Using HAProxy as an API Gateway, Part 1 [Introduction]

An API gateway handles load balancing, security, rate limiting, monitoring, and other cross-cutting concerns for API services. Read on to learn how HAProxy Enterprise excels as an API gateway.

Using HAProxy as an API Gateway, Part 2 [Authentication]
Using HAProxy as an API Gateway, Part 3 [Health Checks]
Using HAProxy as an API Gateway, Part 4 [Metrics]
Using HAProxy as an API Gateway, Part 5 [Monetization]
Using HAProxy as an API Gateway, Part 6 [Security]

“Danger! Avalanche Area” is a sign you’ll see when you drive through some parts of Colorado. Looking at the surrounding mountains in winter, you feel that you’re creeping by sleeping giants that might pour down a surge of snow at any moment. If that weren’t enough, driving alone through such a place can seem like if you were to be buried, would anyone ever know?

In your work as an IT operator, the weight of managing modern IT systems can give you the same anxious feeling. The complexity that builds up in infrastructure is like snow forming on a mountainside. If you get buried, how long will it take before people know? You’ve got to offload complexity wherever you can!

The specific form of complexity that we’ll address here is of connecting internal microservices to the outside world. Internal functionality, such as authentication, retrieving customer data, fetching product details, and taking payments, must be exposed to your frontend clients. This is accomplished by defining application programming interfaces (APIs) that express these function signatures.

The challenge is that having clients connect directly to backend APIs creates tight coupling between the frontend and backend components, which is difficult to manage and scale. The way to handle this, and even get some observability over who’s calling what, is to consolidate how external clients access your APIs. Combining disparate APIs behind a single, unifying URL is the purview of an API gateway. In this way, clients can maintain a single reference to the gateway, and it will route them to the right place. You also get a smaller area that needs to be exposed to the Internet.

An Open-Source API Gateway

An API gateway is a layer of software between the client and backend services that routes requests intelligently. HAProxy, the world’s fastest and most widely used software load balancer, fills the role as an API gateway extremely well. In addition to routing API calls for /cart or /catalog to the proper backend services, it also handles load balancing, security, rate limiting, monitoring, and other cross-cutting concerns. In essence, by placing all of your APIs behind a shared HAProxy instance, these requirements can be offloaded.

The API gateway becomes the orchestration layer that forwards requests so that frontend code and backend API endpoints are not tightly coupled. All devices can point to a single domain and HAProxy will handle the routing.

api gateway

Due to its speed, high availability and reliability, HAProxy can be used as a API gateway

Here are some of the functions that an HAProxy API gateway will handle for you:

  • High Performance

  • Load balancing

  • HTTP routing

  • Security

  • Rate limiting

  • Observability

  • Connection queuing

  • Circuit-breaking

  • Authentication

  • Device Detection

Let’s explore some of these features in more detail.

HTTP Routing

The primary role of the API gateway is to route an incoming client request to the appropriate internal service. HAProxy can route based on any information found in the HTTP request, including portions of the URL path, query string, and HTTP headers.

In the following example, our HAProxy configuration sets up a frontend that accepts incoming requests on port 443, checks their URL paths for /cart and /catalog, and then forwards them to the correct backend.

frontend api_gateway
bind :443 ssl crt /etc/hapee-1.8/certs/cert.pem
acl PATH_cart path_beg -i /cart
acl PATH_catalog path_beg -i /catalog
use_backend be_cart if PATH_cart
use_backend be_catalog if PATH_catalog
backend be_cart
server s1
backend be_catalog
server s1

If you are managing multiple website domains, then you can check the Host header when determining how to route requests. Here’s an example that will segregate API requests depending on the domain. When accessing, for example,, it will only route requests for /catalog and /cart. B2B partners that access can access the /inventory API.

frontend api_gateway
bind :443 ssl crt /etc/hapee-1.8/certs/cert.pem
acl VHOST_publicapi req.hdr(Host) -i -m dom
acl VHOST_partnersapi req.hdr(Host) -i -m dom
acl PATH_catalog path_beg -i /catalog
acl PATH_cart path_beg -i /cart
acl PATH_inventory path_beg -i /inventory
use_backend be_cart if VHOST_publicapi PATH_cart
use_backend be_catalog if VHOST_publicapi PATH_catalog
use_backend be_inventory if VHOST_partnersapi PATH_inventory
backend be_cart
server s1
backend be_catalog
server s1
backend be_inventory
server s1

Note that HAProxy is extremely flexible and powerful and the examples provided are just simple use cases. HAProxy can apply more complex logic for HTTP routing and request handling.

If an application requires tens, hundreds, or even thousands of paths in a single ACL, then it is better to manage them through a map file. A map file stores key/value associations in memory. In our example, the key would be the “host/path” string and the value would be the name of the backend to route the request to. The map file would contain:

# endpoint backend name be_catalog be_catalog be_cart be_cart be_inventory be_inventory

Our HAProxy configuration would contain:

frontend api_gateway
# …
use_backend %[base,map_beg(“/etc/hapee-1.8/”)]
Did you know?

The base fetch method returns the concatenation of the Host header and the path part of the request, which starts at the first slash and ends before the question mark.

To simplify the process of adding or removing path-to-backend associations, you can enable the HAProxy Enterprise “lb-update” module. This module can read the contents of the map file and refresh the ACLs during runtime, without the need to reload HAProxy.

Load Balancing


With HAProxy, the load balancing algorithm can be adjusted to suit the type of service and protocol

In order to improve the performance and resilience of each API endpoint, it’s recommended to replicate the service over several nodes. Then, the API gateway will balance incoming client requests among them. You can adjust the load balancing algorithm to suit the type of service and protocol.

  • For quick and short API calls, use the roundrobin algorithm

  • For longer-lived websockets, use the leastconn algorithm

  • For services that have backend servers optimized to process particular functions, use the uri algorithm

In the following example, the mobile API backend is balanced across two nodes using the roundrobin algorithm.

backend mobile_api
balance roundrobin
server s1
server s2

Load balancing your API endpoints improves performance and creates redundancy. Note that you can choose the most appropriate balancing algorithm on a per-backend basis.

You can also define active and passive health checks for your servers so that HAProxy automatically reroutes traffic if there’s a problem. In the following example, we monitor the health of our servers by sending GET requests to the /health URL and expecting a successful response.

backend mobile_api
balance roundrobin
option httpchk GET /health
server s1 check
server s2 check

The option httpchk directive sets the method and URL to monitor. If you append \r\n you can add additional HTTP headers to this request. A check parameter is added to each server to enable the feature. Being able to watch a URL endpoint works well with tools like Prometheus that already expose a /metrics web page used for scraping metrics.

HAProxy maxconn

The HAProxy load balancer stands in a strategic position between your clients and services, ensuring that no backend nodes are saturated by spikes in traffic. Without this, all requests would be forwarded to the backend servers, risking high wait times and timeouts.

HAProxy implements queuing mechanisms to prevent sending too many requests at once to a service. Add the maxconn argument to a server directive to queue additional requests within the gateway, as shown:

backend mobile_api
balance roundrobin
server s1 maxconn 100
server s2 maxconn 100

In this case, up to 100 connections can be established at once to a server. Any more than that will be queued. This relieves strain on your servers, allowing them to process requests more efficiently. HAProxy is highly proficient at managing queues of this sort.

Rate Limiting


HAProxy’s Stick Tables can be used to limit the number of requests sent to the API by one client

You may want to limit the number of requests a client can send to your APIs within a period of time. This might be a to enforce a quota for various tiers of customers. To be allowed to send more requests, clients could subscribe to a higher-priced tier.

In HAProxy, stick tables can be used for such a purpose. You can track clients by IP address, cookie or other means, such as API tokens passed in the URL or headers. In this example, the client is expected to pass a URL parameter called apitoken and is limited to 1000 requests within 24 hours. The period is set with the expire parameter on the stick-table directive.

frontend api_gateway
bind :443 ssl crt /etc/hapee-1.8/certs/cert.pem
stick-table type string size 1m expire 24h store http_req_cnt
acl exceeds_limit url_param(apitoken),table_http_req_cnt(api_gateway) gt 1000
http-request track-sc0 url_param(apitoken) unless exceeds_limit
http-request deny deny_status 429 if exceeds_limit

Now, as I make requests to the site, passing the URL parameter, apitoken=abcdefg, the count of HTTP requests is incremented. I can see this by logging into the server via SSH and querying the Runtime API. In the following example, I’ve made 12 requests using my API token.

root@server1:~$ echo "show table api_gateway" | socat UNIX-CONNECT:/var/run/haproxy.sock stdio
# table: api_gateway, type: string, size:1048576, used:1
0x55bd73392fa4: key=abcdefg use=0 exp=86396974 http_req_cnt=12

When clients go past their limit, they’ll receive a 429 Too Many Requests response. Check out our blog post Introduction to HAProxy Stick Tables for more information about defining stick tables and other examples of rate limiting.

Did you know?

Adding deny_status to the http-request deny directive allows you to set a custom response code when rejecting requests. Possible values are 200, 400, 403, 405, 408, 425, 429, 500, 502, 503, 504

Optionally, instead of a daily limit as used above, you can also do it based on the rate of the requests. This acts as a security feature to prevent abuse or clients with runaway processes. The following example allows clients to make no more than one request per second.


While the above and below are separate examples, they could also be combined to provide extra control

frontend api_gateway
bind :443 ssl crt /etc/hapee-1.8/certs/cert.pem
stick-table type string size 1m expire 24h store http_req_rate(10s)
acl rate_abuse url_param(apitoken),table_http_req_rate(api_gateway) gt 10
http-request track-sc0 url_param(apitoken) unless rate_abuse
http-request deny deny_status 429 if rate_abuse

Detailed Monitoring & Statistics

HAProxy is very famous for the level of details it provides on the traffic it processes. There are two main features: the statistic dashboard and the logs.

Statistics Dashboard

You can enable an HTML statistics page within HAProxy and HAProxy Enterprise, which comes as a set of tabular data with many metrics for each frontendbackend, backend server and frontend bind line. The image below shows an example of the HAProxy Enterprise Real Time dashboard:


HAProxy’s Real-Time Dashboard

This data is also available by querying the HAProxy Runtime API. JSON (within the Runtime API) and CSV (within both the dashboard and the Runtime API) are also available for easy integration with third party tools like Prometheus, Grafana, and SNMP. The image below shows an integration of HAProxy with Grafana via the Prometheus exporter:


Integration of HAProxy with Grafana, via the Prometheus exporter


The HAProxy logs are a gold mine of information, since HAProxy can report the following information for each API call:

  • Client IP, port

  • Routing within HAProxy: frontend, backend, server

  • URL endpoint with query string

  • Timers: time for the client to send the request, server connection time, response time, total session duration, etc…

  • Termination status: did the session finished properly or not, if not what happened and at what phase of the session (connection time, header time, data streaming time, etc…)

  • Any custom header or cookie you want to capture

  • Any SSL / TLS information

Based on all the information provided above, it is possible to build reports and to figure out when and where problems are occurring (e.g. Is it networking? The application? A particular server?).


We highly recommend watching this video to get tips on observability from the creator of HAProxy, Willy Tarreau, to understand the power of HAProxy in this area:


In this article, you learned what an API gateway is and how it can simplify connecting microservices to external clients. You also saw how HAProxy load balancer works as an API gateway: managing routing, load balancing, rate limiting and other complexities.

Please share your experiences using HAProxy as an API gateway in the section below! Contact us to learn more about HAProxy Enterprise or sign up for a free trial.

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