82 lines
2.8 KiB
Markdown
82 lines
2.8 KiB
Markdown
> **Warning**
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> These architecture notes are spectacularly old, and date back
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> to when Synapse was just federation code in isolation. This should be
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> merged into the main spec.
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# Server to Server
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## Server to Server Stack
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To use the server to server stack, homeservers should only need to
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interact with the Messaging layer.
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The server to server side of things is designed into 4 distinct layers:
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1. Messaging Layer
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2. Pdu Layer
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3. Transaction Layer
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4. Transport Layer
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Where the bottom (the transport layer) is what talks to the internet via
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HTTP, and the top (the messaging layer) talks to the rest of the Home
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Server with a domain specific API.
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1. **Messaging Layer**
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This is what the rest of the homeserver hits to send messages, join rooms,
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etc. It also allows you to register callbacks for when it gets notified by
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lower levels that e.g. a new message has been received.
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It is responsible for serializing requests to send to the data
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layer, and to parse requests received from the data layer.
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2. **PDU Layer**
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This layer handles:
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- duplicate `pdu_id`'s - i.e., it makes sure we ignore them.
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- responding to requests for a given `pdu_id`
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- responding to requests for all metadata for a given context (i.e. room)
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- handling incoming backfill requests
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So it has to parse incoming messages to discover which are metadata and
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which aren't, and has to correctly clobber existing metadata where
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appropriate.
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For incoming PDUs, it has to check the PDUs it references to see
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if we have missed any. If we have go and ask someone (another
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homeserver) for it.
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3. **Transaction Layer**
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This layer makes incoming requests idempotent. i.e., it stores
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which transaction id's we have seen and what our response were.
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If we have already seen a message with the given transaction id,
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we do not notify higher levels but simply respond with the
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previous response.
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`transaction_id` is from "`GET /send/<tx_id>/`"
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It's also responsible for batching PDUs into single transaction for
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sending to remote destinations, so that we only ever have one
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transaction in flight to a given destination at any one time.
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This is also responsible for answering requests for things after a
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given set of transactions, i.e., ask for everything after 'ver' X.
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4. **Transport Layer**
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This is responsible for starting a HTTP server and hitting the
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correct callbacks on the Transaction layer, as well as sending
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both data and requests for data.
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## Persistence
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We persist things in a single sqlite3 database. All database queries get
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run on a separate, dedicated thread. This that we only ever have one
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query running at a time, making it a lot easier to do things in a safe
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manner.
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The queries are located in the `synapse.persistence.transactions` module,
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and the table information in the `synapse.persistence.tables` module.
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