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- So our next basic and our
last basic channel pattern here

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is wait for finished.

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And wait for finished is going to show us

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how we can signal without data.

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Now I just want to make it clear that

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what I'm showing you

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would really be better
served with a wait group.

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A wait group began as
a way of kind of doing

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a signaling without data for
that type of orchestration

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and would be cleaner here

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but I'm going to show you
and walk through this code

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just so you can see the mechanics.

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Again, I'm going to want to stress that.

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A wait group would make this code cleaner

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but I need to show you the mechanics

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so later on when we talk about

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cancellation and deadlines
with the context package,

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it'll give you a little bit more sense.

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Now look on line 75.

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We're using the make again,

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channel empty struct and it's unbuffered.

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So we have an unbuffered
channel which means guarantees,

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except that the data
type isn't string here,

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it's the empty struct.

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And we're going to use the empty struct

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to denote from a code readability,
signaling without data.

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Since the empty struct is about no data,

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we're going to use this.

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And that basically means that

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all we're really going to be
doing is closing the channel

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and there's no real guarantees with this

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as I'm going to show you.

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But anytime you see a
make chan empty struct,

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now we're going to be thinking,
signaling without data.

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We're going to turn the lights off.

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Now, let's go back and
look at what we're doing.

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We are the manager, here we are

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and we're running in our path of execution

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and right away we go and we launch that

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employee path of execution or go routine.

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And once we do that again,

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we're asking this go
routine to do some work.

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So they already know what
they're supposed to do.

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So now we come back and end
up right there on line 83.

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Notice again on line 83,
we're doing a channel receive.

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This is very similar

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to what we were just doing before, right?

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And here we go on the channel receive,

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which means we're now in
a blocking call, right?

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We're right here blocking

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because we're in our
channel receive operation.

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Now I want you to notice something

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about the channel receive operation.

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Up until now, all I've
done is allowed you to see

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that we can receive the
value that's being sent.

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But when we talk about
cancellation and deadlines

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or signaling without data,
there is no data to receive.

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And that's why what you're seeing is

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the second form of the channel receive.

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You can always get data out of it

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if you're signaling with data,

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but we can get a second flag,
which is a buoyant flag,

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that I use WD for, with data.

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This is my with data flag

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and it's a part of the
signaling mechanics.

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And what it's telling me is

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that if I receive
successfully off this channel

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and the with data flag is
true, then I have data.

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We signaled with data.

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But if the with data flag is false,

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then we receive because
the channel is closed,

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because the lights are out.

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Now I don't really need
to store this variable,

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I just wanted to display it on the screen

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if you decided to run this.

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Ideally in these scenarios,

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we're probably doing
nothing more than this,

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which is I'm just going
to block on the receive

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and I don't care what comes out of it.

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But what we're doing,

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I'm just showing you the with data flag.

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So here we are right,
like we talked about here,

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I'm blocked right now waiting, waiting.

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How long am I waiting?

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I don't know, it's unknown
because we're signaling

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even if it's signaling without data,

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we're signaling with
guarantees here right?

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I have no idea how long
we're going to be waiting.

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Eventually the work finishes

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and this time what happens
is once the work finishes,

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we don't signal with data,

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on line 79, we close the channel.

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What we now do here is close the channel.

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We're turning the lights off.

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Now when we close the channel,

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that close is going to happen nano seconds

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before the receive.

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It's kind of like the send
happens before the receive,

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this time the close happens.

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So the close is going to complete first

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and then we're going to
be able to move on second.

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So that channel receive
will immediately unblock

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once the close operation is done.

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So the close is going to happen
before the channel receive

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but what we've kind of done is just said

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the work is done and
you don't need anything

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so turn the lights off.

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Oh, we've seen the state
change, now we get to move on.

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This is still better
done with wait groups,

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and I'd ask us to be
using wait groups here.

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When we start talking about
cancellation and deadlines,

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I'm going to show you
some examples of that.

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We're going to be really using

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this idea of a unbuffered channel,

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where the data is the empty struct,

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where we're really saying here that

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what we're going to want to
do is signal without data

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and we want to be able to do
that to many, many go routines.

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If I had many, many go
routines at one point,

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all sitting on this same channel receive,

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if I had all of these go routines blocked

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on the same channel
receive, at the same time,

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then basically on the close, all of these,

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all of these would then be
able to unblock immediately

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and keep running.

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Okay, so this is our wait for finished.

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So we've looked at our wait for task,

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which is going to be really
like pool related right?

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The go routine gets lost,

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it's now waiting for us
to tell it what to do.

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Then we also have, we
had the wait for task,

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then we're going to
have to wait for result.

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This is like a fan out
pattern or drop pattern,

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where we launch these go routines,

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they know already what they have to do,

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we're waiting for them to reply back with

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whatever the result of their work is

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and I'm going to have
this wait for finished,

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for cancellation and deadlines,

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where we're going to use this to be able

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to tell a go routine,

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"hey, stop doing what you're doing,"

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or be able to like indicate a wide scale,

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that maybe we should be shutting down.