Discussion/Group-Sharing.md

+# Private groups
+Briar offers to types of group messaging: private groups and forums.  
+This article will focus on private groups, how they work in general and how the 
+repeater could be used to improve them.
+
+## How it works
+In centralized networks group messaging is pretty simple. Messages are sent to 
+and retrieved from a server known to all group members:
+
+```mermaid
+graph BT
+ subgraph Group
+ A[Alice]
+ S((Server))
+ style S fill:#f40
+ B[Bob]
+ C[Carol]
+ D[Dave]
+ A-- send and fetch messages -->S
+ B-- send and fetch messages -->S
+ C-- send and fetch messages -->S
+ D-- send and fetch messages -->S
+ end
+```
+
+Without such a central server synchronizing messages between all the group
+members is more difficult.  
+In Briar, the creator of a group is taking on the 
+role of the server for this particular group instead.
+
+Illustration:
+
+Alice wants to create a private group for herself and her friends:
+
+```mermaid
+graph LR
+ A[Alice]
+ subgraph Friends
+ A1[Alice]
+ style A1 fill:#f9f
+ end
+ A-- creates group --> A1
+```
+
+As creator, it's up to Alice to invite new members (regular members are not
+allowed to invite anyone):
+
+```mermaid
+graph LR
+ subgraph Group
+ A[Alice]
+ style A fill:#f9f
+ end
+ subgraph Contacts
+ B[Bob]
+ C[Carol]
+ D[Dave]
+ end
+ A-. invites .->B 
+ A-. invites .->C
+ A-. invites .->D
+ subgraph Group
+ A1[Alice]
+ B1[Bob]
+ C1[Carol]
+ D1[Dave]
+ B-- joins -->B1
+ C-- joins -->C1
+ D-- joins -->D1
+ end
+```
+
+To synchronize the messages between the group members a star topology is formed
+around the creator (Alice in our case).   
+When a member sends a message to the group, they'll send it to Alice and 
+she will forward it to the other group members.
+
+
+```mermaid
+graph TB
+subgraph Group
+ A[Alice]
+ style A fill:#f9f
+ B[Bob]
+ C[Carol]
+ D[Dave]
+ A-- send and receive messages ---B
+ A-- send and receive messages ---C
+ A-- send and receive messages ---D
+ end
+```
+
+You might ask yourself: Why aren't messages synchronized between 
+group members as well ?   
+For example: Bob and Carol are contacts  in Briar, they could 
+exchange group messages without having to wait for Alice to be online:
+
+
+```mermaid
+graph TB
+subgraph Group
+ A[Alice]
+ style A fill:#f9f
+ B[Bob]
+ C[Carol]
+ D[Dave]
+ A-- sync messages ---B
+ A-- sync messages ---C
+ A-- sync messages ---D
+ B-. could sync messages .-C
+ end
+```
+
+There's a good reason Briar doesn't do this by default: metadata.  
+If Bob and Carol sync messages directly, the other group members can learn
+of their relationship.   
+In some situations this might still be acceptable (e.g. the group members know of the relationship already 
+because they are all friends),   
+therefore Briar offers the option to reveal a contact relationship to the group
+and improve the message circulation:
+
+```mermaid
+graph TB
+ B1[Bob]
+
+ subgraph Group
+ A[Alice]
+ style A fill:#f9f
+ B[Bob]
+ C[Carol]
+ D[Dave]
+ A-- sync messages ---B
+ A-- sync messages ---C
+ A-- sync messages ---D
+ B-- sync messages ---C
+ linkStyle 3 stroke:#ff3,stroke-width:4px
+ end
+  subgraph Contact relationship
+ C1[Carol]
+  B1-. are contacts .-C1
+ C1-- reveal relationship to group -->A
+ end
+
+```
+
+## Problems