Logging with Chalice

A friend in need is a friend indeed

Logging is your friend. Without proper logging (and tracing) it can be very difficult to understand why the application broke down at 3:15 in the morning and PagerDuty is screaming at you.

From Logging best practices in Software Development

From an application point of view, logs are designed to enhance communication between users and the application software. Having appropriate logs with exceptional search capabilities is similar to having an artificial lighting appliance in a dark room.

Most software developers neglect the significance of logs in resolving software issues. However, the fact is, logs are indispensable when it comes to addressing software issues and helping the developers to know the health status of the application. Also, logs are pivotal to the performance of an application. If you’re a developer and have no insight into logs as a programming tool, you’re missing an invaluable tool to better your development career.

Logging with Chalice can be easy or very easy.

Very easy logging

AWS Lambda will log anything that is written to stdout, meaning you can simply print() your log messages and be done with it.

from os import getenv
from chalice import Chalice

try:
    from dotenv import load_dotenv

    load_dotenv()
except ImportError:
    pass

first_name = getenv("WORKSHOP_NAME", "ivica")  # replace with your own name of course

app = Chalice(app_name=f"{first_name}-savealife")

@app.route("/donor/signup", methods=["POST"])
def donor_signup():
    body = app.current_request.json_body

    print(f"Received JSON payload: {body}")

    return body

Run the local server if it is not already running with chalice local --autoreload and invoke your function:

http :8000/donor/signup first_name=joe

Output should be similar to:

and in another terminal window/tab

chalice local --autoreload

will output

It is not the prettiest or the most informative log line but

Easy logging

As we saw earlier, Chalice supports print()-style logging which is not the most informative but it gets the job done. If you would like more information from your logs and you are used to the Python logging from stdlib you’ll be pleased to know that Chalice comes with a preconfigured logger that can be accessed via the app.log attribute. You may use it like so:

import logging

from os import getenv
from chalice import Chalice

try:
    from dotenv import load_dotenv

    load_dotenv()
except ImportError:
    pass

first_name = getenv("WORKSHOP_NAME", "ivica")  # replace with your own name of course

app = Chalice(app_name=f"{first_name}-savealife")
app.log.setLevel(logging.DEBUG)

@app.route("/donor/signup", methods=["POST"])
def donor_signup():
    body = app.current_request.json_body

    app.log.debug(f"Received JSON payload: {body}")
    app.log.info("This is a INFO level message")

    return body

Which produces log messages that feel like messages produced by the logging module from the standard library.

http :8000/donor/signup first_name=joe

will output:

Deploy these changes with chalice deploy and invoke the function:

http -b POST $(chalice url)/donor/signup first_name=joe

Output should be the same as when we invoked the function locally::

But how can we view these logs? And where are they?

chalice logs

will output something similar to:

These logs are stored remotely in a service called AWS CloudWatch. Apart from log storage it provides metrics, dashboards and alarms for resources in your account (Lambda functions, DynamoDB tables etc.)