在python中尝试使用参数通过AWS创建EC2时出现jsii错误

Question

下面是代码，这是引用正式文档后拼凑而成的：

    image_id = core.CfnParameter(
        self,
        id='ImageName',
        type='String',
        default='MyTestImage',
        constraint_description="Must be the AMI name not the ID",
        )

    vpc_id = "vpc-XXXXXX"
    # lookup existing VPC
    vpc = ec2.Vpc.from_lookup(
        self,
        "vpc",
        vpc_id=vpc_id,
    )

    # create a new security group
    sec_group = ec2.SecurityGroup(
        self,
        "MyAppSG",
        vpc=vpc,
        allow_all_outbound=True,
    )

    ami_id = ec2.LookupMachineImage(
        name=image_id.value_as_string
    )

    ec2_instance = ec2.Instance(
        self,
        "ec2-instance",
        instance_name='Web',
        instance_type=ec2.InstanceType('t2.micro'),
        machine_image=ami_id,
        vpc=vpc,
        key_name='Mykey',
        security_group=sec_group,
    )

然后运行CDK命令: cdk diff - ImageName=MySecondTestImage参数。

错误：

Searching for AMI in XXXXXXXX:us-east-1
jsii.errors.JavaScriptError: 
  Error: Invalid context key: context keys can't include tokens
      at ConstructNode.setContext (/private/var/folders/w3/b69qmypx72nd9kk9s0vx73hdhdcs6y/T/jsii-kernel-kDU1vU/node_modules/@aws-cdk/core/lib/construct-compat.js:356:19)
      at App.loadContext (/private/var/folders/w3/b69qmypx72nd9kk9s0vx73hdhdcs6y/T/jsii-kernel-kDU1vU/node_modules/@aws-cdk/core/lib/app.js:75:23)
      at new App (/private/var/folders/w3/b69qmypx72nd9kk9s0vx73hdhdcs6y/T/jsii-kernel-kDU1vU/node_modules/@aws-cdk/core/lib/app.js:46:14)
      at /data/CDK/POC/test/.venv/lib/python3.9/site-packages/jsii/_embedded/jsii/jsii-runtime.js:3621:49
      at Kernel._wrapSandboxCode (/data/CDK/POC/test/.venv/lib/python3.9/site-packages/jsii/_embedded/jsii/jsii-runtime.js:4105:16)
      at Kernel._create (/data/CDK/POC/test/.venv/lib/python3.9/site-packages/jsii/_embedded/jsii/jsii-runtime.js:3621:26)
      at Kernel.create (/data/CDK/POC/test/.venv/lib/python3.9/site-packages/jsii/_embedded/jsii/jsii-runtime.js:3356:21)
      at KernelHost.processRequest (/data/CDK/POC/test/.venv/lib/python3.9/site-packages/jsii/_embedded/jsii/jsii-runtime.js:13162:28)
      at KernelHost.run (/data/CDK/POC/test/.venv/lib/python3.9/site-packages/jsii/_embedded/jsii/jsii-runtime.js:13100:14)
      at Immediate._onImmediate (/data/CDK/POC/test/.venv/lib/python3.9/site-packages/jsii/_embedded/jsii/jsii-runtime.js:13103:37)

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/data/CDK/POC/test/app.py", line 9, in <module>
    app = core.App()
  File "/data/CDK/POC/test/.venv/lib/python3.9/site-packages/jsii/_runtime.py", line 83, in __call__
    inst = super().__call__(*args, **kwargs)
  File "/data/CDK/POC/test/.venv/lib/python3.9/site-packages/aws_cdk/core/__init__.py", line 14940, in __init__
    jsii.create(App, self, [props])
  File "/data/CDK/POC/test/.venv/lib/python3.9/site-packages/jsii/_kernel/__init__.py", line 265, in create
    response = self.provider.create(
  File "/data/CDK/POC/test/.venv/lib/python3.9/site-packages/jsii/_kernel/providers/process.py", line 353, in create
    return self._process.send(request, CreateResponse)
  File "/data/CDK/POC/test/.venv/lib/python3.9/site-packages/jsii/_kernel/providers/process.py", line 335, in send
    raise JSIIError(resp.error) from JavaScriptError(resp.stack)
jsii.errors.JSIIError: Invalid context key: context keys can't include tokens
Subprocess exited with error 1

为什么会产生CDK错误？

Answer 1

来自https://docs.aws.amazon.com/cdk/latest/guide/parameters.html

一个CfnParameter实例通过令牌向您的AWS应用程序公开它的值。与所有令牌一样，参数的令牌在合成时被解析，但它解析为对AWS CloudFormation模板中定义的参数的引用，该参数将在部署时被解析，而不是具体的值。

当合成时间尚未被解析时，您正在尝试在合成之前使用该值。对于您想要实现的运行时上下文，https://docs.aws.amazon.com/cdk/latest/guide/context.html更合适。下面是一个示例：

from aws_cdk import core
import aws_cdk.aws_ec2 as ec2


class MyCdkStack(core.Stack):

    def __init__(self, scope: core.Construct, construct_id: str, **kwargs) -> None:
        super().__init__(scope, construct_id, **kwargs)

        image_name = self.node.try_get_context("image_name")
        vpc_id = self.node.try_get_context("vpc_id")

        # lookup existing VPC
        vpc = ec2.Vpc.from_lookup(
            self,
            "vpc",
            vpc_id=vpc_id,
        )

        # create a new security group
        sec_group = ec2.SecurityGroup(
            self,
            "MyAppSG",
            vpc=vpc,
            allow_all_outbound=True,
        )

        ami_id = ec2.LookupMachineImage(
            name=image_name
        )

        ec2_instance = ec2.Instance(
            self,
            "ec2-instance",
            instance_name='Web',
            instance_type=ec2.InstanceType('t2.micro'),
            machine_image=ami_id,
            vpc=vpc,
            key_name='Mykey',
            security_group=sec_group,
        )

可以通过提供上下文来运行它，例如：

cdk synth --context image_name=xxxxx --context vpc_id=xxxxx