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96 lines
4.7 KiB
Markdown
96 lines
4.7 KiB
Markdown
# Client Sync
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#### Organization
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This directory contains modules which compose the `/sync` endpoint content.
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Each module in this directory creates content within some property of the
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`/sync` JSON response to fulfill a certain feature (refer to the [matrix
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c2s spec](https://matrix.org/docs/spec/client_server/r0.5.0#get-matrix-client-r0-sync)).
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`sync::item`'s register themselves at different locations of the `/sync`
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response tree. Items then implement two functions to provide the data at
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that location in the `/sync` tree. The `/sync/` `GET` handler will iterate
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all of the items and call one of these functions to build the response.
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> First note that even though this is a directory of modules, the `/sync`
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resource itself is a single endpoint and not a directory. The `../sync.cc`
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module alone services the `/sync` resource and invokes the modular
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functionality in this directory.
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> There are at least two ways to organize these modules, and this
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directory represents one of those ways. Another way is to disperse them
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throughout `/client/` near the endpoints related to their feature suite.
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That still remains a viable option for debate as this system further matures.
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### Methodology
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As documented elsewhere, events processed by the server receive a unique
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monotonic sequence integer (or index number, type: `m::event::idx`). The
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`next_batch` and `since` tokens are these sequence integers. This single
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number is fundamental for the `/sync` to achieve stateless and stable operation:
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- Nothing is saved about a request being made. The since token alone has enough
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information to fulfill a `/sync` request without hidden server-side state.
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- The same request (same since token) produces a similar result each time,
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again without any server-side state describing that token.
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When a `/sync` request is made, the `since` token is compared with the
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server's current sequence number. The `since` token can be between 0
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and 1 greater than the server's sequence number. The `since` token and
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the server's sequence number form the "range" to be sync'ed. Adding 1
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to the server's sequence number becomes the `next_batch` field returned
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to the client.
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The properties of the sync range determine what happens next as there
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are large ranges, small ranges, a null ranges. For large ranges, a
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_polylog_ sync is conducted; for small ranges a _linear_ sync is used
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instead, and for null ranges a _longpoll_ sync.
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#### Sync Modes
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- **Polylog**: When the since token's "delta" exceeds the threshold for a
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_linear sync_ the client enters _polylog sync_. This is common when no
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`since` token is supplied at all (equal to 0) which is known as an initial
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sync. In this mode, a series of point-lookups are made to compose the response
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content. This involves iterating the rooms relevant to a user and checking if
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the sequence numbers for events in the timeline fall within the `since` window
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so they can be included. This requires a lot of random access in contrast to
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_linear sync_; each room has to be queried at least once for every invocation
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of _polylog sync_. The goal for the threshold between polylog and linear
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is to invoke the cheaper mode: Even though polylog usually involves a
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minimum of many queries, it is more efficient than a linear iteration of all
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events on the server.
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- **Linear**: When the since token's "delta" from the current sequence number
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ranges from 0 to some configurable limit (i.e 1024 or 4096), the client enters
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_linear sync_: an iteration of events between the `since` token and the current
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sequence is made where each event is tested for relevance to the client and
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a response is then made. If nothing was found for the client in this iteration,
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it falls back to _longpoll sync_ until an event comes through or timeout.
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- **Longpoll**: When the since token is 1 greater than the current sequence
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number, the client enters _longpoll sync_: It waits for the next appropriate
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event which is then sent immediately. The `next_batch` will then be 1 greater
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than the sequence number of that event. The implementation of _longpoll sync_
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is a specialization of _linear sync_, using the same handlers.
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### Implementation
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Each `/sync` module implements two primary functions:
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- A complex composer for a response in _polylog_ mode. This requires the
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handler itself to find all the events to include for a satisfying response.
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This handler is invoked once for a `since` token during the single pass and
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is responsible for building JSON inside the object the handler is responsible
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for in the larger `/sync` tree.
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- A simple composer for a response in _linear_ mode. This requires the handler
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to evaluate the event argument being passed. If the event is to be included
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in the `/sync` response, the _linear_ handler builds a complete `/sync` JSON
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object tree around this event, which indicates where it is placed in the `/sync`
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tree.
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