How DNS load balancing works

A load balancer answers A and AAAA queries for one hostname in a zone, such as api.example.com, with the address of one of your servers. Coritan picks the server for each query from origin pools, skips servers that fail their health checks, and can send clients in different places to different pools. You manage both on a domain's Load balancing tab.

Origin

A server that can answer for the hostname, given by its public IPv4 or IPv6 address. An IPv4 origin answers A queries and an IPv6 origin answers AAAA queries.

Origin pool

A named group of origins in one zone, such as the web servers in one data centre. The pool's algorithm decides which of its origins answers.

Load balancer

A hostname in the zone and the pools that answer for it, in order, with the steering that reorders them.

Steering

How the load balancer orders its pools for a query: as listed, at random, by where the client is, or by distance.

Fallback pool

The pool that answers when every pool in the load balancer's order has no healthy origin.

1. Steering puts the load balancer's pools in order for this query.
2. Coritan goes down the order and skips disabled pools and pools with no healthy origin of the query's type.
3. The first pool left chooses one origin, and the answer carries that origin's address with the load balancer's TTL.
4. If no pool in the order can answer, the fallback pool tries.
5. If that fails too, Coritan answers with one of the enabled origins anyway, so the name keeps resolving while it has any.

Each answer holds one address. Visitors then connect straight to that origin: their traffic does not pass through Coritan's edge, its WAF or its certificates. For that, put a web proxy in front of the hostname instead.

Weighted round-robin

Picks a healthy origin at random, in proportion to its Weight from 1 to 100. An origin with weight 20 gets about twice the queries of one with weight 10.

Failover (by priority)

Answers with the healthy origin that has the lowest Priority number. The others answer only while it is unhealthy.

The load balancer's Session affinity can change both. With By client IP, the same resolver gets the same origin while that origin stays healthy. Coritan sees the address of the visitor's DNS resolver, so every visitor behind one resolver gets the same origin. In a Failover (by priority) pool, By client IP spreads resolvers over all the healthy origins, whatever their priority.

An origin can have a Health check of HTTP, HTTPS or TCP connect. Coritan checks every enabled origin that has one once a minute, and a check fails when the origin does not answer within its Timeout (s).

• An HTTP or HTTPS check asks for the check's Path at the origin's address and passes on a status from 200 to 399. The request names the IP address as its host, so the origin must answer a request that does not name your domain.
• An HTTPS check also verifies the origin's certificate against its IP address. A certificate that names only your domain fails it, so use HTTP or TCP connect for such an origin.
• A TCP connect check passes when the port accepts a connection.

An origin shows healthy, unhealthy or unknown. A new origin is unknown until its first check, and Coritan treats it as healthy until then. An origin with no check is always treated as healthy. Run health checks in a pool's menu checks its origins straight away.

Off

The load balancer tries its Default pools in the order you numbered them.

Random

It puts the default pools in a random order for each query.

Geographic

It matches the client's location against rules By PoP, then By country, then By region, and uses the pools of the first rule that matches. A client that no rule matches gets the default pools.

Proximity

It ranks the pools by the distance from the client to each pool's Primary location. A pool without one comes last.

Geographic and Proximity need to know where the client is. Locate the client by sets how:

Answering PoP

The client is where the Coritan nameserver that answered the query is: that PoP, its country and its region.

Resolver IP

Coritan looks up the country of the resolver's IP address, and the region that country is in. By PoP rules do not match, because the resolver has no PoP. Proximity has no coordinates to measure from, so it keeps the pools in their listed order. When the address has no known country, Coritan uses the answering PoP instead.

A load balancer owns its hostname for A, AAAA and CNAME records. You cannot create one on a name that already has such records, and the zone refuses them at the name while the load balancer exists, even when it is off. While it is off, the name gets no A or AAAA answers. Other types at the name, such as TXT or MX, are unaffected. When you delete the load balancer, you can add records at the name again.

• Create an origin pool
• Create a load balancer
• Load balancing reference