The OrderSend() command, which MQL programmers use to open a trade, also tells the MT4 client terminal to await a server response. Everything locks up. The code cannot do anything until the server permits the client terminal to continue. More importantly, the trading terminal does not allow any additional trades to open.
Example
Consider a scenario where you trade EURUSD, AUDUSD and USDJPY currency pairs on H1 charts. MT4 must await an incoming tick before MQL allows it to make a trading decision. I discussed this issue several months ago with time based orders.
The ticks of multiple currencies often group together during liquid hours. If the terminal receives the incoming ticks within a few hundred milliseconds of each other, MetaTrader creates its own bottleneck.
The AUDUSD tick comes in first. The Expert Advisor sees a trading opportunity and decides to open an AUDUSD trade with OrderSend(). 150 ms later a EURUSD tick comes in and creates another trading opportunity. The EURUSD EA, which runs on a completely different chart, is not allowed to trade.
Retry delay for Trade Context is Busy
Our Expert Advisor programmers check for "Trade Context is Busy" errors before sending orders to the broker. When the situation exits, the EA checks the trade context every 50 ms up to 50 times.
It is not uncommon for MetaTrader orders to take a second or more to execute. The original AUDUSD trade from our example is still hanging out there. The EURUSD EA enters a 50 ms cycle where it constantly asks, "Did the trade context open up?"
Then the USDJPY tick comes in 70 ms after the EURUSD tick. USDJPY encounters the same trade context issue, kicking it into a separate, "Is the trade context busy" loop.
When the AUDUSD trade finally executes, it leaves the EURUSD and USDJPY expert advisors in a frantic race to discover the newly opened trading window. The first EA that discovers that the trading context busy error disappeared will send its request first. The second EA, unfortunately, cannot communicate with the first. Although the second EA will discover that the trade context opened up, it will receive a trade conext is busy error when it attempts to trade; the firs EA already sent a trading request. The second expert advisor allots itself a maximum of 10 failures before it gives up on the trade.
Assuming that the EURUSD discovers the new trading window first, then the USDJPY receives its first failed order attempt. The USDJPY once again inquires about the trading context every 50 ms. The EURUSD trade eventually executes, allowing the USDJPY to finally enter the market.
Most of our customers never notice this delay, which means that we did our job. There is, however, a breaking point where MetaTrader simply cannot issue enough orders at the speed the trader desires. Any combination of charts where 10 or more orders go off near-simultaneously will result in missed trading opportunities.
MetaQuotes designed MT4 with the trade context error in order to limit the processing requirements of the brokers. As I've stated repeatedly, the brokers get MetaQuotes' preferential treatment because they are the ones paying MetaQuotes' fee. MetaTrader is not a high frequency trading platform. It's not in the broker's interest to support high speed trading. It usually opens them up to too many traders taking advantage of arbitrage opportunities.
