# Chapter 18: Custom Publishers & Backpressure **URL:** \ **Category:** Combine: Asynchronous Programming with Swift\ **Created:** [March 24, 2020, 12:14am UTC](https://forums.kodeco.com/t/chapter-18-custom-publishers-backpressure/102581 "2020-03-24T00:14:35Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![elichigo](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/elichigo/32/81494_2.png) [@elichigo](https://forums.kodeco.com/u/elichigo)\ **Post date:** [March 24, 2020, 12:14am UTC](https://forums.kodeco.com/t/chapter-18-custom-publishers-backpressure/102581/1 "2020-03-24T00:14:35Z")
I couldn’t figure out this, in the ShareReplay Publisher(Transformer to be more precise) when I finished I’ve got this output: **shareReplay: receive subscription: (PassthroughSubject)** **shareReplay: request unlimited** **shareReplay: receive value: (1)** **+0.04658s: subscription1 received 1** **shareReplay: receive value: (2)** **+0.04791s: subscription1 received 2** **shareReplay: receive value: (3)** **+0.04878s: subscription1 received 3** **+0.05000s: subscription2 received 2** **+0.05009s: subscription2 received 3** **shareReplay: receive value: (4)** **+0.05055s: subscription1 received 4** **+0.05080s: subscription2 received 4** **shareReplay: receive value: (5)** **+0.05166s: subscription1 received 5** **+0.05192s: subscription2 received 5** **shareReplay: receive finished** **+0.05359s: subscription1 completed: finished** **+0.05383s: subscription2 completed: finished** **Subscribing to shareReplay after upstream completed** **+1.05626s: subscription3 received 4** **+1.05640s: subscription3 received 5** **+1.05695s: subscription3 completed: finished** **1. Why didn’t the subscription 2 get value 1, and subscription 3 get 1,2,3**. Even if I set the capacity to 20, it will support all values of the example(we just sent 5 values through the subject). **2. We never delete data from our replay variable inside the ShareReplay class** , so when a new subscription is created we pass this array to it and there when that receives a new input from upstream we can send the values inside the buffer variable(ShareReplaySubscription) + the new value. I don’t get, **why are there some values not delivered to the the subscription** (just the first is the only one that gets all)?. Is it possible to explain in a detailed way? ---
**Author:** ![shogunkaramazov](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/shogunkaramazov/32/108826_2.png) [@shogunkaramazov](https://forums.kodeco.com/u/shogunkaramazov)\ **Post date:** [April 6, 2020, 1:23am UTC](https://forums.kodeco.com/t/chapter-18-custom-publishers-backpressure/102581/2 "2020-04-06T01:23:33Z")
@fpillet Can you please help with this when you get a chance? Thank you - much appreciated! :] ---
**Author:** ![fpillet](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/fpillet/32/97677_2.png) [@fpillet](https://forums.kodeco.com/u/fpillet)\ **Post date:** [April 6, 2020, 7:06am UTC](https://forums.kodeco.com/t/chapter-18-custom-publishers-backpressure/102581/3 "2020-04-06T07:06:17Z")
Hi @elichigo, 1. In the book example and in the code you type all along the `shareReplay` section, the publisher you create replays the _last two_ items upon subscription. Since the example code emits three items before you create `subscription2`, it is normal that this subscription only gets the last two items the publisher emitted before you subscribe. I will double check the code that we have in the book for any mistake, because running the final version of the project yields the correct results (with one gotcha – I mistakenly print “subscription2” instead of “subscription1” in the subscription1 subscription – fixing this for the next iteration of the book) I also tested the publisher with a replay value set to 20 and it worked as expected (make sure you click in the text area and scroll down in your browser to see all output): ```auto shareReplay: receive subscription: (PassthroughSubject) shareReplay: request unlimited shareReplay: receive value: (1) +0.03541s: subscription1 received 1 shareReplay: receive value: (2) +0.03619s: subscription1 received 2 shareReplay: receive value: (3) +0.03671s: subscription1 received 3 +0.03740s: subscription2 received 1 +0.03746s: subscription2 received 2 +0.03751s: subscription2 received 3 shareReplay: receive value: (4) +0.03781s: subscription1 received 4 +0.03798s: subscription2 received 4 shareReplay: receive value: (5) +0.03853s: subscription1 received 5 +0.03870s: subscription2 received 5 shareReplay: receive finished +0.03971s: subscription1 completed: finished +0.03986s: subscription2 completed: finished Subscribing to shareReplay after upstream completed +1.13669s: subscription3 received 1 +1.13679s: subscription3 received 2 +1.13684s: subscription3 received 3 +1.13689s: subscription3 received 4 +1.13694s: subscription3 received 5 +1.13725s: subscription3 completed: finished ``` 1. You effectively do remove data from the replay variable in the `ShareReplay` class. Look for step **// 30** in the book: ```swift // 30 replay.append(value) if replay.count > capacity { replay.removeFirst() } ``` Every time you receive a new value and the buffer is already at capacity, you drop the first item. Hope this helps, Florent ---
**Author:** ![wayfaerer](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/wayfaerer/32/103049_2.png) [@wayfaerer](https://forums.kodeco.com/u/wayfaerer)\ **Post date:** [November 26, 2020, 5:19pm UTC](https://forums.kodeco.com/t/chapter-18-custom-publishers-backpressure/102581/4 "2020-11-26T17:19:29Z")
I think there is also an error in the code, at least in my version (Page 373, I also checked online). The code snippet says: ```auto let subscription1 = publisher.sink( receiveCompletion: { print("subscription2 completed: \($0)", to: &logger) }, receiveValue: { print("subscription2 received \($0)", to: &logger) } ) ``` and it should print “subscription1”. The log above also writes “subscription1”. Was this error perhaps introduced at a later point in time? Or am I getting something wrong?