Attach meta data / custom data to slack messages sent through the API

Question

I am developing a series of Slack apps for my workspace, and some of them are meant to interact with the content (messages) delivered by the other apps : extracting content IDs that may be referred to by other messages

A concrete example :

Suppose I have an app A "FindUser" that is capable of giving me the user profile when a slack user types find [email protected], and it replies in the thread with a formatted view of the user profile

I am developing an app B "EditTags", which basically gives me a right click option with "edit tags" (see Slack's Interactive Components/Actions), the idea being that a user could first ask app A to find a user, and then right click on the reply from App A and click the "edit tags" action given by the other app. What this app B does it actually retrieve the tags for the user mentionned by the previous message from app A, and in another reply to the thread it gives some controls to either delete an existing tag OR it shows a select with autocomplete to add new tags.

The B app needs to retrieve the user ID that the A app mentionned previously. So I need some way to pass that data directly in the slack message. When looking at the examples, slack does not seem to provide a way to add arbitrary "metadata" to a message, am I wrong ? Do you have workaround for this ? I mean I could totally send the user ID say, in the footer, so I can just read the footer, but I was planning to use the footer for something else... Is there a way to pass metadata hrough properties that would be hidden to the end user ?

Although this does not feel relevant, I am building a slack nodeJS app using the node slack sdk (and especially the @slack/interactive-messages package)

Answer 1

For the most part the Slack API does not provide any official means to attach custom data / meta data to messages. But with some simple "hacks" it is still possible. Here is how:

Approach

The basic approach is to use an existing field of the message as container for your data. Obviously you want to pick a field that is not directly linked to Slack functionality.

Some field are not always needed, so you can just use that field as data container. Or if its needed, you can include the functional value of that field along with your custom data in the data container.

For example for message buttons you could use the value field of a button and structure your code in a way that you do not need it in its original function. Usually its sufficient to know which button the user client (via the name field), so the value field is free to be used for your custom data. Or you can include the functional value of your button along with the custom data in a data container (e.g. a JSON string) in that field.

Serialization

All messages are transported through HTTP and mostly encoded as UTF-8 in JSON. So you want to serialize / de-serialize your data accordingly, especially if its binary data. If possible I would recommend to use JSON.

Length

The maximum allowed length of most fields is documented in the official Slack API documentation. e.g. for the value field for message buttons can contain up to 2.000 characters. Keep in mind that you need to consider the length of your data after serialization. e.g. if you convert binary data into Base64 so it can be transported with HTTP you will end up with about 1.33 characters for every byte.

Contents

In general I would recommend to keep your data container as small as possible and not include the actual data, but only IDs. Here are two common approaches:

• Include IDs of your data objects and load the actual objects from a data store when the request is later processed.
• Include the ID of server session and when processing the request you can restore the corresponding server session which contains all data objects. In addition you might need to include functional values so that the functionality of the field you are using still works (e.g. value of a menu option, see below)

Implementation

Dialogs

Dialogs provide an official field for custom data called state. Up to 3.000 characters.

Message buttons

For Message buttons you can use the message action fields / value. Up to 2.000 characters. Its also possible to use the name field, but I would advise against it, because the maximum allowed length of that field is not documented.

Message menus

For Message menus you can use the value field of an option or the name field of the menu action.

Usually the value field is the better approach, since you have a documented max length of 2.000 and it gives you more flexibility. However, you will need to combine you custom data with the actual functional value for each option. Also, this will not work for dynamic select elements (like users), where you can not control the value field.

When using the name field note, keep in mind that the maximum allowed length of name is not documented, so you want to keep you data as short as possible. Also, if you want to use more than one menu per attachment you need to include the actual name of the menu into your data container.

Normal message attachments

Normal message attachments do not contain any suitable field to be used as container for custom data, since all fields are linked to Slack functionality.

Technically you could use the fallback field, but only if you are 100% sure that your app is never used on a client that can not display attachments. Otherwise your data will be displayed to the user.