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Loadbalancing reference


Kong provides multiple ways of load balancing requests to multiple backend services: a straightforward DNS-based method, and a more dynamic ring-balancer that also allows for service registry without needing a DNS server.

DNS-based loadbalancing

When using DNS-based load balancing, the registration of the backend services is done outside of Kong, and Kong only receives updates from the DNS server.

Every Service that has been defined with a host containing a hostname (instead of an IP address) will automatically use DNS-based load balancing if the name resolves to multiple IP addresses, provided the hostname does not resolve to an upstream name or a name in your DNS hostsfile.

The DNS record ttl setting (time to live) determines how often the information is refreshed. When using a ttl of 0, every request will be resolved using its own DNS query. Obviously this will have a performance penalty, but the latency of updates/changes will be very low.

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A records

An A record contains one or more IP addresses. Hence, when a hostname resolves to an A record, each backend service must have its own IP address.

Because there is no weight information, all entries will be treated as equally weighted in the load balancer, and the balancer will do a straight forward round-robin.

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SRV records

An SRV record contains weight and port information for all of its IP addresses. A backend service can be identified by a unique combination of IP address and port number. Hence, a single IP address can host multiple instances of the same service on different ports.

Because the weight information is available, each entry will get its own weight in the load balancer and it will perform a weighted round-robin.

Similarly, any given port information will be overridden by the port information from the DNS server. If a Service has attributes and port=123, and resolves to an SRV record with, then the request will be proxied to, as port 123 will be overridden by 456.

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DNS priorities

The DNS resolver will start resolving the following record types in order:

  1. The last successful type previously resolved
  2. SRV record
  3. A record
  4. CNAME record

This order is configurable through the dns_order configuration property.

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DNS caveats

  • Whenever the DNS record is refreshed a list is generated to handle the weighting properly. Try to keep the weights as multiples of each other to keep the algorithm performant, e.g., 2 weights of 17 and 31 would result in a structure with 527 entries, whereas weights 16 and 32 (or their smallest relative counterparts 1 and 2) would result in a structure with merely 3 entries, especially with a very small (or even 0) ttl value.

  • Some nameservers do not return all entries (due to UDP packet size) in those cases (for example Consul returns a maximum of 3) a given Kong node will only use the few upstream service instances provided by the nameserver. In this scenario, it is possible that the pool of upstream instances will be loaded inconsistently, because the Kong node is effectively unaware of some of the instances, due to the limited information provided by the nameserver. To mitigate this use a different nameserver, use IP addresses instead of names, or make sure you use enough Kong nodes to still have all upstream services being used.

  • When the nameserver returns a 3 name error, then that is a valid response for Kong. If this is unexpected, first validate the correct name is being queried for, and second check your nameserver configuration.

  • The initial pick of an IP address from a DNS record (A or SRV) is not randomized. So when using records with a ttl of 0, the nameserver is expected to randomize the record entries.

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When using the ring-balancer, the adding and removing of backend services will be handled by Kong, and no DNS updates will be necessary. Kong will act as the service registry. Nodes can be added/deleted with a single HTTP request and will instantly start/stop receiving traffic.

Configuring the ring-balancer is done through the upstream and target entities.

  • target: an IP address or hostname with a port number where a backend service resides, eg. "". Each target gets an additional weight to indicate the relative load it gets. IP addresses can be in both IPv4 and IPv6 format.
  • upstream: a 'virtual hostname' which can be used in a Route host field, e.g., an upstream named weather.v2.service would get all requests from a Service with host=weather.v2.service.

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Each upstream gets its own ring-balancer. Each upstream can have many target entries attached to it, and requests proxied to the 'virtual hostname' will be load balanced over the targets. A ring-balancer has a pre-defined number of slots, and based on the target weights the slots get assigned to the targets of the upstream.

Adding and removing targets can be done with a simple HTTP request on the Admin API. This operation is relatively cheap. Changing the upstream itself is more expensive as the balancer will need to be rebuilt when the number of slots change for example.

The only occurrence where the balancer will be rebuilt automatically is when the target history is cleaned; other than that, it will only rebuild upon changes.

Within the balancer there are the positions (from 1 to slots), which are randomly distributed on the ring. The randomness is required to make invoking the ring-balancer cheap at runtime. A simple round-robin over the wheel (the positions) will do to provide a well distributed weighted round-robin over the targets, whilst also having cheap operations when inserting/deleting targets.

The number of slots to use per target should (at least) be around 100 to make sure the slots are properly distributed. Eg. for an expected maximum of 8 targets, the upstream should be defined with at least slots=800, even if the initial setup only features 2 targets.

The tradeoff here is that the higher the number of slots, the better the random distribution, but the more expensive the changes are (add/removing targets)

Detailed information on adding and manipulating upstreams is available in the upstream section of the Admin API reference.

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Because the upstream maintains a history of changes, targets can only be added, not modified nor deleted. To change a target, just add a new entry for the target, and change the weight value. The last entry is the one that will be used. As such setting weight=0 will disable a target, effectively deleting it from the balancer. Detailed information on adding and manipulating targets is available in the target section of the Admin API reference.

The targets will be automatically cleaned when there are 10x more inactive entries than active ones. Cleaning will involve rebuilding the balancer, and hence is more expensive than just adding a target entry.

A target can also have a hostname instead of an IP address. In that case the name will be resolved and all entries found will individually be added to the ring balancer, e.g., adding with weight=100. The name '' resolves to an A record with 2 IP addresses. Then both ip addresses will be added as target, each getting weight=100 and port 123. NOTE: the weight is used for the individual entries, not for the whole!

Would it resolve to an SRV record, then also the port and weight fields from the DNS record would be picked up, and would overrule the given port 123 and weight=100.

The balancer will honor the DNS record's ttl setting and requery and update the balancer when it expires.

Exception: When a DNS record has ttl=0, the hostname will be added as a single target, with the specified weight. Upon every proxied request to this target it will query the nameserver again.

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Balancing algorithms

By default a ring-balancer will use a weighted-round-robin scheme. The alternative would be to use the hash-based algorithm. The input for the hash can be either none, consumer, ip, header, or cookie. When set to none the weighted-round-robin scheme will be used, and hashing will be disabled.

There are two options, a primary and a fallback in case the primary fails (e.g., if the primary is set to consumer, but no consumer is authenticated)

The different hashing options:

  • none: Do not use hashing, but use weighted-round-robin instead (default).

  • consumer: Use the consumer id as the hash input. This option will fallback on the credential id if no consumer id is available (in case of external auth like ldap).

  • ip: The remote (originating) IP address will be used as input. Review the configuration settings for determining the real IP when using this.

  • header: Use a specified header (in either hash_on_header or hash_fallback_header field) as input for the hash.

  • cookie: Use a specified cookie name (in the hash_on_cookie field) with a specified path (in the hash_on_cookie_path field, default "/") as input for the hash. If the cookie is not present in the request, it will be set by the response. Hence, the hash_fallback setting is invalid if cookie is the primary hashing mechanism.

The hashing algorithm is based on 'consistent-hashing' (or the 'ketama principle') which makes sure that when the balancer gets modified by changing the targets (adding, removing, failing, or changing weights) only the minimum number of hashing losses occur. This will maximize upstream cache hits.

For more information on the exact settings see the upstream section of the Admin API reference.

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Balancing caveats

The ring-balancer is designed to work both with a single node as well as in a cluster. For the weighted-round-robin algorithm there isn't much difference, but when using the hash based algorithm it is important that all nodes build the exact same ring-balancer to make sure they all work identical. To do this the balancer must be build in a deterministic way.

  • Do not use hostnames in the balancer as the balancers might/will slowly diverge because the DNS ttl has only second precision and renewal is determined by when a name is actually requested. On top of this is the issue with some nameservers not returning all entries, which exacerbates this problem. So when using the hashing approach in a Kong cluster, add target entities only by their IP address, and never by name.

  • When picking your hash input make sure the input has enough variance to get to a well distributed hash. Hashes will be calculated using the CRC-32 digest. So for example, if your system has thousands of users, but only a few consumers, defined per platform (eg. 3 consumers: Web, iOS and Android) then picking the consumer hash input will not suffice, using the remote IP address by setting the hash to ip would provide more variance in the input and hence a better distribution in the hash output. However, if many clients will be behind the same NAT gateway (e.g. in call center), cookie will provide a better distribution than ip.

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Blue-Green Deployments

Using the ring-balancer a blue-green deployment can be easily orchestrated for a Service. Switching target infrastructure only requires a PATCH request on a Service, to change its host value.

Set up the "Blue" environment, running version 1 of the address service:

# create an upstream
$ curl -X POST http://kong:8001/upstreams \
    --data "name=address.v1.service"

# add two targets to the upstream
$ curl -X POST http://kong:8001/upstreams/address.v1.service/targets \
    --data "target="
    --data "weight=100"
$ curl -X POST http://kong:8001/upstreams/address.v1.service/targets \
    --data "target="
    --data "weight=50"

# create a Service targeting the Blue upstream
$ curl -X POST http://kong:8001/services/ \
    --data "name=address-service" \
    --data "host=address.v1.service" \
    --data "path=/address"

# finally, add a Route as an entry-point into the Service
$ curl -X POST http://kong:8001/services/address-service/routes/ \
    --data "hosts[]"

Requests with host header set to will now be proxied by Kong to the two defined targets; 2/3 of the requests will go to (weight=100), and 1/3 will go to (weight=50).

Before deploying version 2 of the address service, set up the "Green" environment:

# create a new Green upstream for address service v2
$ curl -X POST http://kong:8001/upstreams \
    --data "name=address.v2.service"

# add targets to the upstream
$ curl -X POST http://kong:8001/upstreams/address.v2.service/targets \
    --data "target="
    --data "weight=100"
$ curl -X POST http://kong:8001/upstreams/address.v2.service/targets \
    --data "target="
    --data "weight=100"

To activate the Blue/Green switch, we now only need to update the Service:

# Switch the Service from Blue to Green upstream, v1 -> v2
$ curl -X PATCH http://kong:8001/services/address-service \
    --data "host=address.v2.service"

Incoming requests with host header set to will now be proxied by Kong to the new targets; 1/2 of the requests will go to (weight=100), and the other 1/2 will go to (weight=100).

As always, the changes through the Kong Admin API are dynamic and will take effect immediately. No reload or restart is required, and no in progress requests will be dropped.

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Canary Releases

Using the ring-balancer, target weights can be adjusted granularly, allowing for a smooth, controlled canary release.

Using a very simple 2 target example:

# first target at 1000
$ curl -X POST http://kong:8001/upstreams/address.v2.service/targets \
    --data "target="
    --data "weight=1000"

# second target at 0
$ curl -X POST http://kong:8001/upstreams/address.v2.service/targets \
    --data "target="
    --data "weight=0"

By repeating the requests, but altering the weights each time, traffic will slowly be routed towards the other target. For example, set it at 10%:

# first target at 900
$ curl -X POST http://kong:8001/upstreams/address.v2.service/targets \
    --data "target="
    --data "weight=900"

# second target at 100
$ curl -X POST http://kong:8001/upstreams/address.v2.service/targets \
    --data "target="
    --data "weight=100"

The changes through the Kong Admin API are dynamic and will take effect immediately. No reload or restart is required, and no in progress requests will be dropped.

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