Mechanics underlying a trade

I have searched the net and the forums quite a bit, but I cannot find answers to the following. Could somebody please point me to a place where I can find the answers or answer them here, please.

For simplicities sake, lets say that there are only three traders T1, T2 and T3 with a ECN-broker. T1 has a long of size 1,000,000 EURUSD, and wants to close his position @ 1.5 (the bid price, correct?). T2 and T3 want the full 1,000,000.

a) T2 places an order @ 1.4999. The platform will not fill T2's order, correct?

or b) T2 places an order @ 1.5. The platform will immediately fill T2's order, correct? The new bid price will be set @ 1.5, correct?

or c) T2 places an order @ 1.5001. The platform will immediately fill T2's order at this price, correct? The new bid price will be set @ 1.5001, correct?

or d) T2 places an order @ 1.5, and T3 places an order at exactly the same time @ 1.5002. The platform will immediately fill T3's order at 1.5002, and T2 will be requoted, correct?

I'll try to help with this one. Forgive me if I state things too obvious or that you already know.

Let's limit our ECN scenario with just Limit or Market orders.
A TRUE ECN Broker will have no requotes. Basically, all that is available to Buy or Sell is what is produced from the ECN participants (retail traders using that broker).

Think of each side of the quote as a stack of orders for people wishing to Buy or Sell. The Bid side are the ECN participants and the price at which they want to Buy. The Offer side are the sellers, and the prices that they wish to sell.

If someone places a Limit order (or a Bid) as high as some available quantity that is for sale, it is executed immediately for the amount available. If there is a remainder quantity, a new Bid would appear for the rest of the order.

so, with your examples...

b) is correct...but the Bid price on the ECN would only stay at 1.5, if T3 also had his Bid in the ECN for 1,000,000 Euros. Remember, the quote ONLY displays a price at a certain level, if there are actually real buyers or sellers on an ECN. Otherwise, it changes to another level where ECN participants have orders (different prices).

as for

d) There are no requotes. ECN's only match buyers/sellers. If there is no match then a New Bid/Offer appears in the ECN's quote. Someone is always first, so If T2's order got there first, he would get the 1,000,000 at 1.5 if T1 had it for sale (on the Offer side of the quote). If there is nothing else for sale until 1.5003, then T3's Bid would show up in the ECN's quote. The current Quote might then be 1.5002 (for 1,000,000) - 1.5003 (for some size). If T3's order had arrived first, even at a buying price of 1.5002, he would've bought the Euros for 1.5 if it was offered for sale at that price. (If there were none, he would've bought his quantity up to a price of 1.5002...whatever was available).

Market orders are executed against what is available at the current quotes. The prices may change after that.

MB Trading is an ECN broker and they have a demo available. You can see the depth of the market and the orders "stacked" up. It might be worth a look.

A TRUE ECN only matches buyers and sellers, the price quotes are made up of the traders using the ECN, there should be no requotes "per se", but yes, it is quite possible that you missed the price you were "aiming for". Perhaps that is what you mean by requote.

I hope this helps somewhat.