{"id":2127,"date":"2019-04-17T13:00:39","date_gmt":"2019-04-17T12:00:39","guid":{"rendered":"https:\/\/chewett.co.uk\/blog\/?p=2127"},"modified":"2020-09-26T22:29:48","modified_gmt":"2020-09-26T21:29:48","slug":"raspberry-pi-cluster-node-14-a-simple-webserver","status":"publish","type":"post","link":"https:\/\/chewett.co.uk\/blog\/2127\/raspberry-pi-cluster-node-14-a-simple-webserver\/","title":{"rendered":"Raspberry Pi Cluster Node \u2013 14 A simple webserver"},"content":{"rendered":"\n
\"\"<\/figure>\n\n\n\n

This tutorial focuses on creating a simple webserver that displays the status of the master using python Bottle.<\/p>\n\n\n\n\n\n\n\n

What will the webserver be used for?<\/h2>\n\n\n\n

To interact with the cluster I am planning on making a small set of webpages. Initially these will just display information about the cluster but going forward they will be able to schedule tasks to run on the cluster.<\/p>\n\n\n\n

This will allow more feature rich ways of interacting with the cluster. It will operate like another slave, but this one will be in charge of running the webserver.<\/p>\n\n\n\n

For the first stage I am going to be making the webserver output information about the master.<\/p>\n\n\n\n

What is Python Bottle<\/h2>\n\n\n\n

Python Bottle is a lightweight library which can be used to host a small webserver. This works with a simple WSGI server that reacts to the HTTP requests.<\/p>\n\n\n\n

In production you should really attach it to a full webserver which will handle the requests instead of using the inbuilt WGSI server. However for our purposes this will work fine we we wont need it to handle a large number of requests.<\/p>\n\n\n\n

If this becomes too slow then there are options to move to using a webserver like NGINX.<\/p>\n\n\n\n

Creating the Webserver Slave<\/h2>\n\n\n\n

The basic webserver will revolve around a single webserver slave which will talk to the master to obtain information about the cluster.<\/p>\n\n\n\n

Initially it will just poll the master for information and cache this. Then when a user requests the information via the webserver this cached information will be presented. In a later tutorial the webserver will make direct requests for specific information to the master.<\/p>\n\n\n\n

Below is the basic webserver slave thread class.<\/p>\n\n\n

\nclass RpiWebserverSlaveThread(RpiBasicSlaveThread):\n\n    current_webserver_data = None\n    webserver_data_updated = None\n\n    def perform_action(self):\n        logger.info("Now sending a keepalive to the master")\n        send_message(self.sock, create_payload("I am still alive, client: {num}".format(num=self.client_number)))\n        send_message(self.sock, create_payload("computer_details", "info"))\n        message = get_message(self.sock)\n        RpiWebserverSlaveThread.current_webserver_data = message['payload']\n        RpiWebserverSlaveThread.webserver_data_updated = datetime.datetime.now()\n        time.sleep(5)\n<\/pre><\/div>\n\n\n
This is a subclass of the basic RpiBasicSlaveThread<\/code> class which was refactored in the previous tutorial.  This refactoring has allowed all the base code to be kept the same and just the perform_action<\/code> function to be overridden.<\/p>\n\n\n\n
This overridden method will request the information about the master and save it to a static class variable.<\/p>\n\n\n\n
Creating the simple webserver<\/h2>\n\n\n\n
Python Bottle allows you to create a very simple webserver. Again it must be noted that using the inbuilt server is not appropriate for large applications but for us it works fine.<\/p>\n\n\n
\n@route('\/')\ndef index():\n    return template("templates\/ClusterHomepage.html",\n                    info=json.dumps(RpiWebserverSlaveThread.current_webserver_data, indent=4, sort_keys=True))\n\nrun(host=webserver_host, port=webserver_port)\n<\/pre><\/div>\n\n\n
Here we define an index method and annotate it with a route annotation from Python Bottle. This will associate it to the base webserver and add it as a handler for the “\/” base route.<\/p>\n\n\n\n
When called, it will open the ClusterHomepage.html<\/code> and replace the {{info}}<\/code> text with the value of the info variable. In this case it will be the following.<\/p>\n\n\n
\njson.dumps(RpiWebserverSlaveThread.current_webserver_data, indent=4, sort_keys=True))\n<\/pre><\/div>\n\n\n
This includes the current cached information about the master on the webpage and then serves it to the client.<\/p>\n\n\n\n
The HTML page that is used as a template is reproduced below.<\/p>\n\n\n
\n<!doctype html>\n<html lang="en">\n<head>\n  <title>Raspberry Pi Cluster<\/title>\n  <meta name="description" content="Raspberry Pi Cluster Homepage">\n  <meta name="author" content="Christopher Hewett">\n<\/head>\n\n<body>\n<h1>Raspberry Pi Cluster Homepage<\/h1>\n\n<p>This page shows the status of the Raspberry Pi Cluster<\/p>\n\n<h2>Master Information<\/h2>\n<pre>{{info}}<\/pre>\n\n<\/body>\n<\/html>\n<\/pre><\/div>\n\n\n
The final line then starts up the webserver on the given port and hostname. To associate the webserver to all hostnames you can set it to 0.0.0.0<\/code>. This will make it easily publicly accessible to other computers on the same network.<\/p>\n\n\n\n
Once this is done and loaded it will look like this.<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
Summary of the changes<\/h2>\n\n\n\n
In this tutorial a new slave is made which will continually request data from the master. This slave also starts a small Python Bottle webserver which will allow viewing of this data.<\/p>\n\n\n\n
Currently there is nothing that the webserver can request to be performed so this will be the focus in future tutorials.<\/p>\n\n\n\n
The full code is available on Github<\/a>, any comments or questions can be raised there as issues or posted below. <\/p>\n","protected":false},"excerpt":{"rendered":"
This tutorial focuses on creating a simple webserver that displays the status of the master using python Bottle.<\/p>\n","protected":false},"author":1,"featured_media":2131,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_exactmetrics_skip_tracking":false,"_exactmetrics_sitenote_active":false,"_exactmetrics_sitenote_note":"","_exactmetrics_sitenote_category":0,"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"The #RaspberryPi Cluster project \u2013 14 Adding a simple webserver #code  #python","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[122,1],"tags":[102,184,185,37],"class_list":["post-2127","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-raspberry-pi-cluster","category-uncategorized","tag-distributed-computing","tag-python","tag-rasbian","tag-raspberry-pi"],"wppr_data":{"cwp_meta_box_check":"No"},"jetpack_publicize_connections":[],"jetpack_featured_media_url":"https:\/\/i0.wp.com\/chewett.co.uk\/blog\/wp-content\/uploads\/2019\/04\/rpi_cluster_14_a_simple_webserver.jpg?fit=800%2C800&ssl=1","jetpack_shortlink":"https:\/\/wp.me\/p2toWX-yj","jetpack_sharing_enabled":true,"jetpack-related-posts":[{"id":2179,"url":"https:\/\/chewett.co.uk\/blog\/2179\/raspberry-pi-cluster-node-15-a-more-complex-webserver\/","url_meta":{"origin":2127,"position":0},"title":"Raspberry Pi Cluster Node \u2013 15 A more complex webserver","author":"Chewett","date":"May 8, 2019","format":false,"excerpt":"This tutorial focuses on improving the webserver to display information about the slaves connected to the master using python Bottle. Refactoring the Master Script To start with the changes, I am going to focus on the master script. This is going to move the changes into a new class. class\u2026","rel":"","context":"In "Raspberry Pi Cluster"","block_context":{"text":"Raspberry Pi Cluster","link":"https:\/\/chewett.co.uk\/blog\/category\/raspberry-pi-cluster\/"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/chewett.co.uk\/blog\/wp-content\/uploads\/2019\/05\/rpi_cluster_15_more_complex_webserver.jpg?fit=800%2C800&ssl=1&resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/chewett.co.uk\/blog\/wp-content\/uploads\/2019\/05\/rpi_cluster_15_more_complex_webserver.jpg?fit=800%2C800&ssl=1&resize=350%2C200 1x, https:\/\/i0.wp.com\/chewett.co.uk\/blog\/wp-content\/uploads\/2019\/05\/rpi_cluster_15_more_complex_webserver.jpg?fit=800%2C800&ssl=1&resize=525%2C300 1.5x, https:\/\/i0.wp.com\/chewett.co.uk\/blog\/wp-content\/uploads\/2019\/05\/rpi_cluster_15_more_complex_webserver.jpg?fit=800%2C800&ssl=1&resize=700%2C400 2x"},"classes":[]},{"id":1054,"url":"https:\/\/chewett.co.uk\/blog\/1054\/raspberry-pi-tutorial-website-online-now\/","url_meta":{"origin":2127,"position":1},"title":"Raspberry Pi Tutorial Website online now!","author":"Chewett","date":"March 10, 2018","format":false,"excerpt":"This post talks about the new page on my website with linking to all my Raspberry Pi Cluster tutorials. 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Porting RaspberryPiVcgencmd to Python 3 Originally this library was written for Python 2 but now that has been retired it was time to update it. 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