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Workflow Settings

Wherever workflows are defined, such as in effectors, sensors and policies and in nested workflow, there are a number of properties which can be defined. The most common of these, input, output, and parameters are described in the sections above. Some of the common properties permitted on steps also apply to workflow definitions, including condition, timeout, and on-error.

This rest of this section describes the remaining properties for more advanced use cases including mutex locking and resilient workflows with replay points.

Locks and Mutual Exclusion Behavior

In some cases, it is important to ensure that the same workflow does not run concurrently with itself, or more generally to assign a mutual exclusion “mutex” lock to make sure that at most one executing instance from a group can run at any point in time.

This can be done in Cloudsoft AMP by specifying lock: LOCK-NAME on the workflow. The lock is scoped to the entity, and means that if a workflow instance running at the entity enters this block, it acquires that “lock”, and no other workflow instance at the entity can enter that block until the first one exits the block and releases the lock. Workflow instances at the entity that seek to lock the same LOCK-NAME will block until the lock becomes available.

For example to ensure that start and stop do not run simultaneously, we could write:

brooklyn.initializers:
- type: workflow-effector
  name: start
  lock: start-stop
  steps:
    - ...
- type: workflow-effector
  name: stop
  lock: start-stop
  steps:
    - ...

If stop is run while start is still running, or a second start is run, they will not run until the first start completes and releases the lock. An operator with appropriate access permissions could also manually cancel the start. Details of why the effector is blocked are shown in the UI and available via the API, as part of the workflow data.

To set a lock shared across multiple entities, the lock can be set as map of the form { name: <lock-name>, entity: <entity> }.

Example: Thread-Safe Package Management

Locks can also be used in workflow steps saved as registered types. A good example where this is useful is when working with on-box package managers, most of which do not allow concurrent operation. For instance, an apt-get workflow step might use a lock to ensure that multiple parallel effectors do not try to apt-get on a server at the same time:

id: apt-get
type: workflow
lock: apt-package-management
shorthand: ${package}
parameters:
  package:
    description: package(s) to install
steps:
- ssh sudo apt-get install -y ${package}

A workflow can then do apt-get iputils-ping as a step and AMP will ensure it interacts nicely with any other workflow at the same entity.

Advanced Implementation Details

AMP guarantees that if a workflow is interrupted by server shutdown, it will resume with that lock after startup, so it works well with replayable: automatically described below. Brookilyn does not guarantee that waiters will acquire the lock in the same order they requested the lock, although this behavior can be constructed using a sensor that acts as a queue.

Any on-error handler on the workflow with lock will run with the lock still acquired.

Any timeout period starts once the lock is acquired.

Internally, the lock is acquired by setting a sensor lock-... equal to the ${workflow.id}, where ... is the LOCK-NAME. If a different workflow ID is indicated, the workflow will block. The sensor will always be cleared after the workflow with the lock completes.

Thus if a workflow needs to test whether it can acquire the lock, it can do exactly what the internal lock mechanism does: set that sensor to its ${workflow.id} with a require condition testing that it is blank or already held. This technique can also be used to specify a timeout on the lock with a retry.

This can also be used to force-clear a lock, to allow another workflow to run, either interactively using “Run workflow” in the App Inspector, saying clear-sensor lock-LOCK-NAME, or as below if the lock isn’t available after 1 minute.

These techniques are all illustrated in the following example:

- step: set-sensor lock-checking-first-example = ${workflow.id}
  require:
    any:
      - when: absent
      - equals: ${workflow.id}
        # allowing = ${workflow.id} is recommended in the edge case where the workflow is interrupted 
        # in the split-second between setting the sensor and moving to the next step;
        # a "replay-resuming" as described below will proceed in case this step has already run
  on-error:
    # retry every 10 milliseconds for up to 1 minute
    - step: retry backoff 10ms timeout 1m
      on-error: goto lock-not-available
- type: workflow
  lock: checking-first-example
  steps:
    - log we got the lock
    # ... other steps to be performed while holding the lock
# other steps to be performed after clearing the lock
  next: end
  
- id: lock-not-available
  step: log Lock not available after one minute, force-clearing it and continuing
- clear-sensor lock-checking-first-example
- goto start

Resilience: Replaying and Retention/Expiry

Workflows have a small number of settings that determine how AMP handles workflow metadata.
These allow workflow details to be accessible via the API and in the UI (in addition to whatever is persisted in the logs) and optionally for a user to “replay” a workflow. These are:

  • retention: for how long after completion should details of workflow steps, input, and output be kept for inspection or manual replaying
  • idempotent (for steps): is a step safe to re-run if it is interrupted or fails, if true implying it can be recovered by replaying resuming at that step
  • replayable: from where can a retained workflow be safely replayed, such as replaying from the start or from other defined explicitly defined replay points

Common Replay Settings

Most of the time, there are just a few tweaks to idempotent and replayable needed to let Cloudsoft AMP do the right thing to replay correctly. These simple settings are covered first. The other settings, including changing the retention, are intended for advanced use cases only.

AMP workflows are designed so that most steps are automatically “idempotent”: they can safely be run multiple times in a row, and so long as the last run is successful, the workflow can proceed. This means if a workflow is interrupted or fails, it is safe to attempt a recovery by replaying resuming at that step. This can be used for transient problems (e.g. a flaky network) or where an operator corrects a problem (e.g. fixes the network). It means uncertainty about whether the step completed or not can be ignored, and the step re-run if in doubt. Instructions such as “sleep”, “set-config”, or “wait for sensor X to be true” are obviously idempotent; it also applies to let because AMP records a copy of the value of workflow variables on entry to each step and will restore them on a replay.

However, for some step types, it is impossible for AMP to infer whether they are idempotent: this applies to “external” steps such as http and ssh, and some invoke-effector steps. It can also be the case that even where individual steps are idempotent, a sequence of steps is not. In either of these cases the workflow author should give instructions to AMP about how to “replay”.

There are two common ways for an author to let Cloudsoft AMP know how to replay:

  • individual steps that are idempotent but not obviously so can be marked explicitly as such with idempotent: yes; for example a read-only http or container command

  • explicit “replayable waypoints” can be set with a step workflow replayable from here to indicate the workflow can be replayed from that point, either manually or automatically; if any step is not idempotent, a “replay resuming” request will replay from the last such waypoint; this might be a retry step in the workflow, on failover with a replayable: automatically instruction, or a manual request from an operator; if waypoints are defined, operators will also have the option to select a waypoint to replay from

An example of a non-idempotent step is a container which calls aws ec2 run-instances; this might fail after the command has been received by AWS, but before the response is received by AMP, and simply re-running it in this case would cause further new instances to be created. The solution for this situation is to have a sequence of steps which creates a unique identifier for the request (setting this as a tag), then scans to discover any instances with a matching tag, calling run-instances only if none are found, and then to wait on the instances just created or discovered. An author can specify replayable from here just after the unique identifier is created, so if the workflow is subsequently interrupted on run-instances it will replay from the discovery.

This is also an example where a sequence of individually idempotent steps is not altogether idempotent; once a unique identifier has been used in a subsequent step, it would be invalid to create a new unique identifier. Defining the replay point immediately after this step is a good solution, because AMP’s “replay resuming” will only ever run from the last executed step if that step is idempotent, or from the last explicit replayable from here point. (Alternatively the unique identifier step could use ${entity.id} rather than something random, or store the random value in a sensor with a require instruction there to ensure it is only ever created once per entity.)

Where an external step is known to be idempotent – such as a describe-instances step that does the discovery, or any read-only step – the step can be marked idempotent: yes and AMP will support replay resuming at that step. ( However here, and often, this is unnecessary, if the nearest “replay point” is good enough.)

The internal steps workflow and invoke-effector are by default considered idempotent if AMP can tell they are running nested workflows at idempotent step. All other internal steps are idempotent. Actions such as deploy-application use special techniques internally to guarantee idempotency.

In some cases, it can be convenient to indicate default replayable/idempotency instructions when defining a workflow. As part of any workflow definition, such as workflow-effector or a nested type: workflow step, the entry idempotent: all indicates that all external steps in the workflow are idempotent; replayable: automatically indicates that an automatic on-error handler should resume any workflow interrupted by an AMP server restart or failover; and replayable: from start indicates that the start of the workflow is a replay point.

Thus by default, most steps will allow a manual “replay resuming” picking up at the step that was last run. However without a retry replay step (such as in an error handler), this will not happen automatically, and some steps, external ones (which are often the ones most likely to fail), cannot safely permit a “replay resuming” and so require extra attention. The following is a summary of the common settings used:

  • as a step
    • workflow replayable from here to indicate that a step is a valid replay point, with the option of appending the word only at the end to clear other replay points
  • on a step
    • idempotent: yes to indicate that if the workflow is interrupted or fails at that step, it can be resumed at that step (only needed for external steps which are not automatically inferrable as idempotent) when defining a workflow
    • replayable: from start to indicate that the start of the workflow is a valid replay point
    • replayable: automatically to indicate that on an unhandled AMP failover (DanglingWorkflowException), the workflow should attempt to “replay resuming”, either from the last executed step if it is resumable, or from the last replay point
    • idempotent: all to indicate that all steps are idempotent and if interrupted there, the workflow can resume there, unless explicitly indicated otherwise (by default steps such as http and container are not; only internal steps known to be safely re-runnable are resumable)

Finally, it is worth elaborating the differences between the three types of retry behavior, as described on the retry step:

  • A “replay resuming” attempts to resume flow at the last incomplete executed step. If that step is idempotent, it is replayed with special arguments to resume where it left off: this means skipping any condition check, using the workflow variables as at that point, using any previous resolved values for input, and if the step launched sub-workflows (such as workflow or switch, or an invoke-effector calling directly to a workflow effector), those sub-workflows are resumed if possible. If the step is not idempotent, it attempts to “replay from” the last replayable step, which might be the same step, but the condition and inputs will be re-evaluated. If there is no last replayable step, it will fail.

  • A “replay from” looks at a given step to see if it is replayable, and if not, searches the flow backwards until it finds one. If there are none, or it cannot backtrack, it will fail. If it finds one, execution replays from that step, using the workflow variables as they were at that point, but re-checking any condition, re-evaluating inputs, and re-creating any sub-workflows.

  • A retry step can specify replay and/or a next step. If a next step is specified without replay, it will do a simple goto to that step and so will use the most recent value of workflow variables. In all other cases it will do some form of replay: if replay with next is specified, it will replay from that point, with last an alias for the last replay point and end an alias for replay resuming; if replay is specified without next, it will replay from the last replay point; if neither next nor replay is specified, it will replay resuming where that makes sense (in an error handler) and otherwise replay from last. In all cases, retry options including limit and backoff are respected.

Only replay is permitted through the API or UI, either “from” a specific step, from “start”, from the “last” replayable step, or resuming from the “end”. Users with suitable entitlement also have the ability to force a replay from a given step or resuming, which will proceed irrespective of the idempotent or replayable status of the step.

Example: Mutex Atomic Increment

Consider an atomic increment:

- let x = ${entity.sensor.count}
- step: let x = ${x} + 1
  replayable: from here only
- set-sensor count = ${x}

If this is interrupted on step 3 after setting the sensor, a replay from start will retrieve the new sensor value and increment it again. By saying here only on step two, we remove all previous workflow points and ensure the workflow is only ever replayed from the one safe place.

The above assumes no other instances of workflow might use the sensor; if two workflows run concurrently they both might get the same initial value of count, so the result would be a single increment. Wrapping this in a workflow with a lock block as described above will prevent this problem, with Cloudsoft AMP ensuring that on interruption the workflow with the lock is replayed first. Again we need to set a replay point before the incremented value is written, and for good measure we put a replay point after the sensor is updated.

- type: workflow
  lock: single-entry-workflow-incrementing-count
  replayable: from start
  steps:
    # ... various steps
    - let x = ${entity.sensor.count} ?? 0
    - step: let x = ${x} + 1
      replayable: from here only       # do not allow replays from the start
    - set-sensor count = ${x}
    - workflow replayable from here    # could say only, but previous replay point also okay
    # ... various steps
  on-error:
    - retry limit 2 in 5s              # allow it to retry replaying on any error
                                       # (if we just wanted AMP server failover errors to recover,
                                       # we could have said `replayable: automatically from start`)

Advanced: Replay/Resume Settings

There are additional options for idempotent and replayable useful in special cases, and the retention can be configured. As noted above, this section and the next can be skipped on first reading and returned to if there are complicated replay or retention needs.

  • idempotent
    • when defining a workflow, as idempotent: <value>
      • all: means that all external steps in this workflow will be resumable unless explicitly declared otherwise (by default, per below, external steps are not resumable); this includes steps in explicit sub-workflows (where the workflow definition has a workflow with steps) but not sub-workflows which are references (effectors or registered workflow types)
      • the empty string or null (the default): means normal idempotency determination applies
    • on a step as a key, as idempotent: <value>
      • yes: the step is idempotent and the workflow can replay resuming at this step if interrupted there
      • no: the step is not idempotent and should not resumed at this step; if interrupted there, replay resuming will start from the previous replay point
      • default (the default): no for fail (because there is no point in resuming from a fail step), no for external steps (eg http, ssh) except where the surrounding workflow definition is all, computed based on the state of sub-workflows if at a workflow step, and yes otherwise
  • replayable
    • when defining a workflow, as replayable: <value>
      • enabled (the default): is is permitted to replay resuming wherever the workflow fails on idempotent steps or where there are explicit replay points
      • disabled: it is not permitted for callers to replay the workflow, whether operator-driven or automatic; resumable steps and replay points in the workflow are not externally visible (but may still be used by replays triggered within the workflow)
      • from start: the workflow start is a replay point
      • automatically: indicates that on an unhandled AMP failover (DanglingWorkflowException), the workflow should attempt to replay resuming; implies enabled, can be combined with from start
    • as a step, as workflow replayable <value> (or { type: workflow, replayable: <value> })
      • reset: to invalidate all previous replay points in the workflow
      • from here: this step is a valid replay point; on workflow failure, any “retry replay” or “resumable: automatically” handler will replay from this point if the workflow is non-resumable; operators will have the option to replay from this point
      • from here only: like a reset followed by a from here
    • on a step, as a key
      • from here or from here only: as for workflow replayable
    • on workflow step with sub-steps, as a key
      • any of the key values for defining a workflow, with their semantics applied to the nested workflow(s) only (replayable: disabled is equivalent to idempotent: no and will override an idempotent: yes there)
      • any of the key values for a step, with semantics applied to the containing workflow

Advanced: Retention Settings

Cloudsoft AMP stores details of workflows as part of its persistence while a workflow is executing and for a configurable period of time after completion. This allows workflows to be resumed even in the case of an AMP server restart or failover, and it allows operators to manually explore or replay workflows for as long as they are retained.

If needed, it is possible to specify that a workflow should be kept for a period of time (including forever) or up to a maximum number of invocations. The specification can also refer to the loosest (“max”) or tightest (“min”) of a list of values. This can be set as part of a workflow’s definition, where some workflows are more interesting than others, and/or as part of a workflow step, if the retention period should be changed depending how far the workflow progresses.

Where not explicitly set, a system-wide retention default is used. This can be configured in brooklyn.properties using the key workflow.retention.default. If no supplied, AMP defaults to 3, meaning it will keep the three most recent invocations of a workflow, with no time limit.

Workflows may be kept in-memory for a longer period than persisted to disk, depending on the memory available. This allows, for example, disabled and 0 to be indicated to minimize persistence requirements, while maintaining UI and API access to workflow state “softly”, that is to say if memory permits. The key workflow.retention.default.soft can be configured in brooklyn.properties to override the default limit of such workflows kept in memory, from the default value of 3, or the expression soft <soft_retention_value> can be used as part of a retention expression, typically at the end, to customize it per-workflow. If the soft limit is less than or the same as the standard limit there is no apparent effect, as workflow state can be retrieved from in-memory or on-disk. Active workflows are always kept in memory.

Workflow retention is done on a per-entity basis based by default on a hash of the workflow name. Typically workflow definitions for effectors, sensors, and policies all get unique names for that definition, so the retention applies separately to each of the different defined workflows on an entity. However each definition typically assigns the same name to each instance, so any retention count limit applies to completed runs in that group of workflows. Thus any(2, 24h) on an effector will keep all runs for 24 hours but only the 2 most recent completed invocations for longer, in addition to ongoing instances.

A custom hash can be specified for a workflow to use a key different to the name. This can be used to apply the retention limit to instances across multiple workflow definitions, for instance if only the last 2 of any start, stop, or restart command should be kept, the instruction retention: 2 hash start-stop can be included in the definition for each of the start, stop, and restart workflows. This can also be used to specify that a workflow might go into different retention classes depending where it is in its execution; if workflow failures should be kept for longer, the fail step might say retention: forever hash ${workflow-name} failed, causing the workflow to be retained with a different hash (“ failed") and for it to apply a different period ("forever") when it checks expiry on that hash.

Formally, the syntax for retention is:

  • when defining a workflow, as retention: <value>
  • as a step, as workflow retention <value> (or { type: workflow, retention: <value> })

Permitted <value> expressions in either case are:

  • a number, to indicate how many instances of a workflow should be kept
  • a duration, to indicate for how long workflows should be kept
  • forever, to never expire
  • context, to use the previous retention values (often used together with max)
  • parent, to use the value of any parent workflow or else the system default; this is the default for workflows, they inherit their parent workflow’s retention if it is a nested workflow, otherwise it takes the system default
  • system, to use the system default (from brooklyn.properties)
  • min(<value>, <value>, ...) or max(<value>, <value>, ...) where <value> is any of the expressions on this line or above (but not disabled or hash); in particular a max or a min or vice versa is useful, and also to refer to the parent value
    • min means completed workflow instances must only be retained if they meet all the constraints implied by the <value> arguments, i.e. min(2, 3, 1h, 2h) means only the most recent two instances need to be kept and only if it has been less than an hour since they completed
    • max means completed workflow instances must be retained if they meet any of the constraints implied by the <value> arguments, i.e. max(2, 3, 1h, 2h) means to keep the 3 most recent instances irrespective of when they run, and to keep all instances for up to two hours
  • <value> soft <soft_value> where <soft_value> can be any of the above, to specify an explicit in-memory soft-retention limit, and <value> is any retention expression indicating the normal on-disk retention (where <value> must not indicate an additional soft or hard expression)
  • <value> hard as per soft but indicating that the <value> is both the on-disk and in-memory limit
  • disabled, to prevent persistence of a workflow, causing less work for the system where workflows don’t need to be stored; such workflows will not be replayable by an operator or recoverable on failover; this should not be used with workflows that acquire a lock unless the entity has special handlers to clear locks
  • hash <hash> to change the retention key; useful if some instances of a workflow should be kept for a longer duration than others; unlike most values, this can be a ${...} variable expression; this can optionally be preceded by any of the other expressions listed

Advanced Example

This defines an effector with idempotent workflow that can be replayed from most steps, and from the beginning if failing on a step which isn’t resumable, and details of the last 5 invocations will always be kept, and all invocations in the past 24 hours will be kept:

brooklyn.initializers:
- type: workflow-effector
  retention: max(24h,5)
  replayable: yes
  steps:
    - ...

As a more interesting example, consider provisioning a VM where approval is needed and where unlike the aws case above, tags cannot be used to make the actual call idempotent. The call to the actual provisioner needs to fail hard so an operator can review it, but the rest of the workflow should be as robust as possible. (Of course it is recommended to try to make workflows idempotent, as discussed in this section, but in some cases that may be difficult.) Specifically here, any cancellation or failure prior to sending the request might be uninteresting for operators and fine for a user to replay; however once provisioning begins, all details should be kept, and the provisioning step itself should not be replayable; finally once the machine details are known locally it is no longer important to keep workflow details. In this case the workflow might look like:

type: workflow
retention: max(context,6h)   # keep all for at least 6h, and longer/more if parent or system workflow says so
replayable: from start       # allow replay from the start (until changed)
on-error:
- retry limit 10    # automatically replay on any error (unless no replay points)
  steps:

# get and wait for approval
- http approvals.acme.com/request/infrastructure/vm?<details_omitted>
- let request_id = ${content.request_id}
- id: wait_for_approval
  step: http approvals.acme.com/check?request_id=${request_id}
  # assume returns a map { completed: boolean, approved: boolean, details: string }
- step: retry from wait_for_approval limit 7d backoff 10s increasing 2x up to 5m
  condition:
    target: ${content.completed}
    when: falsy
- step: fail message Provisioning request denied by approvals system: ${content.details}
  # the 'fail' step type is not resumable so replay will not be permitted here,
  # but it would be allowed from the start, so we have to disable it
  replayable: reset
  condition:
    target: ${content.approved}
    not:
      equals: true

# now provision, don't allow replays and keep details for cleanup
- workflow replayable reset
- workflow retention forever
- http cloud.acme.com/provision/vm?<details_omitted>
# once the request is made we can allow replays again
# but continue to keep details for cleanup
- workflow replayable from here
- let provision_id = ${content.provision_id}
- http cloud.acme.com/check?provision_id=${provision_id}
  # assume returns a map with { completed: boolean, id: string, ip_address: string }
- step: retry limit 1h backoff 10s increasing 2x max 1m
  condition:
    target: ${content.completed}
    equals: false
- set-sensor vm_id = ${content.id}
- set-sensor ip_address = ${content.ip_address}

# finally restore default retention per parent or system, as details are now stored on the entity
- workflow retention parent

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