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Reservation

Mesos provides mechanisms to reserve resources in specific slaves. The concept was first introduced with static reservation in 0.14.0 which enabled operators to specify the reserved resources on slave startup. This was extended with dynamic reservation in 0.23.0 which enabled operators and authorized frameworks to dynamically reserve resources in the cluster.

In both types of reservations, resources are reserved for a role.

Static Reservation

An operator can configure a slave with resources reserved for a role. The reserved resources are specified via the --resources flag. For example, suppose we have 12 CPUs and 6144 MB of RAM available on a slave and that we want to reserve 8 CPUs and 4096 MB of RAM for the ads role. We start the slave like so:

    $ mesos-slave \
      --master=<ip>:<port> \
      --resources="cpus:4;mem:2048;cpus(ads):8;mem(ads):4096"

We now have 8 CPUs and 4096 MB of RAM reserved for ads on this slave.

CAVEAT: In order to modify a static reservation, the operator must drain and restart the slave with the new configuration specified in the --resources flag.

NOTE: This feature is supported for backwards compatibility. The recommended approach is to specify the total resources available on the slave as unreserved via the --resources flag and manage reservations dynamically via the master HTTP endpoints.

Dynamic Reservation

As mentioned in Static Reservation, specifying the reserved resources via the --resources flag makes the reservation static. That is, statically reserved resources cannot be reserved for another role nor be unreserved. Dynamic reservation enables operators and authorized frameworks to reserve and unreserve resources after slave-startup.

By default, frameworks and operators can reserve resources for any role, and can unreserve any dynamically reserved resources. Authorization allows this behavior to be limited so that only particular roles can be reserved for, and only particular resources can be unreserved. For these operations to be authorized, the framework or operator should provide a principal to identify itself. To use authorization with reserve/unreserve operations, the Mesos master must be configured with the appropriate ACLs. For more information, see the authorization documentation.

  • Offer::Operation::Reserve and Offer::Operation::Unreserve messages are available for frameworks to send back via the acceptOffers API as a response to a resource offer.
  • /reserve and /unreserve HTTP endpoints allow operators to manage dynamic reservations through the master.

In the following sections, we will walk through examples of each of the interfaces described above.

If two dynamic reservations are made for the same role at a single slave (using the same labels, if any; see below), the reservations will be combined by adding together the resources reserved by each request. This will result in a single reserved resource at the slave. Similarly, “partial” unreserve operations are allowed: an unreserve operation can release some but not all of the resources at a slave that have been reserved for a role. In this case, the unreserved resources will be subtracted from the previous reservation and any remaining resources will still be reserved.

Dynamic reservations cannot be unreserved if they are still being used by a running task or if a persistent volume has been created using the reserved resources. In the latter case, the volume should be destroyed before unreserving the resources.

Labeled Reservations

Dynamic reservations can optionally include a list of labels, which are arbitrary key-value pairs. Labels can be used to associate arbitrary metadata with a resource reservation. For example, frameworks can use labels to identify the intended purpose for a portion of the resources that have been reserved at a given slave. Note that two reservations with different labels will not be combined together into a single reservation, even if the reservations are at the same slave and use the same role.

Framework Scheduler API

Offer::Operation::Reserve

A framework can reserve resources through the resource offer cycle. Suppose we receive a resource offer with 12 CPUs and 6144 MB of RAM unreserved.

    {
      "id": <offer_id>,
      "framework_id": <framework_id>,
      "slave_id": <slave_id>,
      "hostname": <hostname>,
      "resources": [
        {
          "name": "cpus",
          "type": "SCALAR",
          "scalar": { "value": 12 },
          "role": "*",
        },
        {
          "name": "mem",
          "type": "SCALAR",
          "scalar": { "value": 6144 },
          "role": "*",
        }
      ]
    }

We can reserve 8 CPUs and 4096 MB of RAM by sending the following Offer::Operation message. Offer::Operation::Reserve has a resources field which we specify with the resources to be reserved. We must explicitly set the resources' role field with the framework’s role. The required value of the principal field depends on whether or not the framework provided a principal when it registered with the master. If a principal was provided, then the resources' principal field must be equal to the framework’s principal. If no principal was provided during registration, then the resources' principal field can take any value, or can be left unset. Note that the principal field determines the “reserver principal” when authorization is enabled, even if authentication is disabled.

    {
      "type": Offer::Operation::RESERVE,
      "reserve": {
        "resources": [
          {
            "name": "cpus",
            "type": "SCALAR",
            "scalar": { "value": 8 },
            "role": <framework_role>,
            "reservation": {
              "principal": <framework_principal>
            }
          },
          {
            "name": "mem",
            "type": "SCALAR",
            "scalar": { "value": 4096 },
            "role": <framework_role>,
            "reservation": {
              "principal": <framework_principal>
            }
          }
        ]
      }
    }

If the reservation is successful, a subsequent resource offer will contain the following reserved resources:

    {
      "id": <offer_id>,
      "framework_id": <framework_id>,
      "slave_id": <slave_id>,
      "hostname": <hostname>,
      "resources": [
        {
          "name": "cpus",
          "type": "SCALAR",
          "scalar": { "value": 8 },
          "role": <framework_role>,
          "reservation": {
            "principal": <framework_principal>
          }
        },
        {
          "name": "mem",
          "type": "SCALAR",
          "scalar": { "value": 4096 },
          "role": <framework_role>,
          "reservation": {
            "principal": <framework_principal>
          }
        },
      ]
    }

Offer::Operation::Unreserve

A framework can unreserve resources through the resource offer cycle. In Offer::Operation::Reserve, we reserved 8 CPUs and 4096 MB of RAM on a particular slave for our role. The master will continue to only offer these resources to our role. Suppose we would like to unreserve these resources. First, we receive a resource offer (copy/pasted from above):

    {
      "id": <offer_id>,
      "framework_id": <framework_id>,
      "slave_id": <slave_id>,
      "hostname": <hostname>,
      "resources": [
        {
          "name": "cpus",
          "type": "SCALAR",
          "scalar": { "value": 8 },
          "role": <framework_role>,
          "reservation": {
            "principal": <framework_principal>
          }
        },
        {
          "name": "mem",
          "type": "SCALAR",
          "scalar": { "value": 4096 },
          "role": <framework_role>,
          "reservation": {
            "principal": <framework_principal>
          }
        },
      ]
    }

We can unreserve the 8 CPUs and 4096 MB of RAM by sending the following Offer::Operation message. Offer::Operation::Unreserve has a resources field which we can use to specify the resources to be unreserved.

    {
      "type": Offer::Operation::UNRESERVE,
      "unreserve": {
        "resources": [
          {
            "name": "cpus",
            "type": "SCALAR",
            "scalar": { "value": 8 },
            "role": <framework_role>,
            "reservation": {
              "principal": <framework_principal>
            }
          },
          {
            "name": "mem",
            "type": "SCALAR",
            "scalar": { "value": 4096 },
            "role": <framework_role>,
            "reservation": {
              "principal": <framework_principal>
            }
          }
        ]
      }
    }

The unreserved resources may now be offered to other frameworks.

Operator HTTP Endpoints

As described above, dynamic reservations can be made by a framework scheduler, typically in response to a resource offer. However, dynamic reservations can also be created and deleted by sending HTTP requests to the /reserve and /unreserve endpoints, respectively. This capability is intended for use by operators and administrative tools.

/reserve (since 0.25.0)

Suppose we want to reserve 8 CPUs and 4096 MB of RAM for the ads role on a slave with id=<slave_id> (note that it is up to the user to find the ID of the slave that hosts the desired resources; the request will fail if sufficient unreserved resources cannot be found on the slave). In this case, the principal that must be included in the reservation field of the reserved resources depends on the status of HTTP authentication on the master. If HTTP authentication is enabled, then the principal in the reservation should match the authenticated principal provided in the request’s HTTP headers. If HTTP authentication is disabled, then the principal in the reservation can take any value, or can be left unset. Note that the principal field determines the “reserver principal” when authorization is enabled, even if HTTP authentication is disabled.

We send an HTTP POST request to the master’s /reserve endpoint like so:

    $ curl -i \
      -u <operator_principal>:<password> \
      -d slaveId=<slave_id> \
      -d resources='[
        {
          "name": "cpus",
          "type": "SCALAR",
          "scalar": { "value": 8 },
          "role": "ads",
          "reservation": {
            "principal": <operator_principal>
          }
        },
        {
          "name": "mem",
          "type": "SCALAR",
          "scalar": { "value": 4096 },
          "role": "ads",
          "reservation": {
            "principal": <operator_principal>
          }
        }
      ]' \
      -X POST http://<ip>:<port>/master/reserve

The user receives one of the following HTTP responses:

  • 202 Accepted: Request accepted (see below).
  • 400 BadRequest: Invalid arguments (e.g., missing parameters).
  • 401 Unauthorized: Unauthenticated request.
  • 403 Forbidden: Unauthorized request.
  • 409 Conflict: Insufficient resources to satisfy the reserve operation.

This endpoint returns the 202 ACCEPTED HTTP status code, which indicates that the reserve operation has been validated successfully by the master. The request is then forwarded asynchronously to the Mesos slave where the reserved resources are located. That asynchronous message may not be delivered or reserving resources at the slave might fail, in which case no resources will be reserved. To determine if a reserve operation has succeeded, the user can examine the state of the appropriate Mesos slave (e.g., via the slave’s /state HTTP endpoint).

/unreserve (since 0.25.0)

Suppose we want to unreserve the resources that we dynamically reserved above. We can send an HTTP POST request to the master’s /unreserve endpoint like so:

    $ curl -i \
      -u <operator_principal>:<password> \
      -d slaveId=<slave_id> \
      -d resources='[
        {
          "name": "cpus",
          "type": "SCALAR",
          "scalar": { "value": 8 },
          "role": "ads",
          "reservation": {
            "principal": <reserver_principal>
          }
        },
        {
          "name": "mem",
          "type": "SCALAR",
          "scalar": { "value": 4096 },
          "role": "ads",
          "reservation": {
            "principal": <reserver_principal>
          }
        }
      ]' \
      -X POST http://<ip>:<port>/master/unreserve

Note that reserver_principal is the principal that was used to make the reservation, while operator_principal is the principal that is attempting to perform the unreserve operation—in some cases, these principals might be the same. The operator_principal must be authorized to unreserve reservations made by reserver_principal.

The user receives one of the following HTTP responses:

  • 202 Accepted: Request accepted (see below).
  • 400 BadRequest: Invalid arguments (e.g., missing parameters).
  • 401 Unauthorized: Unauthenticated request.
  • 403 Forbidden: Unauthorized request.
  • 409 Conflict: Insufficient resources to satisfy the unreserve operation.

This endpoint returns the 202 ACCEPTED HTTP status code, which indicates that the unreserve operation has been validated successfully by the master. The request is then forwarded asynchronously to the Mesos slave where the reserved resources are located. That asynchronous message may not be delivered or unreserving resources at the slave might fail, in which case no resources will be unreserved. To determine if an unreserve operation has succeeded, the user can examine the state of the appropriate Mesos slave (e.g., via the slave’s /state HTTP endpoint).

Listing Reservations

Information about the reserved resources at each slave in the cluster can be found by querying the /slaves master endpoint (under the reserved_resources_full key).

The same information can also be found in the /state endpoint on the agent (under the reserved_resources_full key). The agent endpoint is useful to confirm if a reservation has been propagated to the agent (which can fail in the event of network partition or master/agent restarts).

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