WCF Service Failover

Introduction

Failover - The standby server takes over, when the main server fails.

In this article, I will explain the Failover functionality to be achieved, using WCF concepts. The general idea behind going into Failover is 24/7 availability of the service to the critical client applications. The client application should always be in a position to consume the Backup Service though the Main Service, which is not available due to some reason. Let us discuss this in detail.

Failover

Failover is a method of protecting the Applications/communication from failure, where there is a backup Service. IT automatically takes over when the Main Server fails. In a real world scenario, you can see that ticket booking websites take more time to book the tickets (to get the Server response) during peak hours. The Server takes more time due to Server’s unavailability, or if the number of requests in the queue is higher, or for any other reason/s. Whatever the reason is, the client application should not be impacted, another Server should take it up when the main Server is not available.

Figure1.0 Work Flow Diagram

In the snapshot, given above, you can find that the client communicates with the main router. It is not aware of the Primary Service or Backup Service. When the router receives the request from the client, it usually tries to process it with the primary Server. If the primary Server is not reachable for any reasons, the main router then automatically tries to connect with the Backup Service 1. If the Backup Service 1 is available, the request would be processed and send the response to the client. If the Backup Service 1 is also not available, it would send the request to Backup Service 2 to get it processed.

The client does not need to worry about the availability of the Servers. The main router will re-route the request to the back-up services and get it processed, if the Primary Servers are not available.

Need for Backup

Let us discuss, in detail, about the need of a back-up service.

Critical Applications Availability

Critical applications are the ones, where large number of users always want to perform many transactions at a time. For example, bank transfers, where a large number of people need to do fund transfers and other transactions as part of their day-to-day activities. If this kind of Application Server gets affected, the impact will be huge. To overcome these issues, Backup Server is always recommended to take care of the client’s transactions, if the Primary Server is not available.

Load Balancing

There are situations when the Service gets more requests/transactions only during the peak time on any single day. Let us take the ticket booking websites. A large number of users will try to book the tickets once the tatkal period gets started. If the Server is not capable enough to process all the requests, the clients will not be able to book the tatkal tickets. To overcome this, the Main Server should load balance the requests to the backup services accordingly. Let's say, if the Primary Server is already processing 1000 messages, the Main Server would reroute the other 1000 messages to the Backup Services. This would increase the processing time and the client’s request will not affect.

Routing Configuration

Let us take the Figure 1.0 as a reference for configuring primary and backup services accordingly. As a first step, create the Primary Service library and the host application. As usual, in the Primary Service library, you can define the contracts and service implementation. Afterwards, create the host application and refer to the Primary Service library, define the Service, endpoint, and host it. In our example, I have done it in self-hosting. If you need an assistance in configuring and hosting the Services, please look at my previous articles Configuring WCF Routing, Hosting in WCF.

Primary Service


Figure 1.2 Primary Service

From Figure 1.2, you can find that Primary Service is hosted on net.tcp://localhost:8523/PrimaryServiceLib/Service. Afterwards, you need to repeat the same configuration for the Backup Services 1 and 2. In our case, the address of Backup Service 1 and 2 are given below.

“net.tcp://localhost:8524/BackupService1/Service/”,net.tcp://localhost:8525/BackupService2/Service/.