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chore(kb/aws/ecs): clarify tasks placement with capacity provider strategy weights

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Michele Cereda
2025-07-04 20:08:42 +02:00
parent 61b5642913
commit 2e215f4028
1 changed files with 19 additions and 11 deletions
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30
knowledge base/cloud computing/aws/ecs.md
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@@ -326,19 +326,25 @@ can only contain either Fargate or Auto Scaling group capacity providers, but **
One **cannot** update a service that is using an Auto Scaling Group capacity provider to use a Fargate one, and
vice versa.
A strategy's capacity provider can have a defined `base` value. This determines the **guaranteed minimum** number of
tasks that that provider will host. If no `base` value is specified for the provider, it defaults to `0`.<br/>
A strategy's capacity provider can have a defined `base` value. This determines how many **guaranteed** tasks that
provider will be given **as minimum** when enough replicas are requested.<br/>
Setting the `base` value higher than the service or standalone task's `desiredCount` only results in `desiredCount`
tasks being placed on that provider. If no `base` value is specified for a provider, it defaults to `0`.<br/>
When multiple capacity providers are specified within a strategy, only **one** of them can have a defined `base` value.
The `weight` value determines **the relative ratio** of tasks to place over the long run, **after the `base` values are
satisfied**.<br/>
The `weight` value determines **the relative ratio** of tasks to execute over the long run.<br/>
This value is taken into account **only after the `base` values are satisfied**.<br/>
When multiple capacity providers are specified within a strategy, at least one of the providers **must** have a `weight`
value greater than zero (`0`).
Capacity providers with a `weight` value of zero are **not** used to run tasks. Should _all_ providers in a strategy
have a weight of `0`, any RunTask or CreateService actions using that strategy will fail.
Aside from their `base` value (if not `0`), capacity providers with a `weight` value of `0` are **not** considered when
the scheduler decides where to place tasks. Should _all_ providers in a strategy have a weight of `0`, any `RunTask` or
`CreateService` actions using that strategy will fail.
The `weight` ratio is computed by summing up all providers' weights, then determining the percentage per provider.
The `weight` ratio is computed by:
1. Summing up all providers' weights.
1. Determining the percentage per provider.
<details style='padding: 0 0 0 1rem'>
<summary>Simple example</summary>
@@ -387,7 +393,7 @@ Provider 3 is `some-custom-ec2-capacity-provider`, with a weight of `0`.
{
"capacityProvider": "FARGATE",
"weight": 1,
"base": 1
"base": 2
},
{
"capacityProvider": "FARGATE_SPOT",
@@ -401,15 +407,17 @@ Provider 3 is `some-custom-ec2-capacity-provider`, with a weight of `0`.
}
```
`some-custom-ec2-capacity-provider` will just be ignored due to its weight being `0`.<br/>
`FARGATE` will maintain tasks just for having the `base` value between the bunch.<br/>
It will get at least 2 of them since the `base` value is `2`.
`some-custom-ec2-capacity-provider` will just be ignored during scheduling due to its weight being `0`.<br/>
Sum of the remaining weights: `1 + 19 = 20`.<br/>
Percentage per provider:
- `FARGATE`: `1 / 20 = 0.05`.
- `FARGATE_SPOT`: `19 / 20 = 0.95`.
`FARGATE` will receive 2 task for the `base` value being `2`, then 5% of the remaining tasks.<br/>
`FARGATE_SPOT` will receive the remaining 95% of the remaining tasks.
`FARGATE` will receive 5% of the remaining tasks, while `FARGATE_SPOT` will receive 95% of them.
</details>