Introduction
Important: This applies only to Visual Adelia applications generated with a Windows client and to Adelia Web applications. Load balancing is not supported for Visual Adelia applications compiled with a Java client.
The load balancer supports the use of server clusters, which can be defined in the MWCLIENT.INI file.
It provides the following features:
- Transparent load-balancing across multiple servers.
- High availability: As long as at least one of the servers in the cluster is accessible, client applications will be able to establish a connection.
- Fallback to a backup system in case of failure of the main servers in a server cluster.
The load balancer can be implemented in two different ways:
- For Visual Adelia programs, the load balancer is implemented as a daemon running on the client workstations (i.e. the Adelia - Cluster Manager service).
- For Adelia Web programs, the load balancer is implemented transparently as part of the Web application.
As a rule, all the servers in a cluster must have compatible configurations (with the same connection profiles and the same database names). Ideally, all the servers should be the same type (C / Java / AS400), although this is not essential.
As the server group's configuration is loaded directly from the application's MWCLIENT.INI file, it is not possible to modify the physical servers associated with the cluster when the configuration is set (host name and port). The only modifiable parameters are the profile and password of the cluster or physical servers, and any associations.
Operation of the backup cluster
The load balancer can be used to define a backup system to take over processing in the event of failure of the servers in the main cluster.
Where a backup cluster is configured, the system operates as follows:
- If none of the servers in the main cluster is responding, the system switches automatically to the backup cluster.
- If the backup cluster is already in use, the system automatically switches back to the main cluster as soon as one of its servers begins responding again.
Use of the load balancer in Windows mode
The load balancer must be started on the client workstations that connect to a server cluster.
To run the load balancer, you must either:
- Start the Windows Adelia – Cluster Manager service.
- Execute the mwdispatcher command, which starts the load balancer in a console.
The load balancer management service must read the client configuration file (MWCLIENT.INI). Therefore, you must start the service using a profile with the necessary authorities to read the appropriate configuration file(s).
The command line options are as follows:
-i <ligne commande> |
Installs the Adelia – Cluster Manager service in manual start mode. |
-a <ligne commande> |
Installs the Adelia – Cluster Manager service in autostart mode. |
-r |
Uninstalls the Adelia – Cluster Manager service. |
-v |
Verbose mode: displays additional diagnostic messages when executed. |
-q |
Silent mode (no messages). |
-ts |
Displays the current date and time in message headers. |
-o:<nom_fichier> |
Copies the messages to the specified file. |
-refresh:<s> |
Refreshes the servers every <n> seconds (every 30 seconds by default). |
-enable-ramp |
Activates a reconnection attempt-limiting algorithm when a server is restarted following an incident (in "by number of connections" mode). |
-ramp-time:<ms> |
Minimum time between two connection attempts when a server becomes available again after an incident. This mechanism prevents all new connection attempts from being redirected to a server that has just returned to the cluster. |
-ramp-percent:<p> |
Percentage of total load above which the limiting algorithm is disabled (default setting: 0.8). By default, the connection limiting algorithm is disabled when the monitored server's workload reaches 80% of that of the most heavily-loaded server in the cluster. |
Examples:
- The service is installed with the default parameters, and writes timestamped messages to a log file:
Mwdispatch -i -ts
- Startup in a console in verbose mode:
mwdispatch
Use of the load balancer in Web mode
The load balancer is started automatically in the Web application. No configuration operations or manual actions are required.