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- What's New
- Function Overview
-
Service Overview
- What Is ELB?
- Product Advantages
- How ELB Works
- Application Scenarios
- Differences Between Dedicated and Shared Load Balancers
- Load Balancing on a Public or Private Network
- Network Traffic Paths
- Specifications of Dedicated Load Balancers
- Quotas and Constraints
- Billing (Dedicated Load Balancers)
- Permissions
- Product Concepts
- How ELB Works with Other Services
- Change History
- Getting Started
-
User Guide
-
Load Balancer
- Overview
- Preparations for Creating a Load Balancer
- Creating a Dedicated Load Balancer
- Creating a Shared Load Balancer
- Modifying the Bandwidth
- Changing the Specifications of a Dedicated Load Balancer
- Changing an IP Address
- Binding an IP Address to or Unbinding an IP Address from a Load Balancer
- Adding to or Removing from an IPv6 Shared Bandwidth
- Enabling or Disabling a Load Balancer
- Exporting the Load Balancer List
- Deleting a Load Balancer
- Listener
- Advanced Features of HTTP/HTTPS Listeners
- Backend Server Group
- Backend Server (Dedicated Load Balancers)
- Backend Server (Shared Load Balancers)
- Certificate
- Access Control
- TLS Security Policy
- Tag
- Access Logging
- Monitoring
- Auditing
- Permissions Management
- Quotas
- Appendix
- Change History
-
Load Balancer
- Best Practices
-
API Reference
- Before You Start
- API Overview
- Calling APIs
- APIs (V3)
- Examples
- Permissions and Supported Actions
- Appendix
- SDK Reference
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FAQs
- Popular Questions
-
ELB Use
- Service Abnormality
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ELB Functionality
- Can ELB Be Used Separately?
- Does ELB Support Persistent Connections?
- Does ELB Support FTP on Backend Servers?
- Can ELB Block DDoS Attacks and Secure Web Code?
- Is an EIP Assigned Exclusively to a Load Balancer?
- How Many Load Balancers and Listeners Can I Have?
- What Types of APIs Does ELB Provide? What Are Permissions of ELB?
- Can I Adjust the Number of Backend Servers When a Load Balancer is Running?
- Can Backend Servers Run Different OSs?
- Can I Configure Different Backend Ports for a Load Balancer?
- Are There Any Restrictions on the Frequency of Access from an IP Address?
- Can ELB Be Used Across Accounts or VPCs?
- Can Backend Servers Access the Ports of a Load Balancer?
- Can I Bind a Public IP Address Purchased from a Third-Party Cloud Provider to My Load Balancer?
- Can Both the Listener and Backend Server Group Use HTTPS?
- Can I Change the VPC and Subnet for My Load Balancer?
- Does ELB Support IPv6 Networks?
- Load Balancing Performance
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Load Balancers
- What Is Quota?
- How Does ELB Distribute Traffic?
- How Can I Access a Load Balancer Across VPCs?
- How Can I Configure Load Balancing for Containerized Applications?
- Why Can't I Delete My Load Balancer?
- Do I Need to Configure Bandwidth for My Load Balancers?
- Can I Bind Multiple EIPs to a Load Balancer?
- Why Multiple IP Addresses Are Required When I Create or Enable a Load Balancer?
- Why Are Requests from the Same IP Address Routed to Different Backend Servers When the Load Balancing Algorithm Is Source IP Hash?
- Can Backend Servers Access the Internet Using the EIP of the Load Balancer?
- Will Traffic Routing Be Interrupted If the Load Balancing Algorithm Is Changed?
- What Is the Difference Between the Bandwidth Included in Each Specification of a Dedicated Load Balancer and the Bandwidth of an EIP?
- How Do I Combine ELB and WAF?
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Listeners
- What Are the Relationships Between Load Balancing Algorithms and Sticky Session Types?
- Can I Bind Multiple Certificates to a Listener?
- Do HTTP and HTTP Listeners Support the X-Forwarded-Host Header?
- Will ELB Stop Distributing Traffic Immediately After a Listener Is Deleted?
- Does ELB Have Restrictions on the File Upload Speed and Size?
- Can Multiple Load Balancers Route Requests to One Backend Server?
- How Is WebSocket Used?
- What Are the Three Timeouts of a Listener and What Are the Default Durations?
- Why Can't I Select the Target Backend Server Group When Adding or Modifying a Listener?
- Why Cannot I Add a Listener to a Dedicated Load Balancer?
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Backend Servers
- Why Is the Interval at Which Backend Servers Receive Health Check Packets Different from What I Have Configured?
- Can Backend Servers Access the Internet After They Are Associated with a Load Balancer?
- Can ELB Distribute Traffic Across Servers That Are Not Provided by Huawei Cloud?
- Can ELB Route Traffic Across Regions?
- Does Each Backend Server Need an EIP to Receive Requests from a Public Network Load Balancer?
- How Do I Check the Network Conditions of a Backend Server?
- How Can I Check the Network Configuration of a Backend Server?
- How Do I Check the Status of a Backend Server?
- How Long Is the Timeout Duration of Connections Between a Load Balancer and Backend Servers?
- When Is a Backend Server Considered Healthy?
- How Do I Check Whether a Backend Server Can Be Accessed Through an EIP?
- Why Is the Number of Active Connections Monitored by Cloud Eye Different from the Number of Connections Established with the Backend Servers?
- Why Can I Access Backend Servers After a Whitelist Is Configured?
- When Will Modified Weights Take Effect?
- How Much Time Is Required for a Load Balancer to Disconnect from Backend Servers After The Servers Are Removed?
- Why Must the Subnet Where the Load Balancer Resides Have at Least 16 Available IP Addresses for Enabling IP as a Backend?
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Health Checks
- How Do I Troubleshoot an Unhealthy Backend Server?
- Why Is the Interval at Which Backend Servers Receive Health Check Packets Different from the Configured Interval?
- How Does ELB Perform UDP Health Checks? What Are the Precautions for UDP Health Checks?
- Why Does ELB Frequently Send Requests to Backend Servers During Health Checks?
- When Does a Health Check Start?
- Do Maximum Retries Include Health Checks That Consider Backend Servers Unhealthy?
- What Do I Do If a Lot of Access Logs Are Generated During Health Checks?
- What Status Codes Will Be Returned If Backend Servers Are Identified as Healthy?
- Obtaining Source IP Addresses
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HTTP/HTTPS Listeners
- Which Protocol Should I Select for the Backend Server Group When Adding an HTTPS Listener?
- Why Is There a Security Warning After a Certificate Is Configured?
- Why Is a Forwarding Policy in the Faulty State?
- Why Can't I Add a Forwarding Policy to a Listener?
- Why Cannot I Select an Existing Backend Server Group When Adding a Forwarding Policy?
- Sticky Sessions
- Certificates
- Monitoring
- Billing
- Videos
Show all
Overview
A backend server is a cloud server added to a backend server group associated with a load balancer. When you add a listener to a load balancer, you can create or select a backend server group to receive requests from the load balancer by using the port and protocol you specify for the backend server group and the load balancing algorithm you select.
After a new server is added to the associated backend server group for which the health check is configured, the load balancer will check its running status. If the backend server responds normally, it is declared healthy. If the backend server does not respond normally, the load balancer periodically checks its health for multiple times. Only after the backend server is considered healthy, it can receive requests from the load balancer.
You can adjust the number of backend servers to ensure stable and reliable service with the minimum budget. Load balancers can distribute requests across backend servers in different AZs to prevent SPOFs. You must ensure that at least one backend server is working normally in each AZ.
On the ELB console, you can view the load balancer that the backend server is associated with by the IP address or ID of the backend server.
If a backend server is stopped or restarted, connections established with the server will be disconnected, and data being transmitted over these connections will be lost. Configure the retry function on the clients to prevent data loss.
Notes
When you add backend servers, note the following:
- Backend servers must be in the same VPC as the load balancer if IP as a backend is not enabled.
- For ease of management and maintenance, backend servers must run the same OS.
- You can set a weight for each server in the backend server group. The larger the weight is, the higher proportion of requests the backend server receives.
- If you enable sticky sessions, the proportions of requests processed by backend servers may become unbalanced. In this case, disable sticky sessions and check the requests received by each backend server.
- Security groups for backend servers must have inbound rules to allow traffic from both backend server ports and health check ports. Otherwise, health checks will be affected, and backend servers cannot receive requests from the load balancer. For details, see Configuring Security Group Rules for Backend Servers.
- If you select only network load balancing, a server cannot serve as both a backend server and a client.
Slow Start
After you enable slow start, the load balancer linearly increases the proportion of requests to send to backend servers in this mode. When the slow start duration elapses, the load balancer sends full share of requests to backend servers and exits the slow start mode.
- Slow start can only be enabled for HTTP and HTTPS backend server groups.
- If slow start is enabled, only the weighted round robin algorithm can be used.
- Slow start can be enabled only for new backend servers. If a backend server group has no backend servers, slow start will not take effect for the first backend server added to the backend server group.
- When the health check function is enabled, slow start takes effect after backend servers are detected healthy. The slow start duration will not stop even if the health check is abnormal due to network exceptions or other reasons.
- When the health check function is disabled, slow start takes effect immediately.
- After the slow start duration elapses, backend servers will not enter the slow start mode again.
The slow start function applies only to dedicated load balancers.
IP as Backend Servers
If you enable IP as a backend, you can add backend servers that are not in the VPC of the load balancer, using their private IP addresses. The backend servers can be in a VPC connected through a VPC peering connection, in a VPC in another region and connected through a cloud connection, or in an on-premises data center at the other end of a Direct Connect or VPN connection.
- If you do not enable the function when you create a load balancer, you can still enable it on the Basic Information page of the load balancer.
- IP as backend servers must use IPv4 addresses.
- Configure the VPC routes correctly to ensure that backend servers are reachable. For details, see Adding or Removing Backend Servers.
- If you enable IP as a backend for a load balancer, you can add only TCP, HTTP, and HTTPS listeners to the load balancer.
- The subnet where the load balancer works must have sufficient IP addresses (at least 16 IP addresses). You can add more subnets for more IP addresses on the Basic Information page of the load balancer.
- Security group rules of IP as backend servers must allow traffic from the subnet of the load balancer. Otherwise, health checks will fail.
- IP as backend cannot be disabled after it is enabled.
- A maximum of 492 servers can be associated with a load balancer.
- Source IP addresses of the clients cannot be passed to IP as backend servers. Install the TOA module to obtain source IP addresses.
The IP as a backend function applies only to dedicated load balancers.
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