Open Event API Server: Implementing FAQ Types

In the Open Event Server, there was a long standing request of the users to enable the event organisers to create a FAQ section.

The API of the FAQ section was implemented subsequently. The FAQ API allowed the user to specify the following request schema

{
 "data": {
   "type": "faq",
   "relationships": {
     "event": {
       "data": {
         "type": "event",
         "id": "1"
       }
     }
   },
   "attributes": {
     "question": "Sample Question",
     "answer": "Sample Answer"
   }
 }
}

 

But, what if the user wanted to group certain questions under a specific category. There was no solution in the FAQ API for that. So a new API, FAQ-Types was created.

Why make a separate API for it?

Another question that arose while designing the FAQ-Types API was whether it was necessary to add a separate API for it or not. Consider that a type attribute was simply added to the FAQ API itself. It would mean the client would have to specify the type of the FAQ record every time a new record is being created for the same. This would mean trusting that the user will always enter the same spelling for questions falling under the same type. The user cannot be trusted on this front. Thus the separate API made sure that the types remain controlled and multiple entries for the same type are not there.

Helps in handling large number of records:

Another concern was what if there were a large number of FAQ records under the same FAQ-Type. Entering the type for each of those questions would be cumbersome for the user. The FAQ-Type would also overcome this problem

Following is the request schema for the FAQ-Types API

{
 "data": {
   "attributes": {
     "name": "abc"
   },
   "type": "faq-type",
   "relationships": {
     "event": {
       "data": {
         "id": "1",
         "type": "event"
       }
     }
   }
 }
}

 

Additionally:

  • FAQ to FAQ-type is a many to one relation.
  • A single FAQ can only belong to one Type
  • The FAQ-type relationship will be optional, if the user wants different sections, he/she can add it ,if not, it’s the user’s choice.

Related links

Open Event Server: Getting The Identity From The Expired JWT Token In Flask-JWT

The Open Event Server uses JWT based authentication, where JWT stands for JSON Web Token. JSON Web Tokens are an open industry standard RFC 7519 method for representing claims securely between two parties. [source: https://jwt.io/]

Flask-JWT is being used for the JWT-based authentication in the project. Flask-JWT makes it easy to use JWT based authentication in flask, while on its core it still used PyJWT.

To get the identity when a JWT token is present in the request’s Authentication header , the current_identity proxy of Flask-JWT can be used as follows:

@app.route('/example')
@jwt_required()
def example():
   return '%s' % current_identity

 

Note that it will only be set in the context of function decorated by jwt_required(). The problem with the current_identity proxy when using jwt_required is that the token has to be active, the identity of an expired token cannot be fetched by this function.

So why not write a function on our own to do the same. A JWT token is divided into three segments. JSON Web Tokens consist of three parts separated by dots (.), which are:

  • Header
  • Payload
  • Signature

The first step would be to get the payload, that can be done as follows:

token_second_segment = _default_request_handler().split('.')[1]

 

The payload obtained above would still be in form of JSON, it can be converted into a dict as follows:

payload = json.loads(token_second_segment.decode('base64'))

 

The identity can now be found in the payload as payload[‘identity’]. We can get the actual user from the paylaod as follows:

def jwt_identity(payload):
   """
   Jwt helper function
   :param payload:
   :return:
   """
   return User.query.get(payload['identity'])

 

Our final function will now be something like:

def get_identity():
   """
   To be used only if identity for expired tokens is required, otherwise use current_identity from flask_jwt
   :return:
   """
   token_second_segment = _default_request_handler().split('.')[1]
   missing_padding = len(token_second_segment) % 4
   payload = json.loads(token_second_segment.decode('base64'))
   user = jwt_identity(payload)
   return user

 

But after using this function for sometime, you will notice that for certain tokens, the system will raise an error saying that the JWT token is missing padding. The JWT payload is base64 encoded, and it requires the payload string to be a multiple of four. If the string is not a multiple of four, the remaining spaces can pe padded with extra =(equal to) signs. And since Python 2.7’s .decode doesn’t do that by default, we can accomplish that as follows:

missing_padding = len(token_second_segment) % 4

# ensures the string is correctly padded to be a multiple of 4
if missing_padding != 0:
   token_second_segment += b'=' * (4 - missing_padding)

 

Related links:

Image Loading in Open Event Organizer Android App using Glide

Open Event Organizer is an Android App for the Event Organizers and Entry Managers. Open Event API Server acts as a backend for this App. The core feature of the App is to scan a QR code from the ticket to validate an attendee’s check in. Other features of the App are to display an overview of sales and ticket management. As per the functionality, the performance of the App is very important. The App should be functional even on a weak network. Talking about the performance, the image loading part in the app should be handled efficiently as it is not an essential part of the functionality of the App. Open Event Organizer uses Glide, a fast and efficient image loading library created by Sam Judd. I will be talking about its implementation in the App in this blog.

First part is the configuration of the glide in the App. The library provides a very easy way to do that. Your app needs to implement a class named AppGlideModule using annotations provided by the library and it generates a glide API which can be used in the app for all the image loading stuff. The AppGlideModule implementation in the Orga App looks like:

@GlideModule
public final class GlideAPI extends AppGlideModule {

   @Override
   public void registerComponents(Context context, Glide glide, Registry registry) {
       registry.replace(GlideUrl.class, InputStream.class, new OkHttpUrlLoader.Factory());
   }

   // TODO: Modify the options here according to the need
   @Override
   public void applyOptions(Context context, GlideBuilder builder) {
       int diskCacheSizeBytes = 1024 * 1024 * 10; // 10mb
       builder.setDiskCache(new InternalCacheDiskCacheFactory(context, diskCacheSizeBytes));
   }

   @Override
   public boolean isManifestParsingEnabled() {
       return false;
   }
}

 

This generates the API named GlideApp by default in the same package which can be used in the whole app. Just make sure to add the annotation @GlideModule to this implementation which is used to find this class in the app. The second part is using the generated API GlideApp in the app to load images using URLs. Orga App uses data binding for layouts. So all the image loading related code is placed at a single place in DataBinding class which is used by the layouts. The class has a method named setGlideImage which takes an image view, an image URL, a placeholder drawable and a transformation. The relevant code is:

private static void setGlideImage(ImageView imageView, String url, Drawable drawable, Transformation<Bitmap> transformation) {
       if (TextUtils.isEmpty(url)) {
           if (drawable != null)
               imageView.setImageDrawable(drawable);
           return;
       }
       GlideRequest<Drawable> request = GlideApp
           .with(imageView.getContext())
           .load(Uri.parse(url));

       if (drawable != null) {
           request
               .placeholder(drawable)
               .error(drawable);
       }
       request
           .centerCrop()
           .transition(withCrossFade())
           .transform(transformation == null ? new CenterCrop() : transformation)
           .into(imageView);
   }

 

The method is very clear. First, the URL is checked for nullability. If null, the drawable is set to the imageview and method returns. Usage of GlideApp is simpler. Pass the URL to the GlideApp using the method with which returns a GlideRequest which has operators to set other required options like transitions, transformations, placeholder etc. Lastly, pass the imageview using into operator. By default, Glide uses HttpURLConnection provided by android to load the image which can be changed to use Okhttp using the extension provided by the library. This is set in the AppGlideModule implementation in the registerComponents method.

Links:
1. Documentation for Glide, an Image Loading Library
2. Documentation for Okhttp, an HTTP client for Android and Java Applications

Checking Whether Migrations Are Up To Date With The Sqlalchemy Models In The Open Event Server

In the Open Event Server, in the pull requests, if there is some change in the sqlalchemy model, sometimes proper migrations for the same are missed in the PR.

The first approach to check whether the migrations were up to date in the database was with the following health check function:

from subprocess import check_output
def health_check_migrations():
   """
   Checks whether database is up to date with migrations, assumes there is a single migration head
   :return:
   """
   head = check_output(["python", "manage.py", "db", "heads"]).split(" ")[0]
   
   if head == version_num:
       return True, 'database up to date with migrations'
   return False, 'database out of date with migrations'

 

In the above function, we get the head according to the migration files as following:

head = check_output(["python", "manage.py", "db", "heads"]).split(" ")[0]


The table alembic_version contains the latest alembic revision to which the database was actually upgraded. We can get this revision from the following line:

version_num = (db.session.execute('SELECT version_num from alembic_version').fetchone())['version_num']

 

Then we compare both of the given heads and return a proper tuple based on the comparison output.While this method was pretty fast, there was a drawback in this approach. If the user forgets to generate the migration files for the the changes done in the sqlalchemy model, this approach will fail to raise a failure status in the health check.

To overcome this drawback, all the sqlalchemy models were fetched automatically and simple sqlalchemy select queries were made to check whether the migrations were up to date.

Remember that a raw SQL query will not serve our purpose in this case as you’d have to specify the columns explicitly in the query. But in the case of a sqlalchemy query, it generates a SQL query based on the fields defined in the db model, so if migrations are missing to incorporate the said change proper error will be raised.

We can accomplish this from the following function:

def health_check_migrations():
   """
   Checks whether database is up to date with migrations by performing a select query on each model
   :return:
   """
   # Get all the models in the db, all models should have a explicit __tablename__
   classes, models, table_names = [], [], []
   # noinspection PyProtectedMember
   for class_ in db.Model._decl_class_registry.values():
       try:
           table_names.append(class_.__tablename__)
           classes.append(class_)
       except:
           pass
   for table in db.metadata.tables.items():
       if table[0] in table_names:
           models.append(classes[table_names.index(table[0])])

   for model in models:
       try:
           db.session.query(model).first()
       except:
           return False, '{} model out of date with migrations'.format(model)
   return True, 'database up to date with migrations'

 

In the above code, we automatically get all the models and tables present in the database. Then for each model we try a simple SELECT query which returns the first row found. If there is any error in doing so, False, ‘{} model out of date with migrations’.format(model) is returned, so as to ensure a failure status in health checks.

Related:

Implementing Health Check Endpoint in Open Event Server

A health check endpoint was required in the Open Event Server be used by Kubernetes to know when the web instance is ready to receive requests.

Following are the checks that were our primary focus for health checks:

  • Connection to the database.
  • Ensure sql-alchemy models are inline with the migrations.
  • Connection to celery workers.
  • Connection to redis instance.

Runscope/healthcheck seemed like the way to go for the same. Healthcheck wraps a Flask app object and adds a way to write simple health-check functions that can be used to monitor your application. It’s useful for asserting that your dependencies are up and running and your application can respond to HTTP requests. The Healthcheck functions are exposed via a user defined flask route so you can use an external monitoring application (monit, nagios, Runscope, etc.) to check the status and uptime of your application.

Health check endpoint was implemented at /health-check as following:

from healthcheck import HealthCheck
health = HealthCheck(current_app, "/health-check")

 

Following is the function for checking the connection to the database:

def health_check_db():
   """
   Check health status of db
   :return:
   """
   try:
       db.session.execute('SELECT 1')
       return True, 'database ok'
   except:
       sentry.captureException()
       return False, 'Error connecting to database'

 

Check functions take no arguments and should return a tuple of (bool, str). The boolean is whether or not the check passed. The message is any string or output that should be rendered for this check. Useful for error messages/debugging.

The above function executes a query on the database to check whether it is connected properly. If the query runs successfully, it returns a tuple True, ‘database ok’. sentry.captureException() makes sure that the sentry instance receives a proper exception event with all the information about the exception. If there is an error connecting to the database, the exception will be thrown. The tuple returned in this case will be return False, ‘Error connecting to database’.

Finally to add this to the endpoint:

health.add_check(health_check_db)

Following is the response for a successful health check:

{
   "status": "success",
   "timestamp": 1500915121.52474,
   "hostname": "shubham",
   "results": [
       {
           "output": "database ok",
           "checker": "health_check_db",
           "expires": 1500915148.524729,
           "passed": true,
           "timestamp": 1500915121.524729
       }
   ]
}

If the database is not connected the following error will be shown:

{
           "output": "Error connecting to database",
           "checker": "health_check_db",
           "expires": 1500965798.307425,
           "passed": false,
           "timestamp": 1500965789.307425
}

Related:

Supporting Dasherized Attributes and Query Params in flask-rest jsonapi for Open Event Server

In the Open Event API Server project attributes of the API are dasherized.

What was the need for dasherizing the attributes in the API ?

All the attributes in our database models are separated by underscores i.e first name would be stored as first_name. But most of the API client implementations support dasherized attributes by default. In order to attract third party client implementations in the future and making the API easy to set up for them was the primary reason behind this decision.Also to quote the official json-api spec recommendation for the same:

Member names SHOULD contain only the characters “a-z” (U+0061 to U+007A), “0-9” (U+0030 to U+0039), and the hyphen minus (U+002D HYPHEN-MINUS, “-“) as separator between multiple words.

Note: The dasherized version for first_name will be first-name.

flask-rest-jsonapi is the API framework used by the project. We were able to dasherize the API responses and requests by adding inflect=dasherize to each API schema, where dasherize is the following function:

def dasherize(text):
   return text.replace('_', '-')

 

flask-rest-jsonapi also provides powerful features like the following through query params:

But we observed that the query params were not being dasherized which rendered the above awesome features useless 🙁 . The reason for this was that flask-rest-jsonapi took the query params as-is and search for them in the API schema. As Python variable names cannot contain a dash, naming the attributes with a dash in the internal API schema was out of the question.

For adding dasherizing support to the query params, change in the QueryStringManager located at querystring.py of the framework root are required. A config variable named DASHERIZE_APIwas added to turn this feature on and off.

Following are the changes required for dasherizing query params:

For Sparse Fieldsets in the fields function, replace the following line:

result[key] = [value]
with
if current_app.config['DASHERIZE_API'] is True:
    result[key] = [value.replace('-', '_')]
else:
    result[key] = [value]

 

For sorting, in the sorting function, replace the following line:

field = sort_field.replace('-', '')

with

if current_app.config['DASHERIZE_API'] is True:
   field = sort_field[0].replace('-', '') + sort_field[1:].replace('-', '_')
else:
   field = sort_field[0].replace('-', '') + sort_field[1:]

 

For Include related objects, in include function, replace the following line:

return include_param.split(',') if include_param else []

with

if include_param:
   param_results = []
   for param in include_param.split(','):
       if current_app.config['DASHERIZE_API'] is True:
           param = param.replace('-', '_')
       param_results.append(param)
   return param_results
return []

Related links:

Documenting Open Event API Using API-Blueprint

FOSSASIA‘s Open Event Server API documentation is done using an api-blueprint. The API Blueprint language is a format used to describe API in an API blueprint file, where a blueprint file (or a set of files) is such that describes an API using the API Blueprint language. To follow up with the blueprint, an apiary editor is used. This editor is responsible for rendering the API blueprint and printing the result in user readable API documented format. We create the API blueprint manually.

Using API Blueprint:-
We create the API blueprint by first adding the name and metadata for the API we aim to design. This step looks like this :-

FORMAT: V1
HOST: https://api.eventyay.com

# Open Event API Server

The Open Event API Server

# Group Authentication

The API uses JWT Authentication to authenticate users to the server. For authentication, you need to be a registered user. Once you have registered yourself as an user, you can send a request to get the access_token.This access_token you need to then use in Authorization header while sending a request in the following manner: `Authorization: JWT <access_token>`


API blueprint starts with the metadata, here FORMAT and HOST are defined metadata. FORMAT keyword specifies the version of API Blueprint . HOST defines the host for the API.

The heading starts with # and the first heading is regarded as the name of the API.

NOTE – Also all the heading starts with one or more # symbol. Each symbol indicates the level of the heading. One # symbol followed by heading serves as the top level i.e. one # = Top Level. Similarly for  ## = second level and so on. This is in compliance with normal markdown format.
        Following the heading section comes the description of the API. Further, headings are used to break up the description section.

Resource Groups:
—————————–
    By using group keyword at the starting of a heading , we create a group of related resources. Just like in below screenshot we have created a Group Users.

# Group Users

For using the API you need(mostly) to register as an user. Registering gives you access to all non admin API endpoints. After registration, you need to create your JWT access token to send requests to the API endpoints.


| Parameter | Description | Type | Required |
|:----------|-------------|------|----------|
| `name`  | Name of the user | string | - |
| `password` | Password of the user | string | **yes** |
| `email` | Email of the user | string | **yes** |

 

Resources:
——————
    In the Group Users we have created a resource Users Collection. The heading specifies the URI used to access the resource inside of the square brackets after the heading. We have used here parameters for the resource URI which takes us into how to add parameters to the URI. Below code shows us how to add parameters to the resource URI.

## Users Collection [/v1/users{?page%5bsize%5d,page%5bnumber%5d,sort,filter}]
+ Parameters
    + page%5bsize%5d (optional, integer, `10`) - Maximum number of resources in a single paginated response.
    + page%5bnumber%5d (optional, integer, `2`) - Page number to fetchedfor the paginated response.
    + sort (optional, string, `email`) - Sort the resources according to the given attribute in ascending order. Append '-' to sort in descending order.
    + filter(optional, string, ``) - Filter according to the flask-rest-jsonapi filtering system. Please refer: http://flask-rest-jsonapi.readthedocs.io/en/latest/filtering.html for more.

 

Actions:
————–
    An action is specified with a sub-heading inside of  a resource as the name of Action, followed by HTTP method inside the square brackets.
    Before we get on further, let us discuss what a payload is. A payload is an HTTP transaction message including its discussion and any additional assets such as entity-body validation schema.

There are two payloads inside an Action:

  1. Request: It is a payload containing one specific HTTP Request, with Headers and an optional body.
  2. Response: It is a payload containing one HTTP Response.

A payload may have an identifier-a string for a request payload or an HTTP status code for a response payload.

+ Request

    + Headers

            Accept: application/vnd.api+json

            Authorization: JWT <Auth Key>

+ Response 200 (application/vnd.api+json)


Types of HTTP methods for Actions:

  • GET – In this action, we simply send the header data like Accept and Authorization and no body. Along with it we can send some GET parameters like page[size]. There are two cases for GET: List and Detail. For example, if we consider users, a GET for List helps us retrieve information about all users in the response, while Details, helps us retrieve information about a particular user.

The API Blueprint examples implementation of both GET list and detail request and response are as follows.

### List All Users [GET]
Get a list of Users.

+ Request

    + Headers

            Accept: application/vnd.api+json

            Authorization: JWT <Auth Key>

+ Response 200 (application/vnd.api+json)

        {
            "meta": {
                "count": 2
            },
            "data": [
                {
                    "attributes": {
                        "is-admin": true,
                        "last-name": null,
                        "instagram-url": null,

 

### Get Details [GET]
Get a single user.

+ Request

    + Headers

            Accept: application/vnd.api+json

            Authorization: JWT <Auth Key>

+ Response 200 (application/vnd.api+json)

        {
            "data": {
                "attributes": {
                    "is-admin": false,
                    "last-name": "Doe",
                    "instagram-url": "http://instagram.com/instagram",

 

  • POST – In this action, apart from the header information, we also need to send a data. The data must be correct with jsonapi specifications. A POST body data for an users API would look something like this:
### Create User [POST]
Create a new user using an email, password and an optional name.

+ Request (application/vnd.api+json)

    + Headers

            Authorization: JWT <Auth Key>

    + Body

            {
              "data":
              {
                "attributes":
                {
                  "email": "[email protected]",
                  "password": "password",


A POST request with this data, would create a new entry in the table and then return in jsonapi format the particular entry that was made into the table along with the id assigned to this new entry.

  • PATCH – In this action, we change or update an already existing entry in the database. So It has a header data like all other requests and a body data which is almost similar to POST except that it also needs to mention the id of the entry that we are trying to modify.
### Update User [PATCH]
+ `id` (integer) - ID of the record to update **(required)**

Update a single user by setting the email, email and/or name.

Authorized user should be same as user in request body or must be admin.

+ Request (application/vnd.api+json)

    + Headers

            Authorization: JWT <Auth Key>

    + Body

            {
              "data": {
                "attributes": {
                  "password": "password1",
                  "avatar_url": "http://example1.com/example1.png",
                  "first-name": "Jane",
                  "last-name": "Dough",
                  "details": "example1",
                  "contact": "example1",
                  "facebook-url": "http://facebook.com/facebook1",
                  "twitter-url": "http://twitter.com/twitter1",
                  "instagram-url": "http://instagram.com/instagram1",
                  "google-plus-url": "http://plus.google.com/plus.google1",
                  "thumbnail-image-url": "http://example1.com/example1.png",
                  "small-image-url": "http://example1.com/example1.png",
                  "icon-image-url": "http://example1.com/example1.png"
                },
                "type": "user",
                "id": "2"
              }
            }

Just like in POST, after we have updated our entry, we get back as response the new updated entry in the database.

  • DELETE – In this action, we delete an entry from the database. The entry in our case is soft deleted by default. Which means that instead of deleting it from the database, we set the deleted_at column with the time of deletion. For deleting we just need to send header data and send a DELETE request to the proper endpoint. If deleted successfully, we get a response as “Object successfully deleted”.
### Delete User [DELETE]
Delete a single user.

+ Request

    + Headers

            Accept: application/vnd.api+json

            Authorization: JWT <Auth Key>

+ Response 200 (application/vnd.api+json)

        {
          "meta": {
            "message": "Object successfully deleted"
          },
          "jsonapi": {
            "version": "1.0"
          }
        }


How to check after manually entering all these? We can use the
apiary website to render it, or simply use different renderer to do it. How? Checkout for my next blog on apiary and aglio.

Learn more about api blueprint here: https://apiblueprint.org/

Open Event Server: No (no-wrap) Ellipsis using jquery!

Yes, the title says it all i.e., Enabling multiple line ellipsis. This was used to solve an issue to keep Session abstract view within 200 characters (#3059) on FOSSASIA‘s Open Event Server project.

There is this one way to ellipsis a paragraph in html-css and that is by using the text-overflow property:

.div_class{
white-space: nowrap;
overflow: hidden;
text-overflow: ellipsis;
}’’

But the downside of this is the one line ellipis. Eg: My name is Medozonuo. I am…..

And here you might pretty much want to ellipsis after a few characters in multiple lines, given that your div space is small and you do want to wrap your paragraph. Or maybe not.

So jquery to the rescue.

There are two ways you can easily do this multiple line ellipsis:

1) Height-Ellipsis (Using the do-while loop):

//script:
if ($('.div_class').height() > 100) {
    var words = $('.div_class').html().split(/\s+/);
    words.push('...');

    do {
        words.splice(-2, 1);
        $('.div_class').html( words.join(' ') );
    } while($('.div_class').height() > 100);
}

Here, you check for the div content’s height and split the paragraph after that certain height and add a “…”, do- while making sure that the paragraphs are in multiple lines and not in one single line. But checkout for that infinite loop.

2) Length-Ellipsis (Using substring function):  

//script:
$.each($('.div_class'), function() {
        if ($(this).html().length > 100) {
               var cropped_words = $(this).html();
               cropped_words = cropped_words.substring(0, 200) + "...";
               $(this).html(cropped_words);
        }
 });

Here, you check for the length/characters rather than the height, take in the substring of the content starting from 0-th character to the 200-th character and then add in extra “…”.

This is exactly how I used it in the code.

$.each($('.short_abstract',function() {
   if ($(this).html().length > 200) {
       var  words = $(this).html();
       words = words.substring(0,200 + "...";
       $(this).html(words);
    }
});


So ellipsing paragraphs over heights and lengths can be done using jQuery likewise.

DetachedInstanceError: Dealing with Celery, Flask’s app context and SQLAlchemy in the Open Event Server

In the open event server project, we had chosen to go with celery for async background tasks. From the official website,

What is celery?

Celery is an asynchronous task queue/job queue based on distributed message passing.

What are tasks?

The execution units, called tasks, are executed concurrently on a single or more worker servers using multiprocessing.

After the tasks had been set up, an error constantly came up whenever a task was called

The error was:

DetachedInstanceError: Instance <User at 0x7f358a4e9550> is not bound to a Session; attribute refresh operation cannot proceed

The above error usually occurs when you try to access the session object after it has been closed. It may have been closed by an explicit session.close() call or after committing the session with session.commit().

The celery tasks in question were performing some database operations. So the first thought was that maybe these operations might be causing the error. To test this theory, the celery task was changed to :

@celery.task(name='lorem.ipsum')
def lorem_ipsum():
    pass

But sadly, the error still remained. This proves that the celery task was just fine and the session was being closed whenever the celery task was called. The method in which the celery task was being called was of the following form:

def restore_session(session_id):
    session = DataGetter.get_session(session_id)
    session.deleted_at = None
    lorem_ipsum.delay()
    save_to_db(session, "Session restored from Trash")
    update_version(session.event_id, False, 'sessions_ver')


In our app, the app_context was not being passed whenever a celery task was initiated. Thus, the celery task, whenever called, closed the previous app_context eventually closing the session along with it. The solution to this error would be to follow the pattern as suggested on http://flask.pocoo.org/docs/0.12/patterns/celery/.

def make_celery(app):
    celery = Celery(app.import_name, broker=app.config['CELERY_BROKER_URL'])
    celery.conf.update(app.config)
    task_base = celery.Task

    class ContextTask(task_base):
        abstract = True

        def __call__(self, *args, **kwargs):
            if current_app.config['TESTING']:
                with app.test_request_context():
                    return task_base.__call__(self, *args, **kwargs)
            with app.app_context():
                return task_base.__call__(self, *args, **kwargs)

    celery.Task = ContextTask
    return celery

celery = make_celery(current_app)


The __call__ method ensures that celery task is provided with proper app context to work with.

 

Event-driven programming in Flask with Blinker signals

Setting up blinker:

The Open Event Project offers event managers a platform to organize all kinds of events including concerts, conferences, summits and regular meetups. In the server part of the project, the issue at hand was to perform multiple tasks in background (we use celery for this) whenever some changes occurred within the event, or the speakers/sessions associated with the event.

The usual approach to this would be applying a function call after any relevant changes are made. But the statements making these changes were distributed all over the project at multiple places. It would be cumbersome to add 3-4 function calls (which are irrelevant to the function they are being executed) in so may places. Moreover, the code would get unstructured with this and it would be really hard to maintain this code over time.

That’s when signals came to our rescue. From Flask 0.6, there is integrated support for signalling in Flask, refer http://flask.pocoo.org/docs/latest/signals/ . The Blinker library is used here to implement signals. If you’re coming from some other language, signals are analogous to events.

Given below is the code to create named signals in a custom namespace:


from blinker import Namespace

event_signals = Namespace()
speakers_modified = event_signals.signal('event_json_modified')

If you want to emit a signal, you can do so by calling the send() method:


speakers_modified.send(current_app._get_current_object(), event_id=event.id, speaker_id=speaker.id)

From the user guide itself:

“ Try to always pick a good sender. If you have a class that is emitting a signal, pass self as sender. If you are emitting a signal from a random function, you can pass current_app._get_current_object() as sender. “

To subscribe to a signal, blinker provides neat decorator based signal subscriptions.


@speakers_modified.connect
def name_of_signal_handler(app, **kwargs):

 

Some Design Decisions:

When sending the signal, the signal may be sending lots of information, which your signal may or may not want. e.g when you have multiple subscribers listening to the same signal. Some of the information sent by the signal may not be of use to your specific function. Thus we decided to enforce the pattern below to ensure flexibility throughout the project.


@speakers_modified.connect
def new_handler(app, **kwargs):
# do whatever you want to do with kwargs['event_id']

In this case, the function new_handler needs to perform some task solely based on the event_id. If the function was of the form def new_handler(app, event_id), an error would be raised by the app. A big plus of this approach, if you want to send some more info with the signal, for the sake of example, if you also want to send speaker_name along with the signal, this pattern ensures that no error is raised by any of the subscribers defined before this change was made.

When to use signals and when not ?

The call to send a signal will of course be lying in another function itself. The signal and the function should be independent of each other. If the task done by any of the signal subscribers, even remotely affects your current function, a signal shouldn’t be used, use a function call instead.

How to turn off signals while testing?

When in testing mode, signals may slow down your testing as unnecessary signals subscribers which are completely independent from the function being tested will be executed numerous times. To turn off executing the signal subscribers, you have to make a small change in the send function of the blinker library.

Below is what we have done. The approach to turn it off may differ from project to project as the method of testing differs. Refer https://github.com/jek/blinker/blob/master/blinker/base.py#L241 for the original function.


def new_send(self, *sender, **kwargs):
    if len(sender) == 0:
        sender = None
    elif len(sender) > 1:
        raise TypeError('send() accepts only one positional argument, '
                        '%s given' % len(sender))
    else:
        sender = sender[0]
    # only this line was changed
    if not self.receivers or app.config['TESTING']:
        return []
    else:
        return [(receiver, receiver(sender, **kwargs))
                for receiver in self.receivers_for(sender)]
                
Signal.send = new_send

event_signals = Namespace
# and so on ....

That’s all for now. Have some fun signaling 😉 .