Dispatching containers to cloud VMs¶

(Draft)

Component name / purpose¶

crunch-dispatch-cloud runs Arvados user containers on generic public cloud infrastructure by automatically creating and destroying VMs of various sizes according to demand, preparing the VMs' runtime environments, and running containers on them.

Overview¶

The dispatcher waits for containers to appear in the queue, and runs them on appropriately sized cloud VMs. When there are no idle cloud VMs with the desired size, the dispatcher brings up more VMs using the cloud provider's API. The dispatcher also shuts down idle VMs that exceed the configured idle timer -- and sooner if the provider refuses to create new VMs.

Interaction with other components¶

API server (backed by PostgreSQL) supplies the container queue: which containers the system should be trying to execute (or cancel) at any given time.

The cloud provider's API supplies a list of VMs that exist (or are being created) at a given time and their network addresses, accepts orders to create new VMs, updates instance tags, and (optionally, depending on the driver) obtains the VMs' SSH server public keys.

The SSH server on each cloud VM allows the dispatcher to authenticate with a private key and execute shell commands as root.

Configuration¶

Arvados configuration (currently a file in /etc) supplies cloud provider credentials, allowed node types, spending limits/policies, etc.

    CloudVMs:
      BootProbeCommand: "docker ps -q" 
      SyncInterval: 1m    # get list of 
      TimeoutIdle: 1m     # shutdown if idle longer than this
      TimeoutBooting: 10m # shutdown if exists longer than this without running BootProbeCommand successfully
      TimeoutProbe: 2m    # shutdown if (after booting) communication fails longer than this, even if ctrs are running
      TimeoutShutdown: 1m # shutdown again if node still exists this long after shutdown
      Driver: Amazon
      DriverParameters:   # following configs are driver dependent
        Region: us-east-1
        APITimeout: 20s
        EC2Key: abcdef
        EC2Secret: abcdefghijklmnopqrstuvwxyz
        StorageKey: abcdef
        StorageSecret: abcdefghijklmnopqrstuvwxyz
        ImageID: ami-0123456789abcdef0
        SubnetID: subnet-01234567
        SecurityGroups: sg-01234567
    Dispatch:
      StaleLockTimeout: 1m     # after restart, time to wait for workers to come up before abandoning locks from previous run
      PollInterval: 1m         # how often to get latest queue from arvados controller
      ProbeInterval: 10s       # how often to probe each instance for current status/vital signs
      MaxProbesPerSecond: 1000 # limit total probe rate for dispatch process (across all instances)
      PrivateKey: |            # SSH key able to log in as root@ worker VMs
        -----BEGIN RSA PRIVATE KEY-----
        MIIEowIBAAKCAQEAqYm4XsQHm8sBSZFwUX5VeW1OkGsfoNzcGPG2nzzYRhNhClYZ
        0ABHhUk82HkaC/8l6d/jpYTf42HrK42nNQ0r0Yzs7qw8yZMQioK4Yk+kFyVLF78E
        GRG4pGAWXFs6pUchs/lm8fo9zcda4R3XeqgI+NO+nEERXmdRJa1FhI+Za3/S/+CV
        mg+6O00wZz2+vKmDPptGN4MCKmQOCKsMJts7wSZGyVcTtdNv7jjfr6yPAIOIL8X7
        LtarBCFaK/pD7uWll/Uj7h7D8K48nIZUrvBJJjXL8Sm4LxCNoz3Z83k8J5ZzuDRD
        gRiQe/C085mhO6VL+2fypDLwcKt1tOL8fI81MwIDAQABAoIBACR3tEnmHsDbNOav
        Oxq8cwRQh9K2yDHg8BMJgz/TZa4FIx2HEbxVIw0/iLADtJ+Z/XzGJQCIiWQuvtg6
        exoFQESt7JUWRWkSkj9JCQJUoTY9Vl7APtBpqG7rIEQzd3TvzQcagZNRQZQO6rR7
        p8sBdBSZ72lK8cJ9tM3G7Kor/VNK7KgRZFNhEWnmvEa3qMd4hzDcQ4faOn7C9NZK
        dwJAuJVVfwOLlOORYcyEkvksLaDOK2DsB/p0AaCpfSmThRbBKN5fPXYaKgUdfp3w
        70Hpp27WWymb1cgjyqSH3DY+V/kvid+5QxgxCBRq865jPLn3FFT9bWEVS/0wvJRj
        iMIRrjECgYEA4Ffv9rBJXqVXonNQbbstd2PaprJDXMUy9/UmfHL6pkq1xdBeuM7v
        yf2ocXheA8AahHtIOhtgKqwv/aRhVK0ErYtiSvIk+tXG+dAtj/1ZAKbKiFyxjkZV
        X72BH7cTlR6As5SRRfWM/HaBGEgED391gKsI5PyMdqWWdczT5KfxAksCgYEAwXYE
        ewPmV1GaR5fbh2RupoPnUJPMj36gJCnwls7sGaXDQIpdlq56zfKgrLocGXGgj+8f
        QH7FHTJQO15YCYebtsXWwB3++iG43gVlJlecPAydsap2CCshqNWC5JU5pan0QzsP
        exzNzWqfUPSbTkR2SRaN+MenZo2Y/WqScOAth7kCgYBgVoLujW9EXH5QfXJpXLq+
        jTvE38I7oVcs0bJwOLPYGzcJtlwmwn6IYAwohgbhV2pLv+EZSs42JPEK278MLKxY
        lgVkp60npgunFTWroqDIvdc1TZDVxvA8h9VeODEJlSqxczgbMcIUXBM9yRctTI+5
        7DiKlMUA4kTFW2sWwuOlFwKBgGXvrYS0FVbFJKm8lmvMu5D5x5RpjEu/yNnFT4Pn
        G/iXoz4Kqi2PWh3STl804UF24cd1k94D7hDoReZCW9kJnz67F+C67XMW+bXi2d1O
        JIBvlVfcHb1IHMA9YG7ZQjrMRmx2Xj3ce4RVPgUGHh8ra7gvLjd72/Tpf0doNClN
        ti/hAoGBAMW5D3LhU05LXWmOqpeT4VDgqk4MrTBcstVe7KdVjwzHrVHCAmI927vI
        pjpphWzpC9m3x4OsTNf8m+g6H7f3IiQS0aiFNtduXYlcuT5FHS2fSATTzg5PBon9
        1E6BudOve+WyFyBs7hFWAqWFBdWujAl4Qk5Ek09U2ilFEPE7RTgJ
        -----END RSA PRIVATE KEY-----

Scheduling policy¶

The container priority field determines the order in which resources are allocated.

• If container C1 has priority P1,
• ...and C2 has higher priority P2,
• ...and there is no pending/booting/idle VM suitable for running C2,
• ...then C1 will not be started.

However, containers that run on different VM types don't necessarily start in priority order.

• If container C1 has priority P1,
• ...and C2 has higher priority P2,
• ...and there is no idle VM suitable for running C2,
• ...and there is a pending/booting VM that will be suitable for running C2 when it comes up,
• ...and there is an idle VM suitable for running C1,
• ...then C1 will start before C2.

Synchronizing state¶

When first starting up, dispatcher inspects API server’s container queue and the cloud provider’s list of dispatcher-tagged cloud nodes, and restores internal state accordingly.

Often, at startup there will be some containers with state=Locked. To avoid breaking priority order, the dispatcher won't schedule any new containers until all such locked containers are matched up with crunch-run processes on existing VMs (typically preparing a docker image) or all of the existing VMs have been probed successfully (meaning the locked containers aren't running anywhere and need to be rescheduled).

When a user cancels a container request with state=Locked or Running, the container priority changes to 0. On its next poll, the dispatcher notices this and kills any corresponding crunch-run processes (or, if there is no such process, just unlocks the container).

When a crunch-run process ends without finalizing its container's state, the dispatcher notices this and sets state to Cancelled.

Operator view¶

Management status endpoint provides:

• list of cloud VMs, each with
  • provider's instance ID
  • hourly price (from configuration file)
  • instance type (from configuration file)
  • instance type (from provider's menu)
  • UUID of the current / most recent container attempted (if known)
  • time last container finished (or boot time, if nothing run yet)
• list of queued/running containers, each with
  • UUID
  • state (queued/locked/running/complete/cancelled)
  • desired instance type
  • time appeared in queue
  • time started (if started)

Metrics endpoint tracks:

• (each VM) time elapsed between VM creation and first successful SSH connection
• (each VM) time elapsed between first successful SSH connection and ready to run a container
• total hourly price of all existing VMs
• total VCPUs and memory allocated to containers
• number of containers running
• number of containers allocated to VMs but not started yet (because VMs are pending/booting)
• number of containers not allocated to VMs (because provider quota is reached)

SSH keys¶

Each worker node has a public key in /root/.ssh/authorized_keys. Dispatcher has the corresponding private key.

(Future) Dispatcher generates its own keys and installs its public key on new VMs using cloud provider bootstrapping/metadata features.

Probes¶

Sometimes (on the happy path) the dispatcher knows the state of each worker, whether it's idle, and which container it's running. In general, it's necessary to probe the worker node itself.

Probe:

• Check whether the SSH connection is alive; reopen if needed.
• Run the configured "ready?" command (e.g., "grep /encrypted-tmp /etc/mtab"); if this fails, conclude the node is still booting.
• Run "crunch-run --list" to get a list of crunch-run supervisors (pid + container UUID)

Dispatcher, after a successful probe, should tag the cloud node record with the dispatcher's ID and probe timestamp. (In case the tagging API fails, remember the probe time in memory too.)

Detecting dead/lame nodes¶

If a node has been up for N seconds without a successful probe, despite at least M attempts, shut it down. (M handles the case where the dispatcher restarts during a time when the "update tags" operation isn't effective, e.g., provider is rate-limiting API calls.)

Multiple dispatchers¶

Not supported in initial version.