# Guidelines for AWS modules

The Ansible AWS modules and these guidelines are maintained by the Ansible AWS Working Group.  For
further information see 
[the AWS working group community page](https://github.com/ansible/community/tree/master/group-aws).  
If you are planning to contribute AWS modules to Ansible then getting in touch with the the working
group will be a good way to start, especially because a similar module may already be under
development.

## Choose Boto3 not Boto

Starting from Ansible 2.0, it has been required that all new AWS modules are written to use boto3.
Please do not add new dependencies on the old boto library.

Prior to 2.0, modules may have been written in boto or boto3. The effort to port all modules to
boto3 has begun.  From Ansible 2.4 it is permissible for modules which previously required boto to
start to require boto3 in order to deliver the functionality previously supported with boto and the
boto dependency can be deleted.

## Bug fixing

Bug fixes to code that relies on boto will still be accepted. When possible, the code should be
ported to use boto3.

## Naming your module

Base the name of the module on the part of AWS that you actually use. (A good rule of thumb is to
take whatever module you use with boto as a starting point).  Don't further abbreviate names - if
something is a well known abbreviation due to it being a major component of AWS, that's fine, but
don't create new ones independently (e.g. VPC, ELB, and similar abbreviations are fine).

Unless the name of your service is quite unique, please consider using "aws_" as a prefix.  For
example "aws_lambda".

## Adding new features

Try to keep backward compatibility with relatively recent versions of boto3. That means that if you
want to implement some functionality that uses a new feature of boto3, it should only fail if that
feature actually needs to be run, with a message stating the missing feature and minimum required
version of boto3.

Use feature testing (e.g. `hasattr('boto3.module', 'shiny_new_method')`) to check whether boto3
supports a feature rather than version checking

e.g. from the `ec2` module:
```python
if boto_supports_profile_name_arg(ec2):
    params['instance_profile_name'] = instance_profile_name
else:
    if instance_profile_name is not None:
        module.fail_json(msg="instance_profile_name parameter requires boto version 2.5.0 or higher")
```

## Using botocore and boto3

### Importing

The `ansible.module_utils.ec2` module and `ansible.module_utils.core.aws` modules will both
automatically import boto3 and botocore.  If boto3 is missing from the system then the variable
HAS_BOTO3 will be set to false.  Normally, this means that modules don't need to import either
botocore or boto3 directly.

If you want to import the modules anyway (for example `from botocore.exception import
ClientError`) Wrap import statements in a try block and fail the module later using HAS_BOTO3 if
the import fails.

#### boto

```python
try:
    import boto.ec2
    from boto.exception import BotoServerError
    HAS_BOTO = True
except ImportError:
    HAS_BOTO = False

def main():

    if not HAS_BOTO:
        module.fail_json(msg='boto required for this module')
```

#### boto3

```python
from ansible.module_utils.aws.core import HAS_BOTO3

def main():

    if not HAS_BOTO3:
        module.fail_json(msg='boto3 and botocore are required for this module')
```

#### boto and boto3 combined

Ensure that you clearly document if a new parameter requires requires a specific version. Import
boto3 at the top of the module as normal and then use the HAS_BOTO3 bool when necessary, before the
new feature.

```python
from ansible.module_utils.ec2 import HAS_BOTO3

try:
    import boto
    HAS_BOTO = True
except ImportError:
    HAS_BOTO = False

if my_new_feature_Parameter_is_set:
    if HAS_BOTO3:
        # do feature
    else:
        module.fail_json(msg="boto3 is required for this feature")
```

### Connecting to AWS

To connect to AWS, you should use `get_aws_connection_info` and then `boto3_conn`.

These functions handle some of the more esoteric connection options, such as security tokens and
boto profiles.

Some boto services require that the region is specified. You should check for the region parameter
if required.

#### boto

An example of connecting to ec2:

```python
region, ec2_url, aws_connect_params = get_aws_connection_info(module)
if region:
    try:
        connection = connect_to_aws(boto.ec2, region, **aws_connect_params)
    except (boto.exception.NoAuthHandlerFound, AnsibleAWSError) as e:
        module.fail_json(msg=str(e))
else:
    module.fail_json(msg="region must be specified")
```

#### boto3

An example of connecting to ec2 is shown below.  Note that there is no `NoAuthHandlerFound`
exception handling like in boto.  Instead, an `AuthFailure` exception will be thrown when you use
'connection'. See exception handling.

```python
region, ec2_url, aws_connect_params = get_aws_connection_info(module, boto3=True)
if region:
    connection = boto3_conn(module, conn_type='client', resource='ec2', region=region, endpoint=ec2_url, **aws_connect_params)
else:
    module.fail_json(msg="region must be specified")
```

### Exception Handling for boto

You should wrap any boto call in a try block. If an exception is thrown, it is up to you decide how
to handle it but usually calling fail_json with the error or helpful message and traceback will
suffice.

#### boto

```python
# Import BotoServerError
try:
    import boto.ec2
    from boto.exception import BotoServerError
    HAS_BOTO = True
except ImportError:
    HAS_BOTO = False

# Connect to AWS
...

# Make a call to AWS
try:
    result = connection.aws_call()
except BotoServerError as e:
    module.fail_json(msg="helpful message here", exception=traceback.format_exc(),
                     **camel_dict_to_snake_dict(e.message))
```

### Exception Handling for boto3 and botocore

You should wrap any boto3 or botocore call in a try block. If an exception is thrown, then there
are a number of possibilities for handling it.

* use aws_module.fail_json_aws() to report the module failure in a standard way
* retry using AWSRetry
* use fail_json() to report the failure without using `ansible.module_utils.aws.core`
* do something custom in the case where you know how to handle the exception

For more information on botocore exception handling see [the botocore error documentation](http://botocore.readthedocs.org/en/latest/client_upgrades.html#error-handling).

#### using fail_json_aws()

_fail_json_aws() is a new method and may be subject to change.  You can use it in modules which are
being contributed back to Ansible, however if you are publishing your module separately please
don't use it before the start of 2018 / Ansible 2.4_

In the AnsibleAWSModule there is a special method, `module.fail_json.aws()` for nice reporting of
exceptions.  Call this on your exception and it will report the error together with a traceback for
use in Ansible verbose mode.

```python
from ansible.module_utils.aws.core import HAS_BOTO3, AnsibleAWSModule

# Set up module parameters
...

# Connect to AWS
...

# Make a call to AWS
try:
    result = connection.aws_call()
except Exception as e:
    module.fail_json_aws(e, msg="trying to do aws_call")
```

Note that it should normally be acceptable to catch all normal exceptions here, however if you
expect anything other than botocore exceptions you should test everything works as expected.

If you need to perform an action based on the error boto3 returned, use the error code.

```python
# Make a call to AWS
try:
    result = connection.aws_call()
except ClientError, e:
    if e.response['Error']['Code'] == 'NoSuchEntity':
        return None
    else:
        module.fail_json_aws(e, msg="trying to do aws_call")
```

#### using fail_json() and avoiding ansible.module_utils.aws.core

Boto3 provides lots of useful information when an exception is thrown so pass this to the user
along with the message.

```python
# Import ClientError from botocore
try:
    from botocore.exceptions import ClientError
    HAS_BOTO3 = True
except ImportError:
    HAS_BOTO3 = False

# Connect to AWS
...

# Make a call to AWS
try:
    result = connection.aws_call()
except ClientError as e:
    module.fail_json(msg=e.message, exception=traceback.format_exc(),
                     **camel_dict_to_snake_dict(e.response))
```

If you need to perform an action based on the error boto3 returned, use the error code.

```python
# Make a call to AWS
try:
    result = connection.aws_call()
except ClientError as e:
    if e.response['Error']['Code'] == 'NoSuchEntity':
        return None
    else:
        module.fail_json(msg=e.message, exception=traceback.format_exc(),
                         **camel_dict_to_snake_dict(e.response))
```

### API throttling and pagination

For methods that return a lot of results, boto3 often provides
[paginators](http://boto3.readthedocs.io/en/latest/guide/paginators.html). If the method
you're calling has `NextToken` or `Marker` parameters, you should probably
check whether a paginator exists (the top of each boto3 service reference page has a link
to Paginators, if the service has any). To use paginators, obtain a paginator object,
call `paginator.paginate` with the appropriate arguments and then call `build_full_result`.

Any time that you are calling the AWS API a lot, you may experience API throttling,
and there is an `AWSRetry` decorator that can be used to ensure backoff. Because
exception handling could interfere with the retry working properly (as AWSRetry needs to
catch throttling exceptions to work correctly), you'd need to provide a backoff function
and then put exception handling around the backoff function.

You can use `exponential_backoff` or `jittered_backoff` strategies - see
the [cloud module_utils](/tree/devel/lib/ansible/module_utils/cloud.py)
and [AWS Architecture blog](https://www.awsarchitectureblog.com/2015/03/backoff.html)
for more details.

The combination of these two approaches is then

```
@AWSRetry.exponential_backoff(tries=5, delay=5)
def describe_some_resource_with_backoff(client, **kwargs):
     paginator = client.get_paginator('describe_some_resource')
     return paginator.paginate(**kwargs).build_full_result()['SomeResource']


def describe_some_resource(client, module):
    filters = ansible_dict_to_boto3_filter_list(module.params['filters'])
    try:
        return describe_some_resource_with_backoff(client, Filters=filters)
    except botocore.exceptions.ClientError as e:
        module.fail_json_aws(e, msg="Could not describe some resource")
```

### Returning Values

When you make a call using boto3, you will probably get back some useful information that you
should return in the module.  As well as information related to the call itself, you will also have
some response metadata.  It is OK to return this to the user as well as they may find it useful.

Boto3 returns all values CamelCased.  Ansible follows Python standards for variable names and uses
snake_case. There is a helper function in module_utils/ec2.py called `camel_dict_to_snake_dict`
that allows you to easily convert the boto3 response to snake_case.

You should use this helper function and avoid changing the names of values returned by Boto3.
E.g. if boto3 returns a value called 'SecretAccessKey' do not change it to 'AccessKey'.  

```python
# Make a call to AWS
result = connection.aws_call()

# Return the result to the user
module.exit_json(changed=True, **camel_dict_to_snake_dict(result))
```

### Dealing with IAM JSON policy

If your module accepts IAM JSON policies then set the type to 'json' in the module spec. For
example:

```python
argument_spec.update(
    dict(
        policy=dict(required=False, default=None, type='json'),
    )
)
```

Note that AWS is unlikely to return the policy in the same order that is was submitted. Therefore,
a helper function has been created to order policies before comparison.

```python
# Get the policy from AWS
current_policy = aws_object.get_policy()

# Compare the user submitted policy to the current policy but sort them first
if sort_json_policy_dict(user_policy) == sort_json_policy_dict(current_policy):
    # Nothing to do
    pass
else:
    # Update the policy
    aws_object.set_policy(user_policy)
```

### Dealing with tags

AWS has a concept of resource tags. Usually the boto3 API has separate calls for tagging and
untagging a resource.  For example, the ec2 API has a create_tags and delete_tags call.

It is common practice in Ansible AWS modules to have a `purge_tags` parameter that defaults to
true.

The `purge_tags` parameter means that existing tags will be deleted if they are not specified by
the Ansible task.

There is a helper function `compare_aws_tags` to ease dealing with tags. It can compare two dicts
and return the tags to set and the tags to delete.  See the Helper function section below for more
detail.

### Helper functions

Along with the connection functions in Ansible ec2.py module_utils, there are some other useful
functions detailed below.

#### camel_dict_to_snake_dict

boto3 returns results in a dict.  The keys of the dict are in CamelCase format. In keeping with
Ansible format, this function will convert the keys to snake_case.

#### ansible_dict_to_boto3_filter_list

Converts a an Ansible list of filters to a boto3 friendly list of dicts.  This is useful for any
boto3 `_facts` modules.

### boto_exception

Pass an exception returned from boto or boto3, and this function will consistently get the message from the exception.

```
import traceback
from ansible.module_utils.ec2 import boto_exception
try:
    ...
except boto.exception.BotoServerError as err:
    error_msg = boto_exception(err)
    module.fail_json(msg=error_msg, exception=traceback.format_exc())
```


#### boto3_tag_list_to_ansible_dict

Converts a boto3 tag list to an Ansible dict. Boto3 returns tags as a list of dicts containing keys
called 'Key' and 'Value' by default.  This key names can be overridden when calling the function.
For example, if you have already camel_cased your list of tags you may want to pass lowercase key
names instead i.e. 'key' and 'value'.

This function converts the list in to a single dict where the dict key is the tag key and the dict
value is the tag value.

#### ansible_dict_to_boto3_tag_list

Opposite of above. Converts an Ansible dict to a boto3 tag list of dicts. You can again override
the key names used if 'Key' and 'Value' is not suitable.

#### get_ec2_security_group_ids_from_names

Pass this function a list of security group names or combination of security group names and IDs
and this function will return a list of IDs.  You should also pass the VPC ID if known because
security group names are not necessarily unique across VPCs.

#### sort_json_policy_dict

Pass any JSON policy dict to this function in order to sort any list contained therein. This is
useful because AWS rarely return lists in the same order that they were submitted so without this
function, comparison of identical policies returns false.

### compare_aws_tags

Pass two dicts of tags and an optional purge parameter and this function will return a dict
containing key pairs you need to modify and a list of tag key names that you need to remove.  Purge
is True by default.  If purge is False then any existing tags will not be modified.

This function is useful when using boto3 'add_tags' and 'remove_tags' functions. Be sure to use the
other helper function `boto3_tag_list_to_ansible_dict` to get an appropriate tag dict before
calling this function. Since the AWS APIs are not uniform (e.g. EC2 versus Lambda) this will work
without modification for some (Lambda) and others may need modification before using these values
(such as EC2, with requires the tags to unset to be in the form `[{'Key': key1}, {'Key': key2}]`).
