Reading and Writing Application Logs
Overview
When a request is sent to your app, a request log is automatically written byApp Engine. During the handling of the request, your app can also writeapplication logs. In this page, you'll learn how to write application logsfrom your application, how toview logging in theGoogle Cloud console, and how to understand the request log data that App Enginewrites during the request.
For information about downloading log data, seeOverview of logs exports.
Important: Be aware that logs by default do not persist when you use thedevelopment app server. For information on how to make logs persist, seeThe development server and Logs API.Request logs vs application logs
There are two categories of log data: request logs and application logs. A request log is automatically written by App Engine for each request handled by your app, and contains information such as the project ID, HTTP version, and so forth. For a complete list of available properties for request logs, seeRequestLog. Seealso therequest log table for descriptions of the request log fields.
Each request log contains a list of application logs (AppLog) associated with that request, returned in theRequestLog.app_logs property. Each app log contains the time the log was written, the log message, and the log level.
Writing application logs
Important: Individual log entries from an application have a 16KB limit.Nothing beyond this limit will be written to the app log.You may find it useful to read the documentation for the standard Python loggingmodule atPython.org.The Python logging module allows a developer to log 5 levels of severity. :
- Debug
- Info
- Warning
- Error
- Critical
The following example shows how to use the different logging levels:
importloggingimportwebapp2classMainPage(webapp2.RequestHandler):defget(self):logging.debug('This is a debug message')logging.info('This is an info message')logging.warning('This is a warning message')logging.error('This is an error message')logging.critical('This is a critical message')try:raiseValueError('This is a sample value error.')exceptValueError:logging.exception('A example exception log.')self.response.out.write('Logging example.')app=webapp2.WSGIApplication([('/',MainPage)],debug=True)Log URL format in the Google Cloud console
See the following sample URL for an example of the log URL format in the Google Cloud console:
https://console.cloud.google.com/logs?filters=request_id:000000db00ff00ff827e493472570001737e73686966746361727331000168656164000100
filter_type.Reading logs in the console
Note: the logs described here are shown as visible in the Log Viewer.To view logs written by apps running in the standard environment, use theLogs Explorer.
A typical App Engine log contains data in theApache combined log format,along with some special App Engine fields, as shown in the following sample log:
192.0.2.0-test[27/Jun/2014:09:11:47-0700]"GET / HTTP/1.1"200414"http://www.example.com/index.html"Mozilla/5.0(X11;Linuxx86_64)AppleWebKit/537.36(KHTML,likeGecko)Chrome/35.0.1916.153Safari/537.36""1-dot-calm-sylph-602.appspot.com"ms=195cpu_ms=42cpm_usd=0.000046loading_request=1instance=00c61b117cfeb66f973d7df1b7f4ae1f064dapp_engine_release=Understanding request log fields
The following table lists the fields in order of occurrence along with adescription:
| Field Order | Field Name | Always Present? | Description |
|---|---|---|---|
| 1 | Client address | Yes | Client IP address. Example:192.0.2.0 |
| 2 | RFC 1413 identity | No | RFC 1413 identity of the client. This is nearly always the character- |
| 3 | User | No | Present only if the app uses the Users API and the user is logged in. This value is the "nickname" portion of the Google Account, for example, if the Google Account istest@example.com, the nickname that is logged in this field istest. |
| 4 | Timestamp | Yes | Request timestamp. Example:[27/Jun/2014:09:11:47 -0700] |
| 5 | Request querystring | Yes | First line of the request, containing method, path, and HTTP version. Example:GET / HTTP/1.1 |
| 6 | HTTP Status Code | Yes | Returned HTTP status code. Example:200 |
| 7 | Response size | Yes | Response size in bytes. Example:414 |
| 8 | Referrer path | No | If there is no referrer, the log contains no path, but only-. Example referrer path:"http://www.example.com/index.html". |
| 9 | User-agent | Yes | Identifies the browser and operating system to the web server. Example:Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/35.0.1916.153 Safari/537.36 |
| 10 | Hostname | Yes | The hostname used by the client to connect to the App Engine application. Example : (1-dot-calm-sylph-602.appspot.com) |
| 11 | Wallclock time | Yes | Total clock time in milliseconds spent by App Engine on the request. This time duration does not include time spent between the client and the server running the instance of your application. Example:ms=195. |
| 12 | CPU milliseconds | Yes | CPU milliseconds required to fulfill the request. This is the number of milliseconds spent by the CPU actually executing your application code, expressed in terms of a baseline 1.2 GHz Intel x86 CPU. If the CPU actually used is faster than the baseline, the CPU milliseconds can be larger than the actual clock time defined above. Example:cpu_ms=42 |
| 13 | Exit code | No | Only present if the instance shut down after getting the request. In the formatexit_code=XXX whereXXX is a 3 digit number corresponding to the reason the instance shut down. The exit codes are not documented since they are primarily intended to help Google spot and fix issues. |
| 14 | Estimated cost | Yes | DEPRECATED. Estimated cost of 1000 requests just like this one, in USD. Example:cpm_usd=0.000046 |
| 15 | Queue name | No | The name of the task queue used. Only present if request used a task queue. Example:queue_name=default |
| 16 | Task name | No | The name of the task executed in the task queue for this request. Only present if the request resulted in the queuing of a task. Example:task_name=7287390692361099748 |
| 17 | Pending queue | No | Only present if a request spent some time in a pending queue. If there are many of these in your logs and/or the values are high, it might be an indication that you need more instances to serve your traffic. Example:pending_ms=195 |
| 18 | Loading request | No | Only present if the request is a loading request. This means an instance had to be started up. Ideally, your instances should be up and healthy for as long as possible, serving large numbers of requests before being recycled and needing to be started again. Which means you shouldn't see too many of these in your logs. Example:loading_request=1. |
| 19 | Instance | Yes | Unique identifier for the instance that handles the request. Example:instance=00c61b117cfeb66f973d7df1b7f4ae1f064d |
| 20 | Version | Yes | The current App Engine release version used in production App Engine: |
Quotas and limits
Your application is affected by the followinglogs-related quotas:
- Logs data retrieved via the Logs API.
- Log ingestion allotment and retention.
Quota for data retrieved
The first 100 megabytes of logs data retrieved per day via the Logs API callsare free. Data in excess of 100 megabytes results in charges of$0.12/GB.
Logs ingestion allotment
Logging for App Engine apps is provided byGoogle Cloud Observability.SeeGoogle Cloud Observability Pricing formore information on logging costs and limits. For long-term storage of logs, youcanexport logs from Google Cloud Observabilityto Cloud Storage, BigQuery, and Pub/Sub.
The development server and Logs API
Important:- Before running the local development server, follow thesetup instructions.Regardless of your app's runtime or language version, you must run
dev_appserver.pywith Python 3 and set theCLOUDSDK_DEVAPPSERVER_PYTHONenvironment variable in your shell to the path of your Python 2 interpreter. - The
dev_appservertool does not support development of Python 3 appson Windows.
By default, logs are stored in memory only in thedevelopment server and are accessible if you wish to test the Logs API feature. If you wish topersist logs from the development server to disk at a location of your ownchoosing, supply the desired path and filename to the--logs_path command lineoption as follows:
python2[DEVAPPSERVER_ROOT]/google_appengine/dev_appserver.py--logs_path=your-path/your-logfile-nameyour-app-directoryExcept as otherwise noted, the content of this page is licensed under theCreative Commons Attribution 4.0 License, and code samples are licensed under theApache 2.0 License. For details, see theGoogle Developers Site Policies. Java is a registered trademark of Oracle and/or its affiliates.
Last updated 2025-12-15 UTC.