Set rules for generating sequential and unique hostnames - Auto Scaling

Background information

Auto Scaling can create one or more ECS instances during a single scale-out event in a scaling group based on scaling rules. Auto Scaling can also create multiple ECS instances during multiple scale-out events. You can use one of the following methods to set rules for hostnames in a scaling configuration or launch template:

If you want to make the hostnames of ECS instances in a scaling group sequential and unique, you must set rules for the hostnames in a scaling configuration instead of in a launch template. For more information, see (Recommended) Sorting at a fixed-value increment and Dynamic sorting based on extended sequential values.

Note The hostnames of ECS instances in a scaling group are sequentially generated but not necessarily consecutive. For example, the hostnames of created ECS instances in a scaling group are ess-node-0999, ess-node-1000, and ess-node-1002. This indicates that the ess-node-1001 ECS instance fails to start normally. Auto Scaling considers this instance unhealthy, removes it from the scaling group, and then creates another instance whose hostname is ess-node-1002.
If you want to make the hostnames of only ECS instances that are created during a single scale-out event sequential and unique, you can set the hostnames based on the specified sorting rule. For more information, see Batch configure sequential names or hostnames for multiple instances.
If you have no naming requirements for hostnames, you can use regular hostnames without the need to set hostnames based on the preceding rules. For example, if you set the hostname to hostname for a scaling group, the hostnames of all created ECS instances in the scaling group are hostname.

In this topic, two example scenarios are used to describe how to set rules for generating sequential and unique hostnames by using the Auto Scaling console and by calling API operations.

Scenario 1: Set sequential and unique hostnames in the console

Create a scaling group.
For more information, see Create a scaling group.
Create a scaling configuration and enable it.

In the System Configurations (Optional) step, specify the naming convention in the Host section. In this example, enter ess-node-(AUTO_INCREMENT)[0,3]-ecshost.

Note In this example, the naming convention specifies to increment the hostname by a fixed value for each created ECS instance. For more information, see (Recommended) Sorting at a fixed-value increment.

For more information, see Create a scaling configuration.
Enable the scaling group.

For more information, see Enable a scaling group.
Create and manually execute a scaling rule.
1. Create a scaling rule. For more information, see Create a scaling rule.
  In this example, set Rule Type to Simple Scaling Rule and Operation to Add 3 Instances.
2. Manually execute the scaling rule. For more information, see Execute a scaling rule.
  After the scaling rule is executed, the hostnames of the three ECS instances are displayed as ess-node-000-ecshost, ess-node-001-ecshost, and ess-node-002-ecshost.

Scenario 2: Set sequential and unique hostnames by calling API operations

Call the CreateScalingGroup operation to create a scaling group.
Call the CreateScalingConfiguration operation to create a scaling configuration.

Set the HostName parameter to ess-node-(AUTO_INCREMENT)[0,3]-ecshost.

Note In this example, the naming convention specifies to increment the hostname by a fixed value for each created ECS instance. For more information, see (Recommended) Sorting at a fixed-value increment.
Call the EnableScalingGroup operation to enable the scaling group.
Create and manually execute a scaling rule.
1. Call the CreateScalingRule operation to create a scaling rule.
  In this example, a simple scaling rule is created to add three ECS instances.
2. Call the ExecuteScalingRule operation to execute the scaling rule.
  After the scaling rule is executed, the hostnames of the three ECS instances are displayed as ess-node-000-ecshost, ess-node-001-ecshost, and ess-node-002-ecshost.

(Recommended) Sorting at a fixed-value increment

Hostnames are in the name_prefix(AUTO_INCREMENT)[begin_number,bits]name_suffix format.

Table 1. Hostname description
Segment	Required	Description	Example
name_prefix	Yes	The prefix of the hostname.	ess-node-
(AUTO_INCREMENT)	Yes	The fixed value used to indicate the sorting method.	(AUTO_INCREMENT)
[begin_number,bits]	Yes	The sequential value of the hostname. After this segment is specified, the sequential values of hostnames are incremented. Note By default, the system sequentially increments the values. If a created ECS instance in a scaling group cannot be started, Auto Scaling removes the instance from the scaling group and creates another instance. Therefore, the hostnames of ECS instances in the scaling group may not be consecutively incremented. begin_number: the start value of the sequential value. Valid values: 0 to 999999. When Auto Scaling creates instances in a scaling group for the first time, the start value that you specified takes effect. If the start value is not specified, 0 is used. When Auto Scaling creates instances in a scaling group not for the first time, the start value increments starting from the maximum sequential value of existing hostnames in the scaling group. bits: the number of digits of the sequential value. Valid values: 1 to 6. Default value: 6. Note We recommend that you set bits to at least 3. Otherwise, the upper limit of the sequential values may be reached in a short period of time. If Auto Scaling needs to create more ECS instances after the upper limit is reached, an error is reported and the scale-out event is suspended. In this case, you must set the rules for generating hostnames again. The [begin_number,bits] segment cannot contain spaces. By default, when the number of digits of the specified begin_number value is greater than the bits value, bits is set to 6.	[0,6]
name_suffix	No	The suffix of the instance name or hostname.	-ecshost

Table 2. Hostname examples
Example	Existing hostname with the maximum sequential value in a scaling group	Hostname (three created ECS instances)	Description
ess-node-(AUTO_INCREMENT)[0,3]-ecshost	N/A	ess-node-000-ecshost, ess-node-001-ecshost, and ess-node-002-ecshost.	The number of digits of all sequential values is the bits value. When Auto Scaling creates instances in a scaling group for the first time, the sequential value starts from the begin_number value and sequentially increments based on the number of created instances.
ess-node-(AUTO_INCREMENT)[]-ecshost ess-node-(AUTO_INCREMENT)[,]-ecshost	N/A	ess-node-000000-ecshost, ess-node-000001-ecshost, and ess-node-000002-ecshost.	If begin_number is not specified, begin_number is set to 0. If bits is not specified, bits is set to 6.
ess-node-(AUTO_INCREMENT)[99,1]-ecshost	ess-node-000099-ecshost	ess-node-000100-ecshost, ess-node-000101-ecshost, and ess-node-000102-ecshost.	When Auto Scaling creates instances in a scaling group not for the first time, the sequential value increments starting from the maximum sequential value of existing hostnames in the scaling group. By default, when the number of digits of the specified begin_number value is greater than the bits value, bits is set to 6.
ess-node-(AUTO_INCREMENT)[0,2]-ecshost	ess-node-99-ecshost	An error is reported and the scale-out event is suspended.	When Auto Scaling creates instances in a scaling group not for the first time, the sequential value increments starting from the maximum sequential value of existing hostnames in the scaling group. If Auto Scaling needs to create more ECS instances after the upper limit is reached, an error is reported and the scale-out event is suspended. In this case, you must set the rules for generating hostnames again.
ess-node-(AUTO_INCREMENT)[0,4]	ess-node-0998-ecshost	ess-node-0999, ess-node-1000, and ess-node-1002.	When Auto Scaling creates instances in a scaling group not for the first time, the sequential value increments starting from the maximum sequential value of existing hostnames in the scaling group. By default, the system sequentially increments the values. If a created ECS instance in a scaling group cannot be started, Auto Scaling removes the instance from the scaling group and creates another instance. Therefore, the hostnames of ECS instances in the scaling group may not be consecutively incremented. In this example, the hostname of the ECS instance that fails to start is ess-node-1001.

Dynamic sorting based on extended sequential values

Hostnames are in the name_prefix(ess_extend_begin,ess_extend_bits)[begin_number,bits]name_suffix format.

Table 3. Hostname description
Segment	Required	Description	Example
name_prefix	Yes	The prefix of the hostname.	ess-node-
(ess_extend_begin,ess_extend_bits)	Yes	The extended sequential value of the hostname. When the base sequential value of existing hostnames in a scaling group is equal to the maximum sequential value, one value is added to the extended sequential value. Then, the base sequential value increments from 0 again until the upper limit is reached. ess_extend_begin: the start value of the extended sequential value. Valid values: 0 to ZZZ. Each valid value ranges from 0 to 9, a to z, and A to Z. For example, if one value is added to 9, a is obtained. If one value is added to z, A is obtained. When Auto Scaling creates instances in a scaling group for the first time, the start value that you specified takes effect. If the start value is not specified, 0 is used. When Auto Scaling creates instances in a scaling group not for the first time, the start value increments starting from the maximum sequential value of existing hostnames in the scaling group. ess_extend_bits: the number of digits of the extended sequential value. Valid values: 1 to 3. Default value: 3. Notice If Auto Scaling needs to create more ECS instances after the upper limits of extended sequential values and base sequential values are both reached, an error is reported and the scale-out event is suspended. In this case, you must set the rules for generating hostnames again. The (ess_extend_begin,ess_extend_bits) segment cannot contain spaces. By default, when the number of digits of the specified ess_extend_begin value is greater than the bits value, bits is set to 3.	(0,3)
[begin_number,bits]	Yes	The base sequential value of the hostname. After this segment is specified, the base sequential values of hostnames are incremented to the maximum value. Then, one value is added to the extended sequential value and the base sequential value increments from 0 again until the upper limit is reached. Notice By default, the system sequentially increments the values. If a created ECS instance in a scaling group cannot be started, Auto Scaling removes the instance from the scaling group and creates another instance. Therefore, the hostnames of ECS instances in the scaling group may not be consecutively incremented. begin_number: the start value of the base sequential value. Valid values: 0 to 999999. When Auto Scaling creates instances in a scaling group for the first time, the start value that you specified takes effect. If the start value is not specified, 0 is used. When Auto Scaling creates instances in a scaling group not for the first time, the start value increments starting from the maximum sequential value of existing hostnames in the scaling group. bits: the number of digits of the base sequential value. Valid values: 1 to 6. Default value: 6. Notice When the sum of the maximum base sequential value of existing hostnames and the number of instances to be created in a scaling group is greater than or equal to the maximum base sequential value, the hostnames of instances in the scaling group may not be consecutively incremented. To ensure that the hostnames of ECS instances in the scaling group are consecutively incremented, we recommend that you set the number of digits of base sequential values to at least 3. If Auto Scaling needs to create more ECS instances after the upper limits of extended sequential values and base sequential values are both reached, an error is reported and the scale-out event is suspended. In this case, you must set the rules for generating hostnames again. The [begin_number,bits] segment cannot contain spaces. By default, when the number of digits of the specified begin_number value is greater than the bits value, bits is set to 6.	[0,6]
name_suffix	No	The suffix of the instance name or hostname.	-ecshost

Table 4. Hostname examples
Example	Existing hostname with the maximum sequential value in a scaling group	Hostname (three created ECS instances)	Description
ess-node-(0,3)[0,3]-ecshost	N/A	ess-node-000000-ecshost, ess-node-000001-ecshost, and ess-node-000002-ecshost.	When Auto Scaling creates instances in a scaling group for the first time, the sequential values for hostnames of the instances are set based on the following rules: Extended sequential value: The number of digits of all extended sequential values is the ess_extend_bits value. The start value of all extended sequential values is the ess_extend_begin value. If the base sequential value reaches the maximum value, one value is added to the extended sequential value and the base sequential value increments from 0 again. Base sequential value: The number of digits of all base sequential values is the bits value. The start value of all base sequential values is the begin_number value. The base sequential value is incremented based on the number of ECS instances to be created. If the base sequential value reaches the maximum value, one value is added to the extended sequential value and the base sequential value increments from 0 again.
ess-node-()[]-ecshost ess-node-(,)[,]-ecshost	N/A	ess-node-000000000-ecshost, ess-node-000000001-ecshost, and ess-node-000000002-ecshost.	Extended sequential value: If ess_extend_begin is not specified, ess_extend_begin is set to 0. If ess_extend_bits is not specified, ess_extend_bits is set to 3. Base sequential value: If begin_number is not specified, begin_number is set to 0. If bits is not specified, bits is set to 6.
ess-node-(0,1)[0,1]-ecshost	ess-node-08-ecshost	ess-node-10-ecshost, ess-node-11-ecshost, and ess-node-12-ecshost.	When Auto Scaling creates instances in a scaling group not for the first time, the base sequential value increments starting from the maximum sequential value of existing hostnames in the scaling group. When the sum of the maximum base sequential value of existing hostnames and the number of instances to be created in a scaling group is greater than or equal to the maximum base sequential value, the hostnames of instances in the scaling group may not be consecutively incremented. To ensure that the hostnames of ECS instances in the scaling group are consecutively incremented, we recommend that you set the number of digits of base sequential values to at least 3.
ess-node-(0,1)[0,1]-ecshost	ess-node-Z9-ecshost	An error is reported and the scale-out event is suspended.	When Auto Scaling creates instances in a scaling group not for the first time, the base sequential value increments starting from the maximum sequential value of existing hostnames in the scaling group. If Auto Scaling needs to create more ECS instances after the upper limits of extended sequential values and base sequential values are both reached, an error is reported and the scale-out event is suspended. In this case, you must set the rules for generating hostnames again.
ess-node-(0,1)[0,3]	ess-node-0099-ecshost	ess-node-0100, ess-node-0101, and ess-node-0103.	When Auto Scaling creates instances in a scaling group not for the first time, the base sequential value increments starting from the maximum sequential value of existing hostnames in the scaling group. By default, the system sequentially increments the values. If a created ECS instance in a scaling group cannot be started, Auto Scaling removes the instance from the scaling group and creates another instance. Therefore, the hostnames of ECS instances in the scaling group may not be consecutively incremented. In this example, the hostname of the ECS instance that fails to start is ess-node-0102.
ess-node-(0,1)[99,1]-ecshost	ess-node-0000099-ecshost	ess-node-0000100-ecshost, ess-node-0000101-ecshost, and ess-node-0000102-ecshost.	When Auto Scaling creates instances in a scaling group not for the first time, the base sequential value increments starting from the maximum sequential value of existing hostnames in the scaling group. By default, when the number of digits of the specified begin_number value is greater than the bits value, bits is set to 6.