139 lines
7.5 KiB
Markdown
139 lines
7.5 KiB
Markdown
---
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layout: "docs"
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page_title: "Sessions"
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sidebar_current: "docs-internals-sessions"
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description: |-
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Consul provides a session mechanism which can be used to build distributed locks. Sessions act as a binding layer between nodes, health checks, and key/value data. They are designed to provide granular locking, and are heavily inspired by The Chubby Lock Service for Loosely-Coupled Distributed Systems.
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---
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# Sessions
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Consul provides a session mechanism which can be used to build distributed locks.
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Sessions act as a binding layer between nodes, health checks, and key/value data.
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They are designed to provide granular locking, and are heavily inspired
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by [The Chubby Lock Service for Loosely-Coupled Distributed Systems](http://research.google.com/archive/chubby.html).
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~> **Advanced Topic!** This page covers technical details of
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the internals of Consul. You don't need to know these details to effectively
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operate and use Consul. These details are documented here for those who wish
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to learn about them without having to go spelunking through the source code.
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## Session Design
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A session in Consul represents a contract that has very specific semantics.
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When a session is constructed a node name, a list of health checks, behavior,
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TTL and a `lock-delay` may be provided. The newly constructed session is provided with
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a named ID which can be used to refer to it. This ID can be used with the KV
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store to acquire locks, which are advisory mechanisms for mutual exclusion.
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Below is a diagram showing the relationship between these components:
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<div class="center">
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</div>
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The contract that Consul provides is that under any of the following
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situations the session will be *invalidated*:
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* Node is deregistered
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* Any of the health checks are deregistered
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* Any of the health checks go to the critical state
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* Session is explicitly destroyed
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* TTL expires, if applicable
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When a session is invalidated, it is destroyed and can no longer
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be used. What happens to the associated locks depends on the
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behavior specified at creation time. Consul supports a `release`
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and `delete` behavior. The `release` behavior is the default
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if not specified.
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If the `release` behavior is being used, any of the locks held in
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association with the session are released, and the `ModifyIndex` of the key is incremented.
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Alternatively, if the `delete` behavior is used, the key corresponding
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to any of the held locks is simply deleted. This can be used to create
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ephemeral entries that are automatically deleted by Consul.
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While this is a simple design, it enables a multitude of usage
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patterns. By default, the [gossip based failure detector](/docs/internals/gossip.html)
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is used as the associated health check. This failure detector allows
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Consul to detect when a node that is holding a lock has failed, and
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to automatically release the lock. This ability provides **liveness** to
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Consul locks, meaning under failure the system can continue to make
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progress. However, because there is no perfect failure detector, it's possible
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to have a false positive (failure detected) which causes the lock to
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be released even though the lock owner is still alive. This means
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we are sacrificing some **safety**.
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Conversely, it is possible to create a session with no associated
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health checks. This removes the possibility of a false positive,
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and trades liveness for safety. You can be absolutely certain Consul
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will not release the lock even if the existing owner has failed.
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Since Consul APIs allow a session to be force destroyed, this allows
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systems to be built that require an operator to intervene in the
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case of a failure, but preclude the possibility of a split-brain.
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A third health checking mechanism is session TTLs. When creating
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a session a TTL can be specified. If the TTL interval expires without
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being renewed, the session has expired and an invalidation is triggered.
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This type of failure detector is also known as a heartbeat failure detector.
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It is less scalable than the gossip based failure detector as it places
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an increased burden on the servers, but may be applicable in some cases.
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The contract of a TTL is that it represents a lower bound for invalidation,
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meaning Consul will not expire the session before the TTL is reached, but it
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is allowed to delay the expiration past the TTL. The TTL is renewed on
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session creation, on session renew, and on leader failover. When a TTL
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is being used, clients should be aware of clock skew issues, namely that
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time may not progress at the same rate on the client as on the Consul servers.
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It is best to set conservative TTL values, and to renew in advance of the TTL
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to account for network delay and time skew.
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The final nuance is that sessions may provide a `lock-delay`. This
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is a time duration, between 0 and 60 seconds. When a session invalidation
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takes place, Consul prevents any of the previously held locks from
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being re-acquired for the `lock-delay` interval; this is a safeguard
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inspired by Google's Chubby. The purpose of this delay is to allow
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the potentially still live leader to detect the invalidation and stop
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processing requests that may lead to inconsistent state. While not a
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bulletproof method, it does avoid the need to introduce sleep states
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into application logic, and can help mitigate many issues. While the
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default is to use a 15 second delay, clients are able to disable this
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mechanism by providing a zero delay value.
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## KV Integration
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Integration between the Key/Value store and sessions are the primary
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place where sessions are used. A session must be created prior to use,
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and is then referred to by its ID.
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The Key/Value API is extended to support an `acquire` and `release` operation.
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The `acquire` operation acts like a Check-And-Set operation, except it
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can only succeed if there is no existing lock holder. On success, there
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is a normal key update, but there is also an increment to the `LockIndex`,
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and the `Session` value is updated to reflect the session holding the lock.
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Once held, the lock can be released using a corresponding `release` operation,
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providing the same session. Again, this acts like a Check-And-Set operations,
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since the request will fail if given an invalid session. A critical note is
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that the lock can be released without being the creator of the session.
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This is by design, as it allows operators to intervene and force terminate
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a session if necessary. As mentioned above, a session invalidation will also
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cause all held locks to be released or deleted. When a lock is released, the `LockIndex`
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does not change, however the `Session` is cleared and the `ModifyIndex` increments.
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These semantics (heavily borrowed from Chubby), allow the tuple of (Key, LockIndex, Session)
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to act as a unique "sequencer". This `sequencer` can be passed around and used
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to verify if the request belongs to the current lock holder. Because the `LockIndex`
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is incremented on each `acquire`, even if the same session re-acquires a lock,
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the `sequencer` will be able to detect a stale request. Similarly, if a session is
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invalided, the Session corresponding to the given `LockIndex` will be blank.
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To be clear, this locking system is purely *advisory*. There is no enforcement
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that clients must acquire a lock to perform any operation. Any client can
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read, write, and delete a key without owning the corresponding lock. It is not
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the goal of Consul to protect against misbehaving clients.
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## Leader Election
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The primitives provided by sessions and the locking mechanisms of the KV
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store can be used to build client-side leader election algorithms.
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These are covered in more detail in the [Leader Election guide](/docs/guides/leader-election.html).
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