Multi-tier load-balancing with Linux

Vincent Bernat

A common solution to provide a highly-available and scalable service is to insert a load-balancing layer to spread requests from users to backend servers.1 We usually have several expectations for such a layer:

scalability
It allows a service to scale by pushing traffic to newly provisioned backend servers. It should also be able to scale itself when it becomes the bottleneck.
availability
It provides high availability to the service. If one server becomes unavailable, the traffic should be quickly steered to another server. The load-balancing layer itself should also be highly available.
flexibility
It handles both short and long connections. It is flexible enough to offer all the features backends generally expect from a load-balancer like TLS or HTTP routing.
operability
With some cooperation, any expected change should be seamless: rolling out a new software on the backends, adding or removing backends, or scaling up or down the load-balancing layer itself.

The problem and its solutions are well known. From recently published articles on the topic, “Introduction to modern network load-balancing and proxying” provides an overview of the state of the art. Google released “Maglev: A Fast and Reliable Software Network Load Balancer” describing their in-house solution in details.2 However, the associated software is not available. Basically, building a load-balancing solution with commodity servers consists of assembling three components:

  • ECMP routing
  • stateless L4 load-balancing
  • stateful L7 load-balancing

In this article, I describe and support a multi-tier solution using Linux and only open-source components. It should offer you the basis to build a production-ready load-balancing layer.

Update (2018.05)

Facebook just released Katran, an L4 load-balancer implemented with XDP and eBPF and using consistent hashing. It could be inserted in the configuration described below.

Last tier: L7 load-balancing🔗

Let’s start with the last tier. Its role is to provide high availability, by forwarding requests to only healthy backends, and scalability, by spreading requests fairly between them. Working in the highest layers of the OSI model, it can also offer additional services, like TLS-termination, HTTP routing, header rewriting, rate-limiting of unauthenticated users, and so on. Being stateful, it can leverage complex load-balancing algorithm. Being the first point of contact with backend servers, it should ease maintenances and minimize impact during daily changes.

L7 load-balancers
The last tier of the load-balancing solution is a set of L7 load-balancers receiving user connections and forwarding them to the backends.

It also terminates client TCP connections. This introduces some loose coupling between the load-balancing components and the backend servers with the following benefits:

  • connections to servers can be kept open for lower resource use and latency,
  • requests can be retried transparently in case of failure,
  • clients can use a different IP protocol than servers, and
  • servers do not have to care about path MTU discovery, TCP congestion control algorithms, avoidance of the TIME-WAIT state and various other low-level details.

Many pieces of software would fit in this layer and an ample literature exists on how to configure them. You could look at HAProxy, Envoy or Træfik. Here is a configuration example for HAProxy:

# L7 load-balancer endpoint
frontend l7lb
  # Listen on both IPv4 and IPv6
  bind :80 v4v6
  # Redirect everything to a default backend
  default_backend servers
  # Healthchecking
  acl dead nbsrv(servers) lt 1
  acl disabled nbsrv(enabler) lt 1
  monitor-uri /healthcheck
  monitor fail if dead || disabled

# IPv6-only servers with HTTP healthchecking and remote agent checks
backend servers
  balance roundrobin
  option httpchk
  server web1 [2001:db8:1:0:2::1]:80 send-proxy check agent-check agent-port 5555
  server web2 [2001:db8:1:0:2::2]:80 send-proxy check agent-check agent-port 5555
  server web3 [2001:db8:1:0:2::3]:80 send-proxy check agent-check agent-port 5555
  server web4 [2001:db8:1:0:2::4]:80 send-proxy check agent-check agent-port 5555

# Fake backend: if the local agent check fails, we assume we are dead
backend enabler
  server enabler [::1]:0 agent-check agent-port 5555

This configuration is the most incomplete piece of this guide. However, it illustrates two key concepts for operability:

  1. Healthchecking of the web servers is done both at HTTP-level (with check and option httpchk) and using an auxiliary agent check (with agent-check). The later makes it easy to put a server to maintenance or to orchestrate a progressive rollout. On each backend, you need a process listening on port 5555 and reporting the status of the service (UP, DOWN, MAINT). A simple socat process can do the trick:3

    socat -ly \
      TCP6-LISTEN:5555,ipv6only=0,reuseaddr,fork \
      OPEN:/etc/lb/agent-check,rdonly
    

    Put UP in /etc/lb/agent-check when the service is in nominal mode. If the regular healthcheck is also positive, HAProxy will send requests to this node. When you need to put it in maintenance, write MAINT and wait for the existing connections to terminate. Use READY to cancel this mode.

  2. The load-balancer itself should provide an healthcheck endpoint (/healthcheck) for the upper tier. It will return a 503 error if either there is no backend servers available or if put down the enabler backend through the agent check. The same mechanism as for regular backends can be used to signal the unavailability of this load-balancer.

Additionally, the send-proxy directive enables the proxy protocol to transmit the real clients’ IP addresses. This protocol also works for non-HTTP connections and is supported by a variety of servers, including nginx:

http {
  server {
    listen [::]:80 default ipv6only=off proxy_protocol;
    root /var/www;
    set_real_ip_from ::/0;
    real_ip_header proxy_protocol;
  }
}

As is, this solution is not complete. We have just moved the availability and scalability problem somewhere else. How do we load-balance the requests between the load-balancers?

First tier: ECMP routing🔗

On most modern routed IP networks, redundant paths exist between clients and servers. For each packet, routers have to choose a path. When the cost associated to each path is equal, incoming flows4 are load-balanced among the available destinations. This characteristic can be used to balance connections among available load-balancers:

ECMP routing
ECMP routing is used as a first tier. Flows are spread among available L7 load-balancers. Routing is stateless and asymmetric. Backend servers are not represented.

There is little control over the load-balancing but ECMP routing brings the ability to scale horizontally both tiers. A common way to implement such a solution is to use BGP, a routing protocol to exchange routes between network equipments. Each load-balancer announces to its connected routers the IP addresses it is serving.

If we assume you already have BGP-enabled routers available, ExaBGP is a flexible solution to let the load-balancers advertise their availability. Here is a configuration for one of the load-balancers:

# Healthcheck for IPv6
process service-v6 {
  run python -m exabgp healthcheck -s --interval 10 --increase 0 --cmd "test -f /etc/lb/v6-ready -a ! -f /etc/lb/disable";
  encoder text;
}

template {
  # Template for IPv6 neighbors
  neighbor v6 {
    router-id 192.0.2.132;
    local-address 2001:db8::192.0.2.132;
    local-as 65000;
    peer-as 65000;
    hold-time 6;
    family {
      ipv6 unicast;
    }
    api services-v6 {
      processes [ service-v6 ];
    }
  }
}

# First router
neighbor 2001:db8::192.0.2.254 {
  inherit v6;
}

# Second router
neighbor 2001:db8::192.0.2.253 {
  inherit v6;
}

If /etc/lb/v6-ready is present and /etc/lb/disable is absent, all the IP addresses configured on the lo interface will be announced to both routers. If the other load-balancers use a similar configuration, the routers will distribute incoming flows between them. Some external process should manage the existence of the /etc/lb/v6-ready file by checking for the healthiness of the load-balancer (using the /healthcheck endpoint for example). An operator can remove a load-balancer from the rotation by creating the /etc/lb/disable file.

To get more details on this part, have a look at “High availability with ExaBGP.” If you are in the cloud, this tier is usually implemented by your cloud provider, either using an anycast IP address or a basic L4 load-balancer.

Unfortunately, this solution is not resilient when an expected or unexpected change happens. Notably, when adding or removing a load-balancer, the number of available routes for a destination changes. The hashing algorithm used by routers is not consistent and flows are reshuffled among the available load-balancers, breaking existing connections:

Stability of ECMP routing 1/2
ECMP routing is unstable when a change happens. An additional load-balancer is added to the pool and the flows are routed to different load-balancers, which do not have the appropriate entries in their connection tables.

Moreover, each router may choose its own routes. When a router becomes unavailable, the second one may route the same flows differently:

Stability of ECMP routing 2/2
A router becomes unavailable and the remaining router load-balances its flows differently. One of them is routed to a different load-balancer, which do not have the appropriate entry in its connection table.

If you think this is not an acceptable outcome, notably if you need to handle long connections like file downloads, video streaming or websocket connections, you need an additional tier. Keep reading!

Second tier: L4 load-balancing🔗

The second tier is the glue between the stateless world of IP routers and the stateful land of L7 load-balancing. It is implemented with L4 load-balancing. The terminology can be a bit confusing here: this tier routes IP datagrams (no TCP termination) but the scheduler uses both destination IP and port to choose an available L7 load-balancer. The purpose of this tier is to ensure all members take the same scheduling decision for an incoming packet.

There are two options:

  • stateful L4 load-balancing with state synchronization accross the members, or
  • stateless L4 load-balancing with consistent hashing.

The first option increases complexity and limits scalability. We won’t use it.5 The second option is less resilient during some changes but can be enhanced with an hybrid approach using a local state.

We use IPVS, a performant L4 load-balancer running inside the Linux kernel, with Keepalived, a frontend to IPVS with a set of healthcheckers to kick out an unhealthy component. IPVS is configured to use the Maglev scheduler, a consistent hashing algorithm from Google. Among its family, this is a great algorithm because it spreads connections fairly, minimizes disruptions during changes and is quite fast at building its lookup table. Finally, to improve performance, we let the last tier—the L7 load-balancers—sends back answers directly to the clients without involving the second tier—the L4 load-balancers. This is referred to as direct server return (DSR) or direct routing (DR).

Second tier: L4 load-balancing
L4 load-balancing with IPVS and consistent hashing as a glue between the first tier and the third tier. Backend servers have been omitted. Dotted lines represent the path for the return packets.

With such a setup, we expect packets from a flow to be able to move freely between the components of the two first tiers while sticking to the same L7 load-balancer.

Configuration🔗

Assuming ExaBGP has already been configured like described in the previous section, let’s start with the configuration of Keepalived:

virtual_server_group VS_GROUP_MH_IPv6 {
  2001:db8::198.51.100.1 80
}
virtual_server group VS_GROUP_MH_IPv6 {
  lvs_method TUN  # Tunnel mode for DSR
  lvs_sched mh    # Scheduler: Maglev
  sh-port         # Use port information for scheduling
  protocol TCP
  delay_loop 5
  alpha           # All servers are down on start
  omega           # Execute quorum_down on shutdown
  quorum_up   "/bin/touch /etc/lb/v6-ready"
  quorum_down "/bin/rm -f /etc/lb/v6-ready"

  # First L7 load-balancer
  real_server 2001:db8::192.0.2.132 80 {
    weight 1
    HTTP_GET {
      url {
        path /healthcheck
        status_code 200
      }
      connect_timeout 2
    }
  }

  # Many others...
}

The quorum_up and quorum_down statements define the commands to be executed when the service becomes available and unavailable respectively. The /etc/lb/v6-ready file is used as a signal to ExaBGP to advertise the service IP address to the neighbor routers.

Additionally, IPVS needs to be configured to continue routing packets from a flow moved from another L4 load-balancer. It should also continue routing packets from unavailable destinations to ensure we can drain properly a L7 load-balancer.

# Schedule non-SYN packets
sysctl -qw net.ipv4.vs.sloppy_tcp=1
# Do NOT reschedule a connection when destination
# doesn't exist anymore
sysctl -qw net.ipv4.vs.expire_nodest_conn=0
sysctl -qw net.ipv4.vs.expire_quiescent_template=0

The Maglev scheduling algorithm will be available with Linux 4.18, thanks to Inju Song. For older kernels, I have prepared a backport.6 Use of source hashing as a scheduling algorithm will hurt the resilience of the setup.

DSR is implemented using the tunnel mode. This method is compatible with routed datacenters and cloud environments. Requests are tunneled to the scheduled peer using IPIP encapsulation. It adds a small overhead and may lead to MTU issues. If possible, ensure you are using a larger MTU for communication between the second and the third tier.7 Otherwise, it is better to explicitely allow fragmentation of IP packets:

sysctl -qw net.ipv4.vs.pmtu_disc=0

You also need to configure the L7 load-balancers to handle encapsulated traffic:8

# Setup IPIP tunnel to accept packets from any source
ip tunnel add tunlv6 mode ip6ip6 local 2001:db8::192.0.2.132
ip link set up dev tunlv6
ip addr add 2001:db8::198.51.100.1/128 dev tunlv6

Evaluation of the resilience🔗

As configured, the second tier increases the resilience of this setup for two reasons:

  1. The scheduling algorithm is using a consistent hash to choose its destination. Such an algorithm reduces the negative impact of expected or unexpected changes by minimizing the number of flows moving to a new destination. “Consistent Hashing: Algorithmic Tradeoffs” offers more details on this subject.

  2. IPVS keeps a local connection table for known flows. When a change impacts only the third tier, existing flows will be correctly directed according to the connection table.

If we add or remove a L4 load-balancer, existing flows are not impacted because each load-balancer takes the same decision, as long as they see the same set of L7 load-balancers:

L4 load-balancing instability 1/3
Loosing a L4 load-balancer has no impact on existing flows. Each arrow is an example of flow. The dots are flow endpoints bound to the associated load-balancer. If they had moved to another load-balancer, connection would have been lost.

If we add a L7 load-balancer, existing flows are not impacted either because only new connections will be scheduled to it. For existing connections, IPVS will look at its local connection table and continue to forward packets to the original destination. Similarly, if we remove a L7 load-balancer, only existing flows terminating at this load-balancer are impacted. Other existing connections will be forwarded correctly:

L4 load-balancing instability 2/3
Loosing a L7 load-balancer only impacts the flows bound to it.

We need to have simultaneous changes on both levels to get a noticeable impact. For example, when adding both a L4 load-balancer and a L7 load-balancer, only connections moved to a L4 load-balancer without state and scheduled to the new load-balancer will be broken. Thanks to the consistent hashing algorithm, other connections will stay bound to the right L7 load-balancer. During a planned change, this disruption can be minimized by adding the new L4 load-balancers first, waiting a few minutes, then adding the new L7 load-balancers.

L4 load-balancing instability 3/3
Both a L4 load-balancer and a L7 load-balancer come back to life. The consistent hash algorithm ensures that only one fifth of the existing connections would be moved to the incoming L7 load-balancer. Some of them continue to be routed through their original L4 load-balancer, which mitigates the impact.

Additionally, IPVS correctly routes ICMP messages to the same L7 load-balancers as the associated connections. This ensures notably path MTU discovery works and there is no need for smart workarounds.

Tier 0: DNS load-balancing🔗

Optionally, you can add DNS load-balancing to the mix. This is useful either if your setup is spanned accross multiple datacenters, or multiple cloud regions, or if you want to break a large load-balancing cluster into smaller ones. It is not intended to replace the first tier as it doesn’t share the same characteristics: load-balancing is unfair (it is not flow-based) and recovery from a failure is slow.

Complete load-balancing solution
A complete load-balancing solution spanning accross two datacenters.

gdnsd is an authoritative-only DNS server with integrated healthchecking. It can serve zones from master files using the RFC 1035 zone format:

@ SOA ns1 ns1.example.org. 1 7200 1800 259200 900
@ NS ns1.example.com.
@ NS ns1.example.net.
@ MX 10 smtp

@     60 DYNA multifo!web
www   60 DYNA multifo!web
smtp     A    198.51.100.99

The special RR type DYNA will return A and AAAA records after querying the specified plugin. Here, the multifo plugin implements an all-active failover of monitored addresses:

service_types => {
  web => {
    plugin => http_status
    url_path => /healthcheck
    down_thresh => 5
    interval => 5
  }
  ext => {
    plugin => extfile
    file => /etc/lb/ext
    def_down => false
  }
}

plugins => {
  multifo => {
    web => {
      service_types => [ ext, web ]
      addrs_v4 => [ 198.51.100.1, 198.51.100.2 ]
      addrs_v6 => [ 2001:db8::198.51.100.1, 2001:db8::198.51.100.2 ]
    }
  }
}

In nominal state, an A request will be answered with both 198.51.100.1 and 198.51.100.2. An healthcheck failure will update the returned set accordingly. It is also possible to administratively remove an entry by modifying the /etc/lb/ext file. For example, with the following content, 198.51.100.2 will not be advertised anymore:

198.51.100.1 => UP
198.51.100.2 => DOWN
2001:db8::c633:6401 => UP
2001:db8::c633:6402 => UP

You can find all the configuration files and the setup of each tier in the GitHub repository. If you want to replicate this setup at a smaller scale, it is possible to collapse the second and the third tiers by using either localnode or network namespaces. Even if you don’t need its fancy load-balancing services, you should keep the last tier: while backend servers come and go, the L7 load-balancers bring stability, which translates to resiliency.


  1. In this article, “backend servers” are the servers behind the load-balancing layer. To avoid confusion, we will not use the term “frontend.” ↩︎

  2. A good summary of the paper is available from Adrian Colyer. From the same author, you may also have a look at the summary for “Stateless datacenter load-balancing with Beamer.” ↩︎

  3. If you feel this solution is fragile, feel free to develop your own agent. It could coordinate with a key-value store to determine the wanted state of the server. It is possible to centralize the agent in a single location, but you may get a chicken-and-egg problem to ensure its availability. ↩︎

  4. A flow is usually determined by the source and destination IP and the L4 protocol. Alternatively, the source and destination port can also be used. The router hashes these information to choose the destination. For Linux, you may find more information on this topic in “Celebrating ECMP in Linux.” ↩︎

  5. On Linux, it can be implemented by using Netfilter for load-balancing and conntrackd to synchronize state. IPVS only provides active/backup synchronization. ↩︎

  6. The backport is not strictly equivalent to its original version. Be sure to check the README file to understand the differences. Briefly, in Keepalived configuration, you should:

    • not use inhibit_on_failure
    • use sh-port
    • not use sh-fallback
    ↩︎
  7. At least 1520 for IPv4 and 1540 for IPv6. ↩︎

  8. As is, this configuration is a insecure. You need to ensure only the L4 load-balancers will be able to send IPIP traffic. ↩︎


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