MQL5 EA lifecycle ordering during account change/reconnect

[Kuna Shoo]

We need clarification on the documented ordering guarantees for an

already-running Expert Advisor when the trading account changes or a

REASON_ACCOUNT reconnect occurs.

Assume an EA uses a two-event protocol:

Event handler A:

- reads AccountInfo*/TerminalInfo* data

- prepares a response in private memory

- returns without publishing it

Later event handler B:

- rechecks the same account/server context

- verifies IsStopped() == false

- immediately commits/publishes the previously prepared response

Question:

If an account change or REASON_ACCOUNT-related reconnect occurs after

handler B's final context check but before its response publication,

does MetaTrader 5 guarantee that the EA's old instance will receive

deinitialization / REASON_ACCOUNT before it can complete that

publication?

More specifically:

1. Does deinitialization caused by account change/reconnect have

ordering priority over an already queued EA event handler?

2. Can the currently executing handler continue running and publish

data after the terminal account context has changed but before

OnDeinit(REASON_ACCOUNT) is delivered?

3. Is there any documented MQL5 mechanism that provides a lifecycle

fence guaranteeing that no response from the old EA instance can be

committed after the account context has changed?

We are not asking whether events are processed sequentially in general.

We specifically need the ordering guarantee between account lifecycle

changes, OnDeinit(REASON_ACCOUNT), and publication by the old EA

instance.

If no such guarantee exists, please confirm that explicitly.

Thank you.

[Alain Verleyen]

Kuna Shoo:

We need clarification on the documented ordering guarantees for an

already-running Expert Advisor when the trading account changes or a

REASON_ACCOUNT reconnect occurs.

Assume an EA uses a two-event protocol:

Event handler A:

- reads AccountInfo*/TerminalInfo* data

- prepares a response in private memory

- returns without publishing it

Later event handler B:

- rechecks the same account/server context

- verifies IsStopped() == false

- immediately commits/publishes the previously prepared response

Question:

If an account change or REASON_ACCOUNT-related reconnect occurs after

handler B's final context check but before its response publication,

does MetaTrader 5 guarantee that the EA's old instance will receive

deinitialization / REASON_ACCOUNT before it can complete that

publication?

1. There is no "old" instance, it's the same EA that continues to run.

2. If the EA is already running an event handler it will go to the end of it. An event handler processing is never stopped to run an other event handler.

More specifically:

1. Does deinitialization caused by account change/reconnect have

ordering priority over an already queued EA event handler?

That's a good question.

2. Can the currently executing handler continue running and publish

data after the terminal account context has changed but before

OnDeinit(REASON_ACCOUNT) is delivered?

Yes without a doubt as I explained already above.

3. Is there any documented MQL5 mechanism that provides a lifecycle

fence guaranteeing that no response from the old EA instance can be

committed after the account context has changed?

We are not asking whether events are processed sequentially in general.

We specifically need the ordering guarantee between account lifecycle

changes, OnDeinit(REASON_ACCOUNT), and publication by the old EA

instance.

If no such guarantee exists, please confirm that explicitly.

Thank you.

In case an event handler is already running, there is no such guarantee.

For the subsequent events I don't know, I asked MetaQuotes.