Deconstructing Software

Spring 2.5 Architecture Diagrams

I have updated the architecture diagrams for the just-released Spring 2.5. Any new or changed packages are highlighted (since 2.0.6). The diagrams are also online - if you pointed your IDE plugin at these after my previous entry, you will be seeing the updated diagrams in your IDE already, and any compile time messages about architecture violations will be based on the new versions.

Here's the new top-level architecture:

And here are the internals of the larger subsystems:

org.springframework.aop:

org.springframework.beans:

org.springframework.jdbc:

org.springframework.jms:

org.springframework.orm:

org.springframework.web:

Structure101 V3 Released, Adds Architecture Control for Teams

Released today, the new version 3 capabilities make Structure101 a nicely rounded architectural control solution in addition to the previous structural analysis and complexity measurement capabilities.

For example:

• You can now define layering constraints on your code-base using simple, intuitive architecture block diagrams
• Communicate these architecture diagrams to the development team through IDE plug-ins
• Developers get warned immediately if they make code changes that are inconsistent with the architecture
• RSS activity feeds let you know if new architecture violations make it into a build

Also, there's a new online demo (about 13 minutes, with audio (me!)) and the version 3 Help is available online.

Full press release

Enjoy!

Package design matters - Part 1

Java packages are often used like file-system folders to organize source. But source files differ from "normal" files in that they are highly inter-dependent. Considering this interdependence as a package hierarchy evolves can have significant productivity benefits.

Packages as Folders

Java packages provide an ideal way to organize code into a scalable, hierarchical structure that helps us find specific code.

In this sense, packages can be used like folders in a file-system: 

• We place files with something in common in the same folder. 
• When a folder grows too big and we find we’re having trouble finding files, we split the folder into sub-folders according to some criteria that makes sense to us.
• We share files by placing them in a common area on company network, in which case the structure evolves according to the varying criteria of different people.
• We often have trouble deciding which folder a file best belongs, and make an arbitrary decision.

Often Packages are only used as a kind of filesystem equivallent.  However the package hierarchy can also be used to reinforce the intended design and associated development activities.

Packages as Design Abstractions

Source files differ from other files typically stored in the file-system:

• They depend on the detailed contents of other source files. 
• Are created and edited in groups of multiple files. 
• Are subject to a high number of relatively small changes.
• Are edited by a team of developers rather than individuals. 
• Should be reusable on future projects.
• Should be easy to change without impacting other files too widely.
• Must support the defined deployment environment.
• Are subject to a QA, version control and release processes.

The aim of a package design should be to support these characteristics.  For example, the design could explicitly support Martin's “Reuse/Release Equivalence Principle (REP)” (article, book) whereby packages are developed, built, tested and released against released versions of the packages upon which they depend. 

Design is not something that happens once at the start of the project – it is an activity that spans the life of an application or product.  This fact has become explicit with the iterative and Agile development models.  As the code-level design continues, the package-level design emerges. 
Unfortunately, the emergent design is often invisible and so forgotten.  Not only does the original design degrade, but the overall structure tends to become excessively complex.  As the supporting structure dissolves, development activities become harder and the cost of each new feature increases.

This priciple of emergent design is important here. Clearly when I have a project of 50 classes in a half-dozen packages, the overhead of a sub-optimal package dependencies isn't going to slaughter me. But if my project is going to grow to 5000 classes, then putting in minimal effort from the start can save huge effort when things get more complicated.

In part 2 I'll take a closer look at cyclic package dependencies and why they matter.

An Overview of Structure101 Architecture Diagrams

Structure101 lets you work with both structure (the whole code-base as it is) and architecture (the subset of the structure that you really care about, and how it should be). It lets you define the architecture in the context of the physical structure and diseminate this to the team. Architecture diagrams are what makes this possible.

Layering and composition

Structure101 architecture diagrams use a concise visual notation for representing architectural layering and composition. Here is an example of one of the architecture diagrams that we use for the structure101 code-base.

The principle is simple; components (“cells”) should only depend on components at lower levels, not in the same or higher levels.

Layering Overrides

Sometimes a top-down dependency structure is too simple to capture the intent of an architecture. "Overrides” allow you to override the default layering of a diagram. For example we may decide to allow a specific dependency from a cell to a higher-level cell. The override is shown as a green (“allowed”) arrow on the architecture diagram. (Note that enabling this “upward” dependencies practically  merges the “hiView”, “xbase” and “graph” components from the perspective of testing, reuse, development, etc.)

A more common example is where we wish to enforce a more strict layering. For example we may want one layer to only use the next layer down, but not layers below that. Such an override is shown as red (“disallowed”) on the architecture diagram.

Combining diagrams

It is not necessary to include all aspects of an architecture on a single structure101 architecture diagram.

A common scenario is where a number of “add-ins” are distributed across several packages. For example, this diagram shows part of the structure101 architecture.

It is correct, but incomplete. Classes in assemblies.X should never depend on classes in lang.Y.  We could express this by adding several overrides, but it is much cleaner to use a separate diagram for this aspect of the architecture.

The next diagram defines a number of “language packs” that do not have a direct equivalent in the physical structure (they are “pure” architecture components), but express the architectural constraint that was missing above.

The combination of the 2 diagrams defines the intended architecture.

Mapping the architecture to physical code

In order to understand how a physical code-base conforms to an intended architecture, we need to map the architectural components (cells) to physical code. 

Simple patterns are used to establish this mapping. This has a number of benefits:

• If a diagram contains a component mapped to com.headway.lang.* and the team creates a new package com.headway.lang.cobol, then the diagram is not rendered obsolete – all the classes in the new package map to the intended component.
• I can create components with more complex mappings with expressions such as com.headway.*.test.?
• I can create and show a component for which no code has yet been created, either by specifying no pattern or specifying the paths where I expect the new code to be implemented.
• I can effectively “hide” physical entities from a diagram. For example any code in com.headway.lang.cobol will simply map to a component with the expression com.headway.lang.* - I do  not need to show package cobol on the diagram if I don’t want to.

Another flexibility is that a physical entity maps to the component with the most specific pattern.  For example if I include 2 components, one with com.headway.lang.* and the other with the expression com.headway.lang.java.*, then the class com.headway.lang.java.myClass will map to the latter. The effects of this can be at the same time subtle and powerful. For example I could move the component that maps to com.headway.lang.java.* into another “parent” altogether. 

Finally, each diagram has a (possibly empty) expression that maps to  “excluded” items. This is useful if some physical entities would otherwise undesirably map to a component in the diagram.

Once the mapping is established, any dependencies that violate the architecture is shown on the diagram as a curved dotted line as shown here between component “graph” and the higher-level package “hiView”.

It is easy to discover the code-level cause of a violation by selecting it on the diagram within a structure101 client or IDE plug-in.

Structure101 IntelliJ Plug-in Build 104

This update now checks for architecture violations automatically when you do a build (previous version was "on-demand" only).  More on structure101 IDE plug-ins.

Spring Framework 2.1 M3 Architecture

Here are some architecture diagrams for Spring Framework 2.1 M3 (released yesterday). You can point the (free) structure101 plug-in at these and get IDE warnings if your customizations break Jeurgen's architecture.

Here is the top level breakout of org.springframwork:

Structure101 created this from the physical code-base. All the cells in the diagram use only lower-level cells. With such a clean structure, I did no further editing of the diagram, other than to adjust the level of nesting.

Below is a further breakout of some of the larger modules.

org.springframework.aop:

org.springframework.beans:

org.springframework.jdbc:

org.springframework.jms:

org.springframework.orm:

org.springframework.web

You can view these online here (I'll update later today), and if you have a Spring-based project, you could install the structure101 Eclipse or IntelliJ plug-in (free from here) and point it to the Spring project in the online repository (use this url: http://www.structure101.com/java/data in the plug-in properties) and the diagrams will be visible inside your IDE, any existing violations flagged (i.e. if you have created any upward dependencies), and you will be warned if and when you make code-changes that are inconsistent with the layering.

This is a new-ish feature - please email me directly and let me know how you got on or if you have questions.

Code Organization Guidelines for Large Code Bases

In an excellent on-line presentation Juergen Hoeller gives rationale and guidelines for controlling the structure of large, evolving code-bases. Juergen is the chief architect of the Spring framework, which as I have previously pointed out is structurally almost perfect. This didn't happen by accident.

If you don't have time go though the 88 minute presentation, here is a nice sysnopsis by Mike Nereson.

Structure101 IntelliJ Plug-in

This has just been made available for download. It displays architecture diagrams within the IDE, and warns if any code changes are inconsistent with the target architecture.

It works more or less like the structure101 Eclipse plug-in except that for now it checks for architecture violations on demand rather than automatically when you do a build.

Structure101 Supports Java 1.5

We have overhauled the byte-code parser to now pick up Java 1.5 constructs (e.g. generics and annotations), plus it is much faster.

You can download the first build that includes the new parser here.

If you currently use the structure101 IDE plug-in you may find that you get bogus warnings on "new" architectural violation when you start using the new parser. This is  because the parser is now finding a few more dependencies related to the Java 1.5 language updates, and thinks they are new compared to your reference snapshot. If it's giving you a headache, you should just republish your reference snapshot to the structure101 repository with the new parser.

Architecture warnings in Eclipse

The new version of structure101 lets you define concise, layered architecture diagrams and publish them so that the whole team sees them in Eclipse and gets warnings if they make code changes that are out of whack with the architecture.

Here's an example architecture diagram:

It defines layering (cells should only depend on lower cells) and composition. What you can't see is that each cell is mapped to the code by a simple expression (e.g. 'com.headway.assemblies.*) - this means that violations can be easily discovered. Structure101 can initialize diagrams from code which you can then edit.

Once you publish architecture diagrams to a structure101 repository, the all new Eclipse plug-in gets to work. It displays the diagrams (they're compact and don't need much screen space) overlaid with any violations, to which you can naturally navigate. The plug-in also generate warnings or errors (you specify) when you build.

Jeez that's going to be a helluva lot of errors on my project, I hear you say. But here's the cool bit - the plug-in can be told to only warn you about any new violations that you create. That way each developer just makes sure they don't make things any worse, and in so-doing, will occasionally need to refactor out prior violations. With almost no effort, the code-base will evolve towards the intended architecture (the KALOI principle).

So now your team has all the info they need to respect your architecture, how do you know if they're using it? An RSS feed of course! You can get a notification any time a new violation makes it into the mainline. Sweet.