mirror of https://github.com/status-im/consul.git
115 lines
5.8 KiB
Markdown
115 lines
5.8 KiB
Markdown
---
|
|
layout: "docs"
|
|
page_title: "Sessions"
|
|
sidebar_current: "docs-internals-sessions"
|
|
---
|
|
|
|
# Sessions
|
|
|
|
Consul provides a session mechansim 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](http://research.google.com/archive/chubby.html).
|
|
|
|
<div class="alert alert-block alert-warning">
|
|
<strong>Advanced Topic!</strong> This page covers technical details of
|
|
the internals of Consul. You don't need to know these details to effectively
|
|
operate and use Consul. These details are documented here for those who wish
|
|
to learn about them without having to go spelunking through the source code.
|
|
</div>
|
|
|
|
## Session Design
|
|
|
|
A session in Consul represents a contract that has very specific semantics.
|
|
When a session is constructed a node name, a list of health checks, and a
|
|
`lock-delay` are provided. The newly constructed session is provided with
|
|
a named ID which can be used to refer to it. This ID can be used with the KV
|
|
store to acquire locks, which are advisory mechanisms for mutual exclusion.
|
|
Below is a diagram showing the relationship between these components:
|
|
|
|
|
|
|
|
The contract that Consul provides is that under any of the folllowing
|
|
situations the session will be *invalidated*:
|
|
|
|
* Node is deregistered
|
|
* Any of the health checks are deregistered
|
|
* Any of the health checks go to the critical state
|
|
* Session is explicitly destroyed
|
|
|
|
When a session is invalidated, any of the locks held in association
|
|
with the session are released, and the `ModifyIndex` of the key is
|
|
incremented. The session is also destroyed during an invalidation
|
|
and can no longer be used to acquire further locks.
|
|
|
|
While this is a simple design, it enables a multitude of usage
|
|
patterns. By default, the [gossip based failure detector](/docs/internals/gossip.html)
|
|
is used as the associated health check. This failure detector allows
|
|
Consul to detect when a node that is holding a lock has failed, and
|
|
to automatically release the lock. This ability provides **liveness** to
|
|
Consul locks, meaning under failure the system can continue to make
|
|
progress. However, because there is no perfect failure detector, it's possible
|
|
to have a false positive (failure detected) which causes the lock to
|
|
be released even though the lock owner is still alive. This means
|
|
we are sacrificing some **safety**.
|
|
|
|
Conversely, it is possible to create a session with no associated
|
|
health checks. This removes the possibility of a false positive,
|
|
and trades liveness for safety. You can be absolutely certain Consul
|
|
will not release the lock even if the existing owner has failed.
|
|
Since Consul APIs allow a session to be force destroyed, this allows
|
|
systems to be built that require an operator to intervene in the
|
|
case of a failure, but preclude the possibility of a split-brain.
|
|
|
|
The final nuance is that sessions may provide a `lock-delay`. This
|
|
is a time duration, between 0 and 60 second. When a session invalidation
|
|
takes place, Consul prevents any of the previously held locks from
|
|
being re-acquired for the `lock-delay` interval; this is a safe guard
|
|
inspired by Google's Chubby. The purpose of this delay is to allow
|
|
the potentially still live leader to detect the invalidation and stop
|
|
processing requests that may lead to inconsistent state. While not a
|
|
bulletproof method, it does avoid the need to introduce sleep states
|
|
into application logic, and can help mitigate many issues. While the
|
|
default is to use a 15 second delay, clients are able to disable this
|
|
mechanism by providing a zero delay value.
|
|
|
|
## KV Integration
|
|
|
|
Integration between the Key/Value store and sessions are the primary
|
|
place where sessions are used. A session must be created prior to use,
|
|
and is then refered to by it's ID.
|
|
|
|
The Key/Value API is extended to support an `acquire` and `release` operation.
|
|
The `acquire` operation acts like a Check-And-Set operation, except it
|
|
can only succeed if there is no existing lock holder. On success, there
|
|
is a normal key update, but there is also an increment to the `LockIndex`,
|
|
and the `Session` value is updated to reflect the session holding the lock.
|
|
|
|
Once held, the lock can be released using a corresponding `release` operation,
|
|
providing the same session. Again, this acts like a Check-And-Set operations,
|
|
since the request will fail if given an invalid session. A critical note is
|
|
that the lock can be released without being the creator of the session.
|
|
This is by design, as it allows operators to intervene and force terminate
|
|
a session if necessary. As mentioned above, a session invalidation will also
|
|
cause all held locks to be released. When a lock is released, the `LockIndex`,
|
|
does not change, however the `Session` is cleared and the `ModifyIndex` increments.
|
|
|
|
These semantics (heavily borrowed from Chubby), allow the tuple of (Key, LockIndex, Session)
|
|
to act as a unique "sequencer". This `sequencer` can be passed around and used
|
|
to verify if the request belongs to the current lock holder. Because the `LockIndex`
|
|
is incremented on each `acquire`, even if the same session re-acquires a lock,
|
|
the `sequencer` will be able to detect a stale request. Similarly, if a session is
|
|
invalided, the Session corresponding to the given `LockIndex` will be blank.
|
|
|
|
To make clear, this locking system is purely *advisory*. There is no enforcement
|
|
that clients must acquire a lock to perform any operation. Any client can
|
|
read, write, and delete a key without owning the corresponding lock. It is not
|
|
the goal of Consul to protect against misbehaving clients.
|
|
|
|
## Leader Election
|
|
|
|
The primitives provided by sessions and the locking mechanisms of the KV
|
|
store can be used to build client-side leader election algorithms.
|
|
These are covered in more detail in the [Leader Election guide](/docs/guides/leader-election.html).
|
|
|