The MT Integration Suite (MTIS) provides an HTTP and websocket API around MT4 and MT5 (server, not client terminal). The API is designed to be used by both individual clients and also on a back-office/managerial basis, behaving identically regardless of whether the connection is to MT4 or MT5.
We're in the 3rd decade of the 21st century. The days of only offering FIX as an API option - and only to VIP accounts - are long gone.
The MTIS is built on top of the MT4/5 Manager API, not a server plugin. As a result, any load on the API can be kept on a separate server and prevented from affecting the MT4/5 server itself. Any number of clients using the API share a single streaming connection to the MT4/5 server, transferring load away from MT4/5.
The API caches data such as candles, preventing these queries from hitting your MT4/5 server, and the API can be configured to offer multiple international points-of-presence to minimise latency and maximise speed for your clients.
The API has minimal processor and memory usage, doesn't depend on or require any other software such as a web server, and can offer advanced features such as ZIP compression over websockets to minimise bandwidth usage.
The MTIS API harmonises the differences between MT4 and MT5. You and your clients can collect data and write tools identically regardless of whether the connection is to MT4 or MT5.
For example, a traders' room which you build for MT4 can work without modification on MT5 as well, and anything which clients build will similarly continue to work if their account is migrated from MT4 to MT5.
As well as simply offering login using the MT4/5 account number and password, you can also do the following:
The API also offers facilities such as one-time keys, letting you verify a client's credentials server-side and then securely pass that authentication to the client-side. For example, once your client portal has verified the user's identity, you can securely transfer authentication to the browser so that a trader's room page can make a client-side websocket connection for that client directly to the API.
The API doesn't just let you offer clients what they have come to expect from market leaders in the industry. It's also flexible, letting clients work in the way that they want rather than forcing them to use particular technologies or programming languages.