cloudsoft.io

Persistence

By default AMP persists its state to storage so that a server can be restarted without loss or so a high availability standby server can take over.

AMP can persist its state to one of two places: the file system, or to an object store of your choice.

Configuration

To configure persistence, edit the file org.apache.brooklyn.osgilauncher.cfg directory of your AMP instance. The following options are available:

persistMode - This is the mode in which persistence is running, in and is set to AUTO by default. The possible values are:

  • AUTO - will rebind to any existing state, or start up fresh if no state;
  • DISABLED - will not read or persist any state;
  • REBIND - will rebind to the existing state, or fail if no state available;
  • CLEAN - will start up fresh (removing any existing state)

persistenceDir - This is the directory to which AMP reads and writes its persistence data. The default location depends on your installation method. Checkout this page for more information.

persistenceLocation - This is the location for an object store to read and write persisted state.

persistPeriod - This is an interval period which can be set to reduce the frequency with which persistence is carried out, for example 1s.

File-based Persistence

AMP starts with file-based persistence by default, saving data in the persisted state folder. For the rest of this document we will refer to this location as %persistence-home%.

If there is already data at %persistence-home%/data, then a backup of the directory will be made. This will have a name like %persistence-home%/backups/%date%-%time%-jvyX7Wis-promotion-igFH. This means backups of the data directory will be automatically created each time AMP is restarted (or if a standby AMP instances takes over as master).

The state is written to the given path. The file structure under that path is:

  • ./catalog/
  • ./enrichers/
  • ./entities/
  • ./feeds/
  • ./locations/
  • ./nodes/
  • ./plane/
  • ./policies/

In each of those directories, an XML file will be created per item - for example a file per entity in ./entities/. This file will capture all of the state - for example, an entity’s: id; display name; type; config; attributes; tags; relationships to locations, child entities, group membership, policies and enrichers; and dynamically added effectors and sensors.

Object Store Persistence

AMP can persist its state to any Object Store API supported by Apache jclouds including S3, Swift and Azure. This gives access to any compatible Object Store product or cloud provider including AWS-S3, SoftLayer, Rackspace, HP and Microsoft Azure. For a complete list of supported providers, see jclouds.

To configure the Object Store, add the credentials to ~/.brooklyn/brooklyn.properties such as:

brooklyn.location.named.aws-s3-eu-west-1=aws-s3:eu-west-1
brooklyn.location.named.aws-s3-eu-west-1.identity=ABCDEFGHIJKLMNOPQRSTU
brooklyn.location.named.aws-s3-eu-west-1.credential=abcdefghijklmnopqrstuvwxyz1234567890ab/c

or:

brooklyn.location.named.softlayer-swift-ams01=jclouds:openstack-swift:https://ams01.objectstorage.softlayer.net/auth/v1.0
brooklyn.location.named.softlayer-swift-ams01.identity=ABCDEFGHIJKLM:myname
brooklyn.location.named.softlayer-swift-ams01.credential=abcdefghijklmnopqrstuvwxyz1234567890abcdefghijklmnopqrstuvwxyz12
brooklyn.location.named.softlayer-swift-ams01.jclouds.keystone.credential-type=tempAuthCredentials

Then edit the persistenceLocation to point at this object store: softlayer-swift-ams01.

Rebinding to State

When AMP starts up pointing at existing state, it will recreate the entities, locations and policies based on that persisted state.

Once all have been created, AMP will “manage” the entities. This will bind to the underlying entities under management to update the each entity’s sensors (e.g. to poll over HTTP or JMX). This new state will be reported in the web-console and can also trigger any registered policies.

Handling Rebind Failures

If rebind fails fail for any reason, details of the underlying failures will be reported in the amp.debug.log. This will include the entities, locations or policies which caused an issue, and in what way it failed. There are several approaches to resolving problems.

1) Determine Underlying Cause

Go through the log and identify the likely areas in the code from the error message.

2) Seek Help

Help can be found at Cloudsoft support, where folk will be able to investigate issues and suggest workarounds.

By sharing the persisted state (with credentials removed), Cloudsoft support will be able to reproduce and debug the problem.

3) Fix-up the State

The state of each entity, location, policy and enricher is persisted in XML. It is thus human readable and editable.

After first taking a backup of the state, it is possible to modify the state. For example, an offending entity could be removed, or references to that entity removed, or its XML could be fixed to remove the problem.

4) Fixing with Groovy Scripts

The final (powerful and dangerous!) tool is to execute Groovy code on the running AMP instance. If authorized, the REST API allows arbitrary Groovy scripts to be passed in and executed. This allows the state of entities to be modified (and thus fixed) at runtime.

If used, it is strongly recommended that Groovy scripts are run against a disconnected AMP instance. After fixing the entities, locations and/or policies, the AMP instance’s new persisted state can be copied and used to fix the production instance.

Writing Persistable Code

The most common problem on rebind is that custom entity code has not been written in a way that can be persisted and/or rebound.

The rule of thumb when implementing new entities, locations, policies and enrichers is that all state must be persistable. All state must be stored as config or as attributes, and must be serializable. For making backwards compatibility simpler, the persisted state should be clean.

Below are tips and best practices for when implementing an entity in Java (or any other JVM language).

How to store entity state:

  • Config keys and values are persisted.
  • Store an entity’s runtime state as attributes.
  • Don’t store state in arbitrary fields - the field will not be persisted (this is a design decision, because AMP cannot intercept the field being written to, so cannot know when to persist).
  • Don’t just modify the retrieved attribute value (e.g. getAttribute(MY_LIST).add("a") is bad). The value may not be persisted unless setAttribute() is called.
  • For special cases, it is possible to call entity.requestPerist() which will trigger asynchronous persistence of the entity.
  • Overriding (and customizing) of getRebindSupport() is discouraged - this will change in a future version.

How to store policy/enricher/location state:

  • Store values as config keys where applicable.
  • Unfortunately these (currently) do not have attributes. Normally the state of a policy or enricher is transient - on rebind it starts afresh, for example with monitoring the performance or health metrics rather than relying on the persisted values.
  • For special cases, you can annotate a field with @SetFromFlag for it be persisted. When you call requestPersist() then values of these fields will be scheduled to be persisted. Warning: the @SetFromFlag functionality may change in future versions.

Persistable state:

  • Ensure values can be serialized. This (currently) uses xstream, which means it does not need to implement Serializable.
  • Always use static (or top-level) classes. Otherwise it will try to also persist the outer instance!
  • Any reference to an entity or location will be automatically swapped out for marker, and re-injected with the new entity/location instance on rebind. The same applies for policies, enrichers, feeds, catalog items and ManagementContext.

Behaviour on rebind:

  • By extending SoftwareProcess, entities get a lot of the rebind logic for free. For example, the default rebind() method will call connectSensors(). See SoftwareProcess Lifecycle for more details.
  • If necessary, implement rebind. The entity.rebind() is called automatically by the AMP framework on rebind, after configuring the entity’s config/attributes but before the entity is managed. Note that init() will not be called on rebind.
  • Feeds will be persisted if and only if entity.addFeed(...) was called. Otherwise the feed needs to be re-registered on rebind. Warning: this behaviour may change in future version.
  • All functions/predicates used with persisted feeds must themselves be persistable - use of anonymous inner classes is strongly discouraged.
  • Subscriptions (e.g. from calls to subscribe(...) for sensor events) are not persisted. They must be re-registered on rebind. Warning: this behaviour may change in future version.

Below are tips to make backwards-compatibility easier for persisted state:

  • Never use anonymous inner classes - even in static contexts. The auto-generated class names are brittle, making backwards compatibility harder.
  • Always use sensible field names (and use transient whenever you don’t want it persisted). The field names are part of the persisted state.
  • Consider using Value Objects for persisted values. This can give clearer separation of responsibilities in your code, and clearer control of what fields are being persisted.
  • Consider writing transformers to handle backwards-incompatible code changes. AMP supports applying transformations to the persisted state, which can be done as part of an upgrade process.

Persisted State Backup

File system backup

When using the file system it is important to ensure it is backed up regularly.

One could use rsync to regularly backup the contents to another server.

It is also recommended to periodically create a complete archive of the state. A simple mechanism is to run a CRON job periodically (e.g. every 30 minutes) that creates an archive of the persistence directory, and uploads that to a backup facility (e.g. to S3).

Optionally, to avoid excessive load on the AMP server, the archive-generation could be done on another “data” server. This could get a copy of the data via an rsync job.

An example script to be invoked by CRON is shown below:

DATE=`date "+%Y%m%d.%H%M.%S"`
BACKUP_FILENAME=/path/to/archives/back-${DATE}.tar.gz
DATA_DIR=/path/to/base/dir/data

tar --exclude '*/backups/*' -czvf $BACKUP_FILENAME $DATA_DIR
# For s3cmd installation see http://s3tools.org/repositories
s3cmd put $BACKUP_FILENAME s3://mybackupbucket
rm $BACKUP_FILENAME

Object store backup

Object Stores will normally handle replication. However, many such object stores do not handle versioning (i.e. to allow access to an old version, if an object has been incorrectly changed or deleted).

The state can be downloaded periodically from the object store, archived and backed up.

An example script to be invoked by CRON is shown below:

DATE=`date "+%Y%m%d.%H%M.%S"`
BACKUP_FILENAME=/path/to/archives/back-${DATE}.zip
HOSTNAME=localhost
USERNAME=admin
PASSWORD=pa55wOrd

curl -v -u "${USERNAME}:${PASSWORD}" \
        https://${HOSTNAME}:8443/v1/server/ha/persist/export \
        > ${BACKUP_FILENAME}

# For s3cmd installation see http://s3tools.org/repositories
s3cmd put $BACKUP_FILENAME s3://mybackupbucket
rm $BACKUP_FILENAME

PERSISTED STATE IMPORT/EXPORT

Rest API offers capability to export the current persistence state to an archive, which then can be restored by import.

  • /ha/persist/export: (GET) Captures the current state of persistence and saves it as an archive
  • /ha/persist/import: (POST) Takes a previously exported persistence state archive and restores it into the current persistence, by merging the states, adding the catalog bundles, types, locations and deployed applications under the current management plane.

Export/Import functionality is also available from the AMP UI, using the respective options in the About page.

AMP and Maeztro are developed and supported by Cloudsoft Corporation.