One of the reasons we use relational database technology is that existing RDBMS implementations provide extremely mature, scalable and robust concurrency control. This means much more than simple read/write locks. For example, databases that use locking are built to scale efficiently when a particular transaction obtains /many/ locks - this is called /lock escalation/. On the other hand, some databases (for example, Oracle and PostgreSQL) don't use locks at all - instead, they use the multiversion concurrency model. This sophisticated approach to concurrency is designed to achieve higher scalability than is possible using traditional locking models. Databases even let you specify the required level of transaction isolation, allowing you to trade isolation for scalability.

Unfortunately, some Java persistence frameworks (especially CMP engines) assume that they can improve upon the many years of research and development that has gone into these relational systems by implementing their own concurrency control in the Java application. Usually, this takes the form of a comparatively crude locking model, with the locks held in the Java middle tier. There are three main problems with this approach. First, it subverts the concurrency model of the underlying database. If you have spent a lot of money on your Oracle installation, it seems insane to throw away Oracle's sophisticated multiversion concurrency model and replace it with a (less-scalable) locking model. Second, other (non-Java?) applications that share the same database are not aware of the locks. Finally, locks held in the middle tier do not naturally scale to a clustered environment. Some kind of distributed lock will be needed. At best, distributed locking will be implemented using some efficient group communication library like JGroups. At worst (for example, in OJB), the persistence framework will persist the locks to a special database table. Clearly, both of these solutions carry a heavy performance cost. Accordingly, Hibernate was designed to /not require/ any middle-tier locks - even thread synchronization is avoided. This is perhaps the best and least-understood feature of Hibernate and is the key to why Hibernate scales well. So why do other frameworks not just let the database handle concurrency?

Well, the only good justification for holding locks in the middle tier is that we might be using a middle-tier cache. It turns out that the problem of ensuring consistency between the database and the cache is an extremely difficult one and solutions usually do involve some use of middle-tier locking. (Incidently, most applications which use a cache do not solve this problem correctly, even in a non-clustered environment.)

So, for example, when Hibernate integrates with JBoss Cache, the cache implementation must obtain clustered locks internally (again, using JGroups). In Hibernate, we consider it a quality-of-service concern of the cache implementation to provide this kind of functionality. We can do this because Hibernate, unlike many other persistence layers, features a two-level cache architecture. This design separates the transaction-scoped /session cache/ (which does /not/ require middle-tier locking and delegates concurrency concerns to the database) from the process or cluster scoped /second-level cache/ (which /may/ require middle-tier locks). So when the second-level cache is disabled for a particular class, no middle-tier lock is required. Hence, in this case, the scalability of Hibernate is limited only by the scalability of the underlying database. Our design also allows us to consider other, more sophisticated approaches to ensuring consistency between the second-level cache and database - approaches that do not require the use of middle-tier locking. I'll keep this stuff secret for now; it is an active area of investigation!

Jason has pointed out some interesting things about the current release of Hibernate.

First, he notes that Hibernate's query cache may not currently be used with SwarmCache. This is due to SwarmCache not being a /replicated/ cache - it uses clustered eviction. Hibernate's query cache may be used with any replicated clustered cache, as long as clocks are synchronized in the cluster. Our documentation should make this requirement clearer. (For most applications, the query cache is not an especially useful feature, so this issue affects only a small number of users.)

Second, he has noticed that Hibernate currently writes to the second-level cache too often. I had not previously been aware of this issue. Hibernate was optimized with local, perhaps disk-backed, cache implementations in mind, where lookups are at least as expensive (in the case of a disk-backed cache, /more/ expensive) than puts. Jason has pointed out that in the case of a clustered cache, this is reversed and puts are much more expensive than lookups. We will provide a switch in a future point release of Hibernate 2.1 to adjust this behavior.

Finally, Jason is unhappy that we are not supporting JDK dynamic proxies and require CGLIB for lazy association fetching. (CGLIB causes problems if the JVM is running with a restrictive security policy.) Well, as I explained to Jason, Hibernate already provides a hook to allow the proxy implementation to be customized using a custom persister. (Dynamic proxy support could be implemented with a very small amount of code.) This evening I actually refactored this stuff slightly and introduced a ProxyFactory interface to make the hook more obvious.

I'd like to point out that the Hibernate project has many thousands of users and just a handful of active developers. As a result, we must prioritize feature requests by two considerations: first, how many users beg for the feature; second, how strategic a feature is with respect to the future direction of the project. We simply do not implement every requested feature since we have limited resources and are fighting a continual battle against code bloat (bloat is a /very/ serious issue if you are the person maintaining and enhancing the codebase). Hibernate is open source, so users with special requirements can always use a patched version in their application. In addition, we do not usually process feature requests via IM or javablogs.com! The correct process is to submit an enhancement request to JIRA (and let other users comment and vote) or, even better, to start a discussion in the mailing list. We may be an open source project; this does not mean that we are completely unprofessional and disorganized!

There were quite a few comments in response to my post about Criteria queries. I finally get around to responding. A number of people suggest a more tree-oriented approach, where we treat all logical operators as binary. For example, anonymous suggests the following:

session.createCriteria( Project.class,
  or(
      eq("name", "Hibernate"), 
      like("description", "%ORM%")
  )
);

Now, certainly logical operations are binary. But they are also /associative/, and this seems to be denied by the tree approach. We would never, ever write:

( (x=2 and x=1) and y=3 ) and z=4

We always write:

x=2 and x=1 and y=3 and z=4

This is particularly relevant in the case of Criteria queries since the common case is that we compose together many conditions using conjunction. (There were objections to my use of conjunction and disjunction, but I don't know of any other word for a string of expressions composed with and/or.)

Actually, the current API /does/ already allow this alternative. We have Expression.or() and Expression.and(). But to me they seem to be much messier than add().

Carl Rosenberger, the man behind SODA and db4o writes:

In my opinion a clean object querying system is the first and foremost 
basis for any standard on object persistence. 

Enhancers/Reflection/BCEL/code calls to make objects persistent 
all this can be very quickly exchanged, if you want to switch the 
underlying system.

Queries can not be exchanged!

And I couldn't possibly agree more! This is absolutely right. He goes on to say:

Could you maybe bring this thought into JDO 2.0 ? Please ?

Besides, I am positive that a de-facto standard for object querying will 
have a much greater impact on the industry than JDO.
Java is not the only programming language on this planet. 

In fact, I've mentioned the idea of adopting a better query approach to a couple of the guys on the expert group, but didn't get much of a positive response. Besides, I'm not at all convinced that it is even possible to design a nice query language or API in a committee environment. These kind of things need a strong unified vision. Comittees are okay at standardizing mature solutions to well-understood problems, but it is my view that object-oriented querying is a far from well understood problem. Especially, it does not seem to be commonly appreciated that an ORM-oriented query language would look quite different to an object database query language!

James Strachan is pimpin' Groovy , his new JVM-compatible language (I'm not sure if its quite correct to call it a scripting language) that looks quite like Python but features one of the best things about SmallTalk: closures. (Digression: it seems to be widely believed that the JDK Collections API is one of Java's good points. But if you've ever used collections in SmallTalk, you'll know just how impoverished Java actually is when it comes to working with collections. Iterators are far, far uglier than the more functional approach available in languages with closures. If there is /anything/ that I would ask to be fixed in Java, it wouldn't be the lack of generics, the lack of enums, etc, etc - it would be the lack of closures.) Anyway, I'm interested in what James has to say, but I'm skeptical toward the notion that an object-oriented expression language is the right starting point for an ORM query language. Firstly, relational databases implement very different null value semantics to object oriented languages. In fact, SQL's ternary logic is /quite/ different to the binary logic implemented by programming languages. Secondly, equality is a much more slippery concept in the object oriented world than in the database world. For these reasons, and others, we have chosen to base HQL on SQL, not on Java-like syntax. It is my view that this was one of the best decisions we've made.

Razvan describes what (s)he calls an an associative query model (much like the query in JavaSpaces). If I'm not mistaken, this is what we usually call query by example. I'll get round to discussing Hibernate's new Example query API in a future post...

Chris Winters doesn't like object-oriented query APIs. Since Hibernate emphasizes the query /language/ approach, I'm not the best person to disagree with him here. Criteria queries are usually noisier, no doubt about that. And the query languages I've seen tend to be more expressive. Writing arithmetic and even logical expressions is a breeze in a query lanugage, but certainly not in an object-oriented Criteria API.

However, there are a couple of advantages of the Criteria approach. First, some folks like the fact that more can be done to validate the query at compile time. This is no big deal to me because I'm very unit test oriented. Much more importantly, object-oriented query APIs are much better for building queries programmatically. String manipulations suck! Getting your parentheses and whitespace right for all combinations of criteria is a pain. I've seen some truly ugly code that builds HQL strings and it could be rewritten /much/ more neatly using the Criteria API.

Especially, the new query by example stuff could potentially reduce 20 lines of code that builds a query containing the needed properties of Person to:

session.createCriteria(Person.class)
    .add( Example.create(elvis) )
    .list();
    

Well, I suppose query by example is a bit of an extreme case, and you could perhaps implement a similar feature in a query language.

Finally, object oriented query APIs can be more user-extensible. This is what I like best.

P.S. The code example Chris gives is a bit unfair to object APIs. Hibernate's Criteria API is much less verbose than that, partly due to the support for method chaining (which is barely used in Java, so I betray the time I spent with SmallTalk).

Names are important in computing. Really Important. I'm not talking about calling classes sensible things like UpdateUserDetailsCommand in preference to UpdUDetsCd. I mean the names of products themselves. A great name tells us lots about a piece of software: it tells us that the creater has some imagination, a sense of style even. It tells us that this person is serious about the success of the product, that they understand that there is more to making great software than writing great code.

Java is a fantastic name for just about anything. I love the ambiguity; coffee, or the largest island of the Indonesian archipeligo? There is no way I could hear the name Java and not wonder what it is a name for. C# (pronounced cee-hash by those in the know) is twee and derivative. Its pathetic enough trying to impress girls with the fact that I am a Java developer. Imagine if I had to say I was a cee sharp or dot net programmer? Ugggh. But from this we can learn something of the mindset that created these two languages - C# /is/ actually more derivative than Java.

So, if we have eliminated C# as the next step forward in languages, what about the other two possible Java-replacements: Python and Ruby. Well, Python is great, of course. There's that ambiguity once again: funny Englishmen, or snakes. Snakes are cool. Actually, naming things after animals is /always/ cool, even Ants. Ruby grows on me. I don't /love/ it, but as an English name given by a non-native English speaker, I'm happy to give it a pass. I've noticed that non-native speakers tend to name their projects by horrible acronyms.

The Apache project gives us some great examples of both good and bad. Tapestry is a beautiful, evocative name, just perfect for a web presentation framework. Struts is a good (perhaps a little too obvious) pun on framework. Lucene and Avalon are nice enough. Ant we mentioned already. commons-lang is an abomination, being the concatenation of commons (ugh) with that incredibly romantic English word lang. Worse, the concatenation is performed with a hyphen. Names don't have hyphens in them, guys! (Well, apart from Mike's ). Commons could evoke either typical or communal or both; neither interpretation is inspiring in this age. Actually, commons seems to be Jakarta's moniker for stuff we don't have proper names for or, perhaps, stuff that doesn't really deserve to be a real project. Similarly, commons-primitives could /almost/ conjure up some kind of vision of neolithic tribal society, uncorrupted by modern notions of private property .... but it doesn't. To me this indicates something about the respective quality of the projects. Would any truly committed developer, bulding something they love and believe in, possibly call their project commons-lang? Nope. They would call it Tapestry. If they had Howard's sense of theater, that is.

In the realm of distributed computing, we have been given CORBA - a nice, easily pronouncable acronym that sounds like a real English word and even better reminds us of snakes - and the ridiculous WSDL (pronounced whizz-dull, which is an oxymoron). I guess this means that Web Services are just another dead end - surely, nothing useful could ever be named WSDL?

Being a contrarian, I named our project with a verb. Hibernate isn't a thing - it's not even a state of mind; it's something you /do/.

disclaimer: this post is meant in good humour, please don't take offense