Modifying the runtime environment - AWS Lambda

Modifying the runtime environment

You can use internal extensions to modify the runtime process. Internal extensions are not separate processes—they run as part of the runtime process.

Lambda provides language-specific environment variables that you can set to add options and tools to the runtime. Lambda also provides wrapper scripts, which allow Lambda to delegate the runtime startup to your script. You can create a wrapper script to customize the runtime startup behavior.

Language-specific environment variables

Lambda supports configuration-only ways to enable code to be pre-loaded during function initialization through the following language-specific environment variables:

  • JAVA_TOOL_OPTIONS – On Java 11 and Java 8 (java8.al2), Lambda supports this environment variable to set additional command-line variables in Lambda. This environment variable allows you to specify the initialization of tools, specifically the launching of native or Java programming language agents using the agentlib or javaagent options.

  • NODE_OPTIONS – On Node.js 10x and above, Lambda supports this environment variable.

  • DOTNET_STARTUP_HOOKS – On .NET Core 3.1 and above, Lambda can take a path to an assembly (dll) provided by this environment variable.

Using language-specific environment variables is the preferred way to set startup properties.

Example: Intercept Lambda invokes with javaagent

The Java virtual machine (JVM) tries to locate the class that was specified with the javaagent parameter to the JVM, and invoke its premain method before the application's entry point.

The following example uses Byte Buddy, a library for creating and modifying Java classes during the runtime of a Java application without the help of a compiler. Byte Buddy offers an additional API for generating Java agents. In this example, the Agent class intercepts every call of the handleRequest method made to the RequestStreamHandler class. This class is used internally in the runtime to wrap the handler invocations.

import; import net.bytebuddy.agent.builder.AgentBuilder; import net.bytebuddy.asm.Advice; import net.bytebuddy.matcher.ElementMatchers; import java.lang.instrument.Instrumentation; public class Agent { public static void premain(String agentArgs, Instrumentation inst) { new AgentBuilder.Default() .with(new AgentBuilder.InitializationStrategy.SelfInjection.Eager()) .type(ElementMatchers.isSubTypeOf(RequestStreamHandler.class)) .transform((builder, typeDescription, classLoader, module) -> builder .method(ElementMatchers.nameContains("handleRequest")) .intercept( .installOn(inst); } }

The agent in the preceding example uses the TimerAdvice method. TimerAdvice measures how many milliseconds are spent with the method call and logs the method time and details, such as name and passed arguments.

import static net.bytebuddy.asm.Advice.AllArguments; import static net.bytebuddy.asm.Advice.Enter; import static net.bytebuddy.asm.Advice.OnMethodEnter; import static net.bytebuddy.asm.Advice.OnMethodExit; import static net.bytebuddy.asm.Advice.Origin; public class TimerAdvice { @OnMethodEnter static long enter() { return System.currentTimeMillis(); } @OnMethodExit static void exit(@Origin String method, @Enter long start, @AllArguments Object[] args) { StringBuilder sb = new StringBuilder(); for (Object arg : args) { sb.append(arg); sb.append(", "); } System.out.println(method + " method with args: " + sb.toString() + " took " + (System.currentTimeMillis() - start) + " milliseconds "); } }

The TimerAdvice method above has the following dependencies.

*'com.amazonaws'*, *name*: *'aws-lambda-java-core'*, *version*: *'1.2.1'* *'net.bytebuddy'*, *name*: *'byte-buddy-dep'*, *version*: *'1.10.14'* *'net.bytebuddy'*, *name*: *'byte-buddy-agent'*, *version*: *'1.10.14'*

After you create a layer that contains the agent JAR, you can pass the JAR name to the runtime's JVM by setting an environment variable.


After invoking the function with {key=lambdaInput}, you can find the following line in the logs:

public java.lang.Object lambdainternal.EventHandlerLoader$PojoMethodRequestHandler.handleRequest (java.lang.Object, method with args: {key=lambdaInput}, lambdainternal.api.LambdaContext@4d9d1b69, took 106 milliseconds

Example: Adding a shutdown hook to the JVM runtime process

When an extension is registered during a <code>Shutdown</code> event, the runtime process gets up to 500 ms to handle graceful shutdown. You can hook into the runtime process, and when the JVM begins its shutdown process, it starts all registered hooks. To register a shutdown hook, you must register as an extension. You do not need to explicitly register for the <code>Shutdown</code> event, as that is automatically sent to the runtime.

import java.lang.instrument.Instrumentation; public class Agent { public static void premain(String agentArgs, Instrumentation inst) { // Register the extension. // ... // Register the shutdown hook addShutdownHook(); } private static void addShutdownHook() { // Shutdown hooks get up to 500 ms to handle graceful shutdown before the runtime is terminated. // // You can use this time to egress any remaining telemetry, close open database connections, etc. Runtime.getRuntime().addShutdownHook(new Thread(() -> { // Inside the shutdown hook's thread we can perform any remaining task which needs to be done. })); } }

Example: Retrieving the InvokedFunctionArn

@OnMethodEnter static long enter() { String invokedFunctionArn = null; for (Object arg : args) { if (arg instanceof Context) { Context context = (Context) arg; invokedFunctionArn = context.getInvokedFunctionArn(); } } }

Wrapper scripts

You can create a wrapper script to customize the runtime startup behavior of your Lambda function. A wrapper script enables you to set configuration parameters that cannot be set through language-specific environment variables.


Invocations may fail if the wrapper script does not successfully start the runtime process.

The following Lambda runtimes support wrapper scripts:

  • Node.js 12.x

  • Node.js 10.x

  • Python 3.8

  • Ruby 2.7

  • Java 11

  • Java 8 (java8.al2)

  • .NET Core 3.1

When you use a wrapper script for your function, Lambda starts the runtime using your script. Lambda sends to your script the path to the interpreter and all of the original arguments for the standard runtime startup. Your script can extend or transform the startup behavior of the program. For example, the script can inject and alter arguments, set environment variables, or capture metrics, errors, and other diagnostic information.

You specify the script by setting the value of the AWS_LAMBDA_EXEC_WRAPPER environment variable as the file system path of an executable binary or script.

Example: Create and use a wrapper script with Python 3.8

In the following example, you create a wrapper script to start the Python interpreter with the -X importtime option. When you run the function, Lambda generates a log entry to show the duration of the import time for each import.

To create and use a wrapper script with Python 3.8

  1. To create the wrapper script, paste the following code into a file named importtime_wrapper:

    #!/bin/bash # the path to the interpreter and all of the originally intended arguments args=("$@") # the extra options to pass to the interpreter extra_args=("-X" "importtime") # insert the extra options args=("${args[@]:0:$#-1}" "${extra_args[@]}" "${args[@]: -1}") # start the runtime with the extra options exec "${args[@]}"
  2. To give the script executable permissions, enter chmod +x importtime_wrapper from the command line.

  3. Deploy the script as a Lambda layer.

  4. Create a function using the Lambda console.

    1. Open the Lambda console.

    2. Choose Create function.

    3. Under Basic information, for Function name, enter wrapper-test-function.

    4. For Runtime, choose Python 3.8.

    5. Choose Create function.

  5. Add the layer to your function.

    1. On the configuration page, choose Layers.

    2. Choose Add a layer.

    3. Under Choose a layer, choose the Name and Version of the compatible layer that you created earlier.

    4. Choose Add.

  6. Add the code and the environment variable to your function.

    1. In the function code editor, paste the following function code:

      import json def lambda_handler(event, context): # TODO implement return { 'statusCode': 200, 'body': json.dumps('Hello from Lambda!') }
    2. Choose Save.

    3. Under Environment variables, choose Manage environment variables.

    4. Choose Add environment variable.

    5. For Key, enter AWS_LAMBDA_EXEC_WRAPPER.

    6. For Value, enter /opt/importtime_wrapper.

    7. Choose Save.

  7. To run the function, choose Test.

    Because your wrapper script started the Python interpreter with the -X importtime option, the logs show the time required for each import. For example:

    ... 2020-06-30T18:48:46.780+01:00 import time: 213 | 213 | simplejson 2020-06-30T18:48:46.780+01:00 import time: 50 | 263 | simplejson.raw_json ...