In software development we deal with boundaries between systems.
Examples of boundaries are:
- Your application code and a database.
- Your application code and the file system.
- A web server and your server-side application code.
- A client-side application and the browser DOM.
It's important to recognize these boundaries. You want to do things at
the boundaries of our application, just after input has arrived into
your application across an outer boundary, and just before you send
output across an inner boundary.
If you read a file and what's in that file is a string representing a
number, you want to convert the string to a number as soon as possible
after reading it, so that the rest of your codebase can forget about
the file and the string in it, and just deal with the number.
Because if you don't and pass a filename around, you may have to open
that file multiple times throughout your codebase. Or if you read from
the file and leave the value as a string, you may have to convert it
to a number each time you need it. This means duplicated code, and
multiple places where things can go wrong. All that is more work, more
error prone, and less fun.
Boundaries are our friends. So much so that programming languages give
us tools like functions and classes to create new boundaries in
software. With a solid, clear boundary in place in the middle of our
software, both halves can be easier to understand and easier to
One of the most interesting things that happen on the boundaries in
software is conversion and validation of values. I find it very
useful to have a clear understanding of these concepts during software
development. To understand each other better it's useful to share this
understanding out loud. So here is how I define these concepts and how
I use them.
I hope this helps some of you see the boundaries more clearly.
Following a HTML form submit through boundaries
Let's look at an example of a value going across multiple boundaries
in software. In this example, we have a web form with an input field
that lets the user fill in their date of birth as a string in the
I'm going to give examples based on web development. I also give a few
tiny examples in Python. The web examples and Python used here only
exist to illustrate concepts; similar ideas apply in other
contexts. You shouldn't need to understand the details of the web or
Python to understand this, so don't go away if you don't.
Serializing a web form to a request string
In a traditional non-HTML 5 HTTP web form, the input type for dates is
text`. This means that the dates are in fact not interpreted by the
browser as dates at all. It's just a string to the browser, just like
adfdafd. The browser does not know anything about the value
the input is really a date and shows an error message if it's not.
In HTML 5 there is a new input type called date, but for the sake
of this discussion we will ignore it, as it doesn't change all that
much in this example.
So when the user submits a form with the birth date field, the inputs
in the form are serialized to a longer string that is then sent to
the server as the body of a POST request. This serialization happens
according to what's specified in the form tag's enctype
attribute. When the enctype is multipart/form-data, the request to
the server will be a string that looks a lot like this:
POST /some/path HTTP/1.1
Content-type: multipart/form-data, boundary=AaB03x
content-disposition: form-data; name="birthdate"
Note that this serialization of form input to the
multipart/form-data format cannot fail; serialization always
succeeds, no matter what form data was entered.
Converting the request string to a Request object
So now this request arrives at the web server. Let's imagine our web
server is in Python, and that there's a web framework like Django or
Flask or Pyramid or Morepath in place. This web framework takes the
serialized HTTP request, that is, the string, and then converts it
into a request object.
This request object is much more convenient to work with in Python
than the HTTP request string. Instead of having one blob of a string,
you can easily check indidivual aspects of the request -- what request
method was used (POST), what path the request is for, what the
body of the request was. The web framework also recognizes
multipart/form-data and automatically converts the request body
with the form data into a convenient Python dictionary-like data
Note that the conversion of HTTP request text to request object may
fail. This can happen when the client did not actually format the
request correctly. The server should then return a HTTP error, in this
case 400 Bad Request, so that the client software (or the
developer working on the client software) knows something went wrong.
The potential that something goes wrong is one difference between
conversion and serialization; both transform the data, but conversion
can fail and serialization cannot. Or perhaps better said: if
serialization fails it is a bug in the software, whereas conversion
can fail due to bad input. This is because serialization goes from
known-good data to some other format, whereas conversion deals with
input data from an external source that may be wrong in some way.
Thanks to the web framework's parsing of web form into a Python data
structure, we can easily get the field birthdate from our form. If
the request object was implemented by the Webob library (like for
Pyramid and Morepath), we can get it like this:
Converting the string to a date
But the birthdate at this point is still a string 21-10-1985. We
now want to convert it into something more convenient to
Python. Python has a datetime library with a date type, so
we'd like to get one of those.
This conversion could be done automatically by a form framework --
these are very handy as you can declaratively describe what types of
values you expect and the framework can then automatically convert
incoming strings to convenient Python values accordingly. I've written
a few web form frameworks in my time. But in this example we'll do it
it manually, using functionality from the Python datetime library
to parse the date:
>>> from datetime import datetime
>>> birthdate = datetime.strptime(request.POST['birthdate'], '%d-%m-%Y').date()
datetime.date(1985, 10, 21)
Since this is a conversion operation, it can fail if the user gave
input that is not in the right format or is not a proper date Python
will raise a ValueError exception in this case. We need to write
code that detects this and then signal the HTTP client that there was
a conversion error. The client needs to update its UI to inform the
user of this problem. All this can get quite complicated, and here
again a form framework can help you with this.
It's important to note that we should isolate this conversion to one
place in our application: the boundary where the value comes in. We
don't want to pass the birth date string around in our code and only
convert it into a date when we need to do something with it that
requires a date object. Doing conversion "just in time" like that
has a lot of problems: code duplication is one of them, but even worse
is that we would need worry about conversion errors everywhere
instead of in one place.
Validating the date
So now that we have the birth date our web application may want to do
some basic checking to see whether it makes sense. For example, we
probably don't expect time travellers to fill in the form, so we can
safely reject any birth dates set in the future as invalid.
We've already converted the birth date from a string into a convenient
Python date object, so validating that the date is not in the
future is now easy:
>>> from datetime import date
>>> birthdate <= date.today()
Validation needs the value to be in a convenient form, so validation
happens after conversion. Validation does not transform the value;
it only checks whether the value is valid according to additional
There are a lot of possible validations:
- validate that required values are indeed present.
- check that a value is in a certain range.
- relate the value to another value elsewhere in the input
or in the database. Perhaps the birth date is not supposed to be
earlier than some database-defined value, for instance.
If the input passes validation, the code just continues on its merry
way. Only when the validation fails do we want to take special
action. The minimum action that should be taken is to reject the data
and do nothing, but it could also involve sending information about
the cause of the validation failure back to the user interface, just
like for conversion errors.
Validation should be done just after conversion, at the boundary of
the application, so that after that we can stop worrying about all
this and just trust the values we have as valid. Our life is easier if
we do validation early on like this.
Serialize the date into a database
Now the web application wants to store the birth date in a database.
The database sits behind a boundary. This boundary may be clever and
allow you to pass in straight Python date objects and do a conversion
to its internal format afterward. That would be best.
But imagine our database is dumb and expects our dates to be in a
string format. Now the task is up to our application: we need
transform the date to a string before the database boundary.
Let's say the database layer expects date strings in the format
'YYYY-MM-DD'. We then have to serialize our Python date object to
that format before we pass it into the database:
This is serialization and not conversion because this transformation
So we have:
- Transform data from one type to another. Transformation by itself
cannot fail, as it is assumed to always get correct input. It is a
bug in the software if it does not. Conversion and serialization
both do transformation.
- Transform input across a boundary into a more convenient form inside
that boundary. Fails if the input cannot be transformed.
- Transform valid data as output across a boundary into a form
convenient to outside. Cannot fail if there are no bugs in the
- Check whether input across a boundary that is already converted to
convenient form is valid inside that boundary. Can fail. Does not
Conversion just deals with converting one value to another and does
not interact with the rest of the universe. The implementation of a
converter is therefore often reusable between applications.
The behavior of a converter typically does not depend on state or
configuration. If conversion behavior does depend on application
state, for instance because you want to parse dates as 'MM-DD-YYYY'
instead of 'DD-MM-YYYY', it is often a better approach to just swap in
a different converter based on the locale than to have the converter
itself to be aware of the locale.
Validation is different. While some validations are reusable across
applications, a lot of them will be application specific. Validation
success may depend on the state of other values in the input or on
application state. Reusable frameworks that help with validation are
still useful, but they do need additional information from the
application to do their work.
Serialization and parsing
Serialization is transformation of data to a particular type, such
as a string or a memory buffer. These types are convenient for
communicating across the boundary: storing on the file system, storing
data in a database, or passing data through the network.
The opposite of serialization is deserialization and this is done by
parsing: this takes data in its serialized form and transforms it
into a more convenient form. Parsing can fail if its input is not
correct. Parsing is therefore conversion, but not all conversion is
Parsing extracts information and checks whether the input conforms to
a grammar in one step, though if you treat the parser as a black box
you can view these as two separate phases: input validation and
There are transformation operations in an application that do not
serialize but can also not fail. I don't have a separate word for
these besides "transformation", but they are quite common. Take for
instance an operation that takes a Python object and transforms it
into a dictionary convenient for serialization to JSON: it can only
consist of dicts, lists, strings, ints, floats, bools and None.
Some developers argue that data should always be kept in such a format
instead of in objects, as it can encourage a looser coupling between
subsystems. This idea is especially prevalent in Lisp-style homoiconic
language communities, where even code is treated as data. It is
interesting to note that JSON has made web development go in the
direction of more explicit data structures as well. Perhaps it is as
Whoever does not understand LISP is doomed to reinvent it.
We can pick apart conversion and find input validation
inside. Conversion does input validation before transformation, and
serialization (and plain transformation) does not.
Input validation is very different from application-level validation.
Input validation is conceptually done just before the convenient
form is created, and is an inherent part of the conversion. In
practice, a converter typically parses data, doing both in a single
I prefer to reserve the term "validation" for application-level
validation and discuss input validation only when we talk about
implementing a converter.
But sometimes conversion from one perspective is validation from
Take the example above where we want to store a Python date in a
database. What if this operation does not work for all Python date
objects? The database layer could accept dates in a different range
than the one supported by the Python date object. The database may
therefore may therefore be offered a date that is outside of its range
and reject it with an error.
We can view this as conversion: the database converts a date value
that comes in, and this conversion may fail. But we can also view this
in another way: the database transforms the date value that comes in,
and then there is an additional validation that may fail. The database
is a black box and both perspectives work. That comes in handy a
little bit later.
Validation and layers
Consider a web application with an application-level validation layer,
and another layer of validation in the database.
Maybe the database also has a rule to make sure that the birth date
is not in the future. It gives an error when we give a date in the
future. Since validation errors can now occur at the database layer,
we need to worry about properly handling them.
But transporting such a validation failure back to the user interface
can be tricky: we are on the boundary between application code and
database at this point, far from the boundary between application and
user interface. And often database-level validation failure messages
are in a form that is not very informative to a user; they speak in
terms of the database instead of the user.
We can make our life easier. What we can do is duplicate any
validation the database layer does at the outer boundary of our
application, the one facing the web. Validation failures there are
relatively simple to propagate back to the user interface. Since any
validation errors that can be given by the database have already been
detected at an earlier boundary before the database is ever reached,
we don't need to worry about handling database-level validation
messages anymore. We can act as if they don't exist, as we've now
guaranteed they cannot occur.
We treat the database-level validation as an extra sanity check
guarding against bugs in our application-level code. If validation
errors occur on the database boundary, we have a bug, and this should
not happen, and we can just report a general error: on the web this is
a 500 internal server error. That's a lot easier to do.
The general principle is: if we do all validations that the boundary
to a deeper layer already needs at a higher layer, we can effectively
the inner boundary as not having any validations. The validations in
the deeper layer then only exist as extra checks that guard against
bugs in the validations at the outer boundary.
We can also apply this to conversion errors: if we already make sure
we clean up the data with validations at an outer boundary before it
reaches an inner boundary that needs to do conversions, the
conversions cannot fail. We can treat them as transformations again.
We can do this as in a black box we can treat any conversion as a
combination of transformation and validation.
Validation in the browser
In the end, let's return to the web browser.
We've seen that doing validation at an outer boundary can let us
ignore validation done deeper down in our code. We do validation once
when values come into the web server, and we can forget about doing
them in the rest of our server code.
We can go one step further. We can lift our validation out of the
the user inputs values into the web form, we are in the right place to
give really accurate user interface feedback in easiest way
to the browser DOM and that's it. The server is not involved.
We cannot always do this. If our validation code needs information on
the server that cannot be shared securily or efficiently with the
client, the server is still involved in validation, but at least we
can still do all the user interface work in the client.
Even if we do not need server-side validation for the user interface,
we cannot ignore doing server-side validation altogether, as we cannot
information to the server. Through that route, or because of bugs in
invalid. But now if the server detects invalid information, it does
not need do anything complicated to report validation errors to the
client. Instead it can just generate an internal server error.
one that sends information to the server, we could forgo doing
validation on the server altogether. Someone more experienced in the
arts of encryption may be able to say whether this is possible. I
web browsers and encryption.
In any case, we may in fact want to encourage other programs to use
the same web server; that's the whole idea behind offering HTTP
APIs. If this is our aim, we need to handle validation on the server
as well, and give decent error messages.