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| layout | page_title | sidebar_current | description |
|---|---|---|---|
| docs | Service Definition | docs-agent-services | One of the main goals of service discovery is to provide a catalog of available services. To that end, the agent provides a simple service definition format to declare the availability of a service and to potentially associate it with a health check. A health check is considered to be application level if it is associated with a service. A service is defined in a configuration file or added at runtime over the HTTP interface. |
Services
One of the main goals of service discovery is to provide a catalog of available services. To that end, the agent provides a simple service definition format to declare the availability of a service and to potentially associate it with a health check. A health check is considered to be application level if it is associated with a service. A service is defined in a configuration file or added at runtime over the HTTP interface.
Service Definition
A service definition that is a script looks like:
{
"service": {
"name": "redis",
"tags": ["primary"],
"address": "",
"port": 8000,
"enableTagOverride": false,
"checks": [
{
"script": "/usr/local/bin/check_redis.py",
"interval": "10s"
}
]
}
}
A service definition must include a name and may optionally provide an
id, tags, address, port, check, and enableTagOverride. The
id is set to the name if not provided. It is required that all
services have a unique ID per node, so if names might conflict then
unique IDs should be provided.
The tags property is a list of values that are opaque to Consul but
can be used to distinguish between primary or secondary nodes,
different versions, or any other service level labels.
The address field can be used to specify a service-specific IP address. By
default, the IP address of the agent is used, and this does not need to be provided.
The port field can be used as well to make a service-oriented architecture
simpler to configure; this way, the address and port of a service can
be discovered.
Services may also contain a token field to provide an ACL token. This token is
used for any interaction with the catalog for the service, including
anti-entropy syncs and deregistration.
A service can have an associated health check. This is a powerful feature as it allows a web balancer to gracefully remove failing nodes, a database to replace a failed secondary, etc. The health check is strongly integrated in the DNS interface as well. If a service is failing its health check or a node has any failing system-level check, the DNS interface will omit that node from any service query.
The check must be of the script, HTTP, TCP or TTL type. If it is a script type,
script and interval must be provided. If it is a HTTP type, http and
interval must be provided. If it is a TCP type, tcp and interval must be
provided. If it is a TTL type, then only ttl must be provided. The check name
is automatically generated as service:<service-id>. If there are multiple
service checks registered, the ID will be generated as
service:<service-id>:<num> where <num> is an incrementing number starting
from 1.
-> Note: There is more information about checks here.
The enableTagOverride can optionally be specified to disable the
anti-entropy feature for this service. If enableTagOverride is set to
TRUE then external agents can update this service in the
catalog and modify the tags. Subsequent
local sync operations by this agent will ignore the updated tags. For
example, if an external agent modified both the tags and the port for
this service and enableTagOverride was set to TRUE then after the next
sync cycle the service's port would revert to the original value but the
tags would maintain the updated value. As a counter example: If an
external agent modified both the tags and port for this service and
enableTagOverride was set to FALSE then after the next sync cycle the
service's port and the tags would revert to the original value and all
modifications would be lost.
It's important to note that this applies only to the locally registered
service. If you have multiple nodes all registering the same service
their enableTagOverride configuration and all other service
configuration items are independent of one another. Updating the tags
for the service registered on one node is independent of the same
service (by name) registered on another node. If enableTagOverride is
not specified the default value is false. See anti-entropy
syncs for more info.
To configure a service, either provide it as a -config-file option to
the agent or place it inside the -config-dir of the agent. The file
must end in the .json extension to be loaded by Consul. Check
definitions can be updated by sending a SIGHUP to the agent.
Alternatively, the service can be registered dynamically using the HTTP
API.
Multiple Service Definitions
Multiple services definitions can be provided at once using the plural
services key in your configuration file.
{
"services": [
{
"id": "red0",
"name": "redis",
"tags": [
"primary"
],
"address": "",
"port": 6000,
"checks": [
{
"script": "/bin/check_redis -p 6000",
"interval": "5s",
"ttl": "20s"
}
]
},
{
"id": "red1",
"name": "redis",
"tags": [
"delayed",
"secondary"
],
"address": "",
"port": 7000,
"checks": [
{
"script": "/bin/check_redis -p 7000",
"interval": "30s",
"ttl": "60s"
}
]
},
...
]
}
Service and Tag Names with DNS
Consul exposes service definitions and tags over the DNS interface. DNS queries have a strict set of allowed characters and a well-defined format that Consul cannot override. While it is possible to register services or tags with names that don't match the conventions, those services and tags will not be discoverable via the DNS interface. It is recommended to always use DNS-compliant service and tag names.
DNS-compliant service and tag names may contain any alpha-numeric characters, as well as dashes. Dots are not supported because Consul internally uses them to delimit service tags.