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diff --git a/docs/SUMMARY.md b/docs/SUMMARY.md index a8e5ddad9d..31b3032029 100644 --- a/docs/SUMMARY.md +++ b/docs/SUMMARY.md @@ -97,6 +97,7 @@ - [Cancellation](development/synapse_architecture/cancellation.md) - [Log Contexts](log_contexts.md) - [Replication](replication.md) + - [Streams](development/synapse_architecture/streams.md) - [TCP Replication](tcp_replication.md) - [Faster remote joins](development/synapse_architecture/faster_joins.md) - [Internal Documentation](development/internal_documentation/README.md) diff --git a/docs/development/synapse_architecture/streams.md b/docs/development/synapse_architecture/streams.md new file mode 100644 index 0000000000..bee0b8a8c0 --- /dev/null +++ b/docs/development/synapse_architecture/streams.md @@ -0,0 +1,157 @@ +## Streams + +Synapse has a concept of "streams", which are roughly described in [`id_generators.py`]( + https://github.com/matrix-org/synapse/blob/develop/synapse/storage/util/id_generators.py +). +Generally speaking, streams are a series of notifications that something in Synapse's database has changed that the application might need to respond to. +For example: + +- The events stream reports new events (PDUs) that Synapse creates, or that Synapse accepts from another homeserver. +- The account data stream reports changes to users' [account data](https://spec.matrix.org/v1.7/client-server-api/#client-config). +- The to-device stream reports when a device has a new [to-device message](https://spec.matrix.org/v1.7/client-server-api/#send-to-device-messaging). + +See [`synapse.replication.tcp.streams`]( + https://github.com/matrix-org/synapse/blob/develop/synapse/replication/tcp/streams/__init__.py +) for the full list of streams. + +It is very helpful to understand the streams mechanism when working on any part of Synapse that needs to respond to changes—especially if those changes are made by different workers. +To that end, let's describe streams formally, paraphrasing from the docstring of [`AbstractStreamIdGenerator`]( + https://github.com/matrix-org/synapse/blob/a719b703d9bd0dade2565ddcad0e2f3a7a9d4c37/synapse/storage/util/id_generators.py#L96 +). + +### Definition + +A stream is an append-only log `T1, T2, ..., Tn, ...` of facts[^1] which grows over time. +Only "writers" can add facts to a stream, and there may be multiple writers. + +Each fact has an ID, called its "stream ID". +Readers should only process facts in ascending stream ID order. + +Roughly speaking, each stream is backed by a database table. +It should have a `stream_id` (or similar) bigint column holding stream IDs, plus additional columns as necessary to describe the fact. +Typically, a fact is expressed with a single row in its backing table.[^2] +Within a stream, no two facts may have the same stream_id. + +> _Aside_. Some additional notes on streams' backing tables. +> +> 1. Rich would like to [ditch the backing tables](https://github.com/matrix-org/synapse/issues/13456). +> 2. The backing tables may have other uses. + > For example, the events table serves backs the events stream, and is read when processing new events. + > But old rows are read from the table all the time, whenever Synapse needs to lookup some facts about an event. +> 3. Rich suspects that sometimes the stream is backed by multiple tables, so the stream proper is the union of those tables. + +Stream writers can "reserve" a stream ID, and then later mark it as having being completed. +Stream writers need to track the completion of each stream fact. +In the happy case, completion means a fact has been written to the stream table. +But unhappy cases (e.g. transaction rollback due to an error) also count as completion. +Once completed, the rows written with that stream ID are fixed, and no new rows +will be inserted with that ID. + +### Current stream ID + +For any given stream reader (including writers themselves), we may define a per-writer current stream ID: + +> The current stream ID _for a writer W_ is the largest stream ID such that +> all transactions added by W with equal or smaller ID have completed. + +Similarly, there is a "linear" notion of current stream ID: + +> The "linear" current stream ID is the largest stream ID such that +> all facts (added by any writer) with equal or smaller ID have completed. + +Because different stream readers A and B learn about new facts at different times, A and B may disagree about current stream IDs. +Put differently: we should think of stream readers as being independent of each other, proceeding through a stream of facts at different rates. + +**NB.** For both senses of "current", that if a writer opens a transaction that never completes, the current stream ID will never advance beyond that writer's last written stream ID. + +For single-writer streams, the per-writer current ID and the linear current ID are the same. +Both senses of current ID are monotonic, but they may "skip" or jump over IDs because facts complete out of order. + + +_Example_. +Consider a single-writer stream which is initially at ID 1. + +| Action | Current stream ID | Notes | +|------------|-------------------|-------------------------------------------------| +| | 1 | | +| Reserve 2 | 1 | | +| Reserve 3 | 1 | | +| Complete 3 | 1 | current ID unchanged, waiting for 2 to complete | +| Complete 2 | 3 | current ID jumps from 1 -> 3 | +| Reserve 4 | 3 | | +| Reserve 5 | 3 | | +| Reserve 6 | 3 | | +| Complete 5 | 3 | | +| Complete 4 | 5 | current ID jumps 3->5, even though 6 is pending | +| Complete 6 | 6 | | + + +### Multi-writer streams + +There are two ways to view a multi-writer stream. + +1. Treat it as a collection of distinct single-writer streams, one + for each writer. +2. Treat it as a single stream. + +The single stream (option 2) is conceptually simpler, and easier to represent (a single stream id). +However, it requires each reader to know about the entire set of writers, to ensures that readers don't erroneously advance their current stream position too early and miss a fact from an unknown writer. +In contrast, multiple parallel streams (option 1) are more complex, requiring more state to represent (map from writer to stream id). +The payoff for doing so is that readers can "peek" ahead to facts that completed on one writer no matter the state of the others, reducing latency. + +Note that a multi-writer stream can be viewed in both ways. +For example, the events stream is treated as multiple single-writer streams (option 1) by the sync handler, so that events are sent to clients as soon as possible. +But the background process that works through events treats them as a single linear stream. + +Another useful example is the cache invalidation stream. +The facts this stream holds are instructions to "you should now invalidate these cache entries". +We only ever treat this as a multiple single-writer streams as there is no important ordering between cache invalidations. +(Invalidations are self-contained facts; and the invalidations commute/are idempotent). + +### Writing to streams + +Writers need to track: + - track their current position (i.e. its own per-writer stream ID). + - their facts currently awaiting completion. + +At startup, + - the current position of that writer can be found by querying the database (which suggests that facts need to be written to the database atomically, in a transaction); and + - there are no facts awaiting completion. + +To reserve a stream ID, call [`nextval`](https://www.postgresql.org/docs/current/functions-sequence.html) on the appropriate postgres sequence. + +To write a fact to the stream: insert the appropriate rows to the appropriate backing table. + +To complete a fact, first remove it from your map of facts currently awaiting completion. +Then, if no earlier fact is awaiting completion, the writer can advance its current position in that stream. +Upon doing so it should emit an `RDATA` message[^3], once for every fact between the old and the new stream ID. + +### Subscribing to streams + +Readers need to track the current position of every writer. + +At startup, they can find this by contacting each writer with a `REPLICATE` message, +requesting that all writers reply describing their current position in their streams. +Writers reply with a `POSITION` message. + +To learn about new facts, readers should listen for `RDATA` messages and process them to respond to the new fact. +The `RDATA` itself is not a self-contained representation of the fact; +readers will have to query the stream tables for the full details. +Readers must also advance their record of the writer's current position for that stream. + +# Summary + +In a nutshell: we have an append-only log with a "buffer/scratchpad" at the end where we have to wait for the sequence to be linear and contiguous. + + +--- + +[^1]: we use the word _fact_ here for two reasons. +Firstly, the word "event" is already heavily overloaded (PDUs, EDUs, account data, ...) and we don't need to make that worse. +Secondly, "fact" emphasises that the things we append to a stream cannot change after the fact. + +[^2]: A fact might be expressed with 0 rows, e.g. if we opened a transaction to persist an event, but failed and rolled the transaction back before marking the fact as completed. +In principle a fact might be expressed with 2 or more rows; if so, each of those rows should share the fact's stream ID. + +[^3]: This communication used to happen directly with the writers [over TCP](../../tcp_replication.md); +nowadays it's done via Redis's Pubsub. |