What is Weight, and how should it be set?

Short answer

Weight is an integer in the range 1–256 that determines how much traffic a Backend Service receives relative to the others in the same list; its default value is 1. The share of each server is the ratio of its own weight to the total weight: three servers with weights 1, 2 and 3 receive approximately 17%, 33% and 50% of the traffic. The weight is set according to the capacity of the servers. The vService's default algorithm Round Robin ignores the weight; for the weight to take effect, an algorithm that uses the weight, such as Weighted Round Robin, is selected.

How Does It Work?

• Share calculation: share = the server's weight ÷ the sum of the weights of all the servers in the list. Example: 8, 4 and 2 → approximately 57%, 29% and 14%.
• Depends on the algorithm: The weight is applied if an algorithm that uses the weight is selected in the vService's LB Algorithm field.
• The ratio is what matters: If all servers are given the same weight, the distribution is equal; 1-1-1 and 50-50-50 give the same result.
• Per vService: If the same Backend Service is used in more than one vService, the share is calculated separately according to each vService's own list.
• Backups are not counted: Backend Services marked as Backup Service do not receive traffic while the main servers are up; the share is distributed among the main servers.

Which Algorithms Use the Weight?

vService type	Uses the weight	Ignores the weight
HTTP	Weighted Round Robin, Least Connection, Least Response Time, Least Bandwidth, Random, Source, URI, URL Param, HDR	Round Robin (default), First, Fastest, Fastest+
L7 TCP	Weighted Round Robin, Least Connection, Least Response Time, Least Bandwidth, Random, RDP Cookie	Round Robin (default), First, Fastest, Fastest+
Network	Weighted Round-Robin, Weighted Least-Connection, Locality-Based Least-Connection, Source Hash	Round-Robin (default), Least-Connection, Destination Hash

With the First algorithm, server selection is determined not by the weight but by the Connection Limit: the first server in the list takes the entire load until it reaches the connection limit. Fastest and Fastest+ select the server according to the selected measurement (e.g. response time). For Network vService algorithms: Network vService · Algorithms.

How Is It Set?

Situation	Recommendation
The servers are on the same hardware	The same weight for all of them, or an algorithm that ignores the weight
One server is twice as powerful as the other	2 for the more powerful one, 1 for the other
Bringing a new server into service gradually	A low weight first (e.g. 1 while the others are 10); the load is monitored and the weight is increased
A server in a remote data center	A lower weight than the local servers

Weights are chosen in proportion to the number of cores, the memory, or the requests/second capacity measured with a load test.

In TR7

• Individual Backend Service: Load Balancer → Backend Services → Add or Edit → Details switch → Weight. When the field is empty, the default value 1 is shown in it.
• Backend Group: Weight is per server; it is entered separately for each row in the Backends table (Backend Groups · Weight).
• Algorithm: Load Balancer → Load Balancer vService List → vService → Edit → LB Algorithm; the change is saved with Save.
• For a value outside the range, the form gives the warning "Weight must be in range 1-256", and for a value that is not an integer, the warning "Weight must be an integer".
• On the Live Stats screen, the weight of each Backend Service with an assigned weight appears with W= in its name cell; the actual distribution is monitored on the Backend Service's Dashboard screen.
• When Clustering is on and Manual Sync Mode is off (default), a weight change is transferred to the peer device automatically.

Common Misconceptions

• "I entered a Weight, but the distribution did not change." The vService is on the default Round Robin algorithm, which ignores the weight; a switch is made to Weighted Round Robin or to another algorithm that uses the weight.
• "With Weight 0, the server is taken out of traffic." 0 cannot be entered; the lowest value is 1. To stop traffic, Maintenance Mode is used (Maintenance Mode does not apply to a Network vService; the Backend Service is removed from the vService's Backend Services list).
• "A server with Weight 1 receives no traffic at all." Weight 1 is the lowest share, but it is not zero.
• "The weight is the maximum number of connections the server will receive." The weight is a ratio; the upper limit is set with Connection Limit.

How it is done in the documentation

• Backend Services · Weight
• Backend Services · Weight
• Backend Groups · Server-Level Weights
• HTTP vService · Algorithms
• Network vService · Algorithms

Related questions

• What is the difference between Round Robin and Weighted Round Robin?
• Can I give the servers in a Backend Group their own Weight and connection limit?
• How do I temporarily stop all traffic to a server; can I set Weight to 0?