Snapshot Debugger

The Snapshot Debugger lets you inspect the state of a running cloud application, at any code location, without stopping or slowing it down. It’s not your traditional process debugger but rather an always on, whole app debugger taking snapshots from any instance of the app.

You can use the Snapshot Debugger with any deployment of your application, including test, development, and production. The debugger typically adds less than 10ms to the request latency only when the application state is captured. In most cases, this isn’t noticeable by users.

Preview Limitations

During the preview phase, only GCE, and local debugging are supported. Snapshot Debugger only works with the Node.js agent for preview.

Upcoming Features

• Java agent support
• Python 3 agent support
• App Engine support for Java, Python 3, and Node.js

Note:

• All upcoming features will be implemented before the Cloud Debugger shutdown
• Python 2.7 will not be supported

Support Period

Snapshot Debugger and associated agents will be supported until Aug 31, 2023, after which they will be archived and frozen. No bug fixes or security patches will be made after the freeze date. The repository can be forked by users if they wish to maintain it going forward.

CLI Command Reference

See COMMAND_REFERENCE.md.

Installing the Snapshot Debugger CLI

Install the debugger CLI in your local environment, or in your Cloud Shell $HOME directory. See Using Cloud Shell for information on using Cloud Shell.

python3 -m pip install snapshot-dbg-cli

Note: When using the Snapshot Debugger in Cloud Shell you will be asked to Authorize using your account credentials.

Running the Snapshot Debugger CLI

There are two options to run the CLI once the pip install has been completed.

Option 1: Use the installed script

As part of the pip install process, a script, snapshot-dbg-cli will be installed which can be used to run the CLI.

Example running the list_debuggees command:

snapshot-dbg-cli list_debuggees

NOTE: To run the script this way from any directory, you must ensure the script's install directory is in your PATH. Pip should emit a warning if the install location is not in the PATH, and also provide the install location in this case, so that you can add it to your PATH.

Option 2: Run the package directly

Example running the list_debuggees command:

python3 -m snapshot_dbg_cli list_debuggees

Before you begin

Ensure you have the proper permissions

To complete the setup you’ll need to have the following permissions in your Google Cloud project. If you are an Owner or Editor of the Google Cloud project, then you have these permissions.

• firebase.projects.create
• firebase.projects.update,
• firebasedatabase.instances.create
• firebasedatabase.instances.get
• resourcemanager.projects.get
• serviceusage.services.enable
• serviceusage.services.get

For more information on permissions and roles in Google projects, read Understanding roles

Using Snapshot Debugger in Cloud Shell

Snapshot Debugger requires Python 3.6 or above and the gcloud CLI. If you are working in Cloud Shell, you already have Python and gcloud installed.

In addition, the environment should already configured correctly by default. You can verify this by running the following commands:

1. gcloud config get-value project
2. gcloud config get-value account

NOTE: When running the cli, you may encounter a popup warning you that gcloud is requesting your credentials to make a GCP API call. You'll need to click AUTHORIZE to proceed.

Using Snapshot Debugger outside of Cloud Shell

Ensure you have Python 3.6 or above installed

The Snapshot Debugger CLI requires Python 3.6 or newer.

Install Google Cloud gcloud CLI

The Snapshot Debugger CLI depends on the gcloud CLI. To install the gcloud CLI, follow these instructions. If you already have the gcloud CLI installed, run gcloud components update to update all of your installed components to the latest version.

Set up the environment

1. Run gcloud auth login, be sure to use the account that has permissions on the Google Cloud project you are working on.
2. Run gcloud config set project PROJECT_ID. Where PROJECT_ID is the project you want to use. The Snapshot Debugger CLI always acts on the current gcloud configured project.

Enable Firebase for your Google Cloud Project

The Snapshot Debugger CLI and agents use a Firebase Realtime Database (RTDB) to communicate.

If you already use Firebase in your project, skip to the Set up the Firebase RTDB section.

1. Add Firebase to your project:

   https://console.firebase.google.com/?dlAction=MigrateCloudProject&cloudProjectNumber=PROJECT_ID

   Where PROJECT_ID is your project ID

2. Select your Project ID and click Continue. If you have billing enabled in your project, the pay-as-you-go Blaze plan is selected, otherwise the free Spark plan is selected. If you have billing enabled and want to use the free Spark plan, set up a new project without billing enabled.

   Note: The Snapshot Debugger uses the Firebase RTDB service. Most users' usage will be low enough to remain under the free usage limits.

3. If you are using the pay-as-you-go Blaze plan, click Confirm plan. You are not prompted to confirm if you are on the free Spark plan.

4. Read the information under A Few things to remember when adding Firebase to a Google Cloud project then click Continue.

5. Toggle the enable analytics option to enable or disable Google Analytics for Firebase. Google Analytics isn't required for Debugger use.

6. Click Continue.

7. Click Get Started.

8. Click on your project.

Set up the Firebase RTDB

The instructions are slightly different depending on whether you are on the Spark or Blaze billing plan. Follow the steps in the following section for the plan you have.

You can check what billing plan is in effect for your project on the Firebase Usage & Billing page:

https://console.firebase.google.com/project/PROJECT_ID/usage/details

Where PROJECT_ID is your project ID

Blaze plan RTDB setup

This will instruct the debugger CLI to create and use a database with the name PROJECT_ID-cdbg

1. Run snapshot-dbg-cli init.
2. The output resembles the following:

Project 'test-proj' is successfully configured with the Firebase Realtime
Database for use by Snapshot Debugger.

The full database information is below. If you have specified a custom database
ID the url below is the one you'll need to specify when using the other cli
commands.

  name:         projects/23498723497/locations/us-central1/instances/test-proj-cdbg
  project:      projects/23498723497
  database url: https://test-proj-cdbg.firebaseio.com
  type:         USER_DATABASE
  state:        ACTIVE

Note: The information printed by the init command can be accessed from within your Firebase project. It’s safe to run the snapshot-dbg-cli init command multiple times to view this information.

Spark plan RTDB setup

This will instruct the CLI to create and use a database with the name PROJECT_ID-default-rtdb. It will only be created if it does not currently exist.

1. Run snapshot-dbg-cli init --use-default-rtdb
2. The output resembles the following:

Project 'test-proj' is successfully configured with the Firebase Realtime
Database for use by Snapshot Debugger.

The full database information is below. If you have specified a custom database
ID the url below is the one you'll need to specify when using the other cli
commands.

  name:         projects/23498723497/locations/us-central1/instances/default
  project:      projects/23498723497
  database url: https://test-proj-default-rtdb.firebaseio.com
  type:         USER_DATABASE
  state:        ACTIVE

Note: The information printed by the init command can be accessed from within your Firebase project. It’s safe to run the snapshot-dbg-cli init --use-default-rtdb command multiple times to view this information.

Set up Snapshot Debugger in your Google Cloud project

To use the preview Snapshot Debugger, it’s necessary to set a flag to use the Firebase backend.

If you don’t have a project yet and want to try out Snapshot Debugger, follow the steps in Getting started with Node.js on Compute Engine | Google Cloud to create one.

Set up Google Compute Engine

1. Create your Compute Engine instance with the following access scopes:

   • https://www.googleapis.com/auth/userinfo.email grants your cluster access to your email address.
   • https://www.googleapis.com/auth/firebase.database grants your cluster access to the Firebase database.

   See the Firebase information on scopes for Realtime Database and Authentication page for more information on access scopes. To note, the userinfo.email scope is not included when specifying full access to google apis when creating a GCE instance, and will need to be added.

2. Use npm to install the package:

   npm install --save @google-cloud/debug-agent
   

3. Enable the agent at the top of your app's main script or entry point (but after @google/cloud-trace if you are also using it):

   require('@google-cloud/debug-agent').start({
     useFirebase: true,
     serviceContext: {
       service: 'SERVICE',
       version: 'VERSION',
     }
   });
   

   Where:

   • SERVICE is a name for your app, such as MyApp, Backend, or Frontend.
   • VERSION is a version, such as v1.0, build_147, or v20170714.

   We recommend setting these from environment variables so you don’t need to change the source code with each deployment.

The debugger is now ready for use with your app.

Local and elsewhere

1. Use npm to install the package:

   npm install --save @google-cloud/debug-agent
   

2. Download service account credentials from Firebase.

   1. Navigate to your project in the Firebase console service account page. Replace PROJECT_ID with your project’s ID.

   https://console.firebase.google.com/project/PROJECT_ID/settings/serviceaccounts/adminsdk
   

   2. Click Generate new private key and save the key locally.

3. Configure and enable the agent at the top of your app's main script or entry point (but after @google/cloud-trace if you are also using it).

   require('@google-cloud/debug-agent').start({
     useFirebase: true,
     firebaseKeyPath: 'PATH-TO-KEY-FILE',
     // Specify this if you are the Spark billing plan and are using the
     // default RTDB instance.
     // firebaseDbUrl: 'https://RTDB-NAME-default-rtdb.firebaseio.com',
     serviceContext: {
       service: 'SERVICE',
       version: 'VERSION',
     }
   });
   

   Where:

   • PATH-TO-KEY-FILE is the path to your Firebase private key.
   • RTDB-NAME is the name of your Firebase database.
   • SERVICE is a name for your app, such as MyApp, Backend, or Frontend.
   • VERSION is a version, such as v1.0, build_147, or v20170714.

Example workflow

You create a breakpoint (snapshot or logpoint) on debuggees. Debuggees represent instances of the running application. In general all instances of the same version of the application will have the same debuggee ID, and breakpoints set on a debuggee will be installed on all running instances of it.

List Debuggees

Run the following command

snapshot-dbg-cli list_debuggees

The output resembles the following:

Name           ID          Description
-------------  ----------  ----------------------------------------
test-app - v1  d-24abc4f1  node index.js module:test-app version:v1
test-app - v2  d-8dd7f149  node index.js module:test-app version:v2

Set Snapshots

Snapshots capture local variables and the call stack at a specific line location in your app's source code. You can specify certain conditions and locations to return a snapshot of your app's data, and view it in detail to debug your app.

Set snapshots with the following command:

snapshot-dbg-cli set_snapshot index.js:21 --debuggee-id d-8dd7f149

Where:

• index.js:21 is the file:line for the snapshot
• d-8dd7f149 is the debuggee ID

Snapshot conditions (optional)

A snapshot condition is a simple expression in the app's language that must evaluate to true for the snapshot to be taken. Snapshot conditions are evaluated each time the line is executed, by any instance, until the condition evaluates to true or the snapshot times out.

Use of snapshot conditions is optional.

The condition is a full boolean expression that can include logical operators. Conditions are specified using the --condition flag of the set_snapshots command.

Example:

snapshot-dbg-cli set_snapshot index.js:26 --debuggee-id d-8dd7f149 --condition="ultimateAnswer <= 42 && foo==bar"

You can use the following language features to express conditions:

Node.js

Most Javascript expressions are supported, with the following caveat:

Expressions that may have static side effects are disallowed. The debug agent ensures all conditions and watchpoints you add are read-only and have no side effects, however, it doesn’t catch expressions that have dynamic side-effects.

For example, o.f looks like a property access, but dynamically, it may end up calling a getter function. The debugger presently doesn't detect such dynamic-side effects.

Snapshot expressions (optional)

Snapshot Debugger's Expressions feature allows you to evaluate complex expressions or traverse object hierarchies when a snapshot is taken. Expressions support the same language features as snapshot conditions.

Use of expressions is optional.

Typical uses for expressions are:

• To view static or global variables that are not part of the local variable set.
• To easily view deeply nested member variables.
• To avoid repetitive mathematical calculations. For example, calculating a duration in seconds with (endTimeMillis - startTimeMillis) / 1000.0.

Expressions are specified using the --expression flag of the set_snapshots command.

Example:

snapshot-dbg-cli set_snapshot index.js:26 --debuggee-id d-8dd7f149 --expression="histogram.length"

List snapshots

List snapshots with the following command:

snapshot-dbg-cli list_snapshots --debuggee-id d-8dd7f149 --include-inactive

Where:

• d-8dd7f149 is the debuggee ID

The output resembles the following:

Status     Location     Condition    CompletedTime                ID
---------  -----------  -----------  ---------------------------  ------------
ACTIVE     index.js:21                                            b-1648008775
ACTIVE     index.js:21                                            b-1648044994
ACTIVE     index.js:21                                            b-1648045010
COMPLETED  index.js:21               2022-03-23T02:52:23.558000Z  b-1648003845

Get snapshot

Get a snapshot with the following command:

snapshot-dbg-cli get_snapshot b-1649947203 --debuggee-id d-8dd7f149

Where:

• b-1649947203 is the snapshot ID
• d-8dd7f149 is the debuggee ID

The output resembles the following:

--------------------------------------------------------------------------------
| Summary
--------------------------------------------------------------------------------

Location:    index.js:30
Condition:   No condition set.
Expressions: No expressions set.
Status:      Complete
Create Time: 2022-05-13T14:14:01.444000Z
Final Time:  2022-05-13T14:14:02.516000Z

-------------------------------------------------------------------------------
| Evaluated Expressions
--------------------------------------------------------------------------------

There were no expressions specified.

--------------------------------------------------------------------------------
| Local Variables For Stack Frame Index 0:
--------------------------------------------------------------------------------

[
  {
    "req (IncomingMessage) ": {
      "_readableState (ReadableState) ": {
        "objectMode": "false",
        "highWaterMark": "16384",
        "buffer (BufferList) ": {
          "head": null,
          "tail": null,
          "length": "0"
        },
[... snip]

--------------------------------------------------------------------------------
| CallStack:
--------------------------------------------------------------------------------

Function              Location
--------------------  -----------
(anonymous function)  index.js:30

Delete snapshots

Delete snapshots with the following command:

snapshot-dbg-cli delete_snapshots --debuggee-id d-8dd7f149 --include-inactive

Where:

• d-8dd7f149 is the debuggee ID

The output resembles the following:

This command will delete the following snapshots:

Status     Location     Condition    ID
---------  -----------  -----------  ------------
ACTIVE     index.js:28               b-1649959801
ACTIVE     index.js:27               b-1649959807
COMPLETED  index.js:19               b-1649702213
COMPLETED  index.js:22               b-1649702753


Do you want to continue (Y/n)? Y
Deleted 4 snapshots.

Set Logpoints

Adds a debug logpoint to a debug target (debuggee). Logpoints inject logging into running services without changing your code or restarting your application. Every time any instance executes code at the logpoint location, Snapshot Debugger logs a message. Output is sent to the standard log for the programming language of the target (java.logging for Java, logging for Python, etc.)

Logpoints remain active for 24 hours after creation, or until they are deleted or the service is redeployed. If you place a logpoint on a line that receives lots of traffic, Debugger throttles the logpoint to reduce its impact on your application.

Set logpoints with the following command:

snapshot-dbg-cli set_logpoint index.js:21 "a={a} b={b}" --debuggee-id d-8dd7f149

Where:

• index.js:21 is the file:line for the logpoint
• a={a} b={b} is the logpoint message format
• d-8dd7f149 is the debuggee ID

Note: A common issue that users have run into is that logging at INFO level is often suppressed by the default logger and so logpoints will appear to be broken. See the --log-level option for setting a higher priority log level.

Logpoint message format

The format string is the message which will be logged every time the logpoint location is executed. If the string contains curly braces ('{' and '}'), any text within the curly braces will be interpreted as a run-time expression in the debug target's language, which will be evaluated when the logpoint is hit. Some valid examples are {a}, {myObj.myFunc()} or {a + b}. The value of the expression will then replace the {} expression in the resulting log output. For example, if you specify the format string "a={a}, (b+1)={b+1}", and the logpoint is hit when local variable a is 1 and b is 3, the resulting log output would be "a=1, (b+1)=3".

For more detailed information on valid expressions see Snapshot expressions as the rules are the same for logpoint expressions.

Logpoint conditions (optional)

A logpoint condition is a simple expression in the application language that must evaluate to true for the logpoint to be logged. Logpoint conditions are evaluated each time the line is executed, by any instance, until the logpoint expires or is deleted.

Use of logpoint conditions is optional.

For more detailed information on valid conditions see Snapshot conditions as the rules are the same for logpoint conditions.

List logpoints

List logpoints with the following command:

snapshot-dbg-cli list_logpoints --debuggee-id d-8dd7f149 --include-inactive

Where:

• d-8dd7f149 is the debuggee ID

The output resembles the following:

User Email    Location        Condition  Log Level  Log Message Format   ID            Status
------------  --------------  ---------  ---------  -------------------  ------------  -------------------------------------------
foo1@bar.com  Main.java:23               INFO      a={a} b={b}           b-1660681047  EXPIRED
foo2@bar.com  Main.java:25    a == 3     WARNING   Line hit              b-1660932877  EXPIRED
foo2@bar.com  Main.java:9999             INFO      Log msg               b-1661203071  SOURCE_LOCATION: No code found at line 9999

Get logpoint

Get a logpoint with the following command:

snapshot-dbg-cli get_logpoint b-1660681047 --debuggee-id d-8dd7f149

Where:

• b-1660681047 is the logpoint ID
• d-8dd7f149 is the debuggee ID

The output resembles the following:

Logpoint ID:        b-1660681047
Log Message Format: a == 3
Location:           Main.java:23
Condition:          No condition set
Status:             EXPIRED
Create Time:        2022-08-19T18:14:38.240000Z
Final Time:         2022-08-20T18:14:39.618000Z
User Email:         foo1@bar.com

Delete logpoints

Delete logpoints with the following command:

snapshot-dbg-cli delete_snapshots --debuggee-id d-8dd7f149 --include-inactive

Where:

• d-8dd7f149 is the debuggee ID

The output resembles the following:

This command will delete the following logpoints:

Location         Condition  Log Level  Log Message Format  ID
---------------  ---------  ---------  ------------------  ------------
Main.java:25     a == 3     WARNING    Line hit            b-1660927187
Main.java:9999              INFO       Log msg             b-1660927272



Do you want to continue (Y/n)?
Deleted 4 snapshots.

Troubleshooting

If you run into problems with Snapshot Debugger, file an issue.

Your project doesn’t show up when enabling Firebase

Symptom

Your project id is not auto populated and is not present in the project dropdown when you try to Enable Firebase for your GCP Project.

Resolution

Check if the Firebase Management API is already enabled, as if it is, that interferes with the process. Try using the following link to disable the Firebase Management API, then go back and follow the steps in the Enable Firebase for your GCP Project.

https://console.developers.google.com/apis/api/firebase.googleapis.com?project=PROJECT_ID

Where PROJECT_ID is your project ID.

Your database in not displayed in the Firebase Console

Symptom

A blank screen is shown when attempting to view database contents in Firebase Console's Realtime Database section. The project is using the Blaze pricing plan.

Resolution

Rerun the init command to find the database's url. Use that url to view the database's contents. See Blaze plan RTDB setup for details, as noted there, it is safe to run the init command multiple times to view your database's information.

The database's url should resemble https://DATABASE_NAME.firebaseio.com, which should redirect to https://console.firebase.google.com/project/PROJECT_ID/database/DATABASE_NAME/data for the Firebase Console view.