Basic design doc for Bank Plugin lease
design document for Bank Plugin lease Closes-Bug: #1529199 Change-Id: Iaaeb7d50e998f68ba53414932d72cd3a2dbb339c
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This work is licensed under a Creative Commons Attribution 3.0 Unported
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License.
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http://creativecommons.org/licenses/by/3.0/legalcode
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=================
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Bank Plugin Basic
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=================
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Bank Plugin is a component of smaug (an openstack project working as a service for data protection),
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which is responsible for execute CRUD actions in Bank.
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The bank is a backend (such as swift) which is used to store the metadata/data of protection plan.
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Here, we take swift as an bank implementation example.
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*******
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leases
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*******
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Smuag will create a checkpoint when protecting a protection plan. This checkpoint is maintained with
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status, which is a enum type: protecting, available, restoring, deleted, etc.
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The status is used for smaug API layer to control access to one checkpoint from users.
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With the 'protecting' status, there're two cases which we can't tell the difference:
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1. The protection service is working and those 'protecting' protection plan are being executed;
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2. When the Protection Service crashes, those 'protecting' protection plan are actually zombie ones,
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and those checkpoints are zombie ones too;
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In the second case, we need a garbage collection component (GC) to cleanup those zombie checkpoints.
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In order to tell whether the checkpoint is a zombie or not, we introduce a lease mechanism based on
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bank plugin.
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Here, we take swift as an example. The lease is stored as an object in swift with the
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characteristics of auto-deleted.
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The owner of one checkpoint will periodically refresh the expire time of the lease object key.
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When the protection service crashes, the leases of bank plugins will be auto-deleted by the
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swift-object-expirer(one service of swift).
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When GC comes to check whether one checkpoint is a zombie to be collected, GC will first get the
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owner of the checkpoint. Then it will check whether the lease of the owner exists.
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If the lease exists, those 'protecting' checkpoints can not be deleted by the GC; otherwise the GC
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will cleanup them.
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Granularity
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=================
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To avoid flood to bank server, we don't keep one lease for per checkpoint. Instead, we keep one
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lease per checkpoint owner. So the granularity of lease is per bank plugin instance.
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When one protection service instance gets initialized, each bank plugin instance will get
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initialized as well. Each bank plugin will start to maintain its own leases with its corresponding
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bank server.
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Here, every bank plugin will play a role as lease client while the bank server (swift cluster) plays
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as the lease server.
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Functions
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===============
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acquire_lease
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-------------
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Each bank plugin (lease client) will use this function to acquire a lease from bank server (lease
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server).
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For swift specifically, it will create a lease object in swift container and set an expire_window
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for this lease.
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The expire_window represents the validity of this lease from creation(or latest-renew) until being
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auto-deleted by swift server. The value of expire_window should be configurable.
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We use owner_id to identify one instance of bank plugin. The owner_id is a uuid created when bank
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plugin instance is initiated, say, generated from sha256 with parameter as hostname and the
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timestamp instance initiated.
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The key of lease object stored in swift looks like this: /account/leases/owner_id.
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In order to map one checkpoint to its owner, we will create an index like this:
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/account/checkpoints/checkpoint_id/owner when creating a checkpoint.
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- create_owner_id: create a uuid to represent this bank plugin instance
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- put_object: use swift-client to create a lease object in swift, and set 'X-Delete-After' as:
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expire_window
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- set_expire_time in memory in lease client side: set the expire_time as: now+expired_time
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renew_lease
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-----------------
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This function will be called by each lease client in the background periodically.
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The renew_window represents the period with which the lease client will refresh lease frequently.
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This renew_window is configurable as well, where renew_window < expire_window.
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If lease client succeeds to renew lease, this lease has a new expire_window in lease server from now
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on. Then the lease client side will update the expire_time in memory with value as: expire_time =
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now + expired_window.
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If lease client fails to renew, this lease object keeps the old expire_window in lease server side.
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The lease client won't update its expire_time in memory.
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- post_object: use swift-client to reset the 'X-Delete-After' header as: expired_window
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- update_expire_time: if post_object succeeds, update expire_time as: now+expired_window; otherwise,
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don't refresh the expire_time.
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check_lease_validity
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--------------------
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This function is used by the checkpoint owner to check whether there is enough time to execute an
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update operation to one checkpoint (or anything else garded by the lease) before the lease expiring.
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We use validity_window to represent the time window inside which an update operation to a checkpoint
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should complete. This window is configurable and should be estimated by admin.
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This function will check if validity_window <= expire_time - now. If it's true, this function will
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return true and thus allow update operation to go ahead; otherwise, this function will return false
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and the update operation will abort.
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Although the lease may haven't expired when validity_window <= expire_time - now, there might not be
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enough time to finish the update operation. If we allow the update operation to go ahead under this
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situation, there is a risk that while the operation is still on-going, the lease has been recycled
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by lease server during this period.
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check_lease_existence
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---------------------
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This function is used by GC to check whether the lease object exists or not in lease server side.
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Specifically for checkpoints, GC will scan all checkpoints in 'protecting' status. It will first get
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the owner of a checkpoint through its index, and then check the existence of the lease object in
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lease server. If the lease object doesn't exist, it will take this checkpoint as zombie and go
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ahead to recycle it. Otherwise, it will skip this checkpoint and leave it there.
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Configurations
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==============
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renew_window
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------------
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- represents the period with which lease client will renew the lease in background.
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expire_window
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--------------
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- represents how long this lease from creation or latest-renew to expire in lease server side.
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- Note: expired_window > renew_window. To make renew mechanism more robust, we recommend to set
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expired_window = N*renew_window. With this setting, we allow (N-1) times failure to renew lease
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to tolerate unstable network case or IO scheduling issue;
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validity_window
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---------------
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- an optional configuration; The default value it set according to the renew_window, validity_window
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<= renew_window
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- the window estimated by admin, how long one update operation will take at most. The constraint
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here should be: validity_window < expire_window.
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- Note: Same background as renew_window setting, to allow (N-1) times failure of renew lease, we
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recommend to set validity_window <= renew_window.
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@ -33,6 +33,7 @@ Spec Template
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skeleton
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template
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bank-plugin-lease
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Indices and tables
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