Clusters of both types are usually equipped with Frontend Servers. Frontend Servers cannot access Account data directly - they always open connections to other (Backend) Servers to perform any operation with Account data.
Frontend servers accept TCP/IP connections from client computers (usually - from the Internet). In a pure Frontend-Backend configuration no Accounts are created on any Frontend Server, but nothing prohibits you from serving some Domains (with Accounts and mailing lists) directly on the Frontend servers.
When a client establishes a connection with one of the Frontend Servers and sends the authentication information (the Account name), the Frontend server detects on which Backend server the addressed Account can be opened, and establishes a connection with that Backend Server.
The Frontend Servers:
If the Frontend Servers are directly exposed to the Internet, and the security of a Frontend Server operating system is compromised so that someone gets unauthorized access to that Server OS, the security of the site is not totally compromised. Frontend Servers do not keep any Account information (Mailboxes, passwords) on their disks. The "cracker" would then have to go through the firewall and break the security of the Backend Server OS in order to get access to any Account information. Since the network between Frontend and Backend Servers can be disabled for all types of communications except the CommuniGate Pro inter-server communications, breaking the Backend Server OS is virtually impossible.
Both Static and Dynamic Clusters can work without dedicated Frontend Servers. This is called a symmetric configuration, where each Cluster Server implements both Frontend and Backend functions.
In the example below, the domain1.dom and domain2.dom Domain Accounts are distributed between three Static Cluster Servers, and each Server accepts incoming connections for these Domains. If the Server SV1 receives a connection for the Account email@example.com located on the Server SV2, the Server SV1 starts to operate as a Frontend Server, connecting to the Server SV2 as the Backend Server hosting the addressed Account. At the same time, an external connection established with the server SV2 can request access to the firstname.lastname@example.org Account located on the Server SV1. The Server SV2 acting as a Frontend Server will open a connection to the Server SV1 and will use it as the Backend Server hosting the addressed Account.
In a symmetric configuration, the number of inter-server connections can be equal to the number of external (user) access-type (POP, IMAP, HTTP) connections. For a symmetric Static Cluster, the average number of inter-server connections is M*(N-1)/N, where M is the number of external (user) connections, and the N is the number of Servers in the Static Cluster. For a symmetric Dynamic Cluster, the average number of inter-Server conn