Hello juliamccurley,
Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
If your application was built using our retired V1 API, in order to continue use of your integration you or your developer will need to migrate your application to the current (V3) version of our API. Currently, nearly all V1 keys have been fully deprecated & shut down, after which the V1 endpoints being used by the application will be turned off as well.
I would advise against any new development using V2, as while no date is currently set, there will be a point in the near future where our legacy V2 API will begin deprecation, as the deprecation process for our retired V1 API is nearly complete. Due to this, while we will continue to deploy critical bug fixes at this time, we will not be doing any more development for V2 or its endpoints, and strongly recommend upgrading to the V3 API to ensure you're ahead of the curve.
Our current V3 API uses OAuth2.0, which is the current industry-standard for authorization. It’s more secure and does a much better job of keeping your Constant Contact account data safe. When you give their integration permission to access your account, it will show you exactly which permissions the integration is requesting to be granted (rather than just granting all permissions). V3 also uses tokens that are continuously refreshed to keep the connection encrypted and secure, rather than just accessing the account via your username and password.
There are several significant differences between our V1 and V3 APIs, including the use of different endpoints, OAuth 2.0 rather than basic authentication, JSON payloads instead of XML, and a different format for resource identifiers.
A quick start guide for the V3 API can be found here:
https://developer.constantcontact.com/api_guide/getting_started.html
While the rest of our documentation is linked from the quick start guide, the following pages in particular may be useful:
https://developer.constantcontact.com/api_guide/v3_technical_overview.html
https://developer.constantcontact.com/api_guide/developer_portal.html
https://developer.constantcontact.com/api_guide/migration_overview.html
Additionally, you can utilize our API Reference tester to test out the functionality of our available endpoints to see how they will work once programmed within your application:
https://v3.developer.constantcontact.com/api_reference/index.html
Please have a look and let us know if you or your developer(s) have any other questions!
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