125 lines
5.9 KiB
Plaintext
125 lines
5.9 KiB
Plaintext
---
|
|
layout: docs
|
|
page_title: Anti-Entropy Enforcement
|
|
description: >-
|
|
Anti-entropy keeps distributed systems consistent. Learn how Consul uses an anti-entropy mechanism to periodically sync agent states with the service catalog to prevent either from becoming stale.
|
|
---
|
|
|
|
# Anti-Entropy Enforcement
|
|
|
|
Consul uses an advanced method of maintaining service and health information.
|
|
This page details how services and checks are registered, how the catalog is
|
|
populated, and how health status information is updated as it changes.
|
|
|
|
### Components
|
|
|
|
It is important to first understand the moving pieces involved in services and
|
|
health checks: the [agent](#agent) and the [catalog](#catalog). These are
|
|
described conceptually below to make anti-entropy easier to understand.
|
|
|
|
#### Agent
|
|
|
|
Each Consul agent maintains its own set of service and check registrations as
|
|
well as health information. The agents are responsible for executing their own
|
|
health checks and updating their local state.
|
|
|
|
Services and checks within the context of an agent have a rich set of
|
|
configuration options available. This is because the agent is responsible for
|
|
generating information about its services and their health through the use of
|
|
[health checks](/docs/discovery/checks).
|
|
|
|
#### Catalog
|
|
|
|
Consul's service discovery is backed by a service catalog. This catalog is
|
|
formed by aggregating information submitted by the agents. The catalog maintains
|
|
the high-level view of the cluster, including which services are available,
|
|
which nodes run those services, health information, and more. The catalog is
|
|
used to expose this information via the various interfaces Consul provides,
|
|
including DNS and HTTP.
|
|
|
|
Services and checks within the context of the catalog have a much more limited
|
|
set of fields when compared with the agent. This is because the catalog is only
|
|
responsible for recording and returning information _about_ services, nodes, and
|
|
health.
|
|
|
|
The catalog is maintained only by server nodes. This is because the catalog is
|
|
replicated via the [Raft log](/docs/architecture/consensus) to provide a
|
|
consolidated and consistent view of the cluster.
|
|
|
|
### Anti-Entropy
|
|
|
|
Entropy is the tendency of systems to become increasingly disordered. Consul's
|
|
anti-entropy mechanisms are designed to counter this tendency, to keep the
|
|
state of the cluster ordered even through failures of its components.
|
|
|
|
Consul has a clear separation between the global service catalog and the agent's
|
|
local state as discussed above. The anti-entropy mechanism reconciles these two
|
|
views of the world: anti-entropy is a synchronization of the local agent state and
|
|
the catalog. For example, when a user registers a new service or check with the
|
|
agent, the agent in turn notifies the catalog that this new check exists.
|
|
Similarly, when a check is deleted from the agent, it is consequently removed from
|
|
the catalog as well.
|
|
|
|
Anti-entropy is also used to update availability information. As agents run
|
|
their health checks, their status may change in which case their new status
|
|
is synced to the catalog. Using this information, the catalog can respond
|
|
intelligently to queries about its nodes and services based on their
|
|
availability.
|
|
|
|
During this synchronization, the catalog is also checked for correctness. If
|
|
any services or checks exist in the catalog that the agent is not aware of, they
|
|
will be automatically removed to make the catalog reflect the proper set of
|
|
services and health information for that agent. Consul treats the state of the
|
|
agent as authoritative; if there are any differences between the agent
|
|
and catalog view, the agent-local view will always be used.
|
|
|
|
### Periodic Synchronization
|
|
|
|
In addition to running when changes to the agent occur, anti-entropy is also a
|
|
long-running process which periodically wakes up to sync service and check
|
|
status to the catalog. This ensures that the catalog closely matches the agent's
|
|
true state. This also allows Consul to re-populate the service catalog even in
|
|
the case of complete data loss.
|
|
|
|
To avoid saturation, the amount of time between periodic anti-entropy runs will
|
|
vary based on cluster size. The table below defines the relationship between
|
|
cluster size and sync interval:
|
|
|
|
| Cluster Size | Periodic Sync Interval |
|
|
| ------------ | ---------------------- |
|
|
| 1 - 128 | 1 minute |
|
|
| 129 - 256 | 2 minutes |
|
|
| 257 - 512 | 3 minutes |
|
|
| 513 - 1024 | 4 minutes |
|
|
| ... | ... |
|
|
|
|
The intervals above are approximate. Each Consul agent will choose a randomly
|
|
staggered start time within the interval window to avoid a thundering herd.
|
|
|
|
### Best-effort sync
|
|
|
|
Anti-entropy can fail in a number of cases, including misconfiguration of the
|
|
agent or its operating environment, I/O problems (full disk, filesystem
|
|
permission, etc.), networking problems (agent cannot communicate with server),
|
|
among others. Because of this, the agent attempts to sync in best-effort
|
|
fashion.
|
|
|
|
If an error is encountered during an anti-entropy run, the error is logged and
|
|
the agent continues to run. The anti-entropy mechanism is run periodically to
|
|
automatically recover from these types of transient failures.
|
|
|
|
### Enable Tag Override
|
|
|
|
Synchronization of service registration can be partially modified to
|
|
allow external agents to change the tags for a service. This can be
|
|
useful in situations where an external monitoring service needs to be
|
|
the source of truth for tag information. For example, the Redis
|
|
database and its monitoring service Redis Sentinel have this kind of
|
|
relationship. Redis instances are responsible for much of their
|
|
configuration, but Sentinels determine whether the Redis instance is a
|
|
primary or a secondary. Using the Consul service configuration item
|
|
[enable_tag_override](/docs/discovery/services) you can instruct the
|
|
Consul agent on which the Redis database is running to NOT update the
|
|
tags during anti-entropy synchronization. For more information see
|
|
[Services](/docs/discovery/services#enable-tag-override-and-anti-entropy) page.
|