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Board » Technical Discussion » When is my command executed?

warning: I'll explain how serverjumps, commands and the client refresh rate work, but it's complicated and I cannot tell you yet how to take advantage of this knowledge (when rounding a mark for example)

Most of you know about server jumps. For rounding a mark, it's important to turn as early as possible, but absolutely not too early. This is made difficult by the server jumps, and client refresh rate, you rarely see your boat where it actually is. What you see is running a couple of seconds behind, in rare cases it could reach up to about 45 seconds.

So what is exactly happening. It's more than just the server jumps and the client update rate, but let's start with them anyway:

The server continuously checks all boats. For every boat that has not been moved by the server for at least 10 seconds, the server updates its position. This means, your boat will move every 10-12 seconds (when it's busy, it may take 2 seconds before the server notices it hasn't moved your boat yet).

When you haven't issued a command in the last 15 minutes or so, the server may skip you once or twice, and then do one big jump to catch up. This unloads the server a bit from boats that aren't actively steered.

useful tip: send a command 5-10 minutes before rounding a mark (setting a delayed command for 5-10 minutes before the mark also works). It'll make sure the server jumps for your boat are short when you reach the mark.

For the rest of this post, let's assume server jumps of 11 seconds: your boat is moved by the server every 11 seconds. I'm also assuming you have a fast internet connection.

The client polls the server every 15 seconds (roughly). Let's see what can happen with an example:
12:00:00 (jump) server moves your boat
12:00:00 (poll) client polls boat position, you see your boat at its current position (barely any delay!)
12:00:11 (jump)
12:00:15 (poll) your boat moves from its 12:00:00 position to its 12:00:11 position (running 4 second behind)
12:00:22 (jump)
12:00:30 (poll) your boat moves from its 12:00:11 position (19 seconds behind) to its 12:00:22 position (8 seconds behind).
12:00:33 (jump)
12:00:44 (jump)
12:00:45 (poll) your boat moves from its 12:00:22 position (23 seconds behind) to its 12:00:44 position (1 second behind). Note: the 12:00:33 position is not shown, it looks like you made one large jump.

A better understanding of how this works may not easily transfer into being able to steer your boat better. One useful tip is: when you see your boat make a large jump, you know two server jumps happened in the last 15 seconds, meaning the last jump happened less then 4 seconds ago. If you see a short jump, you know only 1 server jump happened in the last 15 seconds, meaning the last jump happened between 4 and 11 seconds ago.

Commands are executed independently from serverjumps. What does this mean? You might have seen your command execute at the start of a new server jump, or at the end. Both can happen. A command will never execute in the middle of a serverjump. All commands due to be executed (of every boat, in every race) are executed in one go, with 4 or 5 seconds between each go. So your command will generally be executed within 5 seconds after the time you set the command for. BUT your boat may still be at a position from 11 seconds ago. This means, your command will have an effect 11 seconds earlier than it was set to execute.

So now there are three things with an interval: the client polls ever 15 seconds, the server moves boats every 11 seconds and turns boats every 4 or 5 seconds. This is complicated, I can't tell you yet how to take advantage of this knowledge.
Up to now I've done one of two things for markroundings:
- wait until the client shows me clear to round, then send the command.
- when I expect a double jump to happen until the next client refresh, and I only need one more jump, I count, to 10. But 11 is probably safer.

Now I've researched how commands work, I might come up with a method that results in better roundings, without the risk of missing the mark. That will be something I'll post on solfans
One advantage I take of your very informative explanation is: Don’t tack (gybe) too short for rounding a waypoint/buoy. To set a tack command in advance may have the effect to round too early. Seems to be better to wait for the visible pass by of the waypoint – then tack or gybe or set new course.
(Apologies for bad english).
This is why roundings are missed even though DC Checker shows the mark as clear. DC Checker assumes that the DC is going to fire exactly at that time with no uncertainty. That is not the case in reality.

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