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S3 to SFTP Transfer Function

This is a simple single-purpose Lambda function, written in Python3, that will transfer a file from S3 to an SFTP server, on upload to S3. If the file transfer from S3 to SFTP is successful, the source S3 file is deleted.

This project contains the source code for the function along with packaging instructions for preparing the function and its dependencies for upload to AWS. The function itself is very simple, and is contained in s3_to_sftp.py. It should be self-explanatory for anyone familiar with Python.

SFTP Authentication

The function supports authentication against the remote SFTP server using a username and either a password or a private key. If authenticating using a private key then the key should be stored in a text file in a secure S3 bucket to which the IAM role under which the function is running has read access.

Configuration

The following environment variables MUST be set:

SSH_HOST - the host address of the destination SFTP server
SSH_USERNAME - the SSH account username
SSH_PASSWORD - the SSH account password, OR
SSH_PRIVATE_KEY - path to a private key file on S3, in 'bucket:key' format

The following environment variables MAY be set:

SSH_PORT - the port number (defaults to 22)
SSH_DIR - a specific directory to upload files to
SSH_FILENAME - the name of SFTP file created - supports {{ bucket|key|current_date }} format options

Packaging and Deployment

Deploying this function to Lambda involves uploaded a zip file that contains the source code and all of the project dependencies (as specified in requirements.txt). The tricky part is that Lambda functions run inside an AmazonLinux environment, and that means that Python dependencies need to be generated / built within the same. The brute force approach to this would be to create an AmazonLinux EC2 instance and package up the project there, but this is hard to do, and impractical during development. A simpler solution is provided by the Dockerfile in this project.

These instructions assume that you have Docker installed.

1. Build the packaging image

The default amazonlinux:latest docker image provided by Amazon has Python2.7 installed - which is not what we are using. In order to support Python3 development we have to create a new image off the base image that has Python3.

Create the new image and give it a sensible name using the -t option:

$ docker build -t lambda-packager .

You should now have a docker image called lambda-packager which can be used to package the function and its dependencies, as well as upload it to AWS.

2. Use the image created to run the make package command inside the container (so that it run within an amazonlinux environment):

# package command builds the package.zip file ready for upload to AWS
$ docker run --rm --volume $(pwd):/lambda lambda-packager package

This command will mount the current directory ($(pwd)) into a new container as /lambda, so that the container has access to requirements.txt and the function file (s3_to_sftp.py), and run the make package command.

The package command does the following, inside the container:

• pip install the requirements into /lambda/.dist
• Copy s3_to_sftp.py source file into /lambda/.dist
• Zip up the directory into a new file called package.zip

3. Once you have generated the package.zip, you can upload it to AWS using the make upload command. As the name suggests, this will only update an existing function's code - so you'll have to create the function initially via the AWS console. Once you have done that you can use the function's 'ARN' to update it:

# NB you will require the aws sdk to be installed and configured for this to work
$ make upload ARN=arn:aws:lambda:us-east-1:account-id:function:s3-to-sftp

Or you can just upload the package.zip file through the console: