
Equalizer Overview
l Active connections - The number of connections a server currently has active and the number of
connections that it tends to have open.
l Connection latency - The amount of time that it takes a server to respond to a client request.
l Health check performance values - Depending on the health checks configured, this may be not used at all,
or it can completely define how the load is calculated.
Once a load is calculated, Equalizer distributes incoming requests using the relative loads as weights.
sv00
Load = 50
sv01
Load = 50
sv02
Load = 50
Equal calculated loads, so the request distribution will be approximately equal
sv00
Load = 100
sv01
Load = 50
sv02
Load = 25
Uneven loads. sv01 is twice as loaded as sv02, so it will receive about half the requests.
The load calculations happen approximately every 10 seconds and server weights are adjusted accordingly.
During that 10 second interval, the relative server loads remain the same, but probe and health check information
is collected about the servers so that it can be used for the next calculation.
The load calculation works the same for Layer 4 and Layer 7 clusters (at the server-pool level – and these can be
shared between all cluster types).
There are two additional variables for load balancing:
l Hot spare - if a server is marked as a hot spare, it is not included in the pool of servers to select from unless
every other non-hot-spare server is down. If a connection persists to this server, it will be placed back on
this server.
l Quiesce - If a server has been quiesced, it will not be included in the pool of servers to select from. Only
previously existing (persistent) connections will be made to this server.
Layer 7 Load Balancing and Server Selection
Equalizer’s support for Layer 7 content-sensitive load balancing enables administrators to define rules for routing
HTTP, HTTPS, and special Layer 7 TCP requests, depending on the content of the request. Layer 7 load
balancing routes requests based on information from the application layer. This provides access to the actual data
payloads of the TCP/UDP packets exchanged between a client and server. For example, by examining the
payloads, a program can base load-balancing decisions for HTTP requests on information in client request headers
and methods, server response headers, and page data.
Equalizer’s Layer 7 load balancing allows administrators to define rules in the administration interface for routing
HTTP and HTTPS requests according to the request content. These rules are called
match rules
. A match rule
might, for example, route requests based on whether the request is for a text file or a graphics file:
l load balance all requests for text files (html, etc.) across servers A and B
l load balance all requests for graphics files across servers C, D, and E
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