended version to run PojoCache 2.x?</para>
</question>
<answer>
<para>PojoCache can be run either in AS4.0.5 (and up) and 5.0. But either way, it will require
JDK5.0 though.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a56">
<para>Can I pre-compile the aop classes such that I don't need to
use the system classloader and jboss-aop configuration xml?</para>
</question>
<answer>
<para>Yes. The latest versions of JBossCache have a pre-compiler
option called <literal>aopc</literal>. You can use this option to
pre-compile your "aspectized" POJO. Once the classes have been byte
code generated, they can be treated as regular class files, i.e.,
you will not need to include any <literal>jboss-aop.xml</literal>
that specifies the advisable POJO and to specify the JBossAop system
class loader.</para>
<para>For an example of how to use <literal>aopc</literal>, please
see 1) <literal>tools</literal> directory for PojoCacheTasks14.xml
and PojoCacheTasks50.xml. Both contain Ant tasks that you can
import to your regular project for
<literal>aopc</literal>. In addition, please also check out the
<literal>examples</literal> directory for concrete examples.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a561">
<para>In PojoCache 2.x release, do I still need <literal>annoc</literal>?</para>
</question>
<answer>
<para>The annoc precompiler is needed for JDK1.4 style annotation. For 2.x release, since
we require the use of JDK5.0, there is no need to use annoc anymore.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a57">
<para>How do I use aopc on multiple module directories?</para>
</question>
<answer>
<para>In aopc, you specify the src path for a specific directory. To
pre-compile multiple ones, you will need to invoke aopc multiple
times.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a58">
<para>What's in the <literal>jboss-aop.xml</literal>
configuration?</para>
</question>
<answer>
<para><literal>jboss-aop.xml</literal> is needed for POJO
instrumentation. In <literal>jboss-aop.xml</literal>, you can
declare your POJO (e.g., <literal>Person</literal>) to be
"prepared", a JBossAop term to denote that the object will be
"aspectized" by the system. After this declaration, JBossAop will
invoke any interceptor that associates with this POJO. PojoCache
will dynamically add an
<literal>org.jboss.cache.pojo.interceptor.dynamic.CacheFieldInterceptor</literal> to this POJO
to perform object mapping and relationship management.</para>
<para>Note that to add your POJO, you should declare all the fields
to be "prepared" as in the example.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a59">
<para>Can I use annotation instead of the xml declaration?</para>
</question>
<answer>
<para>Yes, in release 2.0, you can use JDK5.0 annotation to
instrument your POJO. Check the documentation for details.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a60">
<para>What are the pro and con of xml vs. annotation?</para>
</question>
<answer>
<para>It really depends on your organization environment, I'd say, since this can be turned into a
hot debate. Having said that, I feel strongly that POJO annotation is well suited for PojoCache. This is
because once you specify the annotation, you'd probably change it rarely since there is no parameters to
tune, for example.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a61">
<para>What are the <literal>@org.jboss.cache.pojo.annotation.Transient</literal> and
<literal>@org.jboss.cache.pojo.annotation.Serializable</literal> field level annotations?</para>
</question>
<answer>
<para>In 2.0, we also offer two additional field-level annotations. The first one,
<literal>@org.jboss.cache.pojo.Transient</literal>,
when applied has the same effect as declaring a field <literal>transient</literal>. PojoCache
won't put this field under management.</para>
<para>The second one, <literal>@org.jboss.cache.pojo.Serializable</literal> when applied,
will cause PojoCache to
treat the field as a Serializable object even when it is <literal>@org.jboss.cache.pojo.PojoCacheable</literal>.
</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a62">
<para>What about compile-time vs. load-time instrumentation then?</para>
</question>
<answer>
<para>Again it depends. But my preference is to do compile-time instrumentation via aopc. I prefer this
approach because it is easier to debug (at least at the development stage). In addition, once I generate the
new class, there is no more steps needed.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a63">
<para>Is it possible to store the same object multiple times but
with different Fqn paths? Like /foo/byName and /foo/byId ?</para>
</question>
<answer>
<para>Yes, you can use PojoCache to do that. It supports the
notion of object reference. PojoCache manages the unique object
through association of the dynamic cache interceptor.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a64">
<para>Do I need to declare all my objects "prepared" in
<literal>jboss-aop.xml</literal>?</para>
</question>
<answer>
<para>Not necessarily. If there is an object that you don't need the
cache to manage for you, you can leave it out of the declaration.
The cache will treat this object as a "primitive" type. However, the
object will need to implement <literal>Serializable</literal>
interface for replication.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a65">
<para>Can the cache aop intercept update via reflection?</para>
</question>
<answer>
<para>No. The update via reflection will not be intercepted in
JBossAop and therefore PojoCache will not be able to perform the
necessary synchronization.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a66">
<para>When I declare my POJO to be "aspectized", what happens to the
fields with transient, static, and final modifiers?</para>
</question>
<answer>
<para>PojoCache currently will ignore the fields with these
modifiers. That is, it won't put these fields into the cache (and
thus no replication either).</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a67">
<para>What are those keys such as
<literal>JBoss:internal:class</literal> and
<literal>AOPInstance</literal>?</para>
</question>
<answer>
<para>They are for internal use only. Users should ignore these keys
and values in the node hashmap.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a68">
<para>What about Collection classes? Do I need to declare them
"prepared"?</para>
</question>
<answer>
<para>No. Since the Collection classes such as
<literal>ArrayList</literal> are java util classes, aop by default
won't instrument these classes. Instead, PojoCache will generate
a dynamic class proxy for the Collection classes (upon the
<literal>attach</literal> call is invoked). The proxy will
delegate the operations to a cache interceptor that implements the
actual Collection classes APIs. That is, the system classes won't be
invoked when used in PojoCache.</para>
<para>Internally, the cache interceptor implements the APIs by
direct interaction with respect to the underlying cache store. Note
that this can have implications in performance for certain APIs. For
example, both <literal>ArrayList</literal> and
<literal>LinkedList</literal> will have the same implementation.
Plan is currently underway to optimize these APIs.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a69">
<para>How do I use <literal>List</literal>, <literal>Set</literal>,
and <literal>Map</literal> dynamic proxy?</para>
</question>
<answer>
<para>PojoCache supports classes extending from
<literal>List</literal>, <literal>Set</literal>, and
<literal>Map</literal> without users to declare them "aspectized".
It is done via a dynamic proxy. Here is a code snippet to use an
<literal>ArrayList</literal> proxy class.</para>
<programlisting>ArrayList list = new ArrayList();
list.add("first");
cache.attach("list/test", list); // Put the list under the aop cache
list.add("second"); // Won't work since AOP intercepts the dynamic proxy not the original POJO.
ArrayList myList = (List)cache.find("list/test"); // we are getting a dynamic proxy instead
myList.add("second"); // it works now
myList.add("third");
myList.remove("third");</programlisting>
</answer>
</qandaentry>
<qandaentry>
<question id="a70">
<para>What is the proper way of assigning two different keys with
Collection class object?</para>
</question>
<answer>
<para>Let's say you want to assign a <literal>List</literal> object
under two different names, you will need to use the class proxy to
insert the second time to ensure both are managed by the cache. Here
is the code snippet.</para>
<programlisting>ArrayList list = new ArrayList();
list.add("first");
cache.attach("list", list); // Put the list under the aop cache
ArrayList myList = (List)cache.find("list"); // we are getting a dynamic proxy instead
myList.add("second"); // it works now
cache.attach("list_alias", myList); // Note you will need to use the proxy here!!
myList.remove("second");</programlisting>
</answer>
</qandaentry>
<qandaentry>
<question id="a71">
<para>OK, so I know I am supposed to use proxy when manipulating the
Collection classes once they are managed by the cache. But what
happens to Pojos that share the Collection objects, e.g., a
<literal>List</literal> instance that is shared by 2 Pojos?</para>
</question>
<answer>
<para>Pojos that share Collection instance references will be
handled by the cache automatically. That is, when you ask the Cache
to manage it, the Cache will dynamically swap out the regular
Collection references with the dynamic proxy ones. As a result, it
is transparent to the users.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a72">
<para>What happens when my "aspectized" POJO has field members that
are of Collection class ?</para>
</question>
<answer>
<para>When a user puts a POJO into the cache through the call
<literal>attach</literal>, it will recursively map the field
members into the cache store as well. When the field member is of a
Collection class (e.g., List, Set, or Map), PojoCache will first
map the collection into cache. Then, it will swap out dynamically
the field reference with an corresponding proxy reference.</para>
<para>This is necessary so that an internal update on the field
member will be intercepted by the cache.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a73">
<para>What are the limitation of Collection classes in PojoCache?</para>
</question>
<answer>
<para>Use of Collection class in PojoCache helps you to track fine-grained changes
in your collection fields automatically. However, current implementation has the follow
limitation that we plan to address soon.</para>
<para>Currently, we only support a limited implementation of Collection classes. That is,
we support APIs in List, Set, and Map. However, since the APIs do not stipulate
of constraints like NULL key or value, it makes mapping of user instance to our proxy tricky.
For example, ArrayList would allow NULL value and some other implementation would not.
The Set interface maps to java.util.HashSet implementation. The List interface maps
to java.util.ArrayList implementation. The Map interface maps to java.util.HashMap
implementation.
</para>
<para>Another related issue is the expected performance. For example, the current implementation is ordered, so
that makes insert/delete from the Collection slow. Performance between Set, Map and List collections also vary.
Adding items to a Set is slower than a List or Map, since Set does not allow duplicate entries.
</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a74">
<para>What are the pros and cons of PojoCache?</para>
</question>
<answer>
<para>As mentioned in the reference doc, PojoCache has the following advantages:</para>
<itemizedlist>
<listitem><para>Fine-grained replication and/or persistency. If you use a distributed PojoCache
and once your POJO is put in the cache store, there is no need to use another API to
trigger your changes. Furthermore, the replication are fine-grained field level. Note this
also applies to persistency.</para></listitem>
<listitem><para>Fine-grained replication can have potential performance gain if your POJO is big and
the changes are fine-grained, e.g., only to some selected fields.</para></listitem>
<listitem><para>POJO can posses object relationship, e.g., multiple referenced. Distributed
PojoCache will handle this transparently for you.</para></listitem>
</itemizedlist>
<para>And here are some cases that you may not want to use PojoCache:</para>
<itemizedlist>
<listitem><para>You use only cache. That is you don't need replication or persistency. Then since
everything is operated on the in-memory POJO reference, there is no need for PojoCache.</para></listitem>
<listitem><para>You have simple and small POJOs. Your POJO is small in size and also there is no
object relationship, then PojoCache possess not clear advantage to plain cache.</para></listitem>
<listitem><para>Your application is bounded by memory usage. Because PojoCache need almost twice as much
of memory (the original POJO in-memory space and also the additional cache store for the
primitive fields), you may not want to use PojoCache.</para></listitem>
<listitem><para>Your POJO lifetime is short. That is, you need to create and destroy your POJO often.
Then you need to do "attach" and "detach" often, it will be slow in performance.</para></listitem>
</itemizedlist>
</answer>
</qandaentry>
</qandaset>
<qandaset defaultlabel="qanda">
<title>Passiviation and eviction</title>
<qandaentry>
<question id="a80">
<para>Can I use eviction to evict POJO from the memory?</para>
</question>
<answer>
<para>No. In 2.0 release, we have deprecated the POJO-based eviction policy since it has always been
problematic in earlier release. The main reason is that when we evict a POJO from
the memory, the user has no ways of knowing it. So if the POJO is accessed after the
eviction, there won't be any PojoCache interception (e.g., it will be just like ordinary POJO),
but user may still expect that it will be managed by PojoCache.
</para>
</answer>
</qandaentry>
<qandaentry>
<question id="a81">
<para>So what do I do now?</para>
</question>
<answer>
<para>In order to keep your memory from overflowing, you can use the passivation feature that comes with
the core Cache. Passivation uses the combination of eviction and cache loader such that when the
items are old, it will be evicted from memory and store in a cache store (can be DB or file). Next time,
when the item needs to be accessed again, we will retrieve it from the cache store.
</para>
<para>In this sense, PojoCache level is not aware of the passivation aspect. It is configured through
the underlying cache xml.</para>
</answer>
</qandaentry>
</qandaset>
<qandaset defaultlabel="qanda">
<title>Troubleshooting</title>
<qandaentry>
<question id="a90">
<para>I am having problems getting PojoCache to work, where can I get information on troubleshooting?</para>
</question>
<answer>
<para>Troubleshooting section can be found in the following
<ulink url="http://wiki.jboss.org/wiki/Wiki.jsp?page=PojoCacheTroubleshooting">wiki link</ulink>.
</para>
</answer>
</qandaentry>
</qandaset>
</article>
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