tendermint/docs/architecture/adr-033-pubsub.md

ADR 033: pubsub 2.0

Author: Anton Kaliaev (@melekes)

Changelog

02-10-2018: Initial draft

16-01-2019: Second version based on our conversation with Jae

17-01-2019: Third version explaining how new design solves current issues

Context

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.

Sync vs Async

Now, when publishing a message to subscribers, we can do it in a goroutine:

using channels for data transmission

for each subscriber {
    out := subscriber.outc
    go func() {
        out <- msg
    }
}

by invoking callback functions

for each subscriber {
    go subscriber.callbackFn()
}

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.

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 publish msg in a new goroutine every time.

We can also have a goroutine per subscriber, although we'd need to be careful with the number of subscribers. It's more difficult to implement as well + unclear if we'll benefit from it (cause we'd be forced to create N additional channels to distribute msg to these goroutines).

Non-blocking send

There is also a question whenever we should have a non-blocking send:

for each subscriber {
    out := subscriber.outc
    select {
        case out <- msg:
        default:
            log("subscriber %v buffer is full, skipping...")
    }
}

This fixes the "slow client problem", but there is no way for a slow client to know if it had missed a message. We could return a second channel and close it to indicate subscription termination. On the other hand, if we're going to stick with blocking send, devs must always ensure subscriber's handling code does not block, which is a hard task to put on their shoulders.

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.

Channels vs Callbacks

Yet another question is whether 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.

Go channels are de-facto standard for carrying data between goroutines.

Why Subscribe() accepts an out channel?

Because in our tests, we create buffered channels (cap: 1). Alternatively, we can make capacity an argument.

Decision

Change Subscribe() function to return a Subscription struct:

type Subscription struct {
  // private fields
}

func (s *Subscription) Out() <-chan MsgAndTags
func (s *Subscription) Cancelled() <-chan struct{}
func (s *Subscription) Err() error

Out() returns a channel onto which messages and tags are published. Unsubscribe/UnsubscribeAll does not close the channel to avoid clients from receiving a nil message.

Cancelled() returns a channel that's closed when the subscription is terminated and supposed to be used in a select statement.

If the channel returned by Cancelled() is not closed yet, Err() returns nil. If the channel is closed, Err() returns a non-nil error explaining why: ErrUnsubscribed if the subscriber choose to unsubscribe, ErrOutOfCapacity if the subscriber is not pulling messages fast enough and the channel returned by Out() became full. After Err() returns a non-nil error, successive calls to `Err() return the same error.

subscription, err := pubsub.Subscribe(...)
if err != nil {
  // ...
}
for {
select {
  case msgAndTags <- subscription.Out():
    // ...
  case <-subscription.Cancelled():
    return subscription.Err()
}

Make the Out() channel buffered (with capacity 1) by default. In most cases, we want to terminate the slow subscriber. Only in rare cases, we want to block the pubsub (e.g. when debugging consensus). This should lower the chances of the pubsub being frozen.

// outCap can be used to set capacity of Out channel (1 by default). Set to 0
// for unbuffered channel (WARNING: it may block the pubsub).
Subscribe(ctx context.Context, clientID string, query Query, outCap... int) (Subscription, error) {

Also, the Out() channel should return tags along with a message:

type MsgAndTags struct {
    Msg interface{}
    Tags TagMap
}

to inform clients of which Tags were used with Msg.

How this new design solves the current issues?

#951 (#1880):

Because of non-blocking send, situation where we'll deadlock is not possible anymore. If the client stops reading messages, it will be removed.

#1879:

MsgAndTags is used now instead of a plain message.

Future problems and their possible solutions

https://github.com/tendermint/tendermint/issues/2826

One question I am still pondering about: how to prevent pubsub from slowing down consensus. We can increase the pubsub queue size (which is 0 now). Also, it's probably a good idea to limit the total number of subscribers.

This can be made automatically. Say we set queue size to 1000 and, when it's >= 80% full, refuse new subscriptions.

Status

In review

Consequences

Positive

• more idiomatic interface
• subscribers know what tags msg was published with
• subscribers aware of the reason their subscription was cancelled

Negative

• (since v1) no concurrency when it comes to publishing messages

Neutral