AWS Auto ScalingのAPI Requestの書き方

# はじめに
諸事情により、CLIもSDKも使わないでAWS Auto Scalingに対してAPI操作しなきゃならなくなった。
(EC2 AutoScalingでもApplication AutoScalingでもないやつ。コンソールではAWS Auto ScalingだしCLIとかではauto-scaling-plansとかでややこしい。)
普通に(自分が普通だと思う方法で)書いたらなかなか上手く行かなかったのでめも。

# 完成形のコード

基本はこれのコピペ。
https://docs.aws.amazon.com/ja_jp/general/latest/gr/sigv4-signed-request-examples.html#sig-v4-examples-post

[DescribeScalingPlans](https://docs.aws.amazon.com/autoscaling/plans/APIReference/API_DescribeScalingPlans.html)を叩く。

```python:auto_scaling_plans.py
import sys, os, base64, datetime, hashlib, hmac
import requests
from aws_requests_auth.aws_auth import AWSRequestsAuth
from boto3.session import Session

profile_name = 'default'
method = 'POST'
service = 'autoscaling'
service_alt = 'autoscaling-plans'
region = 'ap-northeast-1'
host = service + '.' + region + '.amazonaws.com'
endpoint = 'https://' + host + '/'

content_type = 'application/x-amz-json-1.1'
amz_target = 'AnyScaleScalingPlannerFrontendService.DescribeScalingPlans'

request_parameters = '{}'


def sign(key, msg):
    return hmac.new(key, msg.encode("utf-8"), hashlib.sha256).digest()


def getSignatureKey(key, date_stamp, regionName, serviceName):
    kDate = sign(('AWS4' + key).encode('utf-8'), date_stamp)
    kRegion = sign(kDate, regionName)
    kService = sign(kRegion, serviceName)
    kSigning = sign(kService, 'aws4_request')
    return kSigning


credentials = Session(profile_name=profile_name).get_credentials()

access_key = credentials.access_key
secret_key = credentials.secret_key
session_token = credentials.token
if access_key is None or secret_key is None:
    print('No access key is available.')
    sys.exit()

t = datetime.datetime.utcnow()
amz_date = t.strftime('%Y%m%dT%H%M%SZ')
date_stamp = t.strftime('%Y%m%d')

canonical_uri = '/'

canonical_querystring = ''

canonical_headers = 'content-type:' + content_type + '\n' + 'host:' + host + '\n' + 'x-amz-date:' + amz_date + '\n' + 'x-amz-security-token:' + session_token + '\n' + 'x-amz-target:' + amz_target + '\n'

signed_headers = 'content-type;host;x-amz-date;x-amz-security-token;x-amz-target'

payload_hash = hashlib.sha256(request_parameters.encode('utf-8')).hexdigest()

canonical_request = method + '\n' + canonical_uri + '\n' + canonical_querystring + '\n' + canonical_headers + '\n' + signed_headers + '\n' + payload_hash

algorithm = 'AWS4-HMAC-SHA256'
credential_scope = date_stamp + '/' + region + '/' + service_alt + '/' + 'aws4_request'
string_to_sign = algorithm + '\n' + amz_date + '\n' + credential_scope + '\n' + hashlib.sha256(
    canonical_request.encode('utf-8')).hexdigest()

signing_key = getSignatureKey(secret_key, date_stamp, region, service_alt)

signature = hmac.new(signing_key, (string_to_sign).encode('utf-8'),
                     hashlib.sha256).hexdigest()

authorization_header = algorithm + ' ' + 'Credential=' + access_key + '/' + credential_scope + ', ' + 'SignedHeaders=' + signed_headers + ', ' + 'Signature=' + signature

headers = {
    'Content-Type': content_type,
    'X-Amz-Date': amz_date,
    'X-Amz-Security-Token': session_token,
    'X-Amz-Target': amz_target,
    'Authorization': authorization_header,
}

print('Request:\n\t' + request_parameters)

response = requests.post(endpoint, data=request_parameters, headers=headers)
print('Response:\n\t' + response.text)
```

# 困ったところ
## x-amz-targetになんて書いたらいいかわからん
DescribeScalingPlansを叩くから
`amz_target = 'DescribeScalingPlans'`って書いたら、↓レスポンスがこんな感じだった。

```
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html class="no-js aws-lng-en_US" lang="en-US" data-static-assets="https://a0.awsstatic.com" data-js-version="1.0.299" data-css-version="1.0.302">
 <head>
  <meta http-equiv="content-type" content="text/html; charset=UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" /> 
  <link rel="dns-prefetch" href="https://a0.awsstatic.com" />
  <link rel="dns-prefetch" href="//d0.awsstatic.com" />
  <link rel="dns-prefetch" href="//d1.awsstatic.com" />
  <title>AWS Auto Scaling</title>
  <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1" />
  <meta name="description" content="Learn how AWS Auto Scaling monitors your applications and automatically adjusts capacity to maintain steady, predictable performance at the lowest possible cost. " />
～以下略～
```

x-amz-targetが正しくないと、AWS Auto ScalingのWebページ (https://autoscaling.ap-northeast-1.amazonaws.com) を取得するようす。

じゃあなにを入れればいいのかと。どこを見ればいいのかと。
ドキュメントを読んでもよくわからなかったので、AWS CLIでAutoScalingPlansのコマンドをdebugモードで叩いて、ログから拾った。

```bash:
$ aws autoscaling-plans describe-scaling-plans --debug
～中略～
2019-09-12 18:52:40,665 - MainThread - botocore.endpoint - DEBUG - Making request for OperationModel(name=DescribeScalingPlans) with params: {'url_path': '/', 'query_string': '', 'method': 'POST', 'headers': {'X-Amz-Target': 'AnyScaleScalingPlannerFrontendService.DescribeScalingPlans', 'Content-Type': 'application/x-amz-json-1.1', 'User-Agent': 'aws-cli/1.16.237 Python/3.7.4 Linux/4.4.0-18362-Microsoft botocore/1.12.227'}, 'body': b'{}', 'url': 'https://autoscaling.ap-northeast-1.amazonaws.com/', 'context': {'client_region': 'ap-northeast-1', 'client_config': <botocore.config.Config object at 0x7f5c9b8fe610>, 'has_streaming_input': False, 'auth_type': None}}
～中略～
```

```bash
'X-Amz-Target': 'AnyScaleScalingPlannerFrontendService.DescribeScalingPlans',
```
AnyScaleScalingPlannerFrontendServiceだって。見たことないよ。

というわけで、こうしたら動いた。
`amz_target = 'AnyScaleScalingPlannerFrontendService.DescribeScalingPlans'`


## InvalidSignatureExceptionが発生する

ここは特にいじらずそのまま書いてたんだけど、

```python
algorithm = 'AWS4-HMAC-SHA256'
credential_scope = date_stamp + '/' + region + '/' + service + '/' + 'aws4_request'
string_to_sign = algorithm + '\n' + amz_date + '\n' + credential_scope + '\n' + hashlib.sha256(
    canonical_request.encode('utf-8')).hexdigest()

signing_key = getSignatureKey(secret_key, date_stamp, region, service)
```

↓これがでた。

```text:エラーメッセージ
{"__type":"InvalidSignatureException","message":"Credential should be scoped to correct service: 'autoscaling-plans'. "}
```

スコープがautoscaling-plansになってないって。

↓ここでホスト名を生成するために`service`を`autoscaling`にしていたが、

```python
service = 'autoscaling'
region = 'ap-northeast-1'
host = service + '.' + region + '.amazonaws.com'
endpoint = 'https://' + host + '/'
```

credential_scopeとsigning_keyのとこは`autoscaling`じゃなくて`autoscaling-plans`じゃないとだめだった。

```python
service_alt = 'autoscaling-plans'

algorithm = 'AWS4-HMAC-SHA256'
credential_scope = date_stamp + '/' + region + '/' + service_alt + '/' + 'aws4_request'
string_to_sign = algorithm + '\n' + amz_date + '\n' + credential_scope + '\n' + hashlib.sha256(
    canonical_request.encode('utf-8')).hexdigest()

signing_key = getSignatureKey(secret_key, date_stamp, region, service_alt)
```


# おわりに
この辺のややこしいのを全部やってくれるCLIさんが好き。