326 lines
16 KiB
Markdown
326 lines
16 KiB
Markdown
---
|
|
layout: api
|
|
page_title: HTTP API
|
|
sidebar_current: api-overview
|
|
description: |-
|
|
Consul exposes a RESTful HTTP API to control almost every aspect of the
|
|
Consul agent.
|
|
---
|
|
|
|
# HTTP API
|
|
|
|
The main interface to Consul is a RESTful HTTP API. The API can perform basic
|
|
CRUD operations on nodes, services, checks, configuration, and more.
|
|
|
|
## Version Prefix
|
|
|
|
All API routes are prefixed with `/v1/`. This documentation is only for the v1 API.
|
|
|
|
## ACLs
|
|
|
|
Several endpoints in Consul use or require ACL tokens to operate. An agent
|
|
can be configured to use a default token in requests using the `acl_token`
|
|
configuration option. However, the token can also be specified per-request
|
|
by using the `X-Consul-Token` request header or Bearer header in Authorization
|
|
header or the `token` query string parameter. The request header takes
|
|
precedence over the default token, and the query string parameter takes
|
|
precedence over everything.
|
|
|
|
For more details about ACLs, please see the [ACL Guide](/docs/guides/acl.html).
|
|
|
|
## Authentication
|
|
|
|
When authentication is enabled, a Consul token should be provided to API
|
|
requests using the `X-Consul-Token` header. This reduces the probability of the
|
|
token accidentally getting logged or exposed. When using authentication,
|
|
clients should communicate via TLS.
|
|
|
|
Here is an example using `curl`:
|
|
|
|
```text
|
|
$ curl \
|
|
--header "X-Consul-Token: abcd1234" \
|
|
http://127.0.0.1:8500/v1/agent/members
|
|
```
|
|
|
|
Previously this was provided via a `?token=` query parameter. This functionality
|
|
exists on many endpoints for backwards compatibility, but its use is **highly
|
|
discouraged**, since it can show up in access logs as part of the URL.
|
|
|
|
## Blocking Queries
|
|
|
|
Many endpoints in Consul support a feature known as "blocking queries". A
|
|
blocking query is used to wait for a potential change using long polling. Not
|
|
all endpoints support blocking, but each endpoint uniquely documents its support
|
|
for blocking queries in the documentation.
|
|
|
|
Endpoints that support blocking queries return an HTTP header named
|
|
`X-Consul-Index`. This is a unique identifier representing the current state of
|
|
the requested resource.
|
|
|
|
On subsequent requests for this resource, the client can set the `index` query
|
|
string parameter to the value of `X-Consul-Index`, indicating that the client
|
|
wishes to wait for any changes subsequent to that index.
|
|
|
|
When this is provided, the HTTP request will "hang" until a change in the system
|
|
occurs, or the maximum timeout is reached. A critical note is that the return of
|
|
a blocking request is **no guarantee** of a change. It is possible that the
|
|
timeout was reached or that there was an idempotent write that does not affect
|
|
the result of the query.
|
|
|
|
In addition to `index`, endpoints that support blocking will also honor a `wait`
|
|
parameter specifying a maximum duration for the blocking request. This is
|
|
limited to 10 minutes. If not set, the wait time defaults to 5 minutes. This
|
|
value can be specified in the form of "10s" or "5m" (i.e., 10 seconds or 5
|
|
minutes, respectively). A small random amount of additional wait time is added
|
|
to the supplied maximum `wait` time to spread out the wake up time of any
|
|
concurrent requests. This adds up to `wait / 16` additional time to the maximum
|
|
duration.
|
|
|
|
### Implementation Details
|
|
|
|
While the mechanism is relatively simple to work with, there are a few edge
|
|
cases that must be handled correctly.
|
|
|
|
* **Reset the index if it goes backwards**. While indexes in general are
|
|
monotonically increasing(i.e. they should only ever increase as time passes),
|
|
there are several real-world scenarios in
|
|
which they can go backwards for a given query. Implementations must check
|
|
to see if a returned index is lower than the previous value,
|
|
and if it is, should reset index to `0` - effectively restarting their blocking loop.
|
|
Failure to do so may cause the client to miss future updates for an unbounded
|
|
time, or to use an invalid index value that causes no blocking and increases
|
|
load on the servers. Cases where this can occur include:
|
|
* If a raft snapshot is restored on the servers with older version of the data.
|
|
* KV list operations where an item with the highest index is removed.
|
|
* A Consul upgrade changes the way watches work to optimize them with more
|
|
granular indexes.
|
|
|
|
* **Sanity check index is greater than zero**. After the initial request (or a
|
|
reset as above) the `X-Consul-Index` returned _should_ always be greater than zero. It
|
|
is a bug in Consul if it is not, however this has happened a few times and can
|
|
still be triggered on some older Consul versions. It's especially bad because it
|
|
causes blocking clients that are not aware to enter a busy loop, using excessive
|
|
client CPU and causing high load on servers. It is _always_ safe to use an
|
|
index of `1` to wait for updates when the data being requested doesn't exist
|
|
yet, so clients _should_ sanity check that their index is at least 1 after
|
|
each blocking response is handled to be sure they actually block on the next
|
|
request.
|
|
|
|
* **Rate limit**. The blocking query mechanism is reasonably efficient when updates
|
|
are relatively rare (order of tens of seconds to minutes between updates). In cases
|
|
where a result gets updated very fast however - possibly during an outage or incident
|
|
with a badly behaved client - blocking query loops degrade into busy loops that
|
|
consume excessive client CPU and cause high server load. While it's possible to just add a sleep
|
|
to every iteration of the loop, this is **not** recommended since it causes update
|
|
delivery to be delayed in the happy case, and it can exacerbate the problem since
|
|
it increases the chance that the index has changed on the next request. Clients
|
|
_should_ instead rate limit the loop so that in the happy case they proceed without
|
|
waiting, but when values start to churn quickly they degrade into polling at a
|
|
reasonable rate (say every 15 seconds). Ideally this is done with an algorithm that
|
|
allows a couple of quick successive deliveries before it starts to limit rate - a
|
|
[token bucket](https://en.wikipedia.org/wiki/Token_bucket) with burst of 2 is a simple
|
|
way to achieve this.
|
|
|
|
### Hash-based Blocking Queries
|
|
|
|
A limited number of agent endpoints also support blocking however because the
|
|
state is local to the agent and not managed with a consistent raft index, their
|
|
blocking mechanism is different.
|
|
|
|
Since there is no monotonically increasing index, each response instead contains
|
|
a header `X-Consul-ContentHash` which is an opaque hash digest generated by
|
|
hashing over all fields in the response that are relevant.
|
|
|
|
Subsequent requests may be sent with a query parameter `hash=<value>` where
|
|
`value` is the last hash header value seen, and this will block until the `wait`
|
|
timeout is passed or until the local agent's state changes in such a way that
|
|
the hash would be different.
|
|
|
|
Other than the different header and query parameter names, the biggest
|
|
difference is that hash values are opaque and can't be compared to see if one
|
|
result is older or newer than another. In general hash-based blocking will not
|
|
return too early due to an idempotent update since the hash will remain the same
|
|
unless the result actually changes, however as with index-based blocking there
|
|
is no strict guarantee that clients will never observe the same result delivered
|
|
before the full timeout has elapsed.
|
|
|
|
## Consistency Modes
|
|
|
|
Most of the read query endpoints support multiple levels of consistency. Since
|
|
no policy will suit all clients' needs, these consistency modes allow the user
|
|
to have the ultimate say in how to balance the trade-offs inherent in a
|
|
distributed system.
|
|
|
|
The three read modes are:
|
|
|
|
- `default` - If not specified, the default is strongly consistent in almost all
|
|
cases. However, there is a small window in which a new leader may be elected
|
|
during which the old leader may service stale values. The trade-off is fast
|
|
reads but potentially stale values. The condition resulting in stale reads is
|
|
hard to trigger, and most clients should not need to worry about this case.
|
|
Also, note that this race condition only applies to reads, not writes.
|
|
|
|
- `consistent` - This mode is strongly consistent without caveats. It requires
|
|
that a leader verify with a quorum of peers that it is still leader. This
|
|
introduces an additional round-trip to all server nodes. The trade-off is
|
|
increased latency due to an extra round trip. Most clients should not use this
|
|
unless they cannot tolerate a stale read.
|
|
|
|
- `stale` - This mode allows any server to service the read regardless of
|
|
whether it is the leader. This means reads can be arbitrarily stale; however,
|
|
results are generally consistent to within 50 milliseconds of the leader. The
|
|
trade-off is very fast and scalable reads with a higher likelihood of stale
|
|
values. Since this mode allows reads without a leader, a cluster that is
|
|
unavailable will still be able to respond to queries.
|
|
|
|
To switch these modes, either the `stale` or `consistent` query parameters
|
|
should be provided on requests. It is an error to provide both.
|
|
|
|
Note that some endpoints support a `cached` parameter which has some of the same
|
|
semantics as `stale` but different trade offs. This behaviour is described in
|
|
[Agent Caching](#agent-caching).
|
|
|
|
To support bounding the acceptable staleness of data, responses provide the
|
|
`X-Consul-LastContact` header containing the time in milliseconds that a server
|
|
was last contacted by the leader node. The `X-Consul-KnownLeader` header also
|
|
indicates if there is a known leader. These can be used by clients to gauge the
|
|
staleness of a result and take appropriate action.
|
|
|
|
## Agent Caching
|
|
|
|
Some read endpoints support agent caching. They are clearly marked in the
|
|
documentation. Agent caching can take two forms, [`simple`](#simple-caching) or
|
|
[`background refresh`](#blocking-refresh-caching) depending on the endpoint's
|
|
semantics. The documentation for each endpoint clearly identify which if any
|
|
form of caching is supported. The details for each are described below.
|
|
|
|
Where supported, caching can be enabled though the `?cached` parameter.
|
|
Combining `?cached` with `?consistent` is an error.
|
|
|
|
### Simple Caching
|
|
|
|
Endpoints supporting simple caching may return a result directly from the local
|
|
agent's cache without a round trip to the servers. By default the agent caches
|
|
results for a relatively long time (3 days) such that it can still return a
|
|
result even if the servers are unavailable for an extended period to enable
|
|
"fail static" semantics.
|
|
|
|
That means that with no other arguments, `?cached` queries might receive a
|
|
response which is days old. To request better freshness, the HTTP
|
|
`Cache-Control` header may be set with a directive like `max-age=<seconds>`. In
|
|
this case the agent will attempt to re-fetch the result from the servers if the
|
|
cached value is older than the given `max-age`. If the servers can't be reached
|
|
a 500 is returned as normal.
|
|
|
|
To allow clients to maintain fresh results in normal operation but allow stale
|
|
ones if the servers are unavailable, the `stale-if-error=<seconds>` directive
|
|
may be additionally provided in the `Cache-Control` header. This will return the
|
|
cached value anyway even it it's older than `max-age` (provided it's not older
|
|
than `stale-if-error`) rather than a 500. It must be provided along with a
|
|
`max-age` or `must-revalidate`. The `Age` response header, if larger than
|
|
`max-age` can be used to determine if the server was unreachable and a cached
|
|
version returned instead.
|
|
|
|
For example, assuming there is a cached response that is 65 seconds old, and
|
|
that the servers are currently unavailable, `Cache-Control: max-age=30` will
|
|
result in a 500 error, while `Cache-Control: max-age=30 stale-if-error=259200`
|
|
will result in the cached response being returned.
|
|
|
|
A request setting either `max-age=0` or `must-revalidate` directives will cause
|
|
the agent to always re-fetch the response from servers. Either can be combined
|
|
with `stale-if-error=<seconds>` to ensure fresh results when the servers are
|
|
available, but falling back to cached results if the request to the servers
|
|
fails.
|
|
|
|
Requests that do not use `?cached` currently bypass the cache entirely so the
|
|
cached response returned might be more stale than the last uncached response
|
|
returned on the same agent. If this causes problems, it is possible to make
|
|
requests using `?cached` and setting `Cache-Control: must-revalidate` to have
|
|
always-fresh results yet keeping the cache populated with the most recent
|
|
result.
|
|
|
|
In all cases the HTTP `X-Cache` header is always set in the response to either
|
|
`HIT` or `MISS` indicating whether the response was served from cache or not.
|
|
|
|
For cache hits, the HTTP `Age` header is always set in the response to indicate
|
|
how many seconds since that response was fetched from the servers.
|
|
|
|
### Background Refresh Caching
|
|
|
|
Endpoints supporting background refresh caching may return a result directly
|
|
from the local agent's cache without a round trip to the severs. The first fetch
|
|
that is a miss will cause an initial fetch from the servers, but will also
|
|
trigger the agent to begin a background blocking query that watches for any
|
|
changes to that result and updates the cached value if changes occur.
|
|
|
|
Following requests will _always_ be a cache hit until there has been no request
|
|
for the resource for the TTL (which is typically 3 days).
|
|
|
|
Clients can perform blocking queries against the local agent which will be
|
|
served from the cache. This allows multiple clients to watch the same resource
|
|
locally while only a single blocking watch for that resource will be made to the
|
|
servers from a given client agent.
|
|
|
|
HTTP `Cache-Control` headers are ignored in this mode since the cache is being
|
|
actively updated and has different semantics to a typical passive cache.
|
|
|
|
In all cases the HTTP `X-Cache` header is always set in the response to either
|
|
`HIT` or `MISS` indicating whether the response was served from cache or not.
|
|
|
|
For cache hits, the HTTP `Age` header is always set in the response to indicate
|
|
how many seconds since that response was fetched from the servers. As long as
|
|
the local agent has an active connection to the servers, the age will always be
|
|
`0` since the value is up-to-date. If the agent get's disconnected, the cached
|
|
result is still returned but with an `Age` that indicates how many seconds have
|
|
elapsed since the local agent got disconnected from the servers, during which
|
|
time updates to the result might have been missed.
|
|
|
|
## Formatted JSON Output
|
|
|
|
By default, the output of all HTTP API requests is minimized JSON. If the client
|
|
passes `pretty` on the query string, formatted JSON will be returned.
|
|
|
|
## HTTP Methods
|
|
|
|
Consul's API aims to be RESTful, although there are some exceptions. The API
|
|
responds to the standard HTTP verbs GET, PUT, and DELETE. Each API method will
|
|
clearly document the verb(s) it responds to and the generated response. The same
|
|
path with different verbs may trigger different behavior. For example:
|
|
|
|
```text
|
|
PUT /v1/kv/foo
|
|
GET /v1/kv/foo
|
|
```
|
|
|
|
Even though these share a path, the `PUT` operation creates a new key whereas
|
|
the `GET` operation reads an existing key.
|
|
|
|
Here is the same example using `curl`:
|
|
|
|
```shell
|
|
$ curl \
|
|
--request PUT \
|
|
--data 'hello consul' \
|
|
http://127.0.0.1:8500/v1/kv/foo
|
|
```
|
|
|
|
## Translated Addresses
|
|
|
|
Consul 0.7 added the ability to translate addresses in HTTP response based on
|
|
the configuration setting for
|
|
[`translate_wan_addrs`](/docs/agent/options.html#translate_wan_addrs). In order
|
|
to allow clients to know if address translation is in effect, the
|
|
`X-Consul-Translate-Addresses` header will be added if translation is enabled,
|
|
and will have a value of `true`. If translation is not enabled then this header
|
|
will not be present.
|
|
|
|
## UUID Format
|
|
|
|
UUID-format identifiers generated by the Consul API use the
|
|
[hashicorp/go-uuid](https://github.com/hashicorp/go-uuid) library.
|
|
|
|
These UUID-format strings are generated using high quality, purely random bytes.
|
|
It is not intended to be RFC compliant, merely to use a well-understood string
|
|
representation of a 128-bit value.
|