What is the difference between Conditional Backend Services and Default Backend Services?

Short answer

Default Backend Services is the main target that exists in every vService: if there is no conditional block, it receives all traffic; if there is, it receives the traffic that fits no block. Conditional Backend Services (Content Switching), on the other hand, are target blocks bound to a condition; if the request meets the condition of a block, it goes to the Backend Services of that block instead of Default Backend Services. Conditional blocks exist only in HTTP and L7 TCP vServices, are evaluated from top to bottom, and the first matching block receives the request.

How Does It Work?

1. The request is first compared with the conditional blocks from top to bottom.
2. It goes to the Backend Services of the first block whose condition is met; the next blocks are not checked.
3. If none of them matches, Default Backend Services is used; if that is also empty, an HTTP vService returns 503, and an L7 TCP vService rejects the connection.

Comparison

	Default Backend Services	Conditional Backend Services
Role	Main target; used if there is no match	Target bound to a condition
vService types	HTTP, L7 TCP, Network	HTTP, L7 TCP
Visibility in the form	Always	Once the first block is added with + Conditional backend services
Condition	None	At most 3 groups per block; AND / OR and the NOT form
Selectable conditions	-	Only conditions of the request stage
Order	-	Matters; the first matching block wins, the order is changed with drag and drop
Target	Backend Services and/or Backend Group	Backend Services and/or Backend Group
LB algorithm and persistence	The settings of the vService	The settings of the vService (blocks do not get a separate algorithm)

In TR7

• A conditional block is added in the vService form with the + Conditional backend services button in the Default Backend Services table; for each block, the condition is selected in the Select Conditions field and the target in the Backend Service selector.
• The conditions used in blocks are defined under Load Balancer → Conditions; the ready-made conditions that come with the product (e.g. If the client is mobile) are also in the list.
• Default Backend Services can be left empty; in this case, requests that do not fit the blocks receive an error.
• Which Backend Service the request went to is seen on the HTTP Logs screen of the vService.

Common Misconceptions

• "Default is the backup server list." Default is the main target of unmatched traffic; backup behavior is provided by the Backup Service setting in the Backend Service.
• "If the Backend Services of a conditional block are unreachable, the request goes to Default." If the Backend Services of the matching block are unreachable, the request is not passed to Default Backend Services.
• "Conditional blocks can be given a different algorithm." Blocks use the LB algorithm and the persistence setting of the vService.
• "Conditional blocks are also added on a Network vService." A Network vService has only Default Backend Services.

How it is done in the documentation

• HTTP (L7) vService · Default Backend Services
• HTTP (L7) vService · Conditional Backend Services
• HTTP (L7) vService · Multiple conditions: AND / OR logic
• L7 TCP vService · Conditional Backend Services

Related questions

• Where does traffic go if no condition matches?
• What is Content Switching?
• How do I change the evaluation order of Conditional Backend Services blocks?