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changefeedccl: webhook sink refactor
Resolves #84676 Epic: https://cockroachlabs.atlassian.net/browse/CRDB-11356 This PR implements the Webhook sink as part of a more general `batchingSink` framework that can be used to make adding new sinks an easier process, making it far more performant than it was previously. A followup PR will be made to use the `batchingSink` for the pubsub client which also suffers performance issues. --- Sink-specific code is encapsulated in a SinkClient interface ```go type SinkClient interface { MakeResolvedPayload(body []byte, topic string) (SinkPayload, error) MakeBatchBuffer() BatchBuffer Flush(context.Context, SinkPayload) error Close() error } type BatchBuffer interface { Append(key []byte, value []byte, topic string) ShouldFlush() bool Close() (SinkPayload, error) } type SinkPayload interface{} ``` Once the Batch is ready to be Flushed, the buffer can be `Close()`'d to do any final formatting (ex: wrap in a json object with extra metadata) of the buffer-able data and obtain a final `SinkPayload` that is ready to be passed to `SinkClient.Flush`. The `SinkClient` has a separate `MakeResolvedPayload` since the sink may require resolved events be formatted differently to a batch of kvs. `Flush(ctx, payload)` encapsulates sending a blocking IO request to the sink endpoint, and may be called multiple times with the same payload due to retries. Any kind of formatting work should be served to run in the buffer's `Close` and stored as a `SinkPayload` to avoid multiple calls to `Flush` repeating work upon retries. --- The `batchingSink` handles all the logic to take a SinkClient and form a full Sink implementation. ```go type batchingSink struct { client SinkClient ioWorkers int minFlushFrequency time.Duration retryOpts retry.Options eventCh chan interface{} pacer *admission.Pacer ... } var _ Sink = (*batchingSink)(nil) ``` It involves a single goroutine which handles: - Creating, building up, and finalizing `BatchBuffer`s to eventually form a `SinkPayload` to emit - Flushing batches when they have persisted longer than a configured `minFlushFrequency` - Flushing deliberately and being able to block until the Flush has completed - Logging all the various sink metrics `EmitRow` calls are thread-safe therefore the use of the `safeSink` wrapper is not required for users of this sink. Events sent through the goroutines would normally need to exist on the heap, but to avoid excessive garbage collection of hundreds of thousands of tiny structs, both the `kvEvents{<data from EmitRow>}` events (sent from the EmitRow caller to the batching wokrer) and the `sinkBatchBuffer{<data about the batch>}` events (sent from the batching worker to the IO routine in the next section) are allocated on object pools. --- For a sink like Cloudstorage where there are large batches, doing the above and just one-by-one flushing the batch payloads on a separate routine is plenty good enough. Unfortunately the Webhook sink can be used with no batching at all with users wanting the lowest latency while still having good throughput. This means we need to be able to have multiple requests in flight. The difficulty here is if a batch with keys [a1,b1] is in flight, a batch with keys [b2,c1] needs to block until [a1,b1] completes as b2 cannot be sent and risk arriving at the destination prior to b1. Flushing out Payloads in a way that is both able to maintain key-ordering guarantees but is able to run in parallel is done by a separate `parallel_io` struct. ```go type parallelIO struct { retryOpts retry.Options ioHandler IOHandler requestCh chan IORequest resultCh chan *ioResult ... } type IOHandler func(context.Context, IORequest) error type IORequest interface { Keys() intsets.Fast } type ioResult struct { request IORequest err error } ``` It involves one goroutine to manage the key ordering guarantees and a configurable number of IO Worker goroutines that simply call `ioHandler` on an `IORequest`. IORequests represent the keys they shouldn't conflict on by providing a `intsets.Fast` struct, which allows for efficient Union/Intersects/Difference operations on them that `parallelIO` needs to maintain ordering guarantees. The request and its error (if one occured despite the retries) are returned on resultCh. --- The webhook sink is therefore formed by: 1. EmitRow is called, creating kvEvents that are sent to a Batching worker 2. The batching worker takes events and appends them to a batch 3. Once the batch is full, its encoded into an HTTP request 4. The request object is then sharded across a set of IO workers to be fully sent out in parallel with other non-key-conflicting requests. With this setup, looking at the CPU flamegraph, at high throughputs most of the `batchingSink`/`parallelIO` work didn't really show up much, the work was largely just step 3, where taking a list of messages and calling `json.Marshal` on it took almost 10% of the time, specifically a call to `json.Compress`. Since this isn't needed, and all we're doing is simply putting a list of already-formatted JSON messages into a surrounding JSON array and small object, I also swapped `json.Marshal` to just stitch together characters manually into a buffer. --- Since Matt's talked about a new significance being placed on Feature flagging new work to avoid need for technical advisories, I placed this new implementation under the changefeed.new_webhook_sink_enabled setting and defaulted it to be disabled. --- Release note (performance improvement): the webhook sink is now able to handle a drastically higher maximum throughput by enabling the "changefeed.new_webhook_sink_enabled" cluster setting.
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