Termipankki
  1. A
    1. API Blueprint
    2. Addressability
    3. Ajax
    4. Anonymous Function
    5. App Context
  2. B
    1. Blueprint
    2. Business Logic
  3. C
    1. CORS
    2. Callback
    3. Client
    4. Column
    5. Column Attribute
    6. Components Object
      Concept
    7. Connectedness
    8. Control
    9. Converter
      Framework
    10. Cookie
      WWW
    11. Credentials
      Concept
  4. D
    1. DOM
    2. Database Schema
    3. Decorator
      Python
  5. E
    1. Element
    2. Entry Point
  6. F
    1. Fixture
    2. Flask App
    3. Foreign Key
  7. G
    1. Generic Client
  8. H
    1. HTTP Method
    2. HTTP Request
    3. Hash
      Concept
    4. Header
    5. Host Part
    6. Hypermedia
  9. I
    1. Idempotent
    2. Info Object
      Concept
    3. Instance Folder
  10. J
    1. JSON
    2. JSON Schema
  11. L
    1. Link Relation
  12. M
    1. MIME Type
    2. Migration
    3. Model Class
  13. N
    1. Namespace
  14. O
    1. ORM
    2. OpenAPI
      OpenAPI
    3. Operation Object
      Concept
  15. P
    1. Pagination
      Concept
    2. Path Parameter
      OpenAPI
    3. Primary Key
    4. Profile
  16. Q
    1. Query
    2. Query Parameter
  17. R
    1. Regular Expression
    2. Request
    3. Request Body
    4. Request Object
    5. Resource
    6. Resource Class
    7. Resource Representation
    8. Response
    9. Response Body
    10. Response Object
    11. Rollback
    12. Routing
    13. Route →
      Routing
    14. Row
  18. S
    1. SQL
    2. Serialization
    3. Static Content
    4. Swagger
      tool
  19. T
    1. Table
    2. Test Setup
    3. Test Teardown
  20. U
    1. URI
    2. URL Template
    3. Uniform Interface
    4. Unique Constraint
  21. V
    1. View Function
    2. Virtualenv
  22. W
    1. Web API
  23. Y
    1. YAML
      Language
Ratkaistu: / tehtävää

REST API Design Example

This page gives you another design example for a REST API. The API designed in this example will also be discussed more in the last two exercises where we add documentation and hypermedia to it, and finally make a machine client in the last exercise.
Please note that this material has been written with the expectation that the reader is familiar with RESTful API concepts from the lecture materials or relevant course book chapters.

API Concept

The API concept in this material is a service that stores metadata about music. The metadata is split into three levels: artist, album and track. The API can be used to enrich music-related data from other sources, and can also be used to fill in partial metadata of a poorly managed music collection. This example is interesting because the problem domain has some peculiar characteristics that require additional design considerations. It's also a good API example because its primary clients are machines.
The data schema is not particularly large: artists are authors of albums which contain tracks. So we have a clear hierarchy that is easy enough to represent in a database.

Challenges

The first challenge related to the problem domain is that names of artists, albums or tracks are not generally not trademarkable. In other words they are not unique. You can find multiple artists - sometimes even from the same country - with the exact same name. The same goes for album names. On the other hand, artists typically don't have multiple albums with the same name. Usually albums don't have multiple tracks with the same name either but there's an exception: there can be multiple untitled tracks on an album. One way or another our API needs to navigate this non-uniqueness mess.
The second challenge is the existence of "various artists" releases (VA for short), i.e. collaborative works. These are albums that have multiple artists, with one or more tracks from each. With these releases each track has to have its artist defined separately unlike normal releases where all tracks on an album are by the same artist. So although these two types of releases are very similar, they are not identical and will require some degree of differing treatment.

Related Services

This example API provides similar data as two free services for music metadata: Musicbrainz and FreeDB. These services are often used when ripping audio files from CDs because they have a CD checksum lookup for metadata. Of course our example is more limited but it is a RESTful API unlike these two. There's also Rate Your Music which offers meta information for human users.
An example of a data source that can be used with this API is last.fm which is a tracking site for your personal music listening. It has a lot of metadata of its own but one tragic failing: it is unable to track listening time accurately as the primary statistics are listening counts per track. This means that the statistics are biased towards artists that have shorter average track length. Not to worry, last.fm has its own API. It would be possible to pull data from there and combine with the length metadata from our proposed API!

Database Design

From our concept we can easily come up with a database that has three
models
: album, artist and track. However we also need to consider the VA exception when designing these models, so we actually have two additional item types to represent: VA album and VA track. We also need to figure out
unique constraints
for each model. Although everything in the database has a unique primary key, you should never use raw database IDs to address resources in an API. First of all they don't mean anything. Second, it introduces vulnerabilities for APIs that don't want unauthorized clients to infer details about the content.
Unique constraint allows us to define more complex definitions of uniqueness than just defining individual columns as unique. A combination of multiple columns can be made into a unique constraint so that a certain combination of values in these columns can only appear once. For example, we can probably assume that the same artist is not going to have multiple albums with the same name (we're not counting separate editions). So while album title by itself cannot be unique, album title combined with the artist ID foreign key can. 
def Album(db.Model):
    __table_args__ = (db.UniqueConstraint("title", "artist_id", name="_artist_title_uc"), )
Please note the comma at the end: this tells Python that this is a 1 item tuple, not a single value in regular parenthesis. You can list as many as column names for the unique constraint as you want. The name at the end doesn't matter, but has to exist, so better make it descriptive. For individual tracks we have an even better unique constraint: each album can only have one track at each disc index (per disc). So the unique constraint for tracks is a combinaton of album ID, track number and disc number.
def Track(db.Model):
    __table_args__ = (db.UniqueConstraint("disc_number", "track_number", "album_id", name="_track_index_uc"), )
We're going to solve the VA problem by allowing an album's artist foreign key to be null, and by adding an optional va_artist field to tracks. We'll make this mandatory for VA tracks on the application logic side later. Overall our database code ends up looking like this:
models.py 
from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from sqlalchemy.engine import Engine
from sqlalchemy import event
from sqlalchemy.exc import IntegrityError, OperationalError

app = Flask(__name__, static_folder="static")
app.config["SQLALCHEMY_DATABASE_URI"] = "sqlite:///development.db"
app.config["SQLALCHEMY_TRACK_MODIFICATIONS"] = False
db = SQLAlchemy(app)

@event.listens_for(Engine, "connect")
def set_sqlite_pragma(dbapi_connection, connection_record):
    cursor = dbapi_connection.cursor()
    cursor.execute("PRAGMA foreign_keys=ON")
    cursor.close()

va_artist_table = db.Table(
    "va_artists", 
    db.Column("album_id", db.Integer, db.ForeignKey("album.id"), primary_key=True),
    db.Column("artist_id", db.Integer, db.ForeignKey("artist.id"), primary_key=True)
)


class Track(db.Model):
    
    __table_args__ = (db.UniqueConstraint(
        "disc_number",
        "track_number",
        "album_id",
        name="_track_index_uc"),
    )
    
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String, nullable=False)
    disc_number = db.Column(db.Integer, default=1)
    track_number = db.Column(db.Integer, nullable=False)
    length = db.Column(db.Time, nullable=False)
    album_id = db.Column(
        db.ForeignKey("album.id", ondelete="CASCADE"),
        nullable=False
    )
    va_artist_id = db.Column(
        db.ForeignKey("artist.id", ondelete="SET NULL"),
        nullable=True
    )
    
    album = db.relationship("Album", back_populates="tracks")
    va_artist = db.relationship("Artist")

    def __repr__(self):
        return "{} <{}> on {}".format(self.title, self.id, self.album.title)
    
    
class Album(db.Model):
    
    __table_args__ = (db.UniqueConstraint(
        "title",
        "artist_id",
        name="_artist_title_uc"),
    )
    
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String, nullable=False)
    release = db.Column(db.Date, nullable=False)
    artist_id = db.Column(
        db.ForeignKey("artist.id", ondelete="CASCADE"),
        nullable=True
    )
    genre = db.Column(db.String, nullable=True)
    discs = db.Column(db.Integer, default=1)
    
    artist = db.relationship("Artist", back_populates="albums")
    va_artists = db.relationship("Artist", secondary=va_artist_table)
    tracks = db.relationship("Track",
        cascade="all,delete",
        back_populates="album",
        order_by=(Track.disc_number, Track.track_number)
    )
    
    sortfields = ["artist", "release", "title"]
    
    def __repr__(self):
        return "{} <{}>".format(self.title, self.id)


class Artist(db.Model):
    
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String, nullable=False)
    unique_name = db.Column(db.String, nullable=False, unique=True)
    formed = db.Column(db.Date, nullable=True)
    disbanded = db.Column(db.Date, nullable=True)
    location = db.Column(db.String, nullable=False)
    
    albums = db.relationship("Album", cascade="all,delete", back_populates="artist")
    va_albums = db.relationship("Album",
        secondary=va_artist_table,
        back_populates="va_artists",
        order_by=Album.release
    )

    def __repr__(self):
        return "{} <{}>".format(self.name, self.id)
From it you can also see how to set default ordering for relationships and a couple of other things that weren't covered in Exercise 1.

Resource Design

We're now ready to design the
resources
provided by our API. An important takeaway from this section is how we turn three database
models
into (way) more than three resources. We will also explain how
HTTP methods
are used in this API by following REST principles.

Resources from Models

A resource should be something that is interesting enough to be given its own
URI
in our service. Likewise each resource must be uniquely identifiable by its URI. It's quite common for an API to have at least twice as many resources as it has database
tables
. This follows from a simple reasoning: for each table, a client might be interested in the table as a collection, or just in an individual
row
in the table. Even if the collection
representation
has all the stored data about each of its items, the item representation must also exist if we want to enable clients to manipulate them.
If we were to follow this very simple reasoning, we'd have 6 resources:
  1. artist collection
  2. artist item
  3. album collection
  4. album item
  5. track collection
  6. track item
It is worth noting that a collection type resource doesn't necessarily have to contain the entire contents of the associated table. For instance contextless track collection makes very little sense; a collection of tracks by album makes more sense. In fact an album is a collection of tracks, so having a separate track collection resource might not even make sense. Artist collection is simple enough because artist is on top of the hierarchy, so it makes sense for the collection to have all artists. What about albums though? Like tracks, it does make sense to have "albums by an artist" as a collection resource. But we also have VA albums to worry about. We can make two collection resources: one for an artist's albums and another for VA albums. We end up with:
  1. artist collection
  2. artist item
  3. albums by artist collection
  4. VA albums collection
  5. album item (incorporates track collection)
  6. track item
However, we have slightly different representation for VA albums compared to normal albums, and same goes for tracks. Even though we chose the same
model
to represent both, they do have ever so slightly different requirements to be valid: for normal albums, we must know the artist; for VA album tracks we must know the track artist. So it would be fair to say that these are in fact separate
representations
that should be added as resources. Finally let's add a collection of all albums so that clients can see what albums our API has data for.
  1. artist collection
  2. artist item
  3. all albums collection
  4. albums by artist collection
  5. VA albums collection
  6. album item (incorporates track collection)
  7. VA album item (incorpotes VA track collection)
  8. track item
  9. VA track item
Bonus consideration: Why are we incorporating track collection into album, but not incorporating album colletion into artist? Mostly because artist as a concept is more than a collection of albums. For example artist could also be a collection of people (band members). The API should state what it means explicitly, and therefore it is better to separate "artist" from "albums by artist".

Routing Resources

After identifying what's considered important enough (and different enough) to be regarded as its own
resource
, we now have to come up with
URIs
so that each can be uniquely identified (
addressability principle
). This also defines our URI hierarchy. We want the URIs to convey the relationships between our resources. For normal albums the hierarchy goes like this:
artist collection
└── artist
    └── album collection
        └── album
            └── track
We decided that album title paired with artist ID is sufficient for
uniqueness
. We also decided that the best way to uniquely identify a track is to use its position an the album as an index consisting of disc and track numbers. Taking all this into account, we end up with a route that looks like this:
/api/artists/{artist_unique_name}/albums/{album_title}/{disc}/{track}/
This uniquely identifies each track, and also clearly shows the hierarchy. All the intermediate resources (both collections and items) can be found by dropping off parts from the end. We will separate VA albums from the rest by using VA to replace {artist}, ending up with this route to identify each VA track:
/api/artists/VA/albums/{album_title}/{disc}/{track}/
Then we need to add one more separate branch to the URI tree for the collection that shows all albums:
/api/albums/
The entire URI tree becomes:
api
├── artists
│   ├── {artist}
│   │   └── albums
│   │       └── {album}
│   │           └── {disc}
│   │               └── {track}
│   └── VA
│       └── albums
│           └── {album}
│               └── {disc}
│                   └── {track}
└── albums

Resource Actions

Following REST principles our API should offer actions as
HTTP methods
targeted at resources. To reiterate, each HTTP method should be used as follows:
Most resources should therefore implement GET. Collection types usually implement POST whereas PUT and DELETE are typically attached to individual items. In our case we make two exceptions: first, as album serves as both an item and as a collection, it actually implements all four; second, the albums resource at the bottom of the URI tree above should not provide POST because there is no way of knowing from the URI which artist is the author. The parent of a new item should always be found from the URI - not in the
request body
. We're not using PATCH in this example.
Gathering everything into a table:
Resource URI GET POST PUT DELETE
artist collection /api/artists/ X X - -
artist item /api/artists/{artist}/ X - X X
albums by artist /api/artists/{artist}/albums/ X X - -
albums by VA /api/artists/VA/albums/ X X - -
all albums /api/albums/ X - - -
album /api/artists/{artist}/albums/{album}/ X X X X
VA album /api/artists/VA/albums/{album}/ X X X X
track /api/artists/{artist}/albums/{album}/{disc}/{track}/ X - X X
VA track /api/artists/VA/albums/{album}/{disc}/{track}/ X - X X
Since we are following the
uniform interface
REST principle and each HTTP method does what it's expected to, this table actually tells a lot about our API: it shows every possible
HTTP
request that can be made and even hints at their meaning: if you send a PUT request to a track resource, it will modify the track's data (even more specifically it will replace all data with what's in the request body). It just doesn't do a very good job of explaining what requests and responses should look like.

Data Representation

Our API communicates in
JSON
. There isn't a whole lot to data representation really, it's a rather straightforward
serialization
process from
model
instance attributes to JSON attributes. If the client sends a GET request to, say, /api/artists/scandal/ the data that is returned would be serialized into this:
{
    "name": "Scandal",
    "unique_name": "scandal",
    "location": "Osaka, JP",
    "formed": "2006-08-21",
    "disbanded": null
}
Likewise if the client wants to add a new artist, they'd send almost an identical JSON document, sans unique_name because it is generated by the API server. A similar serialization process can be applied for all models. Collection type resources will have "items" attribute which is an array containing objects that are part of the collection. Most notably albums have both root level data about the album itself, and an array of tracks. It's also worth noting that collection types don't necessarily have to include all the data about their members. For example in album collections we have deemed it sufficient to show album title and artist name:
{
    "items": [
        {
            "artist": "Scandal",
            "title": "Hello World"
        },
    ]
}
If the client wants more information about the album, it can always send a GET to the album resource itself.

Conclusion

This sort example should have shown you how concepts in the design of an API first become database tables, and ultimately resources that can be offered through the API. In upcoming exercises this API will also be able to give instructions to clients via hypermedia, and we will also give an example of how a machine client can take advantage of it.
?
API Blueprint is a description language for REST APIs. Its primary categories are resources and their related actions (i.e. HTTP methods). It uses a relatively simple syntax. The advantage of using API Blueprint is the wide array of tools available. For example Apiary has a lot of features (interactive documentation, mockup server, test generation etc.) that can be utilized if the API is described in API Blueprint.
Another widely used alteranative for API Blueprint is OpenAPI.
Addressability is one of the key REST principles. It means that in an API everything should be presented as resources with URIs so that every possible action can be given an address. On the flipside this also means that every single address should always result in the same resource being accessed, with the same parameters. From the perspective of addressability, query parameters are part of the address.
Ajax is a common web technique. It used to be known as AJAX, an acronym for Asynchronous Javascript And XML but with JSON largely replacing XML, it become just Ajax. Ajax is used in web pages to make requests to the server without a page reload being triggered. These requests are asynchronous - the page script doesn't stop to wait for the response. Instead a callback is set to handle the response when it is received. Ajax can be used to make a request with any HTTP method.
  1. Kuvaus
  2. Examples
Anonymous functions are usually used as in-place functions to define a callback. They are named such because they are defined just like functions, but don't have a name. In JavaScript function definition returns the function as an object so that it can e.g. passed as an argument to another function. Generally they are used as one-off callbacks when it makes the code more readable to have the function defined where the callback is needed rather than somewhere else. A typical example is the forEach method of arrays. It takes a callback as its arguments and calls that function for each of its members. One downside of anonymous functions is that they function is defined anew every time, and this can cause significant overhead if performed constantly.
  1. Kuvaus
  2. Example
In Flask application context (app context for short) is an object that keeps tracks of application level data, e.g. configuration. You always need to have it when trying to manipulate the database etc. View functions will automatically have app context included, but if you want to manipulate the database or test functions from the interactive Python console, you need to obtain app context using a with statement.
Blueprint is a Flask feature, a way of grouping different parts of the web application in such a way that each part is registered as a blueprint with its own root URI. Typical example could be an admin blueprint for admin-related features, using the root URI /admin/. Inside a blueprint, are routes are defined relatively to this root, i.e. the route /users/ inside the admin blueprint would have the full route of /admin/users/.
Defines how data is processed in the application
Cross Origin Resource Sharing (CORS) is a relaxation mechanism for Same Origin Policy (SOP). Through CORS headers, servers can allow requests from external origins, what can be requested, and what headers can be included in those requests. If a server doesn't provide CORS headers, browsers will browsers will apply the SOP and refuse to make requests unless the origin is the same. Note that the primary purpose of CORS is to allow only certain trusted origins. Example scenario: a site with dubious script cannot just steal a user's API credentials from another site's cookies and make requests using them because the APIs CORS configuration doesn't allow requests from the site's origin. NOTE: this is not a mechanism to protect your API, it's to protect browser users from accessing your API unintentionally.
Callback is a function that is passed to another part of the program, usually as an argument, to be called when certain conditions are met. For instance in making Ajax requests, it's typical to register a callback for at least success and error situations. A typical feature of callbacks is that the function cannot decide its own parameters, and must instead make do with the arguments given by the part of the program that calls it. Callbacks are also called handlers. One-off callbacks are often defined as anonymous functions.
Piece of software that consumes or utilizes the functionality of a Web API. Some clients are controlled by humans, while others (e.g. crawlers, monitors, scripts, agents) have different degree of autonomy.
In databases, columns define the attributes of objects stored in a table. A column has a type, and can have additional properties such as being unique. If a row doesn't conform with the column types and other restrictions, it cannot be inserted into the table.
  1. Kuvaus
  2. Common keywords
In object relational mapping, column attributes are attributes in model classes that have been initialized as columns (e.g. in SQLAlchemy their initial value is obtained by initializing a Column). Each of these attributes corresponds to a column in the database table (that corresponds with the model class). A column attribute defines the column's type as well as additional properties (e.g. primary key).
  1. Kuvaus
  2. Example
In OpenAPI the components object is a storage for reusable components. Components inside this object can be referenced from other parts of the documentation. This makes it a good storage for any descriptions that pop up frequently, including path parameters, various schemas, and request body objects. This also includes security schemes if your API uses authentication.
Connectedness is a REST principle particularly related to hypermedia APIs. It states that there for each resource in the API, there must exist a path from every other resource to get there by following hypermedia links. Connectedness is easiest to analyze by creating an API state diagram.
  1. Kuvaus
  2. Example
A hypermedia control is an attribute in a resource representation that describes a possible action to the client. It can be a link to follow, or an action that manipulates the resource in some way. Regardless of the used hypermedia format, controls include at least the URI to use when performing the action. In Mason controls also include the HTTP method to use (if it's not GET), and can also include a schema that describes what's considered valid for the request body.
  1. Kuvaus
  2. Example
  3. Using
A (URL) converter is a piece of code used in web framework routing to convert a part of the URL into an argument that will be used in the view function. Simple converters are usually included in frameworks by default. Simple converters include things like turning number strings into integers etc. Typically custom converters are also supported. A common example would be turning a model instance's identifier in the URL to the identified model instance. This removes the boilerplate of fetching model instances from view functions, and also moves the handling of Not Found errors into the converter.
The term credentials is used in authentication to indicate the information that identifies you as a specific user from the system's point of view. By far the most common credentials is the combination of username and password. One primary goal of system security is the protection of credentials.
Document Object Model (DOM) is an interface through which Javascript code can interact with the HTML document. It's a tree structure that follows the HTML's hierarchy, and each HTML tag has its own node. Through DOM manipulation, Javascript code can insert new HTML into anywhere, modify its contents or remove it. Any modifications to the DOM are updated into the web page in real time. Do note that since this is a rendering operation, it's very likely one of the most costly operations your code can do. Therefore changing the entire contents of an element at once is better than changing it e.g. one line at a time.
Database schema is the "blueprint" of the database. It defines what tables are contained in the database, and what columns are in each table, and what additional attributes they have. A database's schema can be dumped into an SQL file, and a database can also be created from a schema file. When using object relational mapping (ORM), the schema is constructed from model classes.
  1. Kuvaus
  2. Example
Decorator is a function wrapper. Whenever the decorated function is called, its decorator(s) will be called first. Likewise, when the decorated function returns values, they will be first returned to the decorator(s). In essence, the decorator is wrapped around the decorated function. Decorators are particularly useful in web development frameworks because they can be inserted between the framework's routing machinery, and the business logic implemented in a view function. Decorators can do filtering and conversion for arguments and/or return values. They can also add conditions to calling the view function, like authentication where the decorator raises an error instead of calling the view function if valid credentials are not presented.
In HTML element refers to a single tag - most of the time including a closing tag and everything in between. The element's properties are defined by the tag, and any of the properties can be used to select that element from the document object model (DOM). Elements can contain other elements, which forms the HTML document's hierarchy.
For APIs entry point is the "landing page" of the API. It's typically in the API root of the URL hierarchy and contains logical first steps for a client to take when interacting with the API. This means it typically has one or more hypermedia controls which usually point to relevant collections in the API or search functions.
In software testing, a fixture is a component that satisfies the preconditions required by tests. In web application testing the most common role for fixtures is to initialize the database into a state that makes testing possible. This generally involves creating a fresh database, and possibly populating it with some data. In this course fixtures are implemented using pytest's fixture architecture.
  1. Kuvaus
  2. Creating DB
  3. Starting the App
This term contains basic instructions about setting up and running Flask applications. See the term tabs "Creating DB" and "Starting the App". For all instructions to work you need to be in the folder that contains your app.
In database terminology, foreign key means a column that has its value range determined by the values of a column in another table. They are used to create relationships between tables. The foreign key column in the target table must be unique.
For most hypermedia types, there exists a generic client. This is a client program that constructs a navigatable user interface based on hypermedia controls in the API, and can usually also generate data input forms. The ability to use such clients for testing and prototyping is one of the big advantages of hypermedia.
HTTP method is the "type" of an HTTP request, indicating what kind of an action the sender is intending to do. In web applications by far the most common method is GET which is used for retrieving data (i.e. HTML pages) from the server. The other method used in web applications is POST, used in submitting forms. However, in REST API use cases, PUT and DELETE methods are also commonly used to modify and delete data.
HTTP request is the entirety of the requets made by a client to a server using the HTTP protocol. It includes the request URL, request method (GET, POST etc.), headers and request body. In Python web frameworks the HTTP request is typically turned into a request object.
In computing a hash is a string that is calculated from another string or other data by an algorithm. Hashes have multiple uses ranging from encryption to encoding independent transmission. Hash algorithms can roughly be divided into one- and two-directional. One-directional hashing algorithms are not reversible - the original data cannot be calculated from the hash. They are commonly used to store passwords so that plain text passwords cannot be retrieved even if the database is compromised. Two-directional hashes can be reversed. A common example is the use of base64 to encode strings to use a limited set of characters from the ASCII range to ensure that different character encodings at various transmission nodes do not mess up the original data.
Headers are additional information fields included in HTTP requests and responses. Typical examples of headers are content-type and content-length which inform the receiver how the content should be interpreted, and how long it should be. In Flask headers are contained in the request.headers attribute that works like a dictionary.
Host part is the part of URL that indicates the server's address. For example, lovelace.oulu.fi is the host part. This part determines where (i.e. which IP address) in the world wide web the request is sent.
In API terminology hypermedia means additional information that is added on top of raw data in resource representations. It's derived from hypertext - the stuff that makes the world wide web tick. The purpose of the added hypermedia is to inform the client about actions that are available in relation to the resource they requested. When this information is conveyed in the representations sent by the API, the client doesn't need to know how to perform these actions beforehand - it only needs to parse them from the response.
An idempotent operation is an operation that, if applied multiple times with the same parameters, always has the same result regardless of how many times it's applied. If used properly, PUT is an idempotent operation: no matter how many times you replace the contents of a resource it will have the same contents as it would have if only one request had been made. On the other hand POST is usually not idempotent because it attempts to create a new resource with every request.
  1. Kuvaus
  2. Example
The info object in OpenAPI gives basic information about your API. This basic information includes general description, API version number, and contact information. Even more importantly, it includes license information and link to your terms of service.
Instance folder is a Flask feature. It is intended for storing files that are needed when running the Flask application, but should not be in the project's code repository. Primary example of this is the prodcution configuration file which differs from installation to installation, and generally should remain unchanged when the application code is updated from the repository. The instance path can be found from the application context: app.instance_path. Flask has a reasonable default for it, but it can also be set manually when calling Flask constuctor by adding the instance_path keyword argument. The path should be written as absolute in this case.
  1. Kuvaus
  2. Serializing / Parsing
JavaScript Object Notation (JSON) is a popular document format in web development. It's a serialized representation of a data structure. Although the representation syntax originates from JavaScript, It's almost identical to Python dictionaries and lists in formatting and structure. A JSON document conists of key-value pairs (similar to Python dictionaries) and arrays (similar to Python lists). It's often used in APIs, and also in AJAX calls on web sites.
JSON schema is a JSON document that defines the validity criteria for JSON documents that fall under the schema. It defines the type of the root object, and types as well as additional constraints for attributes, and which attributes are required. JSON schemas serve two purposes in this course: clients can use them to generate requests to create/modify resources, and they can also be used on the API end to validate incoming requests.
  1. Kuvaus
  2. Common MIME types
MIME type is a standard used for indicating the type of a document.In web development context it is placed in the Content-Type header. Browsers and servers the MIME type to determine how to process the request/response content. On this course the MIME type is in most cases application/json.
Database migration is a process where an existing database is updated with a new database schema. This is done in a way that does not lose data. Some changes can be migrated automatically. These include creation of new tables, removal of columns and adding nullable columns. Other changes often require a migration script that does the change in multiple steps so that old data can be transformed to fit the new schema. E.g. adding a non-nullable column usually involves adding it first as nullable, then using a piece of code to determine values for each row, and finally setting the column to non-nullable.
  1. Kuvaus
  2. Example
In ORM terminology, a model class is a program level class that represents a database table. Instances of the class represent rows in the table. Creation and modification operations are performed using the class and instances. Model classes typically share a common parent (e.g. db.Model) and table columns are defined as class attributes with special constuctors (e.g. db.Column).
  1. Kuvaus
  2. Example
In API terminology, namespace is a prefix for names used by the API that makes them unique. The namespace should be a URI, but it doesn't have to be a real address. However, usually it is convenient to place a document that described the names within the namespace into the namespace URI. For our purposes, namespace contains the custom link relations used by the API.
Object relational mapping is a way of abstracting database use. Database tables are mapped to programming language classes. These are usually called models. A model class declaration defines the table's structure. When rows from the database table are fetched, they are represented as instances of the model class with columns as attributes. Likewise new rows are created by making new instances of the model class and committing them to the database. This course uses SQLAlchemy's ORM engine.
OpenAPI (previously: Swagger) is a description language for API documentation. It can be written with either JSON or YAML. An OpenAPI document is a single nested data structure which makes it suitable to be used with various tools. For example, Swagger UI is a basic tool that renders an OpenAPI description into a browsable documentation page. Other kinds of tools include using schemas in OpenAPI description for validation, and generating OpenAPI specification from live code.
  1. Kuvaus
  2. Example
Operation object is one of the main parts of an OpenAPI specification. It describes one operation on a resource (e.g. GET). The operation object includes full details of how to perform the operation, and what kinds of responses can be expected from it. Two of its key parameters are requestBody which shows how to make the request, and responses, which is a mapping of potential responses.
With Flasgger, an operation object can be put into a view method's docstring, or a separate file, to document that particular view method.
Pagination divides a larger dataset into smaller subsets called pages. Search engine results would be the most common example. You usually get 10 or 20 first hits from you search, and then have to request the next page in order to get more. The purpose of pagination is to avoid transferring (and rendering) unnecessary data, and it is particularly useful in scenarios where the relevance of data declines rapidly (like search results where the accuracy drops the further you go). An API that offers paginated data will typically offer access to specific pages with both absolute (i.e. page number) and relative (e.g. "next", "prev", "first" etc.) URLs. These are usually implemented through query parameters.
In OpanAPI a path parameter is a variable placeholder in a path. It is the OpenAPI equivalent for URL parameters that we use in routing. Path parameter typically has a description and a schema that defines what is considered valid for its value. These parameter definitions are often placed into the components object as they will be used in multiple resources. In OpenAPI syntax path parameters in paths are marked with curly braces, e.g. /api/sensors/{sensor}/.
In database terminology primary key refers to the column in a table that's intended to be the primary way of identifying rows. Each table must have exactly one, and it needs to be unique. This is usually some kind of a unique identifier associated with objects presented by the table, or if such an identifier doesn't exist simply a running ID number (which is incremented automatically).
Profile is metadata about a resource. It's a document intended for client developers. A profile gives meaning to each word used in the resource representation be it link relation or data attribute (also known as semantic descriptors). With the help of profiles, client developers can teach machine clients to understand resource representations sent by the API. Note that profiles are not part of the API and are usually served as static HTML documents. Resource representations should always contain a link to their profile.
In database terminology, query is a command sent to the database that can fetch or alter data in the database. Queries use written with a script-like language. Most common is the structured query language (SQL). In object relational mapping, queries are abstracted behind Python method calls.
  1. Kuvaus
  2. Example
Query parameters are additional parameters that are included in a URL. You can often see these in web searches. They are the primary mechanism of passing arbitrary parameters with an HTTP request. They are separated from the actual address by ?. Each parameter is written as a key=value pair, and they are separated from each other by &. In Flask applications they can be found from request.args which works like a dictionary.
  1. Kuvaus
  2. Examples
Regular expressions are used in computing to define matching patterns for strings. In this course they are primarily used in validation of route variables, and in JSON schemas. Typical features of regular expressions are that they look like a string of garbage letters and get easily out of hand if you need to match something complex. They are also widely used in Lovelace text field exercises to match correct (and incorrect) answers.
In this course request referes to HTTP request. It's a request sent by a client to an HTTP server. It consists of the requested URL which identifies the resource the client wants to access, a method describing what it wants to do with the resource. Requests also include headers which provide further context information, and possihby a request body that can contain e.g. a file to upload.
  1. Kuvaus
  2. Accessing
In an HTTP request, the request body is the actual content of the request. For example when uploading a file, the file's contents would be contained within the request body. When working with APIs, request body usually contains a JSON document. Request body is mostly used with POST, PUT and PATCH requests.
  1. Kuvaus
  2. Getting data
Request object is related to web development frameworks. It's a programming language object representation of the HTTP request made to the server. It has attributes that contain all the information contained within the request, e.g. method, url, headers, request body. In Flask the object can be imported from Flask to make it globally available.
in RESTful API terminology, a resource is anything that is interesting enough that a client might want to access it. A resource is a representation of data that is stored in the API. While they usually represent data from the database tables it is important to understand that they do not have a one-to-one mapping to database tables. A resource can combine data from multiple tables, and there can be multiple representations of a single table. Also things like searches are seen as resources (it does, after all, return a filtered representation of data).
Resource classes are introduced in Flask-RESTful for implementing resources. They are inherited from flask_restful.Resource. A resource class has a view-like method for each HTTP method supported by the resource (method names are written in lowercase). Resources are routed through api.add_resource which routes all of the methods to the same URI (in accordance to REST principles). As a consequence, all methods must also have the same parameters.
In this course we use the term representation to emphasize that a resource is, in fact, a representation of something stored in the API server. In particular you can consider representation to mean the response sent by the API when it receives a GET request. This representation contains not only data but also hypermedia controls which describe the actions available to the client.
In this course response refers to HTTP response, the response given by an HTTP server when a request is made to it. Reponses are made of a status code, headers and (optionally) response body. Status code describes the result of the transaction (success, error, something else). Headers provide context information, and response body contains the document (e.g. HTML document) returned by the server.
Response body is the part of HTTP response that contains the actual data sent by the server. The body will be either text or binary, and this information with additional type instructions (e.g. JSON) are defined by the response's Content-type header. Only GET requests are expected to return a response body on a successful request.
Response object is the client side counterpart of request object. It is mainly used in testing: the Flask test client returns a response object when it makes a "request" to the server. The response object has various attributes that represent different parts of an actual HTTP response. Most important are usually status_code and data.
In database terminology, rollback is the cancellation of a database transaction by returning the database to a previous (stable) state. Rollbacks are generally needed if a transaction puts the database in an error state. On this course rollbacks are generally used in testing after deliberately causing errors.
  1. Kuvaus
  2. Routing in Flask
  3. Reverse routing
  4. Flask-RESTful routing
URL routing in web frameworks is the process in which the framework transforms the URL from an HTTP request into a Python function call. When routing, a URL is matched against a sequence of URL templates defined by the web application. The request is routed to the function registered for the first matching URL template. Any variables defined in the template are passed to the function as parameters.
In relational database terminology, row refers to a single member of table, i.e. one object with properties that are defined by the table's columns. Rows must be uniquely identifiable by at least one column (the table's primary key).
SQL (structured query language) is a family of languages that are used for interacting with databases. Queries typically involve selecting a range of data from one or more tables, and defining an operation to perform to it (such as retrieve the contents).
Serialization is a common term in computer science. It's a process through which data structures from a program are turned into a format that can be saved on the hard drive or sent over the network. Serialization is a reversible process - it should be possible to restore the data structure from the representation. A very common serialization method in web development is JSON.
In web applications static content refers to content that is served from static files in the web server's hard drive (or in bigger installations from a separate media server). This includes images as well as javascript files. Also HTML files that are not generated from templates are static content.
Swagger is a set of tools for making API documentation easier. In this course we use it primarily to render easily browsable online documentation from OpenAPI description source files. Swagger open source tools also allow you to run mockup servers from your API description, and there is a Swagger editor where you can easily see the results of changes to your OpenAPI description in the live preview.
In this course we use Flasgger, a Swagger Flask extension, to take render API documentation.
In database terminology, a table is a collection of similar items. The attributes of those items are defined by the table's columns that are declared when the table is created. Each item in a table is contained in a row.
In software testing, test setup is a procedure that is undertaken before each test case. It prepares preconditions for the test. On this course this is done with pytest's fixtures.
In software testing, test teardown is a process that is undertaken after each test case. Generally this involves clearing up the database (e.g. dropping all tables) and closing file descriptors, socket connections etc. On this course pytest fixtures are used for this purpose.
Universal resource identifier (URI) is basically what the name says: it's a string that unambiguously identifies a resource, thereby making it addressable. In APIs everything that is interesting enough is given its own URI. URLs are URIs that specify the exact location where to find the resource which means including protocol (http) and server part (e.g. lovelace.oulu.fi) in addition to the part that identifies the resource within the server (e.g. /ohjelmoitava-web/programmable-web-project-spring-2019).
  1. Kuvaus
  2. Type converters
  3. Custom converters
URL template defines a range of possible URLs that all lead to the same view function by defining variables. While it's possible for these variables to take arbitrary values, they are more commonly used to select one object from a group of similar objects, i.e. one user's profile from all the user profiles in the web service (in Flask: /profile/<username>. If a matching object doesn't exist, the default response would be 404 Not Found. When using a web framework, variables in the URL template are usually passed to the corresponding view function as arguments.
Uniform interface is a REST principle which states that all HTTP methods, which are the verbs of the API, should always behave in the same standardized way. In summary:
  • GET - should return a representation of the resource; does not modify anything
  • POST - should create a new instance that belongs to the target collection
  • PUT - should replace the target resource with a new representation (usually only if it exists)
  • DELETE - should delete the target resource
  • PATCH - should describe a change to the resource
In database terminology, unique constraint is a what ensures the uniqueness of each row in a table. Primary key automatically creates a unique constraint, as do unique columns. A unique constraint can also be a combination of columns so that each combination of values between these columns is unique. For example, page numbers by themselves are hardly unique as each book has a first page, but a combination of book and page number is unique - you can only have one first page in a book.
  1. Kuvaus
  2. Registering
View functions are Python functions (or methods) that are used for serving HTTP requests. In web applications that often means rendering a view (i.e. a web page). View functions are invoked from URLs by routing. A view function always has application context.
  1. Kuvaus
  2. Creation
  3. Activation
A Python virtual environment (virtualenv, venv) is a system for managing packages separately from the operating system's main Python installation. They help project dependency management in multiple ways. First of all, you can install specific versions of packages per project. Second, you can easily get a list of requirements for your project without any extra packages. Third, they can placed in directories owned by non-admin users so that those users can install the packages they need without admin privileges. The venv module which is in charge of creating virtual environments comes with newer versions of Python.
Interface, implemented using web technologies, that exposes a functionality in a remote machine (server). By extension Web API is the exposed functionality itself.
  1. Kuvaus
  2. Example
YAML (YAML Ain't Markup Language) is a human-readable data serialization language that uses a similar object based notation as JSON but removes a lot of the "clutter" that makes JSON hard to read. Like Python, YAML uses indentation to distinguish blocks from each other, although it also supports using braces for this purpose (which, curiously enough, makes JSON valid YAML). It also removes the use of quotation characters where possible. It is one of the options for writing OpenAPI descriptions, and the one we are using on this course.