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# ADR 033: pubsub 2.0 | ||
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Author: Anton Kaliaev (@melekes) | ||
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## Changelog | ||
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02-10-2018: Initial draft | ||
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## Context | ||
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Since the initial version of the pubsub, there's been a number of issues | ||
raised: #951, #1879, #1880. Some of them are high-level issues questioning the | ||
core design choices made. Others are minor and mostly about the interface of | ||
`Subscribe()` / `Publish()` functions. | ||
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### Sync vs Async | ||
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Now, when publishing a message to subscribers, we can do it in a goroutine: | ||
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_using channels for data transmission_ | ||
```go | ||
for each subscriber { | ||
out := subscriber.outc | ||
go func() { | ||
out <- msg | ||
} | ||
} | ||
``` | ||
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_by invoking callback functions_ | ||
```go | ||
for each subscriber { | ||
go subscriber.callbackFn() | ||
} | ||
``` | ||
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This gives us greater performance and allows us to avoid "slow client problem" | ||
(when other subscribers have to wait for a slow subscriber). A pool of | ||
goroutines can be used to avoid uncontrolled memory growth. | ||
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In certain cases, this is what you want. But in our case, because we need | ||
strict ordering of events (if event A was published before B, the guaranteed | ||
delivery order will be A -> B), we can't use goroutines. | ||
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There is also a question whenever we should have a non-blocking send: | ||
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```go | ||
for each subscriber { | ||
out := subscriber.outc | ||
select { | ||
case out <- msg: | ||
default: | ||
log("subscriber %v buffer is full, skipping...") | ||
} | ||
} | ||
``` | ||
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This fixes the "slow client problem", but there is no way for a slow client to | ||
know if it had missed a message. On the other hand, if we're going to stick | ||
with blocking send, **devs must always ensure subscriber's handling code does not | ||
block**. As you can see, there is an implicit choice between ordering guarantees | ||
and using goroutines. | ||
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The interim option is to run goroutines pool for a single message, wait for all | ||
goroutines to finish. This will solve "slow client problem", but we'd still | ||
have to wait `max(goroutine_X_time)` before we can publish the next message. | ||
My opinion: not worth doing. | ||
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### Channels vs Callbacks | ||
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Yet another question is whenever we should use channels for message transmission or | ||
call subscriber-defined callback functions. Callback functions give subscribers | ||
more flexibility - you can use mutexes in there, channels, spawn goroutines, | ||
anything you really want. But they also carry local scope, which can result in | ||
memory leaks and/or memory usage increase. | ||
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Go channels are de-facto standard for carrying data between goroutines. | ||
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Question: Does it worth switching to callback functions? | ||
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### Why `Subscribe()` accepts an `out` channel? | ||
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Because in our tests, we create buffered channels (cap: 1). Alternatively, we | ||
can make capacity an argument. | ||
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## Decision | ||
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Change Subscribe() function to return out channel: | ||
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```go | ||
// outCap can be used to set capacity of out channel (unbuffered by default). | ||
Subscribe(ctx context.Context, clientID string, query Query, outCap... int) (out <-chan interface{}, err error) { | ||
``` | ||
It's more idiomatic since we're closing it during Unsubscribe/UnsubscribeAll calls. | ||
Also, we should make tags available to subscribers: | ||
```go | ||
type MsgAndTags struct { | ||
Msg interface{} | ||
Tags TagMap | ||
} | ||
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// outCap can be used to set capacity of out channel (unbuffered by default). | ||
Subscribe(ctx context.Context, clientID string, query Query, outCap... int) (out <-chan MsgAndTags, err error) { | ||
``` | ||
## Status | ||
In review | ||
## Consequences | ||
### Positive | ||
- more idiomatic interface | ||
- subscribers know what tags msg was published with | ||
### Negative | ||
### Neutral |
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# ADR 000: Template for an ADR | ||
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Author: | ||
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## Changelog | ||
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## Context | ||
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## Decision | ||
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