Introduction
Traefik is a popular modern HTTP reverse proxy and load balancer that simplifies the deployment of microservices. One of its powerful features is its ability to load balance traffic across multiple backend servers using various strategies, including weighted round robin (WRR). This strategy allows administrators to assign different weights to servers based on their capacity or performance, ensuring optimal resource utilization and responsiveness.
However, users have reported an issue where Traefik appears to reset server weights to 1 after a server recovers from a health check failure. This behavior can disrupt the intended load balancing distribution and potentially impact application performance. This article delves into this reported problem, examining the potential cause within Traefik’s source code and discussing its implications.
The Reported Issue: Weight Reset After Recovery
In a scenario involving a weighted round robin configuration, a user reported observing unexpected behavior when a high-weight server experienced a temporary failure and subsequently recovered. The configuration involved two backend servers:
- Server 1: High weight (for better performance)
- Server 2: Weight of 1 (backup server)
The expected behavior is that Server 1, with its higher weight, should handle a significantly larger portion of the traffic. Server 2 would act as a backup and handle a smaller load.
However, the user observed that after Server 1 failed a health check and then recovered, the load balancer seemed to treat both servers as having equal weight (weight of 1). This effectively negated the weighted round robin configuration, distributing traffic evenly instead of according to the defined weights. The user discovered that manually changing the weight of either server would restore the correct weighted load balancing behavior.
Alt text: High-level architecture diagram of Traefik, showcasing its role as a reverse proxy and load balancer managing traffic between clients and backend servers.
Investigating the Root Cause in Traefik’s Source Code
To understand the potential cause of this issue, the user delved into Traefik’s source code, specifically examining the pkg/healthcheck/healthcheck.go file. Within this file, the checkHealth function is responsible for handling server health checks. The user identified a section of code executed when a previously failed server is detected as being back online:
if err := checkHealth(disableURL, backend); err == nil {
log.Warnf("Health check up: Returning to server list. Backend: %q URL: %q", backend.name, disableURL.String())
if err = backend.LB.UpsertServer(disableURL, roundrobin.Weight(1)); err != nil {
log.Error(err)
}
// FIXME serverUpMetricValue = 1
} else {The user pinpointed the following line as the likely culprit:
if err = backend.LB.UpsertServer(disableURL, roundrobin.Weight(1)); err != nil {The suspicion is that this line of code, when re-inserting a recovered server back into the load balancer’s server list, is hardcoding the server’s weight to 1, regardless of its originally configured weight. The roundrobin.Weight(1) argument explicitly sets the weight to 1 during the UpsertServer operation.
Alt text: Icon representing a code snippet, symbolizing the section of Traefik’s source code being discussed regarding server weight reset during health checks.
Confirmation and Potential Solutions
The user’s analysis strongly suggests that the hardcoded weight of 1 in the UpsertServer function is indeed the cause of the observed weight reset issue. Instead of using roundrobin.Weight(1), the code should ideally retrieve and use the server’s originally configured weight when adding it back to the load balancer after recovery.
To address this, the following questions arise:
-
Is the assumption correct? Further investigation and confirmation from Traefik maintainers would be needed to definitively confirm this analysis. Code review and potentially debugging this section of the health check logic would be crucial.
-
How are server weights stored internally and how can it be obtained within the
checkBackendmethod? To fix this issue, developers need to determine how Traefik stores the configured weights for backend servers. ThecheckBackend(or the relevant function containing the identified code) would need to access this stored weight information and use it in theUpsertServercall instead of the hardcoded1. This might involve accessing the backend configuration or server object to retrieve the weight value.
Impact and Implications
This weight reset issue has significant implications for users relying on Traefik’s weighted round robin load balancing, especially in production environments where performance and resource optimization are critical. If server weights are unintentionally reset, it can lead to:
- Uneven load distribution: High-capacity servers might not receive the intended higher traffic load, while lower-capacity servers might be overwhelmed.
- Performance degradation: Overall application performance can suffer if traffic is not efficiently distributed across servers based on their capabilities.
- Manual intervention required: Administrators are forced to manually monitor and adjust server weights after recovery events, increasing operational overhead.
Conclusion
The reported Traefik load balancer weight reset issue after health check recovery appears to stem from a hardcoded weight value in the UpsertServer function within the health check logic. This analysis, based on examination of Traefik’s source code, strongly suggests a potential fix involves retrieving and utilizing the originally configured server weight instead of defaulting to 1 when re-inserting a recovered server into the load balancer.
Addressing this issue is crucial to ensure the reliable and accurate functioning of Traefik’s weighted round robin load balancing, maintaining optimal traffic distribution and application performance. Further investigation, confirmation, and a code fix from the Traefik development team would be highly beneficial for the Traefik user community.
