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69 lines
3.6 KiB
Markdown
69 lines
3.6 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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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 functionality
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offered by the modules in this directory.
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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). Some modules have child modules attached to them in the same way the
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response JSON has objects with child objects and arrays, etc; forming a tree.
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There are at least two ways to organize these modules, and this directory
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represents one of those ways. Another way is to disperse them throughout
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`/client/` near the endpoints related to their feature suite. That still
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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. One of three branches is then taken:
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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.
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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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- **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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### Implementation
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Each `/sync` module implements two primary functions:
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- A simple boolean test of an event which determines if the client should
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receive that event. This is intended to be invoked for the _longpoll_ and
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_linear_ modes.
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- A complex composer for a response in _polylog_ mode. This requires the
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function itself to find all the events to include for a satisfying response.
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