Elisa Shekar

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It wasn't magic. Whether you realized it or not, the model in the code was harmonious with the underlying domain. If that's something you want to happen every time, domain-driven design (DDD) is for you.

I've been an object-oriented software developer since the mid-'80s, and in that time I have seen many methods come and go. While I seldom achieve or sustain that elusive state of coding bliss, it has occurred several times, in different paradigms. The first was in structured programming (with big design up front). More recently, and more often, using DDD. Although anecdotes aren't evidence, I consider DDD to be the current best practice. Unfortunately, it isn't very popular, and those who use it typically don't do it terribly well.

That's a shame.

Every software system ever built has a model at its heart. If this model matches the underlying domain well, the software will accept enhancements more easily, and it has a much better chance of surviving and thriving intact for years. The model may not be obvious; it may exist only in the mind of the developers, but it exists nonetheless. Elegant designs are possible in any paradigm. If the model reflected in your code is aligned with the domain, the code flows.

It's this alignment that matters, and achieving and maintaining alignment is the fundamental purpose of DDD.

Your code has a model within it. If the code isn't a model of the business domain, then what is it? If you're not intentionally modeling the domain, then what are you accidentally modeling instead? Even in the best systems, your code will reflect some aspects of the domain model well in some places, but not so well in others.

Consider this example: Your team is building an application that will support SaaS customers, and you're interested in the sign-up process. The story the team is playing says, "As a customer, I want to be able to sign up for a subscription so that I can use the service." You look at the code and find that the requisite method is in the "customer" class, which makes sense, but that it's named CreateCustomer.

Ouch.

Developers tend to think and talk in development terms, which naturally results in inadvertent models of how instead of what and why. This is common, and it's a tar pit, because developers tend to follow established patterns. What's more, once the true purpose of the code is obscured, it's unlikely that anyone will unearth it.

Yes, the functionality of the method named CreateCustomer does everything required to complete a customer signup, but the language it uses describes its own implementation, not the domain activity. Naming the method SignUpForSaaSSubscription would better reflect the actual domain activity and make the intention of the method clear.

At a trivial level, it's all about the names you use for things. At the level above that, it's about the way that you combine and activate things to produce business value. At the level above that, it's the causal and relationship (semantic) model that keeps everything cohesive, coherent, and aligned with the business. This alignment can't come from an implementation-focused model. You must use a domain model.

A business-applicable model doesn't have to be couched in DDD terms to be good. If your business model is naturally and fully described by a database operations create, read, update, and delete (CRUD) model, then that's good enough. It's the alignment that matters, and continuous alignment is the goal.

Once you've captured a useful piece of the business in the model, you have a framework on which to support multiple applications. It's not a framework in the literal, technical sense, but a framework in the sense that it can guide the thinking and construction of solutions sufficiently well. But this isn't a one-shot activity.

Striving for model-domain harmony never ends, as long as the business keeps changing. But that's alright, because DDD is a continuous process. It keeps the guardrails up, the feedback flowing, and the software on track with the business. DDD does this strategically, tactically, and philosophically.

DDD philosophy

DDD talks a lot about the ubiquitous language of a domain (also called the domain language). Ubiquity is a goal, a guide, and the central organizing theme. Everyone, technical and nontechnical, must speak the same language, use the same terms, and give the same names to the same concepts. If not, group understanding will be inaccurate, the resulting communications will be imprecise, learning will be fractured, and the software will decay over time. The definitive source of names, definitions, and descriptions is the subject matter expert (aka domain expert), not the software developers.

This quasi-Zen emphasis on pure definitions and unified communications, within specific boundaries, is the glue that holds it all together, the philosophical principle that forms the guardrails that keep everything else on track. The strategic, tactical, and technical aspects of DDD are intended to support and enforce this philosophy. You model the business in business terms (and model the domain in domain terms) and then protect that model from corruption in conversation, design, and implementation.

The strategic aspect of DDD aligns software development teams' efforts with the interests of the business. It helps when deciding what to focus on, usually by identifying one core domain. This may be a specific area of business or even a specific slice that's critical. The strategic focus keeps your major efforts devoted to what's most important to the business now. This prevents the sluggishness and paralysis of the everything-we-do-is-most-important blanket that often smothers software development efforts, while effectively accumulating corporate knowledge in the software.

The tactical, technical aspect of DDD guides the implementation process with the fundamental purpose of protecting the model from corruption. The patterns and architectural structures commonly associated with DDD (though not necessarily first invented or discovered by DDD) flow naturally from this constraint, to provide the requisite layers of protection.

Sadly, many developers, when first learning about DDD, scoop up the technical patterns and don't tarry long enough to absorb the philosophy or strategy. Using the patterns of DDD without adherence to the philosophy of DDD reduces the method to a cookie-cutter, mechanical approach to design that can create costly and unnecessary complexity. If you get the context boundaries wrong, it will be difficult to apply and maintain the code in spite of the otherwise desirable patterns. If the model in the code is lax about its use of domain language, it will rapidly lose alignment with the business.

In many ways this is why DDD is considered hard to do right: it takes a certain amount of self-discipline to adhere to the philosophy and requires another level of restraint to resist designing when you should be modeling. It also takes courage to keep asking "What does this really mean?" and "Why does this happen?"until the deeper model is uncovered. Finally, it requires patience to keep refining, refactoring, iterating, and accepting feedback until the model, the code, and the business coalesce into a cooperative synergy.

If you're building simple applications in a simple domain, your model will also be simple, and so shouldn't be excessively time-consuming to discover. That doesn't mean that crafting it will be easy, though. Above all, you must commit to learning about the business. Many developers only have eyes for gadgets, technology, and tools, while business-related things are regarded as irrelevant inconveniences. But you can't help the business develop its capabilities without understanding the business. There's no magic tool to Google the minds of the subject matter experts, and no mobile app will let you skip having those detailed conversations about how the business works.

A little DDD can go a long way. Even if you decide not to continue DDD practices past the initial model, or only go for a few iterations, you'll be able to make that decision from a position of knowledge, instead of guesses and wishes.

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Many people with autosomal dominant polycystic kidney disease (ADPKD) have high blood pressure. Find out here why controlling your blood pressure is important and the treatments available.

High blood pressure is also known as hypertension. It is roughly twice as common in people with ADPKD compared with the general population. At least 5-7 in every 10 adults with ADPKD develop high blood pressure before they have kidney symptoms.

The average age that people with ADPKD are diagnosed with high blood pressure is 30 years old. However, it can develop as early as childhood — between 2 and 4 out of every 10 children with ADPKD have high blood pressure.

High blood pressure may be the first sign you have ADPKD, and it may be diagnosed when your kidney function is normal. However, once you have kidney symptoms, you’re even more likely to have hypertension.

Early and effective treatment is important because high blood pressure increases your risk of having a serious cardiovascular (heart or blood vessel) problem. High blood pressure may also cause your ADPKD to progress faster, meaning your kidney function will decline more quickly.

If your blood pressure is raised, you can lower it with lifestyle changes and medication. By lowering your blood pressure to a healthy range, you’ll reduce your risk of having a cardiovascular complication. It’s not yet clear whether lowering your blood pressure can slow the progress of ADPKD.

Blood pressure (or BP) is the pressure of blood inside your arteries. The higher your blood pressure, the harder your heart needs to work to pump blood around your body.

Blood pressure is measured in millimetres of mercury (mmHg), and is written as two numbers, such as 130/80 mmHg :

So, if your systolic blood pressure is 130 mmHg and your diastolic blood pressure is 80 mmHg, your blood pressure is said to be ‘130 over 80’, written as 130/80 mmHg.

Your doctor will diagnose you with high blood pressure (hypertension) if your blood pressure is:

Diagnosing high blood pressure in children is done differently. An appropriately sized blood pressure cuff is used. Children are diagnosed with high blood pressure if their systolic or diastolic blood pressure is high compared with the usual range for their age, height and sex.

Having high blood pressure means that your heart has to work harder to pump blood around your body. It increases your risk of having serious cardiovascular problems, including a heart attack, heart failure, bleed on the brain (intracranial haemorrhage) or stroke.

If you have an enlarged blood vessel in your brain (called a brain aneurysm), having high blood pressure could slightly increase the risk of it bursting. You can learn more about aneurysms on our website.

The risk of having a cardiovascular problem differs between people, depending on factors including :

The good news is that your risk of cardiovascular disease can be reduced with treatment to lower your blood pressure. Treatment is usually quite straight forward. If you have high blood pressure, your doctor can recommend lifestyle changes and medications to lower your blood pressure and risk (see below).

If your blood pressure is extremely high, it could affect your ability to drive (it can alter your vision). If it’s above 180/100 mmHg, you should not drive until a doctor confirms it has reduced and is well controlled. Speak to the DVLA for advice.

Most people with high blood pressure have no symptoms. It’s spotted and diagnosed through blood pressure monitoring. This is why periodic blood pressure monitoring is recommended for people with ADPKD.

Most people with ADPKD have their blood pressure measured when they go to their kidney clinic for check-ups. This is usually at least once a year. Experts recommend that children with ADPKD (or at risk of ADPKD) have their blood pressure monitored every 2 years from age 5 years.

Adults who haven’t been diagnosed with ADPKD but who are at risk can choose to have their blood pressure checked. This is normally done at least once every 3 years.

Your doctor will use a blood pressure monitor (a small machine with an arm cuff) to measure your blood pressure. If these readings show it is high, you’ll be offered blood pressure monitoring over 24 hours while you go about your normal life. This is called ambulatory blood pressure monitoring. You’ll wear a monitor strapped round your waist and a cuff wrapped around your upper arm. The cuff inflates and deflates automatically throughout 24 hours to take recordings of your blood pressure. Ambulatory blood pressure monitoring is used to confirm that you have high blood pressure.

It’s normal for our blood pressure to lower while we sleep. In some people with ADPKD this doesn’t happen, even if their daytime blood pressure is normal. Ambulatory blood pressure monitoring can be especially useful for spotting night-time increased blood pressure.

If this type of monitoring is not suitable for you, your doctor might offer to lend you a blood pressure machine so you can take your own readings throughout the day. This is called home blood pressure monitoring.

If you’re found to have high blood pressure, your doctor may offer a urine test, blood test, eye test and electrocardiogram (ECG) of your heart. This is to check for other problems that high blood pressure can cause.

Your doctor will also calculate your risk of having cardiovascular problems and explain this to you.

For the general public and adults with ADPKD, the recommended or ‘target’ blood pressure is generally 140/90 mmHg or less. Some experts recommend a lower target than this (e.g. 130/80 mmHg for those with kidney function under 60%, or <110/75 mmHg for those with kidney function over 60%). Your doctor will recommend the best target for you.

Blood pressure targets in children are based on what is normal for their age, sex and height.

Your doctor will suggest lifestyle changes and medications to bring your blood pressure below this target.

Taking the steps below can help to lower your blood pressure and reduce your risk of having a cardiovascular problem :

You’ll find more detailed information and tips on a healthy diet and lifestyle on our website.

If you have high blood pressure and ADPKD, your doctor should discuss options for medication with you. Medications to reduce blood pressure are known as antihypertensive drugs. For people with ADPKD, one medication will probably be enough to control blood pressure. However, some people are advised to take more than one.

Your doctor will probably recommend an angiotensin-converting enzyme (ACE) inhibitor or angiotensin-II receptor blocker (ARB), unless there is a reason you cannot take these.

A common side effect of these medicines is that your blood pressure may become too low. This is called hypotension and can make you feel dizzy or tired. Some people get a persistent dry cough with ACE inhibitors. Other side effects are less common — ask your doctor for more information to help you decide which medication might be best for you.

Other medicines sometimes used to reduce blood pressure are calcium channel blockers and diuretics. If your ACE inhibitor or ARB is not controlling your blood pressure sufficiently, your doctor may suggest adding one of these medications.

If you have any side effects, talk to your doctor. There are many medications to lower blood pressure, so work together with your doctor to find the right one for you.

These medicines work by reducing the amount or action of a hormone in your body called angiotensin II. This hormone can increase blood pressure. By reducing angiotensin II signalling, ACE inhibitors and ARBs reduce your blood pressure.

To keep your blood pressure at a healthy level, you’ll need to continue your healthy lifestyle and your prescribed medication. You’ll have annual blood pressure checks. If your blood pressure is not well controlled, your doctor can recommend different or additional medications for you.

Some people like to check their blood pressure themselves. While you should not rely on this instead of having checks with your doctor, it can be helpful to put your mind at ease. Your kidney specialist or GP might suggest home monitoring, and some chemists offer free blood pressure checks. To find your nearest chemist offering this service, go to bloodpressureuk.org. Alternatively, you could consider buying your own approved blood pressure monitor.

ADPKD can increase blood pressure in two main ways: 1) by altering the lining of your blood vessels, and 2) by activating hormones that control blood pressure.

The faulty genes that cause ADPKD (PKD1 and PKD2) are not only important for your kidneys: they’re also found in the lining of your blood vessels. When these genes don’t work correctly, it can interfere with the way that the vessels tighten and relax. In people with ADPKD, blood vessels tend to be tighter and narrower than usual. Narrower vessels mean the blood has less room to flow, increasing blood pressure. You could think of it like a motorway where a lane has been closed — the traffic becomes busier as it fights for space.

ADPKD can also activate hormones that control blood pressure (e.g. angiotensin II). There are a number of reasons why these hormones can be activated. For example, kidney cysts can squash tiny vessels in the kidney, which makes it harder for blood to flow through them. This can lead to hormones levels rising as the body tries to get more blood to the area.

High blood pressure tends to worsen as kidney cysts grow and the kidneys enlarge.

People whose ADPKD has progressed quite a lot are at increased risk of having additional cardiovascular problems, especially if their blood pressure isn’t controlled. These include:

These conditions increase your chance of having a potentially fatal problem such as abnormal heart rhythm, a heart attack or stroke , so it’s important they’re monitored and treated.

If you have a high risk of having a cardiovascular problem, your doctor might recommend additional medications. These include:

Additional treatments to lower cardiovascular risk are being tested in clinical trials.

Your doctor will be able to explain whether additional medications might be helpful for you and why.