docs/src/contributing_apub_api_outline.md

   1 # Activitypub API outline
   2
   3 This document is targeted at developers who are familiar with the ActivityPub and ActivityStreams protocols. It gives a detailed outline of the actors, objects and activities used by Lemmy.
   4
   5 Before reading this, have a look at our [Federation Overview](contributing_federation_overview.md) to get an idea how Lemmy federation works on a high level.
   6
   7 Lemmy does not yet follow the ActivityPub spec in all regards. For example, we don't set a valid context indicating our context fields. We also ignore fields like `inbox`, `outbox` or `endpoints` for remote actors, and assume that everything is Lemmy. For an overview of deviations, read [#698](https://github.com/LemmyNet/lemmy/issues/698). They will be fixed in the near future.
   8
   9 Lemmy is also really inflexible when it comes to incoming activities and objects. They need to be exactly identical to the examples below. Things like having an array instead of a single value, or an object ID instead of the full object will result in an error.
  10
  11 In the following tables, "mandatory" refers to whether or not Lemmy will accept an incoming activity without this field. Lemmy itself will always include all non-empty fields.
  12
  13 <!-- toc -->
  14
  15 - [Actors](#actors)
  16   * [Community](#community)
  17   * [User](#user)
  18 - [Objects](#objects)
  19   * [Post](#post)
  20   * [Comment](#comment)
  21   * [Private Message](#private-message)
  22 - [Activities](#activities)
  23   * [Follow](#follow)
  24   * [Accept Follow](#accept-follow)
  25   * [Unfollow](#unfollow)
  26   * [Create or Update Post](#create-or-update-post)
  27   * [Create or Update Comment](#create-or-update-comment)
  28   * [Like Post or Comment](#like-post-or-comment)
  29   * [Dislike Post or Comment](#dislike-post-or-comment)
  30   * [Delete Post or Comment](#delete-post-or-comment)
  31   * [Remove Post or Comment](#remove-post-or-comment)
  32   * [Undo](#undo)
  33   * [Announce](#announce)
  34   * [Create or Update Private message](#create-or-update-private-message)
  35   * [Delete Private Message](#delete-private-message)
  36   * [Undo Delete Private Message](#undo-delete-private-message)
  37
  38 <!-- tocstop -->
  39
  40 ## Actors
  41
  42 ### Community
  43
  44 An automated actor. Users can send posts or comments to it, which the community forwards to its followers in the form of `Announce`.
  45
  46 Sends activities to user: `Accept/Follow`, `Announce`
  47
  48 Receives activities from user: `Follow`, `Undo/Follow`, `Create`, `Update`, `Like`, `Dislike`, `Remove` (only admin/mod), `Delete` (only creator), `Undo` (only for own actions)
  49
  50 ```json
  51 {
  52     "@context": "https://www.w3.org/ns/activitystreams",
  53     "id": "https://enterprise.lemmy.ml/c/main",
  54     "type": "Group",
  55     "preferredUsername": "main",
  56     "name": "The Main Community",
  57     "category": {
  58         "identifier": "1",
  59         "name": "Discussion"
  60     },
  61     "sensitive": false,
  62     "attributedTo": [
  63         "https://enterprise.lemmy.ml/u/picard",
  64         "https://enterprise.lemmy.ml/u/riker"
  65     ],
  66     "content": "Welcome to the default community!",
  67     "source": {
  68         "content": "Welcome to the default community!",
  69         "mediaType": "text/markdown"
  70     },
  71     "icon": {
  72         "type": "Image",
  73         "url": "https://enterprise.lemmy.ml/pictrs/image/Z8pFFb21cl.png"
  74     },
  75     "image": {
  76         "type": "Image",
  77         "url": "https://enterprise.lemmy.ml/pictrs/image/Wt8zoMcCmE.jpg"
  78     },
  79     "inbox": "https://enterprise.lemmy.ml/c/main/inbox",
  80     "outbox": "https://enterprise.lemmy.ml/c/main/outbox",
  81     "followers": "https://enterprise.lemmy.ml/c/main/followers",
  82     "endpoints": {
  83         "sharedInbox": "https://enterprise.lemmy.ml/inbox"
  84     },
  85     "published": "2020-10-06T17:27:43.282386+00:00",
  86     "updated": "2020-10-08T11:57:50.545821+00:00",
  87     "publicKey": {
  88         "id": "https://enterprise.lemmy.ml/c/main#main-key",
  89         "owner": "https://enterprise.lemmy.ml/c/main",
  90         "publicKeyPem": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA9JJ7Ybp/H7iXeLkWFepg\ny4PHyIXY1TO9rK3lIBmAjNnkNywyGXMgUiiVhGyN9yU7Km8aWayQsNHOkPL7wMZK\nnY2Q+CTQv49kprEdcDVPGABi6EbCSOcRFVaUjjvRHf9Olod2QP/9OtX0oIFKN2KN\nPIUjeKK5tw4EWB8N1i5HOuOjuTcl2BXSemCQLAlXerLjT8xCarGi21xHPaQvAuns\nHt8ye7fUZKPRT10kwDMapjQ9Tsd+9HeBvNa4SDjJX1ONskNh2j4bqHHs2WUymLpX\n1cgf2jmaXAsz6jD9u0wfrLPelPJog8RSuvOzDPrtwX6uyQOl5NK00RlBZwj7bMDx\nzwIDAQAB\n-----END PUBLIC KEY-----\n"
  91     }
  92 }
  93 ```
  94
  95 | Field Name | Mandatory | Description |
  96 |---|---|---|
  97 | `preferredUsername` | yes | Name of the actor |
  98 | `name` | yes | Title of the community |
  99 | `category` | yes | Hardcoded list of categories, see https://lemmy.ml/api/v1/categories |
 100 | `sensitive` | yes | True indicates that all posts in the community are nsfw |
 101 | `attributedTo` | yes | First the community creator, then all the remaining moderators |
 102 | `content` | no | Text for the community sidebar, usually containing a description and rules |
 103 | `icon` | no | Icon, shown next to the community name |
 104 | `image` | no | Banner image, shown on top of the community page |
 105 | `inbox` | no | ActivityPub inbox URL |
 106 | `outbox` | no | ActivityPub outbox URL, only contains up to 20 latest posts, no comments, votes or other activities |
 107 | `followers` | no | Follower collection URL, only contains the number of followers, no references to individual followers |
 108 | `endpoints` | no | Contains URL of shared inbox |
 109 | `published` | no | Datetime when the community was first created |
 110 | `updated` | no | Datetime when the community was last changed |
 111 | `publicKey` | yes | The public key used to verify signatures from this actor |
 112
 113 ### User
 114
 115 A person, interacts primarily with the community where it sends and receives posts/comments. Can also create and moderate communities, and send private messages to other users.
 116
 117 Sends activities to Community: `Follow`, `Undo/Follow`, `Create`, `Update`, `Like`, `Dislike`, `Remove` (only admin/mod), `Delete` (only creator), `Undo` (only for own actions)
 118
 119 Receives activities from Community: `Accept/Follow`, `Announce`
 120
 121 Sends and receives activities from/to other users: `Create/Note`, `Update/Note`, `Delete/Note`, `Undo/Delete/Note` (all those related to private messages)
 122
 123 ```json
 124 {
 125     "@context": "https://www.w3.org/ns/activitystreams",
 126     "id": "https://enterprise.lemmy.ml/u/picard",
 127     "type": "Person",
 128     "preferredUsername": "picard",
 129     "name": "Jean-Luc Picard",
 130     "content": "The user bio",
 131     "source": {
 132         "content": "The user bio",
 133         "mediaType": "text/markdown"
 134     },
 135     "icon": {
 136         "type": "Image",
 137         "url": "https://enterprise.lemmy.ml/pictrs/image/DS3q0colRA.jpg"
 138     },
 139     "image": {
 140         "type": "Image",
 141         "url": "https://enterprise.lemmy.ml/pictrs/image/XenaYI5hTn.png"
 142     },
 143     "inbox": "https://enterprise.lemmy.ml/u/picard/inbox",
 144     "endpoints": {
 145         "sharedInbox": "https://enterprise.lemmy.ml/inbox"
 146     },
 147     "published": "2020-10-06T17:27:43.234391+00:00",
 148     "updated": "2020-10-08T11:27:17.905625+00:00",
 149     "publicKey": {
 150         "id": "https://enterprise.lemmy.ml/u/picard#main-key",
 151         "owner": "https://enterprise.lemmy.ml/u/picard",
 152         "publicKeyPem": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAyH9iH83+idw/T4QpuRSY\n5YgQ/T5pJCNxvQWb6qcCu3gEVigfbreqZKJpOih4YT36wu4GjPfoIkbWJXcfcEzq\nMEQoYbPStuwnklpN2zj3lRIPfGLht9CAlENLWikTUoW5kZLyU6UQtOGdT2b1hDuK\nsUEn67In6qYx6pal8fUbO6X3O2BKzGeofnXgHCu7QNIuH4RPzkWsLhvwqEJYP0zG\nodao2j+qmhKFsI4oNOUCGkdJejO7q+9gdoNxAtNNKilIOwUFBYXeZJb+XGlzo0X+\n70jdJ/xQCPlPlItU4pD/0FwPLtuReoOpMzLi20oDsPXJBvn+/NJaxqDINuywcN5p\n4wIDAQAB\n-----END PUBLIC KEY-----\n"
 153     }
 154 }
 155 ```
 156
 157 | Field Name | Mandatory | Description |
 158 |---|---|---|
 159 | `preferredUsername` | yes | Name of the actor |
 160 | `name` | no | The user's displayname |
 161 | `content` | no | User bio |
 162 | `icon` | no | The user's avatar, shown next to the username |
 163 | `image` | no | The user's banner, shown on top of the profile |
 164 | `inbox` | no | ActivityPub inbox URL |
 165 | `endpoints` | no | Contains URL of shared inbox |
 166 | `published` | no | Datetime when the user signed up |
 167 | `updated` | no | Datetime when the user profile was last changed |
 168 | `publicKey` | yes | The public key used to verify signatures from this actor |
 169
 170 ## Objects
 171
 172 ### Post
 173
 174 A page with title, and optional URL and text content. The URL often leads to an image, in which case a thumbnail is included. Each post belongs to exactly one community.
 175
 176 ```json
 177 {
 178     "@context": "https://www.w3.org/ns/activitystreams",
 179     "id": "https://voyager.lemmy.ml/post/29",
 180     "type": "Page",
 181     "attributedTo": "https://voyager.lemmy.ml/u/picard",
 182     "to": "https://voyager.lemmy.ml/c/main",
 183     "summary": "Test thumbnail 2",
 184     "content": "blub blub",
 185     "source": {
 186         "content": "blub blub",
 187         "mediaType": "text/markdown"
 188     },
 189     "url": "https://voyager.lemmy.ml:/pictrs/image/fzGwCsq7BJ.jpg",
 190     "image": {
 191         "type": "Image",
 192         "url": "https://voyager.lemmy.ml/pictrs/image/UejwBqrJM2.jpg"
 193     },
 194     "commentsEnabled": true,
 195     "sensitive": false,
 196     "stickied": false,
 197     "published": "2020-09-24T17:42:50.396237+00:00",
 198     "updated": "2020-09-24T18:31:14.158618+00:00"
 199 }
 200 ```
 201
 202 | Field Name | Mandatory | Description |
 203 |---|---|---|
 204 | `attributedTo` | yes | ID of the user which created this post |
 205 | `to` | yes | ID of the community where it was posted to |
 206 | `summary` | yes | Title of the post |
 207 | `content` | no | Body of the post |
 208 | `url` | no | An arbitrary link to be shared |
 209 | `image` | no | Thumbnail for `url`, only present if it is an image link |
 210 | `commentsEnabled` | yes | False indicates that the post is locked, and no comments can be added |
 211 | `sensitive` | yes | True marks the post as NSFW, blurs the thumbnail and hides it from users with NSFW settign disabled |
 212 | `stickied` | yes | True means that it is shown on top of the community |
 213 | `published` | no | Datetime when the post was created |
 214 | `updated` | no | Datetime when the post was edited (not present if it was never edited) |
 215
 216 ### Comment
 217
 218 A reply to a post, or reply to another comment. Contains only text (including references to other users or communities). Lemmy displays comments in a tree structure.
 219
 220 ```json
 221 {
 222     "@context": "https://www.w3.org/ns/activitystreams",
 223     "id": "https://enterprise.lemmy.ml/comment/95",
 224     "type": "Note",
 225     "attributedTo": "https://enterprise.lemmy.ml/u/picard",
 226     "to": "https://enterprise.lemmy.ml/c/main",
 227     "content": "mmmk",
 228     "source": {
 229         "content": "mmmk",
 230         "mediaType": "text/markdown"
 231     },
 232     "inReplyTo": [
 233         "https://enterprise.lemmy.ml/post/38",
 234         "https://voyager.lemmy.ml/comment/73"
 235     ],
 236     "published": "2020-10-06T17:53:22.174836+00:00",
 237     "updated": "2020-10-06T17:53:22.174836+00:00"
 238 }
 239 ```
 240
 241 | Field Name | Mandatory | Description |
 242 |---|---|---|
 243 | `attributedTo` | yes | ID of the user who created the comment |
 244 | `to` | yes | Community where the comment was made |
 245 | `content` | yes | The comment text |
 246 | `inReplyTo` | yes | IDs of the post where this comment was made, and the parent comment. If this is a top-level comment, `inReplyTo` only contains the post |
 247 | `published` | no | Datetime when the comment was created |
 248 | `updated` | no | Datetime when the comment was edited (not present if it was never edited) |
 249
 250 ### Private Message
 251
 252 A direct message from one user to another. Can not include additional users. Threading is not implemented yet, so the `inReplyTo` field is missing.
 253
 254 ```json
 255 {
 256     "@context": "https://www.w3.org/ns/activitystreams",
 257     "id": "https://enterprise.lemmy.ml/private_message/34",
 258     "type": "Note",
 259     "attributedTo": "https://enterprise.lemmy.ml/u/picard",
 260     "to": "https://voyager.lemmy.ml/u/janeway",
 261     "content": "test",
 262     "source": {
 263         "content": "test",
 264         "mediaType": "text/markdown"
 265     },
 266     "mediaType": "text/markdown",
 267     "published": "2020-10-08T19:10:46.542820+00:00",
 268     "updated": "2020-10-08T20:13:52.547156+00:00"
 269 }
 270 ```
 271
 272 | Field Name | Mandatory | Description |
 273 |---|---|---|
 274 | `attributedTo` | ID of the user who created this private message |
 275 | `to` | ID of the recipient |
 276 | `content` | yes | The text of the private message |
 277 | `published` | no | Datetime when the message was created |
 278 | `updated` | no | Datetime when the message was edited (not present if it was never edited) |
 279
 280 ## Activities
 281
 282 ### Follow
 283
 284 When the user clicks "Subscribe" in a community, a `Follow` is sent. The community automatically responds with an `Accept/Follow`.
 285
 286 Sent by: User
 287
 288 Sent to: Community
 289
 290 ```json
 291 {
 292     "@context": "https://www.w3.org/ns/activitystreams",
 293     "id": "https://enterprise.lemmy.ml/activities/follow/2e4784b7-4edf-4fa1-a352-674d5d5f8891",
 294     "type": "Follow",
 295     "actor": "https://enterprise.lemmy.ml/u/picard",
 296     "to": "https://ds9.lemmy.ml/c/main",
 297     "object": "https://ds9.lemmy.ml/c/main"
 298 }
 299 ```
 300
 301 | Field Name | Mandatory | Description |
 302 |---|---|---|
 303 | `actor` | yes | The user that is sending the follow request |
 304 | `object` | yes | The community to be followed |
 305
 306 ### Accept Follow
 307
 308 Automatically sent by the community in response to a `Follow`. At the same time, the community adds this user to its followers list.
 309
 310 Sent by: Community
 311
 312 Sent to: User
 313
 314 ```json
 315 {
 316     "@context": "https://www.w3.org/ns/activitystreams",
 317     "id": "https://ds9.lemmy.ml/activities/accept/5314bf7c-dab8-4b01-baf2-9be11a6a812e",
 318     "type": "Accept",
 319     "actor": "https://ds9.lemmy.ml/c/main",
 320     "to": "https://enterprise.lemmy.ml/u/picard",
 321     "object": {
 322         "@context": "https://www.w3.org/ns/activitystreams",
 323         "id": "https://enterprise.lemmy.ml/activities/follow/2e4784b7-4edf-4fa1-a352-674d5d5f8891",
 324         "type": "Follow",
 325         "object": "https://ds9.lemmy.ml/c/main",
 326         "actor": "https://enterprise.lemmy.ml/u/picard"
 327     }
 328 }
 329 ```
 330
 331 | Field Name | Mandatory | Description |
 332 |---|---|---|
 333 | `actor` | yes | The same community as in the `Follow` activity |
 334 | `to` | no | ID of the user which sent the `Follow` |
 335 | `object` | yes | The previously sent `Follow` activity |
 336
 337 ### Unfollow
 338
 339 Clicking on the unsubscribe button in a community causes an `Undo/Follow` to be sent. The community removes the user from its follower list after receiving it.
 340
 341 Sent by: User
 342
 343 Sent to: Community
 344
 345 ```json
 346 {
 347     "@context": "https://www.w3.org/ns/activitystreams",
 348     "id": "http://lemmy-alpha:8541/activities/undo/2c624a77-a003-4ed7-91cb-d502eb01b8e8",
 349     "type": "Undo",
 350     "actor": "http://lemmy-alpha:8541/u/lemmy_alpha",
 351     "to": "http://lemmy-beta:8551/c/main",
 352     "object": {
 353         "@context": "https://www.w3.org/ns/activitystreams",
 354         "id": "http://lemmy-alpha:8541/activities/follow/f0d732e7-b1e7-4857-a5e0-9dc83c3f7ee8",
 355         "type": "Follow",
 356         "actor": "http://lemmy-alpha:8541/u/lemmy_alpha",
 357         "object": "http://lemmy-beta:8551/c/main"
 358     }
 359 }
 360 ```
 361 ### Create or Update Post
 362
 363 When a user creates a new post, it is sent to the respective community. Editing a previously created post sends an almost identical activity, except the `type` being `Update`. We don't support mentions in posts yet.
 364
 365 Sent by: User
 366
 367 Sent to: Community
 368
 369 ```json
 370 {
 371     "@context": "https://www.w3.org/ns/activitystreams",
 372     "id": "https://enterprise.lemmy.ml/activities/create/6e11174f-501a-4531-ac03-818739bfd07d",
 373     "type": "Create",
 374     "actor": "https://enterprise.lemmy.ml/u/riker",
 375     "to": "https://www.w3.org/ns/activitystreams#Public",
 376     "cc": [
 377       "https://ds9.lemmy.ml/c/main/"
 378     ],
 379     "object": ...
 380 }
 381 ```
 382
 383 | Field Name | Mandatory | Description |
 384 |---|---|---|
 385 | `type` | yes | either `Create` or `Update` |
 386 | `cc` | yes | Community where the post is being made |
 387 | `object` | yes | The post being created |
 388
 389 ### Create or Update Comment
 390
 391 A reply to a post, or to another comment. Can contain mentions of other users. Editing a previously created post sends an almost identical activity, except the `type` being `Update`.
 392
 393 Sent by: User
 394
 395 Sent to: Community
 396
 397 ```json
 398 {
 399     "@context": "https://www.w3.org/ns/activitystreams",
 400     "id": "https://enterprise.lemmy.ml/activities/create/6f52d685-489d-4989-a988-4faedaed1a70",
 401     "type": "Create",
 402     "actor": "https://enterprise.lemmy.ml/u/riker",
 403     "to": "https://www.w3.org/ns/activitystreams#Public",
 404     "tag": [{
 405         "type": "Mention",
 406         "name": "@sisko@ds9.lemmy.ml",
 407         "href": "https://ds9.lemmy.ml/u/sisko"
 408     }],
 409     "cc": [
 410         "https://ds9.lemmy.ml/c/main/",
 411         "https://ds9.lemmy.ml/u/sisko"
 412     ],
 413     "object": ...
 414 }
 415 ```
 416
 417 | Field Name | Mandatory | Description |
 418 |---|---|---|
 419 | `tag` | no | List of users which are mentioned in the comment (like `@user@example.com`) |
 420 | `cc` | yes | Community where the post is being made, the user being replied to (creator of the parent post/comment), as well as any mentioned users |
 421 | `object` | yes | The comment being created |
 422
 423 ### Like Post or Comment
 424
 425 An upvote for a post or comment.
 426
 427 Sent by: User
 428
 429 Sent to: Community
 430
 431 ```json
 432 {
 433     "@context": "https://www.w3.org/ns/activitystreams",
 434     "id": "https://enterprise.lemmy.ml/activities/like/8f3f48dd-587d-4624-af3d-59605b7abad3",
 435     "type": "Like",
 436     "actor": "https://enterprise.lemmy.ml/u/riker",
 437     "to": "https://www.w3.org/ns/activitystreams#Public",
 438     "cc": [
 439         "https://ds9.lemmy.ml/c/main/"
 440     ],
 441     "object": ...
 442 }
 443 ```
 444
 445 | Field Name | Mandatory | Description |
 446 |---|---|---|
 447 | `cc` | yes | ID of the community where the post/comment is |
 448 | `object` | yes | The post or comment being upvoted |
 449
 450 ### Dislike Post or Comment
 451
 452 A downvote for a post or comment.
 453
 454 Sent by: User
 455
 456 Sent to: Community
 457
 458 ```json
 459 {
 460     "@context": "https://www.w3.org/ns/activitystreams",
 461     "id": "https://enterprise.lemmy.ml/activities/dislike/fd2b8e1d-719d-4269-bf6b-2cadeebba849",
 462     "type": "Dislike",
 463     "actor": "https://enterprise.lemmy.ml/u/riker",
 464     "to": "https://www.w3.org/ns/activitystreams#Public",
 465     "cc": [
 466       "https://ds9.lemmy.ml/c/main/"
 467     ],
 468     "object": ...
 469 }
 470 ```
 471
 472 | Field Name | Mandatory | Description |
 473 |---|---|---|
 474 | `cc` | yes | ID of the community where the post/comment is |
 475 | `object` | yes | The post or comment being upvoted |
 476
 477 ### Delete Post or Comment
 478
 479 Deletes a previously created post or comment. This can only be done by the original creator of that post/comment.
 480
 481 Sent by: User
 482
 483 Sent to: Community
 484
 485 ```json
 486 {
 487     "@context": "https://www.w3.org/ns/activitystreams",
 488     "id": "https://enterprise.lemmy.ml/activities/delete/f1b5d57c-80f8-4e03-a615-688d552e946c",
 489     "type": "Delete",
 490     "actor": "https://enterprise.lemmy.ml/u/riker",
 491     "to": "https://www.w3.org/ns/activitystreams#Public",
 492     "cc": [
 493         "https://enterprise.lemmy.ml/c/main/"
 494     ],
 495     "object": "https://enterprise.lemmy.ml/post/32"
 496 }
 497 ```
 498
 499 | Field Name | Mandatory | Description |
 500 |---|---|---|
 501 | `cc` | yes | ID of the community where the post/comment is |
 502 | `object` | yes | ID of the post or comment being deleted |
 503
 504 ### Remove Post or Comment
 505
 506 Removes a post or comment. This can only be done by a community mod, or by an admin on the instance where the community is hosted.
 507
 508 Sent by: User
 509
 510 Sent to: Community
 511
 512 ```json
 513 {
 514     "@context": "https://www.w3.org/ns/activitystreams",
 515     "id": "https://ds9.lemmy.ml/activities/remove/aab93b8e-3688-4ea3-8212-d00d29519218",
 516     "type": "Remove",
 517     "actor": "https://ds9.lemmy.ml/u/sisko",
 518     "to": "https://www.w3.org/ns/activitystreams#Public",
 519     "cc": [
 520         "https://ds9.lemmy.ml/c/main/"
 521     ],
 522     "object": "https://enterprise.lemmy.ml/comment/32"
 523 }
 524 ```
 525
 526 | Field Name | Mandatory | Description |
 527 |---|---|---|
 528 | `cc` | yes | ID of the community where the post/comment is |
 529 | `object` | yes | ID of the post or comment being removed |
 530
 531 ### Undo
 532
 533 Reverts a previous activity, can only be done by the `actor` of `object`. In case of a `Like` or `Dislike`, the vote count is changed back. In case of a `Delete` or `Remove`, the post/comment is restored. The `object` is regenerated from scratch, as such the activity ID and other fields are different.
 534
 535 Sent by: User
 536
 537 Sent to: Community
 538
 539 ```json
 540 {
 541   "@context": "https://www.w3.org/ns/activitystreams",
 542   "id": "https://ds9.lemmy.ml/activities/undo/70ca5fb2-e280-4fd0-a593-334b7f8a5916",
 543   "type": "Undo",
 544   "actor": "https://ds9.lemmy.ml/u/sisko",
 545   "to": "https://www.w3.org/ns/activitystreams#Public",
 546   "cc": [
 547     "https://ds9.lemmy.ml/c/main/"
 548   ],
 549   "object": ...
 550 }
 551 ```
 552
 553 | Field Name | Mandatory | Description |
 554 |---|---|---|
 555 | `object` | yes | Any `Like`, `Dislike`, `Delete` or `Remove` activity as described above |
 556
 557 ### Announce
 558
 559 When the community receives a post or comment activity, it wraps that into an `Announce` and sends it to all followers.
 560
 561 Sent by: Community
 562
 563 Sent to: User
 564
 565 ```json
 566 {
 567   "@context": "https://www.w3.org/ns/activitystreams",
 568   "id": "https://ds9.lemmy.ml/activities/announce/b98382e8-6cb1-469e-aa1f-65c5d2c31cc4",
 569   "type": "Announce",
 570   "actor": "https://ds9.lemmy.ml/c/main",
 571   "to": "https://www.w3.org/ns/activitystreams#Public",
 572   "cc": [
 573     "https://ds9.lemmy.ml/c/main/followers"
 574   ],
 575   "object": ...
 576 }
 577 ```
 578
 579 | Field Name | Mandatory | Description |
 580 |---|---|---|
 581 | `object` | yes | Any `Create`, `Update`, `Like`, `Dislike`, `Delete` `Remove` or `Undo` activity as described above |
 582
 583 ### Remove or Delete Community
 584
 585 ```json
 586 {
 587   "@context": "https://www.w3.org/ns/activitystreams",
 588   "id": "http://ds9.lemmy.ml/activities/remove/e4ca7688-af9d-48b7-864f-765e7f9f3591",
 589   "type": "Remove",
 590   "actor": "http://ds9.lemmy.ml/c/some_community",
 591   "cc": [
 592     "http://ds9.lemmy.ml/c/some_community/followers"
 593   ],
 594   "to": "https://www.w3.org/ns/activitystreams#Public",
 595   "object": "http://ds9.lemmy.ml/c/some_community"
 596 }
 597 ```
 598
 599 | Field Name | Mandatory | Description |
 600 |---|---|---|
 601 | `type` | yes | Either `Remove` or `Delete` |
 602
 603 ### Restore Removed or Deleted Community
 604
 605 ```json
 606 {
 607   "@context": "https://www.w3.org/ns/activitystreams",
 608   "id": "http://ds9.lemmy.ml/activities/like/0703668c-8b09-4a85-aa7a-f93621936901",
 609   "type": "Undo",
 610   "actor": "http://ds9.lemmy.ml/c/some_community",
 611   "to": "https://www.w3.org/ns/activitystreams#Public",
 612   "cc": [
 613     "http://ds9.lemmy.ml/c/testcom/followers"
 614   ],
 615   "object": {
 616     "@context": "https://www.w3.org/ns/activitystreams",
 617     "id": "http://ds9.lemmy.ml/activities/remove/1062b5e0-07e8-44fc-868c-854209935bdd",
 618     "type": "Remove",
 619     "actor": "http://ds9.lemmy.ml/c/some_community",
 620     "object": "http://ds9.lemmy.ml/c/testcom",
 621     "to": "https://www.w3.org/ns/activitystreams#Public",
 622     "cc": [
 623       "http://ds9.lemmy.ml/c/testcom/followers"
 624     ]
 625   }
 626 }
 627
 628 ```
 629 | Field Name | Mandatory | Description |
 630 |---|---|---|
 631 | `object.type` | yes | Either `Remove` or `Delete` |
 632
 633 ### Create or Update Private message
 634
 635 Creates a new private message between two users.
 636
 637 Sent by: User
 638
 639 Sent to: User
 640
 641 ```json
 642 {
 643     "@context": "https://www.w3.org/ns/activitystreams",
 644     "id": "https://ds9.lemmy.ml/activities/create/202daf0a-1489-45df-8d2e-c8a3173fed36",
 645     "type": "Create",
 646     "actor": "https://ds9.lemmy.ml/u/sisko",
 647     "to": "https://enterprise.lemmy.ml/u/riker/inbox",
 648     "object": ...
 649 }
 650 ```
 651
 652 | Field Name | Mandatory | Description |
 653 |---|---|---|
 654 | `type` | yes | Either `Create` or `Update` |
 655
 656 ### Delete Private Message
 657
 658 Deletes a previous private message.
 659
 660 Sent by: User
 661
 662 Sent to: User
 663
 664 ```json
 665 {
 666     "@context": "https://www.w3.org/ns/activitystreams",
 667     "id": "https://ds9.lemmy.ml/activities/delete/2de5a5f3-bf26-4949-a7f5-bf52edfca909",
 668     "type": "Delete",
 669     "actor": "https://ds9.lemmy.ml/u/sisko",
 670     "to": "https://enterprise.lemmy.ml/u/riker/inbox",
 671     "object": "https://ds9.lemmy.ml/private_message/341"
 672 }
 673 ```
 674
 675 ### Undo Delete Private Message
 676
 677 Restores a previously deleted private message. The `object` is regenerated from scratch, as such the activity ID and other fields are different.
 678
 679 Sent by: User
 680
 681 Sent to: User
 682
 683 ```json
 684 {
 685     "@context": "https://www.w3.org/ns/activitystreams",
 686     "id": "https://ds9.lemmy.ml/activities/undo/b24bc56d-5db1-41dd-be06-3f1db8757842",
 687     "type": "Undo",
 688     "actor": "https://ds9.lemmy.ml/u/sisko",
 689     "to": "https://enterprise.lemmy.ml/u/riker/inbox",
 690     "object": ...
 691 }
 692 ```