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1 Exercise 3: Wedding Planner
1.1 General Instructions
IMPORTANT NOTE: The submission deadline for this exercise is a THURSDAY (not a Friday!) on 02/MAY/2024 at 18:00, due to the holiday of the first of May.
This exercise is divided into two main parts. You may choose to develop the first part, referred
to as Session 1, during the initial laboratory session of the exercise. The second part, known as
Session 2, can be developed during the final lab session.
Before submitting your work, ensure that you test it on one of the lab machines. It’s important
to note that your code will be evaluated in the Kilburn’s Linux environment, so make sure it runs
successfully in this setting.
1.2 Introduction
In this section, we describe the problem you are supposed to solve, which represents the motivation
for the development of your SOA-based client-server application. Note that you are being asked to
use Web programming technologies (i.e., Web Services, the HTTP protocol and the JSON markup
language) in the development of your solutions to this exercise.
1.2.1 Problem Description
You need to arrange, for the earliest time possible, a wedding. You are required to make two
reservations — a hotel to host and a band to play at the reception. The aim of this exercise is for
you to experience the inherent difficulties in building distributed applications that need to cope
with concurrency, message delays and failures. Enjoy!
The hotel and the band both advertise slots 1 to 550 where slot 1 represents the earliest slot available while slot 550 represents the latest one.
As you need the band to play at the reception, the reservation for the hotel and the band must
match — that is, if you reserve slot i for the hotel then you must also reserve slot i for the band.
Note that the hotel and band restrict a user to hold at most two reservations at any one time.
Unlike the two previous exercises, for this exercise, you will not need to devise and implement a
protocol to allow your client application to communicate with the server being provided to you.
Instead, you will write your client using the HTTP protocol (for content exchange) and the JSON
language (for content description); your client will compete with other clients (your fellow classmates) to reserve the hotel and the band.
The hotel and the band both provide a reservation service on the Web. They receive JSON messages over HTTP and allow you to send in requests to reserve a slot, cancel a reservation, query
to find available slots and query to find slots reserved by you.
You will need to contend with the service being unavailable to receive your messages, with the fact
that both the hotel and the band may not process your messages in the order in which they are
received and delays before messages are processed. When your client tries to retrieve the results
from a request, the message may not be available so your client will need to try again later. The
services have been configured to randomly make themselves unavailable to incoming requests and
to introduce arbitrary delays in processing requests. However, once the server acknowledges the
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receipt of your message you may assume your message will not be lost.
Finally, your client should not overwhelm the server with requests and should never send more
than one request per second.
1.2.2 Getting Started
Get started by downloading the skeleton code from the Blackboard page of this course unit. And
putting them in a suitably named subdirectory of your ~/COMP28112 directory.
You should have the following files:
• api.ini: a basic configuration file
• exceptions.py: a Python module containing exceptions that are thrown by the ReservationApi.
• reservationapi.py : a simple reservation API wrapper.
• session1.py : an empty skeleton for your client code.
1.3 Session 1 Tasks
1.3.1 Task 3.1: Checking and reserving a slot
Write a simple client using Python3 to send a reservation request to the band (or the hotel).
You must understand ReservationApi (see reservationapi.py) before you start working
on your own code.
Before you can communicate with either the band or hotel APIs, you must obtain an authentication token for each system. These tokens are unique to you, and they are used to identify you in
API requests without you having to store or enter a username or password in your client code. To
obtain a token for the band API, you should open
https://web.cs.manchester.ac.uk/band/
in your web browser. When prompted to log in enter your University of Manchester username and
password. You will then be taken to a page that looks like the following:
Your API Key is shown on that page. All requests to the band API must include your API Key
in the request header field “Authorization” in the following format:
Authorization: Bearer
For example:
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Authorization: Bearer f872be7ee40294....87674b
This header must be created and returned by the ‘headers’ method defined within the
reservationapi.py file.
Note: You can not use the same API Key for the hotel API. You must obtain another API Key
for the hotel API by going to
https://web.cs.manchester.ac.uk/hotel/
in your web browser, logging in as before, and making a note of the API Key you are given by
that system.
Both APIs provide a Swagger interface through which you can try out API calls “by hand” and
this is accessible via the “Swagger” link in the menu bar at the top of both the band and hotel
API Admin Panels. The Swagger page also includes information about the expected arguments
for each API endpoint, and the possible response values you can receive from each endpoint.
To send a reservation request to the band you need to perform a HTTP POST1 operation message
to the URL
https://web.cs.manchester.ac.uk/band/api/reservation/{slotid}
Where {slotid} is the numeric ID (between 1 and 550) of the slot you wish to book. Similarly, if
you wish to send your requests to the hotel then send your POST request to
https://web.cs.manchester.ac.uk/hotel/api/reservation/{slotid}
As noted above, the message must include the “Authorization” header, otherwise you will receive
a permission error when making the request.
There are several possible responses from the server. Each response will contain a HTTP response
code, and a JSON response body containing information about the server’s response to your
request. For a list of defined http response codes see http://www.w3.org/Protocols/rfc2616/r
fc2616-sec10.html - only a subset of possible response codes will be generated by the service:
• If your request has been processed successfully, you will receive a 200 response code, and the
response body will contain JSON — see Listing 1.
• If the service is currently unavailable, you will receive a 503 response code and the response
body will contain JSON — see Listing 2.
• If your request is malformed, you will receive a 400 response code and the response body will
contain JSON — see Listing 3.
• If you have attempted to perform an API request with a missing or invalid token, you will
receive a 401 response code and the response body will contain JSON — see Listing 4.
• If you have requested a slot that does not exist, you will receive a 403 response code and the
response body will contain JSON— see Listing 5.
• If your request has not been processed for some reason, you will receive a 404 response code
and the response body will contain JSON— see Listing 6.
• If the slot you have requested is already held by another user, or is otherwise unavailable, you
will receive a 409 response code and the response body will contain JSON — see Listing 7.
1For more information on how to perform such operations in Python, check this link:
https://requests.readthedocs.io/en/master/
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• If you already hold two reservations on the system, you will receive a 451 response code and
the response body will contain JSON — see Listing 8.
• You may also receive a 500 response code if there has been a server error while processing
your request.
Listing 1: Successful Reservation
{ " i d " : " 5 6 " }
Listing 2: Service not available
{ " code " : 5 0 3 , " message " : " S e r v i c e u n a v ail a bl e " }
Listing 3: Bad request
{ " code " : 4 0 0 , " message " : " Bad r e q u e s t . " }
Listing 4: Invalid or missing token
{ " code " : 4 0 1 , " message " : " The API token was i n v a l i d o r mi s si n g . " }
Listing 5: Slot does not exist
{ " code " : 4 0 3 , " message " : " Sl o t I d d oe s not e x i s t . " }
Listing 6: Request not processed
{ " code " : 4 0 4 , " message " : `The r e q u e s t has not been p r o c e s s e d ' }
Listing 7: Slot is not free
{ " code " : 4 0 9 , " message " : " Sl o t i s not a v a i l a b l e . " }
Listing 8: Maximum permitted number of reservations
{ " code " : 4 5 1 , " message " : " The c l i e n t al r e a d y h ol d s the maximum
number o f r e s e r v a t i o n s . " }
1.3.2 Task 3.2: Cancelling a reservation
Extend your client to request the cancellation of a reservation. This is done by sending a DELETE
request to the URL
https://web.cs.manchester.ac.uk/band/api/reservation/{slotid}
Where {slotid} is the numeric ID (between 1 and 550) of the slot you wish to cancel your reservation for. Similarly, if you wish to send your cancellation requests to the hotel then send your
POST request to
https://web.cs.manchester.ac.uk/hotel/api/reservation/{slotid}
You will receive the same set of responses as before. However, the result of processing your request
is a little different to before. The possible responses are:
• The reservation has been cancelled — code 200;
• The cancellation failed as the slot was not reserved by you — code: 409;
• The cancellation failed as the username password was invalid — code: 401.
• The cancellation failed because the slot does not exist, code 403;
• You may also receive 500, 503, or 400 errors.
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1.3.3 Task 3.3: Finding free slots
Extend your client to find free slots. This time, send a GET request to the URL
https://web.cs.manchester.ac.uk/band/api/reservation/available
As before, if you want to find the free slots available for the hotel, you should use the same endpoint
for the hotel API by sending a GET request to
https://web.cs.manchester.ac.uk/hotel/api/reservation/available
A successful response from this endpoint will contain a JSON array representing the available slots
in the system, an example of which is shown in Listing 9.
Listing 9: Body of a 200 response
[ { " i d " : " 1 0 " } ,
{ " i d " : " 1 5 " } ,
{ " i d " : " 1 7 " } ,
{ " i d " : " 2 2 " } ]
As with the previously discussed endpoints, you may receive several response codes in response to
your request, and you should check whether the response you get is a 200 code or not, and take
appropriate action if not (which may be retrying or exiting with an error message).
1.3.4 Task 3.4: Checking slots reserved by you
You can also send a request to find all the slots that are reserved by you by sending a GET request
to the URL
https://web.cs.manchester.ac.uk/band/api/reservation
As before, if you want to get the list of slots you have reserved for the hotel, you should use the
same endpoint for the hotel API by sending a GET request to
https://web.cs.manchester.ac.uk/hotel/api/reservation
A successful response from this endpoint will contain a JSON array representing the list of slots
you have in the system, an example of which is shown in Listing 10.
Listing 10: Body of a 200 response
[ { " i d " : " 5 6 " } ,
{ " i d " : " 2 2 " } ]
1.4 Session 2 Tasks—Reserving identical slots (as early as possible) for
the hotel and the band
Now the fun and games start. You now need to design and implement a strategy to reserve the
same numbered slot for both the hotel and the band — that is, if you reserve slot 10 for the band
then you also need to reserve slot 10 for the hotel. Remember, you are permitted to have at most
two reservations for the hotel and two reservations for the band.
This strategy should try to reserve the earliest available matching slots. Note, we will sometimes
block slots from reservations and then release them during the labs.
Important:. It is quite easy, by having a number of clients fire requests at the server as quickly as
they can, for this to turn into a denial of service attack! It is strongly recommended that client
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code which loops around interrogating the server should include a deliberate delay (of say one
second) to reduce the danger of this. A code which ignores this advice will be penalised in the
assessment.
To get full marks for Session 2 you need to have a fully operational program (client) that implements a strategy for booking matching slots that relies on availability, does not give up when a
booking request fails, and will cancel unmatched bookings. Furthermore, your strategy should find
the earliest matching slot, not any matching slot; and, once a matching slot is found, it should
also check for a better booking (remember, the availability data for the band and the hotel is outof-date as soon as it is received!); you need to make sure that you cancel bookings after obtaining
better ones, but you need to maintain always at least one booking; also, your code should not give
up if no matching slots are available. Finally, there must be a delay of at least 1 second between
successive requests to the server (even if it is a retry).
To get full marks you should always assume that slot availability changes frequently and messages
may be delayed or lost!
Remember that you are not allowed to give anybody your username and password
(even more you should not put a username/password or any code online). If you
ignore this or the 1-second-delay rule you may be penalised.
1.5 Deliverables (Total marks: 15)
The deliverables for this exercise are:
• Session 1 tasks [5 marks]:
– Complete the code for the following relevant methods that are declared in the file
reservationapi.py:
∗ get_slots_available method: this method should be used to check for free slots
∗ reserve_slot method: this method should be used to reserve a slot
∗ release_slot method: this method should be used to cancel a reserved slot
∗ get_slots_held method : this method should be used to retrieve booked slot(s)
∗ _send_request method: this method should be used by the above-mentioned methods to send requests and also to handle errors and failures from the API sensibly
– Use the code provided in session1.py and create a Python client mysession1.py that
should perform operations as explained in Tasks 3.1, 3.2, 3.3 and 3.4 in an appropriate
sequence by calling the newly defined methods. It should also display messages on the
screen for each of the operations.
• Session 2 tasks [10 marks]:
Create a Python client mysession2.py that should:
– check the availability of common slots and display the first 20 common slots on the
screen
– book earliest common slot, release other booked slots if any and display booked slots
on the screen,
– recheck at least once for better bookings
– displays appropriate messages on the screen
– always adds a one-second delay
– fully working i.e. free of errors and bugs
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1.6 Submission instructions
You should submit your work via Blackboard. You are expected to submit a zip folder with name
comp28112_ex3_username.zip containing :
• api.ini
• exceptions.py
• reservationapi.py
• mysession1.py
• mysession2.py
Don’t forget to replace username with your own username consisting of 8 alphanumeric characters
(e.g. a12345zz) before submitting your work.
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