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Friendica Message Flow
This page attempts to document some of the details of how messages get from one person to another in the Friendica network. There are multiple paths, using multiple protocols and message formats.
Those attempting to understand these message flows should become familiar with (at the minimum) the DFRN protocol document (http://dfrn.org/dfrn.pdf) and the message passing elements of the OStatus stack (salmon and Pubsubhubbub).
Most message passing involves the file include/items.php, which has functions for several feed-related import/export activities.
When a message is posted, all immediate deliveries to all networks are made using include/notifier.php, which chooses how (and to whom) to deliver the message. This file also invokes the local side of all deliveries including DFRN-notify.
mod/dfrn_notify.php handles the remote side of DFRN-notify.
Local feeds are generated by mod/dfrn_poll.php - which also handles the remote side of DFRN-poll protocol.
Salmon notifications arrive via mod/salmon.php.
Push (pubsubhubbub) feeds arrive via mod/pubsub.php
DFRN-poll feed imports arrive via include/poller.php as a scheduled task, this implements the local side of the DFRN-poll protocol.
Scenario #1. Bob posts a public status message
This is a public message with no conversation members so no private transport is used. There are two paths it can take - as a bbcode path to DFRN clients, and converted to HTML with the server's PuSH (pubsubhubbub) hubs notified. When a PuSH hub is operational, dfrn-poll clients prefer to receive their information through the PuSH channel. They will fall back on a daily poll in case the hub has delivery issues (this is quite common when using the default Google reference hub). If there is no specified hub or hubs, DFRN clients will poll at a configurable (per-contact) rate at up to 5-minute intervals. Feeds retrieved via dfrn-poll are bbcode and may also contain private conversations which the poller has permissions to see.
Scenario #2. Jack replies to Bob's public message. Jack is on the Friendica/DFRN network.
Jack uses dfrn-notify to send a direct reply to Bob. Bob then creates a feed of the conversation and sends it to everybody involved in the conversation using dfrn-notify. PuSH hubs are notified that new content is available. The hub or hubs will then retrieve the latest feed and transmit it to all hub subscribers (which may be on different networks).
Scenario #3. Mary replies to Bob's public message. Mary is on the Friendica/DFRN network.
Mary uses dfrn-notify to send a direct reply to Bob. Bob then creates a feed of the conversation and sends it to everybody involved in the conversation (excluding himself, the conversation is now sent to both Jack and Mary). Messages are sent using dfrn-notify. Push hubs are also notified that new content is available. The hub or hubs will then retrieve the latest feed and transmit it to all hub subscribers (which may be on different networks).
Scenario #4. William replies to Bob's public message. William is on the OStatus network.
William uses salmon to notify Bob of the reply. Content is html embedded in salmon magic envelope. Bob then creates a feed of the conversation and sends it to all Friendica participants involved in the conversation using dfrn-notify (excluding himself, the conversation is sent to both Jack and Mary). Push hubs are notified that new content is available. The hub or hubs will then retrieve the latest feed and transmit it to all hub subscribers (which may be on different networks).
Scenario #5. Bob posts a private message to Mary and Jack.
Message is delivered immediately to Mary and Jack using dfrn_notify. Public hubs are not notified. Requeueing is attempted in case of timeout. Replies follow the same flow as the public replies except that hubs are not notified and message is never made available in the public feed. The entire conversation is also made available to Mary and Jack (and nobody else) through their dfrn-poll personalised feed.