Schedule Postings to Mastodon, Twitter and Instagram

This is a simple AWS Lambda function that posts an image with caption to Mastodon, Instagram (via IFTTT) or Twitter (via IFTTT). It is intended to be used with AWS API Gateway and AWS S3. It provides this functionality in one single API call. The Mastodon API itself requires two seperate API calls to post an image with caption (one to upload the image and one to post the image with caption).

This function can be used with automation services like IFTTT to post images with caption to Mastodon, Twitter or Instagram.

Copyright (c) 2023 Michael Kleinhenz michael@kleinhenz.net.

Licensed under the MIT License.

Text Handling

Mastodon posts are limited to 500 characters. If the caption is longer than 500 characters, it will "smart shorten" the text:

• remove all hashtags that are not contained in body text. Criteria is two or more sequential hashtags.
• if the resulting body text still exceeds 500 characters, it is shortened to the end of the last sentence that fits (looking for '.').
• if no '.' is found, the text is shortened to 500 characters and an ellipsis is added.
• if the body text is shorter than 500 characters, hashtags are added back in until the length is approaching 500 characters.

Contents

• function - The Lambda function.
• template.yml - An AWS CloudFormation template that creates an application.

Use the following instructions to deploy the sample application.

Requirements

• Node.js 10 with npm
• The Bash shell. For Linux and macOS, this is included by default. In Windows 10, you can install the Windows Subsystem for Linux to get a Windows-integrated version of Ubuntu and Bash.
• The AWS CLI v1.17 or newer.

If you use the AWS CLI v2, add the following to your configuration file (~/.aws/config):

cli_binary_format=raw-in-base64-out

This setting enables the AWS CLI v2 to load JSON events from a file, matching the v1 behavior.

Make sure your AWS credentials are set correctly, for example using environment variables:

export AWS_ACCESS_KEY_ID=YOUR_ACCESS_KEY
export AWS_SECRET_ACCESS_KEY=YOUR_SECRET
export AWS_DEFAULT_REGION=YOUR_REGION

Setup

Download or clone this repository.

$ git clone https://github.com/michaelkleinhenz/mastodon-post.git
$ cd mastodon-post

Install dependencies.

$ cd function
$ npm install
$ cd ..

To create a new bucket for deployment artifacts, run make createbucket.

mastodon-post$ make createbucket

Deploy

To deploy the application, run make deploy.

mastodon-post$ make deploy
added 16 packages from 18 contributors and audited 18 packages in 0.926s
added 17 packages from 19 contributors and audited 19 packages in 0.916s
Uploading to e678bc216e6a0d510d661ca9ae2fd941  2737254 / 2737254.0  (100.00%)
Successfully packaged artifacts and wrote output template to file out.yml.
Waiting for changeset to be created..
Waiting for stack create/update to complete
Successfully created/updated stack - mastodon-post

The make target uses AWS CloudFormation to deploy the Lambda functions and an IAM role. If the AWS CloudFormation stack that contains the resources already exists, the script updates it with any changes to the template or function code.

Test

To invoke the function with the REST API, run the test make target. This target uses cURL to send a GET request to the API endpoint. Make sure you update the argument values given in testrequest.json.

You can also run the function locally using AWS SAM. Details can be found in the launch configuration in the .vscode folder. Refer also to the AWS SAM documentation.

Note: if using Fedora and want to debug using AWS SAM, you need to disable selinux for docker to work properly. See aws/aws-sam-cli#2360 for details.

Finally, view the application in the Lambda console.

To view the application

1. Open the applications page in the Lambda console.
2. Choose mastodon-post.

Usage

The function expects a JSON POST request with the following body:

{
 'context': 'mastodon',
    'context': 'mastodon',
    'postingHost': 'https://your.mastodon.instance',
    'postingToken': 'YOUR_TOKEN',
    'caption': 'YOUR_TEXT_CAPTION',
    'imageURL': 'IMAGE_URL',
    'postingTime': EPOCH_TIME_IN_SECONDS,      
}

Make sure the postingHost is the full URL of your Mastodon instance or IFTTT service (with key embedded in the URL), including the protocol (https://). The postingToken is the access token of your Mastodon account. You can generate one in the Mastodon web interface under Settings > Development > New Application. The caption is the text that will be posted with the image. The imageURL is the URL of the image that will be posted. The image must be publicly accessible. The postingTime is the time the post should be published, in epoch seconds.

Setting up the Database and Scheduler

The function stores posting requests in a DynamoDB table. The table is created by the CloudFormation template. To schedule the function to run periodically, you can use the AWS EventBridge service. See https://docs.aws.amazon.com/AmazonCloudWatch/latest/events/RunLambdaSchedule.html for details on the scheduler. The service needs to be called with the context schedule to run the scheduler action on the stored posting requests, see test.js for an example.

When setting up the database, create a table scheduled-posts on DynamoDB and an index context-index on the context attribute.