How Persistent Chat Server works in Lync Server 2013

Persistent Chat (Channel) Service

The Persistent Chat (Channel) service is the core service responsible for Persistent Chat Server. This service provides the following functions:

• Accepts incoming messages

• Registers and lists online participants within a Persistent Chat room

• Retransmits messages to other channel subscribers

• Implements logic for channel management, chat room invitation, search, and new content notifications

The Persistent Chat (Channel) service stores and accesses chat room content and other system metadata (authorization rules, and so on) by using the Persistent Chat Store. This service stores files that are uploaded into chat rooms in the Persistent Chat File Store.

Compliance Service

The Compliance service is an optional component of Persistent Chat Server and is responsible for archiving chat content and events to the Persistent Chat Compliance Store. If your organization has regulations that require Persistent Chat activity to be archived, you can deploy the optional Persistent Chat Compliance service. The Compliance service is installed on each Persistent Chat Server in a Persistent Chat pool. When configured, Persistent Chat Server compliance records user activity such as joining and leaving rooms, and posting and reading of messages. The Compliance service stores files that need to be archived in the Persistent Chat Compliance File Store.

Persistent Chat Web Services

On the Lync Server Front End Servers, two services run that depend on Internet Information Services (IIS), and are implemented as web components:

• Persistent Chat Web Services for File Upload/Download Responsible for posting and retrieving files from chat rooms.

• Persistent Chat Web Services for Chat Room Management Responsible for providing users the ability to manage their chat rooms, and create new chat rooms.

Sign-in

The following call flow diagram and steps describe the sign-in process.

Persistent Chat Client Sign-in Call Flow

1. The Persistent Chat client first sends a SIP SUBSCRIBE to retrieve the in-band provisioning document from the server. This document indicates if Persistent Chat is enabled or disabled for the user and the list of SIP URIs for the Persistent Chat Server pool.

2. The Persistent Chat client sends a SIP INVITE message to the SIP URI of the Persistent Chat Server that it obtained in the previous step. The INVITE sequence is followed by 200 OK and ACK, and the Persistent Chat client has now opened a SIP session with a Persistent Chat Server endpoint. Consequently, the Persistent Chat client communicates with Persistent Chat Server by sending SIP INFO messages that contain either chat messages or commands requesting the server to take an action. All of these messages are acknowledged with either 200 OK or 503 Service Unavailable (that is, in the event of heavy server load). If the client receives a 503 response, it will retry the message. (This example does not include a 503 response.) If the server accepts the message or command and sends 200 OK, it provides a response to the client in the form of a separate SIP INFO message. This response includes a reference to the originating command.

3. The Persistent Chat client sends a SIP INFO message that contains the XCCOS getserverinfo command. Persistent Chat Server replies with a new SIP INFO message that contains information about the Persistent Chat service configuration.

4. The Persistent Chat client sends a SIP INFO message that contains the XCCOS getassociations command. Persistent Chat Server replies with a new SIP INFO message that contains the list of rooms of which the user is a member. The Persistent Chat client repeats the command to retrieve the list of rooms of which the user is a manager.

5. The Persistent Chat client gets the list of followed rooms from the "presence" document, where each followed room is represented by a "roomSetting" category. All followed rooms are joined by a single SIP INFO message that contains the XCCOS bjoin command that contains the list of room URIs. Because the list of followed rooms is kept on the server, any client on any computer has the same list of followed rooms for the specified user URI. The Persistent Chat client also keeps the list of opened rooms (if this option is enabled by the user) in the local computer registry, and joins each of these rooms at sign-in by sending a SIP INFO message that contains the XCCOS join command for each opened room. Because this list is kept in the registry, it can be different on two Persistent Chat clients running on different computers.

6. For each room joined, the Persistent Chat client sends a SIP INFO message that contains the XCCOS bccontext command. Persistent Chat Server replies with a new SIP INFO message that contains the most recent chat message in the room.

7. The Persistent Chat client sends a SIP INFO message that contains a XCCOS getinv (that is, get invitation) command to request any new room invitations that the client has not yet seen. In a separate SIP INFO message, Persistent Chat Server returns a list of those rooms.

Subscribe to a Room and Post a Message

The following call flow diagram and steps describe a typical room subscription and message post scenario.

Persistent Chat Client Room Subscription and Message Posting Call Flow

1. From the Persistent Chat client, User1 clicks Join a Chat Room, clicks Search, and then enters some search criteria. The Persistent Chat client sends a SIP INFO message that contains the XCCOS chansrch (room search) command, along with the search criteria. Persistent Chat Server queries the back-end database and replies in a new SIP INFO message that contains a list of available rooms that meet the search criteria.

2. User1 selects the chat room that he or she wants to join, and then clicks Follow this room. The Persistent Chat client sends Persistent Chat Server a SIP INFO message that contains the XCCOS join command and the room ID of the chat room that the user selected. Persistent Chat Server replies with a SIP INFO message that contains the provisioning data.

3. The Persistent Chat client sends Persistent Chat Server a SIP INFO message that contains the XCCOS bccontext (backchat context) command. Persistent Chat Server retrieves the chat history, and returns it to the Persistent Chat client in a separate SIP INFO message. At this point, the user enters the chat room and is ready to participate.

4. User1 enters a new message, and then clicks Send. The Persistent Chat client posts the message to the chat room in a SIP INFO XCCOS grpchat command. Persistent Chat Server stores a copy of this new message in the Persistent Chat back-end database.

5. Persistent Chat Server sends a separate copy of the SIP INFO XCCOS grpchat message to User2, who has already entered the chat room.