This example demonstrates how to implement a group chat with dynamic groups in DAML. All members of a ChatGroup have equal rights -- there is no owner, admin or operator. Message history is maintained as the group changes (ie new members get to see old messages). All messages are signed by all current members so history can't be falsified unless all members decide to do so jointly.
Maintaining the message history and tight linking between messages and group requires the maintenance of an index of all messages. Doing so globally would make the posting of messages racy between group members. To avoid this, the index is sharded by member in MessageIndex.
The message history per group grows indefinitely and changes in membership require all messages to be re-signed by the new mambership. Transactions changing group membership therefore grow in complexity O(#messages * #members).
- Anyone can start a group by creating a
GroupChatcontract. - New group members are onboarded by existing members. Discovery happens off-ledger. Ie
Aliceis assumed to have toldBobandCharlieabout her group "troll-bot" in the included scenarios. - Members can post messages with the
Post_Messagechoice and leave the group using theLeaveGroupchoice as they please.
To compile the project:
daml build
The model is tested in a series of sequential scenarios terminating in archive_charlie. Note how Post_Message is a relatively small transaction, whereas LeaveGroup and AddMember get larger as the group grows.
To test all scenarios:
daml test --color
To load the project into the sandbox and start navigator:
daml start