Cranking (consume events) FAQ

[mihneacalugaru]

Hello,

I want to ask some questions and touch base a bit about what the cranking process is and what are the best practices by which it should be done.

What I know from examining the code:

1. Cranking is done through the consume_events instruction
2. Each invocation of the consume_events instruction needs to know how many items in the queue it should read and process (through the max_iterations parameters)
3. Each invocation of the consume_events instruction needs to provide the DEX program with the open_orders_accounts of all the users that are referenced by the events which are about to be processed. This can be done by loading the event_queue and extracting their open_orders_account from the Event by the following rule: in case it's an EventFill then it's taken from the event_fill.makerCallbackInfo.slice(0, 32); in case it's an EventOut then it's taken from the event_out.callbackInfo.slice(0, 32)
4. A taker doesn't need to have his events consumed because the new_order instruction already takes care of updating the open_orders_account in case there was at least a partial fill for his order
5. A maker has to have his events consumed by a crank in order to get his open_orders_account updated after orders of his have been matched
6. accumulated_royalties and accumulated_fees belonging to the market are updated by consuming events

What I am not sure about and would like to figure out:

1. I thought about ways of embedding the consume_events instruction in different transactions. For example:

• Transaction(new_order, consume_events) so that after a taker will get his order matched, he will also consume the events that would update the open_orders_account of the maker, making the latter able to settle his funds.
• Transaction(consume_events, settle) so that before a maker tries to settle his funds, he makes sure he consumed the events that will update his open_orders_account in order to actually have something to settle

However, I don't think these solutions are good. For example:

• For the first bullet above,: it seems that the events pushed to the queue in the case of a match are not "seen" in the consume_events instruction that belongs to the same transaction. But if I create a second transaction with just the consume_events instruction and execute it after the new_order this will work
• For the second bullet above: this is too sketchy because there could be multiple events waiting in the queue that don't belong to the maker trying to get his open_orders_account updated. Given the fact that he needs to specify a max_iterations, he could consume the next X events that belong to other users and were not consumed yet, so he would end up cranking the market for others and not consume enough events to reach his own events.

All in all, I think trying to embed the consume_events instruction in users' trading transaction is a dead-end. This leads to the point 2) below.

2. There is a cranker folder in the repo. The code looks like something that needs to be run in a background job by the market admin at a particular frequency. The frequency can be set according to the liquidity in the market I guess.

Here are the questions:

• Is the background job the way to go regarding cranking/consuming events of a market?
• If it is, any suggestions about the number of iterations it should consume at each run? Is there a limit for how many events can be consumed?

[ellttBen]

Hi @mihneacalugaru, thanks for your questions. In the general case it's essentially impossible to bundle a new_order instruction with a consume_events and expect that the new order will be consistently cranked. This is due to the fact that you can't reliably predict what order you will be matched against. In practice, for smaller markets in which one maker dominates, predictions might be more successful though. This is why the recommended approach is indeed to run a cranking daemon on a third-party server. This can be done by the market admin or anyone else for that matter. When creating a market we do recommend a creator-run cranking service though. With regards to the number of iterations to consume, the idea is that you want that number to be as high as possible while not exceeding the compute budget. In theory, the cranking server could self-optimize based on metrics such as orderbook depth, but this isn't something we have worked on ourselves. In the meantime, a value of 10 should be safe enough.

[mihneacalugaru]

Hi @ellttBen, thanks for your answer, as always!

Understood everything you said and also implemented it. Works well!

More as a FAQ-sided note, I wanted to talk a bit about the AAOB's event queue model. As it can be implied from my previous comment, initially I was wondering why we even need an event queue from which to pop and process on the caller program. I was wondering why we can't consume the event instead of pushing it to the queue.

But I think I understand now:

• AAOB was not made for dex-v4, but dex-v4 is just a client that calls AAOB.
• AAOB (as it names implies) it doesn't care for what underlying asset it facilitates the trading. Which means it doesn't know what to do when an order match happens. It just knows it happened and lets the calling program to decide what to do about that via the callback information mechanism
• Also, we can't consume the event in the dex-v4's new_order instruction right after the AAOB's new_order instruction returned because the queue it's FIFO, so we don't know what we are consuming and there is no guarantee that through the events we consume there is also the event in which the maker for this order gets his funds.

Correct me If I'm wrong with any of my above findings and understanding.

Thank you very much for your time and for working with me on building this early version of FAQ!