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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.

Advanced Topic! 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.

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 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.