One of the most important concepts in computer science is that of indirection, permeating all areas of software development. It is so important that a famous computer scientist named David Wheeler (as recounted by Butler Lampson, who envisioned the personal computer) once said that all problems in computer science can be solved by another level of indirection. Without understanding indirection, your ability to write computer programs with any complexity or utility will be extremely limited.

What is indirection? Quite simply it means not direct. If there is a direct connection between two things, indirection means that something is placed in the middle so that another level of indirection is created.

Go ask your mother!

When you were a child, you might have gone to your father and asked, Dad, may I stay late at the beach today? He might have responded, Go ask your mother! Every family is different; perhaps your mother sometimes told you to go ask your father. Either way, this is an example of indirection—rather than give you an answer directly, you were told to go ask someone else.

Perhaps you work in a small restaurant, and you are given this schedule of chores. It is a small restaurant with few workers.

  1. 12:00 p.m.: Set the tables
  2. 1:00 p.m.: Wait on customers
  3. 3:00 p.m.: Take a break
  4. 4:00 p.m.: Clean the bathrooms
  5. 5:00 p.m.: Wait on more customers

However some of the days at 4:00 p.m. the manager wants you to take out the trash, and another worker to clean the bathrooms. The manager might make several sets of chore lists. Or the manager might simply substitute step #4 with indirection:

  1. 12:00 p.m.: Set the tables
  2. 1:00 p.m.: Wait on customers
  3. 3:00 p.m.: Take a break
  4. 4:00 p.m.: Ask manager for chore
  5. 5:00 p.m.: Wait on more customers

Changing Paths

Simply Explained: Indirection
Indirection, simply explained by Geek & Poke

Indirection is about adding intermediate steps to the procedure. It allows us to change parts of the procedure without changing the original plan. Consider a very old children's rhyme from the 1800s by Lydia Maria Child:

Over the river, and through the wood,
    To grandfather's house we go; …

If we consider the first lines as instructions for getting to grandfather's house, we might write them down like this:

  1. Cross the river.
  2. Go through the woods.
  3. Arrive at grandfather's house!

But what if the river is frozen? What if the woods are on fire? These instructions aren't very flexible for the changing circumstances! Instead we could place signposts along the way, pointing to the next signpost. If we need to go around the woods because they are dangerous, we simply change the sign on the other side of the river to point to the detour.

  1. Follow the first signpost in front of the river.

The original instructions can now simply say follow the first signpost in front of the river. Through the power of indirection, we never need to change these initial directions, as we can now change the intermediate steps if we need to.


A simple example of indirection in real life can be seen in managing a store. If a person owns a small business, many times the owner has many roles to fill. The owner of a store may need to determine prices and purchase new stock, yet still find time to open the store and greet customers. The owner has a direct relationship with managing the store, and it looks like this:

Owner and store.

As the business grows and responsibilities increase, the owner may hire a manager to unlock the store and greet customers. By using an agent (which is a general term for someone who acts on the behalf of someone else) to manage the store, the owner now has an indirect relationship withe day-to-day management duties.

Owner, manager, and store.

Using indirection provides the owner with two big benefits, which you will see in other areas of software development:

The owner can delegate duties to the manager, who may do a better job at managing the store than the owner.
If the manager is not doing a good job, the owner can replace the manager with a better manager.

Thus this single added level of indirection brings flexibility to the arrangement, potentially improving the job that gets done.

Localizing Changes

A large part of the flexibility of indirection is that it localizes changes. As you saw with the manager scenario, using an indirect relationship provides a single point where changes can be made. If the manager is not doing a good job, he/she can be replaced with a better one. The owner and the store do not need to be changed.

Imagine if a store owner has a safe with all the store's valuables. The safe is very strong and has a key that is very expensive to copy. If the owner has several employees, it will be costly to make so many copies of the key.

Employees with keys to a safe.

The problem grows bigger if the key is somehow compromised. Perhaps one employee leaves to work somewhere else and forgets to return the key. Now what? The owner has no choice but change the key to the safe, which is a huge expenditure—not to mention the high cost of making copies of the new key for all the employees.

Using a level of indirection makes the problem of key compromise easier to surmount. The owner can have two sets of keys: one key for the central safe, and many duplicate keys for a smaller safe that will hold the key to the main safe. The smaller safe need only be large enough to store a single key, and its keys are inexpensive to duplicate.

Employees with keys to a smaller safe holding a single key to a large safe..

To gain access to the main safe, an employee must user his/her key (Key1) to open the smaller safe to gain access to the key to the main safe (Key2). This second key, Key2, may only be used on the premises and must be placed back in the smaller safe when finished accessing the main safe.

Now if one of the employees leaves the company or loses his/her copy of Key1, the owner merely needs to change the key to the smaller safe and make new copies for the employees—all at a much lower cost than changing Key2.

Preventing Redundancy

The other side of the coin of localizing change is preventing redundancy. A layer of indirection can gather together information in one place that might normally be spread out in several locations.

Let's imagine that a store keeps a list of its current stock of toys and furniture, along with the supplier it uses to procure additional items when stock runs low.

Item Stock
Item Stock Supplier
table 20 Acme Furniture Company, 123 Some Street
chair 15 Acme Furniture Company, 123 Some Street
balloon 99 Bozo Toy Company, 321 Other Street
kite 5 Bozo Toy Company, 321 Other Street
doll 10 Bozo Toy Company, 321 Other Street

Notice that several items are sourced from the same suppliers, resulting in duplicated information in the table. What if Acme Furniture changed its address? The store owner would need to go through every item in this list that is purchased from Acme Furniture and update the address.

How can indirection help the owner here? By introducing a unique code for each supplier, the owner can maintain a separate table with information on each supplier with no information duplicated.

Item Stock
Item Stock Supplier
table 20 ACME
chair 15 ACME
balloon 99 BOZO
kite 5 BOZO
doll 10 BOZO
ID Name Address
ACME Acme Furniture Company 123 Some Street
BOZO Bozo Toy Company 321 Other Street
TOP Top Toys Corporation 456 Far Avenue

Because of the extra indirection of the supplier code, finding a supplier name and address takes the extra lookup step of following the supplier code from one table to the other. But the gain is that there is less duplicated information. Consequently if one of the suppliers has to change its address, this modification can be made on just a single line of the Suppliers list rather than multiple lines in the Item Stock list.

There is another benefit: the store owner now can keep track of suppliers even for which there are not yet any products. Here the store owner has added the name and address of Top Toys Corporation. If in the future there are toys purchased from Top Toys Corporation, the owner can simply add the code TOP to the Item Stock list for those new items.

DNS Resolution

The Internet Domain Name System (DNS) provides a good example of how indirection can be used to make connections flexible between computers. Every computer on the Internet is assigned a unique number called an IP address that (as of version 4) looks something like, for example But it would be tedious (not to mention hard to remember) if you were required to enter these number directly into your browser when you wanted to surf the web. Furthermore, you would have problems accessing the web site if the site's owners had to move the content to a different computer with a different IP address.

The DNS provides levels of indirection that address this problem. Simplified greatly, the DNS lives on a separate computer called a DNS server, and is essentially a big table containing domain names (the names used e.g. by web site servers) and their corresponding IP addresses. Conceptually in part it looks like this:

DNS Server
Domain Name IP Address

When you ask your computer to browse to

  1. Your computer first goes out to a DNS server and asks for the IP address of
  2. The DNS server responds that the IP address of is
  3. Your computer then makes a connection directly to the computer with IP address (the computer running the web site of
DNS lookup diagram for resolving to IP address
Simplified DNS lookup (Dyn).

Although this indirection results in an extra lookup step for your computer, it provides a tremendous flexibility:



Indirection makes the relationship between two things less direct by adding some additional lookup layer between them. While this makes the system more complex; it also brings flexibility by promoting delegation and consolidating redundant information, and providing for localized changes. Indirection is one of the fundamental concepts that permeates computer software development, but its overuse can make a system hard to understand and raise the risk of errors.


In the Real World

Think About It

Self Evaluation


Imagine that you are the owner of the store described in the lesson. You decide to change the toy supplier for the toys and purchase them from Top Toy Corporation.

With the current design of the Item Stock list, you would have to look at all the toys in the list and change each of their suppliers, one for each line. With many toys, this could take some time and require changing many lines. But you have noticed in the Item Stock list that all of the furniture comes from one furniture supplier, and that all the toys come from one toy supplier. You would like to be able to change the furniture supplier for all the furniture items; or the toy supplier for all the toy items; just by changing one of the lines in one of the lists.

How might you introduce another level of indirection to make it possible, with your new set of lists, to change the toy supplier for all the toys from Bozo Toy Company to Top Toy Corporation by changing only one line in the Item Stock list? Think about the indirection used by DNS that allows server IP addresses to be changed without changing a web site's domain name.

See Also