Limiting Component Instances During Restart

The max_in_flight Variable

The max_in_flight variable limits how many instances of a component can restart simultaneously during updates or upgrades. It can make deployment upgrades run faster and prevent system overload. For PCF deployments with hundreds of jobs, upgrading sequentially can take hours. With max_in_flight, you can upgrade jobs in batches, which substantially decreases upgrade time.

Values for max_in_flight can be any integer between 1 and 100, or a percentage of the total number of instances (for example, a max_in_flight value of 20% in a deployment with 10 instances would only allow 2 instances to start at once).

Setting the Variable

The max_in_flight is a system-wide value with optional component-specific overrides. You can override the default value for individual jobs using an API endpoint.

Using the max_in_flight API Endpoint

Use the max_in_flight API endpoint to configure the maximum value for component instances that can start at a given time. This endpoint overrides product defaults. You can specify values as a percentage or an integer.

Use the string “default” as the max_in_flight value to force the component to use the deployment’s default value.

Note: The example below lists three JOB_GUIDs. These three GUIDs are examples of the three different types of values you can use to configure max_in_flight. The endpoint only requires one GUID.

    curl "" \
    -X PUT \
    -H "Authorization: Bearer UAA_ACCESS_TOKEN" \
    -H "Content-Type: application/json" \
    -d '{
          "max_in_flight": {
            "JOB_1_GUID": 1,
            "JOB_2_GUID": "20%",
            "JOB_3_GUID": "default"

Best Practices

Set the max_in_flight variable high enough that the remaining component instances are not overloaded by typical use. If component instances are overloaded during updates, upgrades, or typical use, users may experience downtime.

Some more precise guidelines include:

  • For jobs with high resource usage, set max_in_flight low. For example, for Diego cells, max_in_flight allows non-migrating cells to pick up the work of cells stopping and restarting during migration. If resource usage is already close to 100%, scale up your jobs before making any updates.
  • For quorum-based components (these are components with odd-numbered settings in the manifest), such as etcd, consul, and Diego BBS, set max_in_flight to 1. This preserves quorum and prevents a split-brain scenario from occurring as jobs restart.
  • For other components, set max_in_flight to the number of instances that you can afford to have down at any one time. The best values for your deployment vary based on your capacity planning. In a highly redundant deployment, you can make the number high so that updates run faster. If your components are at high utilization, however, you should keep the number low to prevent downtime.
  • Never set max_in_flight to a value greater than or equal to the number of instances you have running for a component.

For more information about max_in_flight, see the following topics:

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