Match day, 10,000 fans

Your football app is quiet all week. On match day, 10,000 fans open it at once.

A good app can add more copies of itself in seconds. When the busy time ends, it removes them. It keeps a clear record of what happened. It starts fast and stops without losing work.

These are the last six factors.

Key words

Word	What it means	Example
Database	An organised store of data that programs can search and change.
Port binding	The app listens on a port number itself and offers its service there.	port 8000
Process	One running copy of a program.
Load balancer	A tool that shares the users between your processes.
Scale out	Handle more users by running more copies of the process.
Disposable	Easy to start and easy to stop at any time.
Graceful shutdown	Finish the work in progress, then stop.
Parity	Being alike. Dev, staging and production should be alike.
Staging	A copy of the live system used for a last check before release.
Log	A record of events, one line for each thing that happened.
Event stream	Events that arrive one after another, in time order.

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Six more habits

In part one you met codebase, dependencies, config, backing services, build and release, and processes.

Now try the idea behind factor 11. A log is a record of events. The app writes one line per event and does not care where it goes.

Try it: logs as a stream

Code: python

def log(level, message):
    print(level, message)   # just print. Something else collects the lines.


log("INFO", "app started")
log("INFO", "fan joined")
log("ERROR", "score service did not answer")

# Add a fourth line of your own.

Factors 7 to 12

Factor	Rule in plain words	Why
7 Port binding	The app listens on a port and offers its own service	It does not need another program to run inside
8 Concurrency	Scale out: run more processes, not a bigger machine	You add copies on match day
9 Disposability	Start fast. Shut down gracefully	A process can come and go without losing work
10 Dev and live alike	Keep development, staging and production as similar as you can	A bug that appears on live also appears in testing
11 Logs	Write events as a stream of lines. Do not manage log files yourself	Tools can collect, search and watch the lines
12 Admin tasks	Run one-off jobs as one-off processes, from the same code	A database fix uses the same code and settings as the app

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Worked example: how many workers?

Fans	Fans one worker can serve	Working	Workers needed
250	100	100 + 100 = 200, not enough. 100 + 100 + 100 = 300, enough	3
300	100	3 workers serve exactly 300	3
301	100	3 workers serve 300, one fan is left	4

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Factor 7 in code: listen on a port

Code: python

from http.server import HTTPServer, BaseHTTPRequestHandler
import os


class Hello(BaseHTTPRequestHandler):
    def do_GET(self):
        self.send_response(200)
        self.end_headers()
        self.wfile.write(b"Scores: Reds 2, Blues 1")


port = int(os.environ.get("PORT", "8000"))   # the port is a setting too
HTTPServer(("0.0.0.0", port), Hello).serve_forever()

# Run this on your own computer, then open http://localhost:8000 in a browser.
# The app is its own web server. It needs no other program.